The role of zinc in forestry. I. Zinc in forest environments, ecosystems and tree nutrition

Forest Ecology and Management, 37 (1990) 167-205 167 Elsevier Science Publishers B.V., Amsterdam

The role of zinc in forestry. I. Zinc in forest environments, ecosystems and tree nutrition

R. Boardman and D.O. McGuire Forest Research Branch, Woods and Forests Department, G.P.O. Box 1604, Adelaide, S.A. 5001

(Australia)

ABSTRACT

Boardman, R. and McGuire, D.O., 1990. The role of zinc in forestry. I. Zinc in forest environments, ecosystems and free'nutrition. For. Ecol. Manage., 37:167-205.

The role of zinc and its importance in forestry is reviewed. The distribution of Zn in soils, rocks, the atmosphere and wastes from Society and the exposure of tree species and forests to Zn are summarized. Zinc is the 'heavy metal' found to occur in the greatest concentration in the majority of wastes arising in modem, industrialized communities. Chemical factors influencing the availability of zinc in the forest environment are related to forestry sites. The biological role of Zn in forest tree species and the biochemistry of zinc deficiency and toxicity are examined. The significance of Zn in plantations and natural ecosystems is discussed. Zinc features strongly in the enhanced levels of pollutants and the wastes being distributed in the environment. There is a growing incidence of plantation forests found in close proximity to urban and metropolitan areas as these spread into the coun- tryside. Recent developments have renewed interest in the management of Zn (and other heavy metals) through forestry and arboriculture. Forests and plantations under intensive management probably will be regarded as having relatively little "nature conservation' value but be suitable places for the disposal of Zn-enriched noxious wastes and sludges in rural locations. Effective management of these developments will require further research into the Zn economy of forest ecosystems.

I N T R O D U C T I O N

The importance of zinc in forestry is a result of its essentiality as a metab- olite for living organisms, its widespread occurrence at trace concentrations ( 30-100 mg kg- ~ ) in the earth's crust, and its relatively low geochemical mobility; different kinds of soil parent material can have characteristic Zn contents. Zinc has been used in increasing quantities as civilizations have developed over the last 2000 years. The many and widespread uses of Zn and its compounds at the present day means that, inevitably, they form a significant component of the wastes and emissions from advanced societies. They have a recognizable capacity to be pollutants in the environment, adding to their ubiquity in Nature (Adriano, 1986). Aerial spread of industrial emissions

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168 R. BOARDMAN AND D.O. McGUIRE

and a search to find innocuous ways to dispose of solid and liquid wastes from society has focussed attention on forests and plantations.

Zinc is an essential trace element for the healthy development of plants and animals. Although it has been well-established that the effect of individual trace nutrients in nature is never the result of a single element acting in iso- lation, and whilst Lepp ( 1981 ) argued that a review of a single trace element in plant systems is of limited value, from a practical point of view, primary causes can be related to plant disorders (Kolari, 1983; Raunemaa et al., 1983 ). This approach will be followed on the grounds that elemental interactions are still being discovered, and some scarcely understood. Only one review of zinc in forestry has been made since Stone ( 1968 ), by Parker and Parker ( 1986 ) who reviewed the literature published between 1965 and 1980 on boron, lead and zinc, with emphasis placed upon contamination from industrial sources. Direct reference to forestry in a broad sense was limited. Zinc was given only very brief mention by Turner and Lambert (1986).

The two most recent comprehensive reviews of zinc in soils and plant nutrition have been prepared by Lindsay ( 1972 ), solely agricultural in context, and Adriano ( 1986 ), similarly specialized but with a brief and selective cov- erage of natural ecosystems. Under these circumstances, a comprehensive review of zinc in plantations and forest ecosystems is timely.

Zinc will be discussed in both beneficial and noxious senses, and the emphasis will be placed on forestry and trees wherever possible. Special attention has been given to recent literature published between 1979 and 1986.

Information on zinc that has a bearing on forestry and tree growth is reviewed from four aspects. Bearing in mind that all metallic elements are potentially toxic to biological systems, these aspects cover the place of Zn in the forest environment, and the role of Zn in tree nutrition, (a) normally as found in healthy plants, (b) in deficiency and (c) with toxicity.

Forest management can suffer a number of constraints as a result of unre- solved nutritional disorders. These are not confined to additional costs or diminished economic returns. An appraisal of the effects of Zn deficiency on the management of a plantation forestry enterprise is described in the com- panion paper (Boardman and McGuire, 1990, this volume) from the experience of the Woods and Forests Department of South Australia over the past 100 years.

ZINC AND FOREST ENVIRONMENTS

Zinc in rocks The natural abundance of Zn in rock shows that magmatites are the pri-

mary global source of Zn which constitutes about 80 mg kg- 1. Sedimentary rocks contain varying amounts, depending on proportional increase or decrease in other components through weathering (Norrish, 1975). Hydroly-

ZINC IN FOREST ENVIRONMENTS, ECOSYSTEMS AND TREE NUTRITION 169

sates (shales) tend to show an increase and precipitates (carbonate and sulphate rocks) show substantial decreases. Aeolian deposits, especially of resistates, are likely to contain lower concentrations than deposits laid down in water. The low concentration found in sea-water, 5 ng Zn L - l , shows its poor geochemical mobility. This is a result of its low solubility and the wide range of 'fixation' processes which hold Zn in the terrestrial environment, which include adsorption and ion-exchange effects on minerals or organic matter, precipitation and co-precipitation, and fixation in biological and mineral substances formed de novo (Matthes, 1984).

Zinc in soils Recent trends in plantation forestry have included the re-afforestation of

agricultural land on economic grounds, and tree-planting for rehabilitation of industrially degraded lands. These developments mean that forest managers have an interest in the distribution of zinc outside existing forests.

The zinc cont¢nts of the great majority of 'naturally' occurring soils, are commonly between 30 and 100 mg Zn kg-l , (oven-dry weight). Exceptions occur which are rich in Zn, such as the calamine soils in Germany and Aus- tria, or peats reported from New York, U.S.A. (Stone, 1968; Allen, 1974).

Baxter et al (1983) have reported total Zn in topsoil in Colorado as 50.8 average, range 39.5-62.5 mg Zn kg-1. Beyer and Cromartie (1987) showed total Zn in topsoil as 44 mg Zn kg- l in a natural, unpolluted woodland. Fried- land et al. (1984) studied Zn in forests remote from emitters in Massachu- setts, Vermont and New Hampshire; they found that the highest concentrations were at depths between 20 and 40 mm at all three widely separated sites, and that the pattern in concentration down the profile was similar in each.

Zinc levels in native soil profiles are rather uniformly distributed, as with all cationic heavy metals which are also micronutrients, and reflect parent materials rather than relocation in pedogenesis (Ellis et al., 1984). However, variations on this general condition have been related to site history of disturbance (Adriano, 1986).

Tiller (1983) included Zn in a review of micronutrients in soils and has assembled a large amount of previously unpublished data relating to Austra- lian surface soils, both in total concentrations and 'extracted' concentrations (using 0.1 M EDTA, pH6 ). Australian soils have been arranged by Great Soil Group, and the remarkably low concentrations found in terra rossa, lateritic podzolic soils, podzolised sands and podzols is apparent. Minimum concentration in five of the Groups is below 0.05 mg Zn kg -1. Most soils from these five groups have been cleared of forest for agricultural use. A number have either been converted to exotic forest plantations, or have reverted to them after agriculture, on economic grounds (Fig. l ).

Comprehensive surveys of soils made in western Europe have often reported 'available Zn' rather than total contents, for a large number of samples


I

ZINC DEFICIENCY IN AUSTRALIA 0

O

m O

/ . m O

m FOREST

]SAVANAH WOODLAND ~

[~.ALLEEWOODLAHO I ~ I ~ DRaGAL~ F~'~EST t~ ~

" • AREAS OF HEATH

FOREST VEGETATION OF AUSTRALIA (Simplified from Specht, 1980)

Fig. 1. The distribution of (a) zinc deficiency in forest and agricultural crops (above), and (b) the distribution of natural forest and woodland formations in Australia (below). (a, after Don- ald and Prescott ( 1975 ); b, after Hallsworth ( 1975 ); redrawn from Specht (1980).


representative of soils in each country. Readers are referred to the original papers for the method of extraction used, as there is no agreed definition of the term 'available Zn' (Beckwith, 1963; Carroll and Loneragan, 1968; Adri- ano, 1986).

A survey of England and Wales showed that 90% of the samples contained < 15 mg Zn kg -1, whereas 94% of Irish samples contained less than this amount. (Dickson and Stevens, 1983). As many as 54% of the Irish samples had <5 mg Zn kg -1. Berrow and Reaves (1983) reported on 1000 Scottish soils; they put 'typical' total content of topsoil at 40 mg Zn kg-1 (air_dry soil ) and considered this as "background in uncontaminated soils". Aichberger et al. ( 1982 ) examined a wide range of soils in Austria and reported limits of 32-220 mg Zn kg- 1, total zinc in topsoil. Lux ( 1982 ) surveyed soils on a grid at various distances south-east of metropolitan Hamburg, West Germany, to check on the effect of city emissions; he found 13-222 mg Zn kg- ~, and that content was correlated with land-use. In contrast, Kick et al. (1980), also in West Germany, had found that Zn contents were more distinctly related to geological origin than to rural management practices, including fertilizer applications.

Relatively undisturbed prairie soils have usually shown a slight gradient of decreasing Zn content as the surface is approached, by a factor of about 0.5- fold compared with the lower B horizon (Baxter et al., 1983). Tilled and cropped soils, when over-exploited, have depleted Zn concentrations in top- soils (Berrow and Mitchell, 1980). Markedly higher Zn content found in the surface soil of undisturbed forest has been attributed to retention near the surface of Zn returned in litterfall (Hibbard, 1940). The study of Kazda and Glatzel ( 1985 ), in beech, showed that the high concentration of zinc found at the base of the trees, 250-500 mg Zn kg -1, total, (soil) was due to the transfer of Zn from the crowns through stemflow. Stemflow has been found to be only 3-6% of precipitation reaching the forest floor (Penman, 1963), but in polluted atmospheres its impact on the accumulation of zinc can be drastic.

Circumstantial evidence showed that that native pecan (Carya illinoiensis (Wangenh.) C. Koch) in woodland adjacent to cultivated pecan orchards, had no symptoms of the disorder which was present - and corrected by Zn sprays - in the orchards established on cleared ground (Finch and Kinnison, 1933). In Western Australia, Hearman (1938) recorded that a radiata pine disorder corrected by application of Zn on sites cleared of native tuart forest (Eucalyptus gomphocephala DC. ), occurred much less severely in radiata pine which had been planted under tuart trees.

Tiller (1957, 1958) found little or no accumulation of zinc under radiata pine plantations at Mr. Burr, South Australia, when he examined four con- trasting soils; two podzolized, aeolian, deep sands overlying buried, ancient basaltic soils; a soil formed on exposed olivine basalt; and a dual basaltic soil


with a modern soil overlying an ancient soil on basaltic tuff. Each of these four soils had been cleared of native forest 30 years before afforestation with pine.

The main factor that influenced geochemical distribution of 18 elements detected in these profiles was attributed to their translocation by plant growth and their relative mobility on weathering. Zinc was not listed among ten elements that had accumulated in the surface horizon.

Both the South- and Western-Australian native forests have been prone to bushfires. Frequent forest fires could be expected to volatilize much of the Zn likely to be accumulated in the organic matter of sub-humid forest areas. For- est clearing, followed by debris burning, the usual practice in these two Aus- tralian examples, would be more severe than the majority of bushfires.

Neither Kessell and Stoate (1938), nor Stoate (1950) recorded soil Zn contents in Western-Australian soils prone to the 'dieback' disorder corrected by applications of zinc, probably because chemical analysis was too insensi- tive at that time. More recently McGrath and Robson (1984a) used topsoil (0-15 cm) from sites at which Zn deficiencies had been established. They recorded low levels of 'available' Zn as < 0.2 mg kg-1 (air-dry soil) extractable with DTPA. Wallace et al. ( 1986 ) examined growth ofjarrah (Eucalyp- tus marginata Donn ex Sm. ) in a 'known Zn-deficient soil'; they did not an- alyse this soil but simply added several amounts of zinc sulphate to it. It can inferred by extrapolation that the min imum concentration of Zn available in 84 days was not less than 0.003 mg kg-1 (air-dry soil), and that total Zn was of the order of 0.03 mg kg- 1. These concentrations reported from Western Australia are remarkably low.

Zinc in wastes from human societies Heavy metals in liquid and solid wastes may become concentrated in sludge

by sewage treatments or in composted residues. Zinc has commonly been the most abundant heavy metal in municipal wastes, composts and sludges reported from a number of countries. Zinc concentration in sludges from a group of US cities with a range of population from 10 000 to 2.2 millions was, on average, highest of all metals present, at 2140 mg kg- ~ dry residue (Mclntyre et al., 1977 ). Most urban-derived sludges reported have been within 1000 mg Zn kg-1 of this level, (Baxter et al., 1983; Abuzkhar et al., 1987 ). Maximum concentrations were five-fold the average.

Zinc in the atmosphere Zinc is a normal component of the atmosphere. The amount detected at

any place depends upon the distance from emittant sources (many of which are removing wastes), prevailing winds and plant cover. The greater leaf area per unit land area and the arborescent form of forest and woodland, compared with pasture of perennial vegetation of low stature, cause increases in


loading per unit area of land. The large differences found have great significance for the management of Zn, the supply of which may also be augmented with waste materials applied to the ground.

The concentrations in the air at seven sites widely spread over Great Brit- ain showed a monthly average of 220 ng Zn kg - l , dry air, and a range of 64- 415 ng kg -1 . The concentration in rain, year average, was 85 ng Zn L- i. Total, deposition was estimated at 10.0 mg m -2, of which dry deposition was only 5%; thus deposition amounted to about 100 g Zn ha- l month- l (Pierson et al., 1973).

The amounts of atmospheric Zn received in climates able to sustain natural forest range from 1.5 kg Zn ha- l year- l in remote rural areas to around 2.7 kg Zn ha-1 year-1 near heavy industrial centres (Mayer, 1983). Open pasture-land may carry a residual of only 0.2 kg Zn ha-~ year- l (Pierson et al., 1973; Scheffer and Schachtshabel, 1976 ).

The fact that interception depends on the 'roughness' of the vegetation has been used by Schlesinger and Reiners (1974) in designing collectors which they describe as "plastic artificial foliage which was selected to structurally resemble balsam fir" (Abies balsamea (L.) Mill.). They found that water catchment averaged 4.5 times greater from foliar collectors than open-mouth 'rainout' collectors. Whilst they do not report any analyses for Zn foliar collectors gathered between 4.9 and 8.3 times as much of five other elements. Comparison of their results for these elements with the results which included zinc (Pierson et al., 1973 ), suggest that interception of Zn by trees would also be of the order 4-6-fold. The ominous effect of Zn in stemflow on soil reaction, discussed below, has its origin in enhanced interception by tree crowns (Kazda and Glatzel, 1984).

Aerial deposition in forests The situation that has developed in forests of central and western Europe,

exposed to high levels of atmospheric pollution, has become the subject of intense research [ Mayer and Heinrichs ( 1980 ); Mayer et al. ( 1980 ); Hofken ( 1983 ); and Ulrich ( 1983 ) for the Soiling Project; Truby ( 1981 ); and Truby and Zottl (1984) for the Rhine Plain and the Black Forest; Mayer ( 1984 ) for Kassel; Kreutzer et al. ( 1983 ) and Zech and Popp ( 1983 ) for Bavaria; Kazda and Glatzel (1984) for Austria; Blume (1981) and Kowalkowski and Szczubialka ( 1981 ) for the Baltic seaboard/P~land; and Hutton (1984) in Great Britain ]. The species investigated have been principally beech (Fagus sylvatica L. ), spruce (Picea abies (L.) Karst. ) and pine (Pinus silvestris L.), and most occur in plantations. Similar studies reported from North America have supported these findings (Harbottle et al., 1982; Miller et al., 1983a,b; Sopper and Seaker, 1983).

Miller et al., (1983a,b) investigated zinc accumulation in oak forests subject to aerial deposition in industrial north-east Indiana. They found that the


uppermost 50 m m of soil was enriched compared to remote forest soils. The top 25 m m contained 1.0 g kg- l total Zn, but adsorption experiments showed that this forest soil could accept as much Zn again before leaching occurred. Tyler (1975a,b) found that soil nitrogen mineralization was impaired by industrial Zn pollution, and concluded that a 3-fold increase over 'background' level, about 120 mg Zn kg- 1, is sufficient to bring about a measurable disturbance in the rate of mineralization in forest soils.

The zinc economy of forests in these reports can be summarized as follows: (a) Aerial Zn input into forest systems is relatively high, accumulating in

crowns and descending in litterfall and stemflow. (b) Precipitation may contain up to l 0 mg Zn kL- 1. (c) Zinc is accumulated in the top-soil to a small extent or is shown to be

in balance with output. (d) Some of the output is absorbed by trees and some is leached; overall,

trace-metal reservoirs are decreasing in most sites at the present time. (e) Zinc is eluviated in soluble form from the A horizon, but tends to be-

come bound in the B horizon; losses detected in deep seepage and groundwater appear to have arisen from mineralized forms of Zn under weathering processes.

(f) Rates of cycling and fluxes through the forest canopy-litter-soil system are determined by both the amount of precipitation (especially 'throughfall' ) and the atmospheric load of Zn.

(g) The balance of bound to mobile Zn compounds in the soil has only a small influence on the rate of cycling.

(h) The study in the Vienna Woods indicates that the effects produced can be very marked but are related to tree distribution; zinc concentration around trunks was 250-500 mg kg-~ of soil and pH3, compared with one-tenth of this amount and pH5 in intervening spaces, which is typical of beech forest.

(i) Copper, zinc and lead concentrations in some top-soils are apparently reaching levels which affect micro-organism activity; in turn, this will reduce rates of litter breakdown and impair nitrogen mineralization.

Z I N C IN N U T R I T I O N

The importance of Zn in nutrit ion will be discussed in four parts: its role as an essential trace element; concentrations which are found in normal, healthy plants and animals; and deficiency and toxicity with the symptoms of disorder they produce in plants and the effects upon metabolic activity. Trace- element concentrations are notably absent from the bulk of forestry literature on fertilizer responses prior to 1975, and many recent biomass and related soil studies have included only the macroelements. A few reports have examined trace-metal abundance in forest ecosystems subject to atmospheric


pollution, and have compared these sites with others more remote (Hall et al., 1975; Mart inet al., 1975).

The role of zinc in nutrition An element is considered 'essential' if its deficiency consistently results in

impairment of plant or animal function from optimal to sub-optimal (Un- derwood, 1975 ). The functions of essential metals in cells are catalytic, generally, and enzymic catalysis is the only rational explanation of how a trace of some substance can produce such profound biological effects. In all cases, the metal must be present in the cell at appropriate concentrations in order to produce the amount of enzymatic activity that fully expresses control of the genetic potential of the cell. Different enzymes have shown differing affinities for the metal available and so a deficiency of the metal will affect the activity of some enzymes more than others (Smith and Gawthorne, 1975). In some enzymes the metal is critical to the catalytic step while in the remainder it appears to maintain the structure of the protein. The trace elements as such are not functionally related to each other and despite close similarity physico- chemically, they are quite dissimilar physiologically (Underwood, 1975 ).

The several roles of Zn as a micronutrient in plant metabolism have been shown by Kolari ( 1983, fig. 1 ). At the same time, knowledge of the functional role of Zn as a component of an enzyme is incomplete (Adriano, 1986). The essentiality of trace elements as components of enzymes has been known for several decades, but it has been appreciated only recently that an essential element may be replaced by other dissimilar elements. Zinc can be replaced by cobalt in carboxypeptidase, causing an increase in peptidase activity. In- teractions involving trace-element substitution in metallo-enzymes are still poorly understood (Suttle, 1975). Although Zn is a component of carbonic anhydrase and is found in chloroplasts, Nicholas ( 1975 ) commented that its function there was not clear. Boardman ( 1975 ) doubted whether this enzyme is essential for photosynthesis. The recent discovery by Van Assche and Clijs- ters (1983) that there is evidence that Zn toxicity impairs photosynthetic transport activity does not contradict Boardman's conclusion.

The primary role of Zn is involvment in fundamental processes of cell replication and gene expression associated with nucleic acid and protein metabolism. Zinc has an essential role in processes leading to DNA synthesis, and DNA polymerase contains firmly bound zinc. These roles help to explain the nature of some of the symptoms characteristic of Zn deficiency in trees, for example, 'rosetting' and shoot tip die-back (Underwood, 1975 ).

Normal concentrations in plants The concentrations of Zn reported in plants are predominantly of 'total

zinc' found in the currently-living parts (Table 1 ). The majority of both primitive and higher plants contain very similar concentrations in assimilat-


TABLE l

The range of concentrations (mg kg - j ) in plants commonly found growing in forest and woodland remote from towns (Allen et al., 1974; Martin et al., 1975 )

Life form Common Unusual

Bryophytes (mosses, etc. ) 20-45 Pteridophytes (ferns) 30-40 Woodland herbs 20-60 Grasses & Monocots 10-60 Low shrubs 15-45 Trees/Shrubs < 5m 15-40 Conifers 15-65 Broadleaftrees > 5m 15-50 Lichens 150

65- l 15 (acid 'damp oakwood' ) 60- 95 (acid soils & heath)

65-200 (acid, damp woodland)

ing tissues, which reflects the several roles of Zn they share in common (Ad- riano, 1986 ).

Stone ( 1968 ) published a comprehensive list of foliage concentrations reported for tree species, most of which have some commercial or forestry significance. He noted that a 'normal range' for most species is between 15 and 125 mg Zn kg -1 (Stone, 1968, table 10). Only a few tree species have been examined across a wide geographical range as well as over a wide range of site types and productivity classes: Pinus radiata D. Don, (Raupach, 1975); Pseudotsuga menziesii (Mirb.) Franco, (Zinke and Strangenberger, 1979; Ballard and Carter, 1985; Carter et al., 1986 ); Pinus ponderosa Dougl. (Zinke and Stangenberger, 1979).

Raupach (1975) summarized the results of foliar analyses of standard samples for radiata pine growing in southern Australia. Zinc concentrations ranged from 5 to 79 mg Zn kg-l; less than 2% were below 10 mg Zn kg -1, whilst 86% were less than 50 mg Zn kg- 1. In the set collected in South Aus- tralia, which included a representative collection of all seven productivity classes and included a balanced, representative set of nutritional disorders, the range of concentrations found was from 11 to 46 mg Zn kg- 1. Two samples, which had not received routine application of Zn fertilizer, contained the least, l 1 and 12 mg Zn kg-J. Will (1985) noted that most radiata pine forests in New Zealand have concentrations of 30-50 mg Zn kg- 1. Zinke and Stangenberger (1979) have taken nutrient-element content and weights of foliage from two conifer species from sites throughout Washington, Oregon and California, U.S.A. and have produced cumulative probability tables for the concentrations of each element. They have done this for both the current season's needles and for second-year foliage. Data has been arranged in per- centile classes based on Weibull cumulative density functions (Table 2 ).

Grey et al. ( 1979 ) reported Zn concentrations in Pinus patula Schlectend. Cham. in a regional survey of the Transkei, South Africa, as 29.4 average, 16-


TABLE 2

Cumulative probability and Zn concentration (mg kg-t ) in two conifer species of the Pacific Coastal Regions of the U.S.A. (Zinke and Stangenberger, 1979)

1% 30% 50% 70% 99%

Ponderosa pine < 1-year 9 15 23 37 148

2-year 6 17 27 39 114 Douglas fir < 1-year 4 10 24 40 148

2-year 1 8 26 45 186

45 mg Zn kg -~. Grey (1988) has recently reported upon Pinus radiata, P. pinaster Air. and P. elliottii Engelm. in the Southern Cape for which the ranges were 3-64, 4-61 and 6-68 mg Zn kg- ~, respectively.

Changes in foliar concentration of Zn over the course of a year have been reported by Forrest ( 1969 ) and Knight ( 1978 ) for radiata pine. These studies are particularly interesting because a number of varieties were examined. Departures from the average over all dates were less than 15%, with no clear seasonal trend. Forrest ( 1969 ) examined three consecutive years on the same calender dates. Two of the years showed similar variations to those reported by Knight ( 1978 ) from New Zealand; in the third year, which was subject to severe drought, there was a decline in concentration of one-third. MacLean and Robertson ( 1981 ) examined the effect of age and seasonal changes on the distribution of zinc in crowns of Picea rubens Sarg.

Large and consistent differences in clones and varieties have been indicated in a wide range of crop plants (Adriano, 1986). Considerable genetic variation in the foliar concentrations of specific nutrient elements have been recorded in radiata pine despite its narrow natural range and the limited genetic source of seed collected for use in the Antipodes until recently (Simp- fendorfer, 1966). Knight noted that the fraction of total variance accounted for by clones with zinc was 37%, a very highly significant result. Clonal re- peatability for zinc at a given sampling date was classed as ' intermediate' at 0.39. Tree-to-tree coefficient of variation was 0.25 for Zn, which indicated that 15-20 sample trees would be needed to establish a difference of 20% between means of two populations with a probability of P < 0.05. Forrest and Ovington ( 1971 ) found similar results with clones in Australia.

Several species of forest trees have been surveyed in the natural range of the Pacific North-West of the U.S.A. and Canada. In 1980, Radwan and DeBell reported zinc concentrations for Tsuga heterophylla (Raf.) Sarg. from eight sites, four in the coastal zone and four from the Cascade Range. The average concentrations of Zn in the two sets were 18.6 and 18.8 mg Zn kg -~, respectively. The range in average concentration across sites was narrow, 15.9-20.3


mg Zn kg- ~. Recently, a broad regional study has been reported by Radwan and Harrington ( 1986 ) on Thuja plicata J. Donn ex D. Don, a species widely associated with hemlock. In the coastal zone, eight sites had an average 20 mg Zn kg- ~ range, 13-26; from the interior zone, eleven sites showed an average 29 mg Zn kg- 1 range 21-48. Alnus rubra Bong., a hardwood species from the Pacific Northwest, was examined by DeBell and Radwan (1984) across a range of stand ages from 6 to 45 years; this showed foliage with an average of of 34 mg Zn kg- l, and no discernable trend was associated with increasing age. Morrison ( 1985 ) studied two hardwood species growing in mixed closed forest on the same soils, sugar maple (Acer saccharum Marsh) and yellow birch (Betula alleghaniensis Britt. ) in Ontario, Canada. Maple showed a range from 24 to 30 mg Zn kg -~ and birch, much greater, 291-388 mg Zn kg -~. A decline of 25-33% in level was apparent from top to bot tom of the crowns.

Raupach et al. (1972) examined the different ages of needles at several distances along the crown from the tip, in radiata pine. The Woods and For- ests Department, South Australia, data for radiata pine (unpublished data, 1987 ) illustrate the general pattern of Zn concentration in foliage by needle age and position in the crown (Table 3 ). Zinc showed consistency that was more stable than for any major nutrient. Translocation from one-year old needles was apparent. Annual variation in rainfall produced small fluctua- tions in concentration.

McGrath and Robson (1984a), who examined concentrations and translocation of Zn within seedling radiata pines, found that concentrations remain stable within one leaf until close to the end of the growing-season when some translocation appeared to take place. Two-year-old foliage showed concentrations of a lower level which also tended to remain stable for most of the growing-season. Lamb ( 1976 ), with Eucalyptus deglupta Blume, and Wallace et al. (1986) with seedlings of Eucalyptus marginata, recorded zinc concentrations at different positions along the shoot from the apex. They found sta- bility in concentration within leaves and, apart from near the apex, consistent

TABLE 3

Zinc concentrations ~ (mg kg -~ ) in the crowns of radiata pine in the south-east of South Australia, site quality I-IV plantations

Crown Needle age class (years) position

Current 1-2 2-3 3-4 Dead on tree

Top 44 Down 1/3 36 28 27 Down 2/3 29 26 23 19 20

~Overall averages for the whole crown have been 30 mg Zn kg ~ for the years 1980-1987, and have been in the range 25 to 40 mg Zn kg- ~.


levels in successive leaves. Both studies found highest concentrations at the shoot apices in trees adequately supplied with Zn.

These results are representative of Zn concentrations of leaves in proximity to where active growth is taking place. The narrow limits recorded consistently, and their similarity with woodland plants in general (Table 1 ), suggest that a similar set of roles is played by Zn in both short- and long-lived perennial plants.

Frederick et al. (1985a,b,c) have reported Zn from New Zealand in plantation-grown Eucalyptus regnans F.J. Muell. aged from 4 to 17 years, and Eu- calyptus saligna sm. and Acacia dealbata Link., both at age 8. Eucalypt and wattle foliage concentrations were found to be about 25-35% that of radiata pine grown on the same soils. The mountain ash (E. regnans) plantations from New Zealand had low concentrations of Zn in foliage, comparable with the lower critical concentration found in jarrah (E. marginata) by Wallace et al., (1986). Crop uptake of zinc per unit area varied between sites, but the mountain-ash stands aged 7 and l0 years were comparable with 8-year-old radiata pine. Expressed as average annual demand for zinc (g tree- 1 year- 1 ) individual requirements were found to be: E. regnans, 0.18 g, avg (range 0.07- 0.34 ); E. saligna, 0.19; A. dealbata much less demanding at 0.08; and, radiata pine, 0.20. Forrest (1969) estimated the net annual accumulation of 0.13 g year- ~ for radiata pines soon after canopy closure in New South Wales, Aus- tralia. About 40% of this amount was deposited in needle litter-faU each year.

Natural high affinity for zinc The general situation is confounded by the existence of so-called 'zinc ac-

cumulator' species, mostly confined to a restricted number of higher plant families or genera (Stone, 1968) but also found recently to occur in some invertebrates (Beyer and Cromartie, 1987 ). These species can accumulate up to 1500 mg Zn kg- ~ without apparent harm.

Selectivity in absorption and accumulation of mineral elements continues to be one of the least understood aspects of plant nutrition. The selective uptake of trace elements and the selectivity shown by strains, varieties or inbreds within particular crop and agronomic species has been receiving attention (Gerloff et al., 1966). Attempts to explain the mechanisms responsible for selective uptake has produced the concept of carriers with varying affinities for the elements so accumulated (Epstein and Jefferies, 1964 ). Malic acid has been the only carrier identified (Ernst, 1975 ). Gerloff et al. (1966) investigated the mineral content in tree species growing naturally at a site in Wiscon- sin. They found that an unusually high or low concentration of one element in a species was not necessarily associated with correspondingly high or low content of other elements. They paid particular attention to the selective absorption of Zn.

The mechanism which stores the considerable quantity of extra Zn above


normal concentrations has not been reported. Schmid et al. (1965; cited by Loneragan 1975 ) who observed that a high proportion of Zn entering roots is adsorbed on cell surfaces, and that when this is taken into account absorption of Zn 2+ was shown to proceed via a metabolically-mediated pathway. This was sensitive to metabolic poisons. It is possible that the reverse situation occurs in the leaves, and excess Zn is once again adsorbed on to cell surfaces.

Gerloff et al. ( 1966 ) studied the selective absorption of Zn in native species growing on a forest site in Wisconsin, U.S.A. Six of the species were Zn- accumulator plants and contained 3-6 X more than in other species. A further six species contained individuals with both 'normal ' and markedly enhanced concentrations. Extending the survey revealed Zn accumulation in two tree genera in the Salicaceae, and Betula in the Corylaceae. Thirteen species of four other genera of the Corylaceae did not show any indication of an unusual capacity to accumulate Zn.

A majority of the 'accumulator species' recognized grow naturally on acid forest soils - e.g. Betula spp. and Taxus spp. - or in 'damp woods' situated on acid clay soils - e.g., Ilex spp. and Corylus spp. In the case of the Salicaeae, however, Salix and Populus spp., are favoured by moist soils with aerated, flowing water available to them for most of the growing-season. Healthy po- plars and Lagunaria patersonia (Andr.) G. Don, Norfolk Island hibiscus (Malvaceae), growing within the atmospheric deposition zone of the zinc smelters at Port Pirie, South Australia, have been found to contain up to 1000 mg Zn kg- ~ compared with levels of 25-40 mg Zn kg- 1 in foliage from central metropolitan Adelaide, the State capital (Woods and Forests Department, South Australia, unpublished data, 1980).

Zinc concentration factors exhibited by plant life-forms which grow in water cab be quite prodigious, despite the low concentration of zinc in sea, lake and river water (Bortels, 1927; Bowen, 1966 ); see Table 4).

The affinity of fresh-water plants for zinc is only exceeded by that of mer- cury [Hg 2+ ] among divalent cations. On the other hand, the affinity for Zn of terrestrial plants among divalent cations is exceeded only by manganese and copper (Bowen, 1966 ). These data indicate that almost all Zn in a form available to plants is likely to be absorbed by them. Zinc is likely to be ab-

TABLE 4

Concentration factors for Zn in aquatic species

Species Concentrations

Plankton - , range 1000-65 000-fold j Brown algae avg. 3400, range 100-13 000-fold 32 freshwater spp. avg. 4600-fold

~Diatoms may accumulate up to 2400 mg Zn kg- i.

ZINC IN FOREST ENVIRONMENTS, ECOSYSTEMS AND TREE NUTRITION

TABLE 5

Zinc concentrations in woodland animals

181

Species Concentrations (mg kg a ~ oven-dry wt )

Vertebrates 110- 180 (herbivores and omnivores) Invertebrates 130- 150 Earthworms ~ 120- 650 avg. 347

200- 950 avg. 523 320-1600 avg. 776

(natural, unpolluted sites) (mining spoil ) (heavy industrial sites)

~Beyer and Cromartie ( 1987 ).

sorbed in greater amounts from waste materials than other divalent cations because it tends to occur in greater amounts.

Woodland animals show some degree of concentration in the food chain (Table 5, Allen, 1974). The soil concentrations of total Zn corresponding to the three types of site with earthworms were 67% < 30, 50% < 150 and 86% < 150 mg Zn kg-l (oven-dry weight) of soil, respectively. By way of comparison, maximum tolerable dietary levels for domestic animals consumed for food have been set by the U.S. National Research Council (Anonymous, 1980) at 500 mg Zn kg- ~.

ZINC DEFICIENCY

Factors associated with zinc deficiency Environmental factors have been found to affect manifestation of defi-

ciency symptoms; these have usually been more severe in high light intensities than in partial shade; more acute in summer than in winter - first discovered in sub-humid areas having warm temperate Mediterranean-type climate or sub-tropical climate. Zinc deficiency, until recently, had been rarely identified in middle to high latitudes (40-60 ° ) in either hemisphere (Nicholas and Egan, 1975 ).

Common factors which have induced Zn deficiency have been: total soil poverty of zinc; low amounts of soluble Zn in the soil solution; high rainfall; calcareous soils; low soil organic matter content; high soil available-phosphate levels; liberal nitrogen fertilizer applications; high levels of mineraliza- ble soil nitrogen; improved pastures; restriction of root expansion; soil deficiency; and low soil temperatures. Zinc deficiency can be induced by increasing tree growth rates on soils marginal for Zn.

Development of zinc deficiency symptoms Zinc deficiency in radiata pine and maritime pine (Pinus pinaster) was

discovered first in Western Australia. Smith and Bayliss ( 1942 ) were the first to study how Zn deficiency symptoms developed in conifers under controlled


conditions and deliberate omission of Zn. First symptoms appeared in less than two months; two weeks later tops began to turn yellow, older needles changed colour gradually, and subsequently some turned bronze. In some, the shoot tip died, soon followed by the whole plant. The remainder continued to live with occasional periods of growth, each of which resulted in a rosette of closely packed, stunted, yellow primary needles. Very few secondary needle fascicles developed. Similar results were found by McGrath and Robson ( 1984b ), whose seedlings were living solely on Zn contained in the seeds.

Field symptoms of the deficiency syndrome have been described compre- hensively by several authors in radiata and Mediterranean pines (Stoate, 1950) and Stone ( 1968 ), who has reviewed many tree species of commercial importance, both broadleaves and conifers.

Recent reports of the diagnosis of Zn deficiency in forests have come from Pinus caribaea Morelet, (Rance et al., 1982) from far-north, tropical Aus- tralia, and Lombin ( 1983 ) has recorded it from tropical savanna in Nigeria. The first report of a district prone to Zn deficiency in radiata pine from New Zealand has been published by Hunter and Skinner (1986). It has occurred on the marginal, podzolic soils in North Island, not afforested earlier because of impeded drainage and low natural fertility. Zech and Popp (1983 ) have reported instances of zinc deficiency which are a recent development in high- altitude spruce (Picea abies) in southwestern Germany. These soils are naturally low in Zn, but the 'acid deposition' phenomenon has increased the acidity of these naturally acid soils, causing leaching and exhaustion of meagre supplies available.

In Douglas fir (Pseudotsuga menziessi) in Washington, Oregon and British Columbia, Carter et al. ( 1986 ), who augmented the regional survey of Zinke and Stangenberger (1979), found that, at much higher latitudes than previously suspected, there was a strong relationship with precipitation during the growing-season. This suggested to them that deficiencies of Zn may be acute rather than chronic, the periodic appearance of acute deficiencies being influenced by soil moisture supply - a point not made by other workers. The increase in osmotic pressure in Zn-deficient leaves with impaired auxin content has been noticed above.

Lower critical levels (poverty~deficiency) Remarkably, as late as 1984, McGrath and Robson were able to point out

that no previous attempt had been made to objectively determine the critical concentration (s) at which deficiency began to manifest itself. They did this for radiata pine (1984b) and established a concentration of 11-12 mg Zn kg- ~ (oven-dry weight ) of apical primary leaves. They adopted the definition of critically devised by Ulrich ( 1952 ): "that nutrient concentration which is just deficient for maximum growth". They used the intersection point of a segmented linear regression fitted to the data, using a method of Hudson


( 1966 ), and also determined the critical concentration for whole seedlings tops, which was similar, but the regression accounted for much less of the variance (63% vs. 85%).

Previously, a lower critical concentration had been suggested empirically as being 10 mg Zn kg -~ (reported by Raupach, 1975). This applied to the results of analysis of fascicle or secondary needles of the uppermost, i.e. youngest, first-order lateral branch bearing fully expanded needles on the leading shoot of plantation trees. This 'standard', obtained from current-season's growth was considered to be applicable to trees over a wide range of ages and usable for diagnostic purposes. It has not been disputed in practice.

Ballard and Carter (1985 ), working in North America, have set tentative critical levels for Douglas fir as lying between 12 and 15 mg Zn kg-~ (oven- dry weight), 'slight'; 9-12 mg Zn kg-l 'moderate'; and < 9 mg Zn kg -~, 'severe' deficiency. However, Carter et al. ( 1986 ) were only able to detect symptoms of disorder for the trees having concentrations < 9 mg Zn kg- ~.

Wallace et al. (1986) published a similar study on an important commercial broadleaf species from Western Australia, jarrah (Eucalyptus marginata). These workers found a critical level of between 10 and 12 mg Zn kg- l (oven-dry weight) at the main shoot apex.

Stone summarised many published results to 1967, but he was unable to find more than a few reports of deficiency concentrations ( 1968, table 10); consequently, he did not attempt a general definition of lower criticality.

Bar-Akiva and Lavon (1969) found that carbonic anhydrase activity was a good indicator of Zn deficiency in Citrus. Improvements in analytical tech- nology are likely to encourage other biochemical tests of criticality. However, there have been objections to this approach. It is not always the enzymes that are most depleted in measurable activity during trace-metal deficiency that have limited and disrupted the processes of normal metabolism because many enzymes are present at activities seemingly in excess of normal needs (Smith and Gawthorne, 1975 ).

Interaction with other plant nutrients in deficiency conditions Interactions between trace and major elements in nutrition have become

better known and of greater significance than many direct effects of individual elements. In the sphere of deficiency, phosphate has been found to lower the availability of Zn probably through the formation of insoluble metal phosphates (Collins, 1981 ). Stone (1968) commented on the consequences of continued heavy application of phosphatic fertilizers to trees and Zn availability in sandy, low phosphate-fixing soils. He noticed that zinc-phosphate interactions at the root surface or within the growing tree were more consequential.

McGrath and Robson (1984a) examined Zn nutrition ofradiata pine seedlings grown in a Zn-deficient soil with and without additions of Zn P and N.


Applying Zn to the seedlings increased the Zn content of shoots at all levels of P and N supply. Results are consistent with the theory that P induced Zn deficiency by diluting whatever Zn was present within the seedlings. Appli- cation of N and P without extra Zn produced additional growth but no increase in Zn content of shoots. This suggests that the primary cause of Zn deficiency was dilution in seedlings due to growth. Subsequently, on soils deficient in Zn, the Zn absorbed was diluted to concentrations below that re- quired for optimum growth through negative feedback resulting from growth itself.

Wallace et al. (1986), in their study of the Zn nutrition ofjarrah, also examined Zn /P interactions. Phosphorus concentration in the stem apex was not affected, remaining at about 2800 mg P kg-~ (oven-dry weight) for no added Zn, and at all rates up to 4.00 mg Zn kg-~ of soil. Seedlings grown in Zn-deficient soil without added Zn had P concentrations increased to < 11 000 mg P kg-~ in the second leaf-pair down the stem. Concentration did not approach 2800 mg P kg- ~ until 0.40 mg Zn kg- 1 of soil had been received, i.e. an adequate supply. High P concentrations, in both shoots and individual leaves, were symptoms of Zn deficiency in Eucalyptus.

Many interactions between elements have been explained on the basis of competition between chemically similar elements for a common metabolic pathway, shown by the iso-electronic ions Zn 2+, Cu 2÷ and Ag 2+. Addition of Zn has been found to induce deficiency of Cu. Addition of cadmium can induce deficiency of Zn. Induction as a cause of deficiency in nature has been rare, because potentially antagonistic ions have usually been found at concentrations different from those likely to be encountered in antagonistic situations (Bowen, 1966).

The antagonistic effects of high concentrations of labile aluminium in the soil solution, on Zn uptake and consequent deficiency have been studied by Schier ( 1985 ), who found inhibition of root elongation in Picea rubens and Abies balsamea at concentrations of 50 mg A1 L- 1 and complete stunting at < 100 mg A1 L- 1. Toxic A1 decreased Zn uptake severely, but phosphate uptake only mildly, as it caused disruption of epidermis and cortical cells.

Biochemical and metabolic effects of zinc deficiency The effect of Zn deficiency at the metabolic and biochemical level is now

moderately well understood. In general, the profound effects of acute Zn deficiency have not been directly attributed to malfunction, but to diminished activity in certain tissues. Under conditions of deficiency, some Zn enzymes (alkaline phosphatase, carbonic anhydrase) have shown this. It has led to decreased starch formation, accumulation of amino acids, decreased synthesis of auxin indolyl-3-acetic (IAA) via impaired tryptophan synthesis; it has decreased carbonate dehydronase activity which has affected the balance of


carbon dioxide and carbonic acid, and so has indirectly influenced the rate of photosynthesis (Kolari, 1983).

Zinc deficiency has led to reduced DNA-synthase and reverse transcriptase activity, suppressed gene replication (but at the same time, has allowed an increase in RNA-synthase production ). Cells programmed to divide were unable to accumulate the necessary quantity of Zn. It is likely that a deficiency of Zn does not depress the rate of DNA synthesis of all cells uniformly; rather it limits the number of cells in which normal DNA synthesis takes place (Ni- cholas, 1975 ). The concept that cells need a 'quantum' of Zn before cells be- gin division is consistent with the fact that Zn contents of most tissues in which cells survive is near normal, even in severely deficient plants and animals. The effects of Zn deficiency are known to be most pronounced in mer- istematic tissue expected to proliferate most rapidly and then differentiate (Smith and Gawthorne, 1975). Radwan and Harrington (1986) have reported that Zn concentration has shown a highly significant correlation (P < 0.01 ) with terminal shoot growth. This has been seen most dramatically in the drastic reduction in flowering, fruit or cone-set and cocommitant reduction in seed numbers in plants diagnosed with Zn deficiency. A definite threshold appears to exist below which vegetative growth may even be satisfactory, but no seeds are produced; for example, Reed (1942) found this threshold to be 0.02 mg Zn kg-~ for Pisum sativum. This information is of considerable importance for tree breeders.

Zinc deficiency has also been found to promote formation of abscissic acid, causing premature abscission of leaves and flower buds.

At the microscopic level, deranged carbon metabolism has been shown by accumulation of oil droplets in chloroplasts, whilst starch grains have usually been absent. Accumulation of calcium-oxalate crystals has been observed in the stunted and chlorotic leaves; accumulation of phenolic substances can occur (Chesters and Rolinson, 1950).

Disruption to cell form and cell division in the mesophyll tissue has been a conspicuous effect of abnormal Zn nutrition of leaves. Palisade mesophyll has shown transverse cell division, with cells adopting a broad form and los- ing their palisade-like disposition and columnar shape; cell enlargement has continued to occur but differentiation has been reduced (Chesters and Rolin- son, 1950). In contrast, Zn toxicity has caused structural changes in the spongy mesophyll and the stomata (Van Assche and Clijsters, 1983).

Skoog ( 1940 ), investigating the failure of stem elongation associated with Zn deficiency, found that incipient deficiency resulted in a drastic reduction of auxin content to < 50% before any signs were seen of diminished growth. Adding Zn restored auxin production to normal within 24 hours. In contrast, copper and manganese deficiencies caused reductions in auxin level after growth had been reduced: (authors' emphasis). Auxin is known to be inacti- vated by the visible-blue/UV part of the spectrum. When Zn-deficient plants


were grown in red-IR light auxin level was not affected. It appears that the low level of auxin in Zn-deficient tissue exposed to light is due to destruction of auxin. IAA is formed from tryptophan by oxidative deamination; as Zn becomes deficient a decrease in tryptophan precedes a decrease in auxin content (Kolari, 1983 ), indicating that Zn is necessary for the synthesis of tryptophan, and that inactivation of such auxin as does exist, aggravates the deficiency, producing dramatic symptoms such as rosetting. Barker (1973) studied fine root production in radiata pine in relation to phosphate and zinc nutrition in deficiency situations.

If many of the effects produced by deficiency of Zn are due to lack of auxin, this may partly explain why mutual shading among trees has been found beneficial, and has reduced the tendency for inactivation of auxin by short-wave light radiation. Citrus leaf-mottle was seen to be more prevalent on the sunny side of crowns. Tracing movement of 6SEn showed that transfer from stem to tip meristem only occurred in the shade (McGrath and Robson, 1984c).

Concommitantly with these other effects, Zn deficiency has profoundly influenced the water relations of the plant. The osmotic pressure of deficient plants, especially shoots, has been seen to increase gradually as severity of deficiency increased (Tsui, 1948 ). Growth resumed within 48 hours after Zn was supplied; water content increased coincidentally with increase in auxin content. Zinc has been shown to be essential for the growth of excised roots, emphasising its role in auxin synthesis.

Remedial treatment of zinc deficiency Substantial economic and practical benefits for forest management accrue

if the cause of major restraints on growth and productivity can be identified and remedied. The effect on productivity, constraints on silvicultural practices and opportunity costs of Zn deficiency are discussed in part II of this review (Boardman and McGuire, 1990), illustrated from experience in South Australia with Pinus radiata.

Dieback disorder was widespread in South- and Western-Australia radiata pines established between the 1880s and the late 1930s. Pathological studies on radiata pine affected by 'dieback', conducted by G. Samuels at the Uni- versity of Adelaide, South Australia, had established by 1952 that "no disease organism was associated" and concluded that "the effect was produced by a soil factor" (Anonymous, 1924/5 ). In Arizona and southern California, different species of orchard trees growing in close proximity on the same soil types, were seen to manifest the symptoms of dieback known variously as 'little-leaf', 'yellows', 'rosette'and 'mottle-leaf' in varying degrees. This evidence also showed that dieback was a soil-based disorder (Chesters and Ro- linson, 1950).

In 1912, Javellier had shown the beneficial results of spraying Zn sulphate. In 1931 and 1932, several groups working independently discovered that


treatment of orchard trees with Zn salts alleviated the symptoms. In 1932 and 1933, the application of Zn as Zn-lime, Zn spray, or by injection of Zn salts was found to be completely effective in removing disease symptoms in deciduous fruit trees. Citrus 'mottle-leaf' was cured this way in 1933-34 (Ches- ters and Rolinson, 1950).

Kessell and Stoate published their first observations on the association between zinc and a dieback syndrome of forest trees in 1938. Observations that symptoms of dieback were prevented in radiata pine trees growing adjacent to galvanized wire rabbit netting fences in pine plantations have been recorded in both Western and South Australia (Stoate, 1950 ). Kessell and Stoate recorded a definite response to a spray application of a soluble Zn salt using a 1% solution. Shortly afterwards, studies made at the University of Western Australia by Hearman ( 1938 ), Smith and Bayliss ( 1942 ) and Smith ( 1943 ) clarified the cause as a deficiency of the trace element Zn. Kessell and Stoate ( 1938 ) and Hearman ( 1938 ) showed that there were several alternative remedial treatments for pines, all involving metallic Zn or Zn compounds. The reliable remedy used in the southeast region of South Australia was to spray a 2.5% solution of Zn sulphate heptahydrate which supplied 6-7 kg Zn ha -~. (Adams, 1946; Fielding, 1947; Stoate, 1950).

Many investigators have found that relief of the symptoms of disorder now associated with Zn deficiency has been readily obtained by direct application of Zn as salts, chelates or metal to the plant stems or foliage (Thorne, 1957 ). Applications of inorganic-Zn fertilizers to soil have been efficacious, especially on mild to strongly acid soils, but the application rate has to be increased above that for foliar application. The min imum useful quantity of Zn applied for many crops is the same as that typically applied to pines in southern Australia, but repeated at about 4-yearly intervals (Adriano, 1986 ). Only a single dose is usually needed by perennial plants able to recycle at least some of the Zn taken up by internal translocation.

Sparks (1987) has examined the management of Zn nutrition in pecan plantations in endemic and non-native areas. He compared the growth rate and yields of pecan plantation trees for the decade prior to Zn application for the cure of ' rosette ' disorder and the decade following.Rate of production increased in non-native areas but not in native trees. This was attributed to the lower liklihood of severe Zn deficiency in native areas on one hand and the general ineffectiveness of soil-based Zn applications on the calcareous soils of native areas on the other.

ZINC TOXICITY

Public perceptions change, and society continually makes changing demands on the uses to which forest and woodland should be put. The concept of multiple-use forestry, with broad public demands including access for rec- reation on the one hand, and a suitably safe place to dispose of noxious waste


products on the other, is causing forest managers to address questions which relate to the regulation of Zn and other heavy metals in greater amounts than have been customary (Comerford and Fiskell, 1983 ). The question of Zn as a nutrient in excessive amounts, ofphytotoxicity and animal toxicity, and the serious problems of leaching into water supplies (Yaron et al., 1984a) are novel for foresters. Likely to remain in the foreseeable future, a review of these questions in a forestry context is timely.

Protracted lack of interest shown in the influence of Zn toxicity on crop species persisted until the early 1970s. This is intriguing, considering that the original view of response to Zn was a stimulatory effect produced by a potential poison, rather than an essential nutrient needed in trace amounts. In an early report of Zn toxicity, Worsnop ( 1955 ) refers to the 'zinc sensitivity' of tree seedlings grown in galvanized metal tubes.

Zinc toxicity remained of very minor interest to forestry until the last ten years. In agriculture, as recently as 1974, the Waite Agricultural Research In- stitute in South Australia celebrated 50 years of trace-element research with a jubilee symposium, 'Trace Elements in Soil-Plant-Animal Systems', in which no reference was made to zinc toxicity in any contribution (Nicholas and Egan, 1975 ).

Symptoms of zinc toxicity That the likelihood of Zn toxicity in plants did occur originally rested on

circumstantial evidence; the absence from Zn-rich soils (calamine soils) of certain species otherwise expected to be present (Antonovics et al., 1971 ). General symptoms of Zn toxicity are retardation of growth, chlorosis and leaf dessication, symptoms closely similar to phosphorus toxicity except that the plants have allegedly high Zn levels (Collins, 1981). Raupach (1975), as a number of other workers, simply attributed the chlorosis to an "iron-deficiency chlorosis", but this appeared to occur without any decrease in foliar Fe levels. The interaction between inorganic phosphates and Zn, investigated by McGrath and Robson (1984a) in radiata pine, has already been mentioned. Jakobsen and Wetselaar (1975) commented that "high phosphate concentration can alleviate zinc toxicity". They also noted the antagonistic effects of iron in reducing Zn toxicity and this too suggests that Zn toxicity is unlikely to induce iron-based chlorosis. The concentrations of other macro- and micronutrient ions in solution may also strongly influence Zn absorption and the appearance of toxicity.

Leeper ( 1972 ) pointed out that liming, especially with magnesium carbonate, "will alleviate zinc toxicity, because the two ions, Zn 2+ and Mg 2+ are approximately the same size"; i.e., the radii of their outer electron shells are 0.72A and 0.66A respectively. This, it was alleged, allowed them to be inter- changed in ionic lattices. The same reason was advanced by Thorne (1957) to explain why magnesite adsorbed more Zn than did dolomite, and dolomite


more than calcite sources of lime. Sopper and McMahon (1988) have reported healthy, vigorous growth in 10 of 11 species of trees on a devegetated site with surface soil having 31 346 mg Zn kg- ~ near a zinc smelter active for 90 years. Foliar Zn concentration well in excess of zinc tolerance (Adriano, 1986) was found following mulching with municipal sewage sludge and fly- ash. It was noteworthy that all nutrient elements were 'optimal' and root systems had penetrated to uncontaminated soils within 6 months. Poplar showed the accumulated levels expected and several volunteer tree species established.

Van Assche and Clijsters ( 1983 ) looked for impairment of photosynthetic activity in relation to levels of Zn. They found that 'high concentrations' induced a decrease in photo-phosphorylation capacity and NADPH production. High concentrations also directly inhibited ribulose 1.5 biphosphate car- boxylase activity. Since both these effects can regulate carbon-dioxide fixation, diffusion from ambient air to chloroplast is reduced. They speculated that the primary site was metabolic. Abnormality induced in the spongy mesophyll cells and stomata aggravated the gas-exchange conditions but were regarded as secondary, though visible, symptoms. Many metabolites, in particular organic acids, have been found to bind Zn strongly (Collins, 1981 ).

High Zn concentrations have also been associated with inhibition of phloem translocation, with concommitant accumulation of sucrose, reducing sugars and starch in the leaves. This has occured, also with sudden-onset phosphorus deficiency in radiata pine (South Australia Woods and Forests Dept., unpublished data, 1977 ). It may, therefore, be due to binding of phosphate by excess labile zinc. Collins ( 1981 ) has remarked on the recent discovery of metallothioneins, low molecular weight proteins which bind specific heavy metals, in plants as well as animals. These may well prove to play an important part in Zn metabolism. R.D. Teasdale, Bond University, personal com- munication, 1985 ) has made Cu-enriched metallothioneins by gene-insertion techniques as part of a study into Cu deficiency in radiata pine in Australia.

Zinc in relation to noxious conditions Prior to 1975, the potential of environmental Zn to cause toxicity appeared

to have been strictly limited. It had been found associated with grazing in pastures on naturally high-Zn content soils. Application of fertilizers, or waste with high concentrations and unnatural balances of antagonistic ions, to soils and crops carries with it the prospect of ~autritional problems and disorders in the future. Forestry is not likely to be immune from these (Sopper and Kardos, 1973; Smith and Evans, 1977 ). In horticulture, research into the uti- lization of municipal composts, various urban and rural sludges and harbour dredgings in vegetable growing and as garden-soil amendments had begun (Garner, 1966; McIntyre et al., 1977; d'Angremond et al., 1978).

Close attention has been given in the last decade to Zn toxicity in soil and crop research, including forestry applications. This has stemmed from an in-


crease in public awareness of a need to dispose of the wastes of modern communities in 'environmentally safe' and economical ways. Forests have been given serious consideration (Huser, 1976; Keller and Beda-Puta, 1976; Fass- bender and Gussone, 1983 ). Concentration of heavy metals in the food chain, particularly that leading to human consumption, but also in wild animals, a conservationists concern, has dominated work on plant uptake. Ways of controlling heavy-metal cycling, otherwise leached into groundwater and water supplies, have received attention. In industrial regions, the contribution of atmospheric metal and 'acid' deposits to the whole cyclic system have been studied (Ellis et al., 1984; Yaron et al., 1984a,b). A major attraction of forest land for the application of wastes has been absorption and retention of noxious metals into the tree perennial biomass. It can be contained there, isolated from the human food chain and insulated from the areas of health concern. The fact that forest litter and humus remain relatively undisturbed masses of organic matter, able to bind significant amounts of transition metals long term, is also being recognised.

Just over 10% of the papers published between 1979 and 1987 on the association between trees and Zn were concerned with species growing in forests. The majority of the reports were about the effect of atmospheric inputs of trace metals on trees and forest soils. Emphasis was placed on two aspects, on the quantity and intensity of trace metals delivered, and in their fate in the forest environment and their effect on the trees themselves.

The second most common theme, one-tenth of the total, dealt with sludge applications in the forest, mostly to plantations. Recent reports have included Zn among the elements being investigated, unlike reports that were published previously. These reports acknowledge a growing awareness that site-specific information on soil properties and hydrology are needed as precursors to any sustained system of waste disposal on land. Several ways of assessing potentially harmful effects have been pursued.

The harmful influence of Zn has been studied through examination of the soluble-Zn concentration in nutrient media and how it was related to decline from opt imum growth or reduction in rate of activity, in this way seeking upper criticality concentrations in both the medium and the plant (Adriano, 1986 ). Chaney and Strickland ( 1984 ) sought the "effective dose which would reduce growth by ten per cent" which they called "EDI0" . They examined the germination of red-pine pollen (Pinus resinosa), and found Zn to be relatively non-toxic as it reduced pollen germination by 10% and tube elongation in Zn z÷ solutions > 10/zM.

Teasdale ( 1986, 1987 ) has shown that Zn 2 ÷ solution of 100/zM is the optimal concentration of zinc for the growth of excised embryos ofradiata pine, based on an exhaustive series of paired nutrient tests, each covering a range of concentrations. He made a very pertinent comment: "benefit is obtained


from concentrations of zinc in excess of the total medium [ EDTA ] such that free Zn 2 ÷ is very substantial, especially in comparison with the requirement for copper....In terms of free cation species, requirements for Cu 2÷ and Co 2÷ are exceedingly low, if indeed any requirement for Co occurs ..... A strong requirement for Fe is readily demonstrated .... Free Co 2÷ and Cu 2÷ are clearly toxic at low levels with EDTA playing a protective role by chelating these; Fe 3÷ appears to be similarly toxic. In contrast, toxicity effects are not incurred until Zn 2÷ and Mn 2+ concentrations several orders o f magnitude higher are attained." This thorough study suggested that an ED 10 is likely to be much greater than 10 gM. Toxic effects are only likely to be produced by [Zn 2÷ ] in solutions > 10 000 #M.

The concentration of Zn (which is relatively loosely bound) in the soil solution, even at pH 6, may be high enough to give rise to critically toxic contents in plants. Special measures are needed to control Zn absorption in these circumstances (Hani and Gupta, 1985). Takkar and Mann, 1978; see Col- lins, 1981 ) found stunting in wheat growing in soil with 7 mg Zn kg-1 of DPTA-extractable zinc and tissue concentration of 60 mg Zn kg-1 and, in maize, stunting at 11 mg Zn kg- 1 of DPTA-extractable zinc and an associated tissue concentration of 81 mg Zn kg -1. Prabhakaran Nair (1984) has advocated a criterion of plant uptake called 'zinc buffer power' based upon a quantity: intensity ratio.

To satisfy the need for specific information, a number of factors of general applicability can be recognized. Significant gaps in knowledge exist before they can be applied to most forest situations. Knowledge of mechanisms which control Zn assimilation into roots and shoots and immobilization, if any, within plants is fundamental to any understanding of toxicity. Certain concentrations in the tissues of animals, particularly domestic animals, have been determined to be toxic. The situation in plants is much less clear. It seems likely that all divalent transition metals are poisonous by virtue of their reac- tivity with proteins. Zinc, being one of the least electro-negative metals, has a relatively low rating which only exceeds that of nickel and manganese. The action of toxic concentrations of Zn is thought only to apply to the poisoning of enzymes, and not to several other modes described by Bowen (1966). Phipps ( 1981 ) commented on the scarcity of studies on the manner of trace metal uptake. Whatever the route, "it cannxgt be stressed too strongly that it is dominated by the principle of co-ordination and redox chemistry. These must be used to account for the availability of the metal at the plant surface, whether it be root, stem or leaf, the primary capture of the metal by the plant and the subsequent mechanism of uptake, which may be described as active, facilitated or passive" (Phipps' italics).

Diffusion was determined to be the dominant uptake mechanism as deple- tion occured in a zone roots some 2-4 m m wide, independent of transpiration rate. Wilkinson et al. (1968) found pH in the immediate vicinity of a root lower than in surrounding soil. When chelating agents were present, increas-


ing pH decreased absorption, by increasing the bonding of Zn 2 +. Changing soil pH frequently induced large changes in the absorption of Zn by plants and the dominant process involved appears to be release of Zn from soil col- loids. The effect of fertilizers has been ascribed to pH effects rather than of carrier cations (Quirk and Posner, 1975; Matzner et al., 1985).

Stefanskii (1984) sought the rates of application of zinc oxide and zinc chloride mixed with soil which reduced growth. He found that depression ( ED 10 ) occurred with ZnO at a concentration of 4030 mg Zn kg- 1 ( dry soil ) and complete suppression with 329 600 mg Zn kg- 1. The Zn concentrations with chloride were lower by an order of magnitude; reduction was probably due to chloride toxicity. Sopper and McMahon (1988) established ten species of trees successfully on soil with total zinc > 31 000 mg Zn kg- 1. Hewitt (1966) mentioned that 6 mg Zn L- 1, 0.09 m M was sufficient to kill green algae.

The actual specification of heavy metals in soils clearly determines the availability of metals for plant uptake and the potential for contamination of groundwater following the application of municipal sludge to land. For the most part, heavy metals of anthropogenic origin have been found to be adsorbed specifically on the surfaces of oxides and humic substances. The sta- bility of these surface complexes decreased rapidly with decreasing soil pH (Hani, 1984). Studies have shown metals to be predominantly associated with the solid phase; soluble and exchangeable species generally have represented less than 10% of total metals (Lake et al., 1984 ). Hence movement of cationic micronutrients, such as Zn is restricted (Ellis et al., 1984).

Heavy-metal movement is most likely to occur with large applications of organic wastes to sandy, acid soil, low in organic matter, which receives high rainfall or irrigation. Even under these conditions movement will be limited. Matzner et al. ( 1985 ) examined the effects of fertilizer and liming on the redistribution of elements, including Zn, in forest soils. Movement through open soil channels in heavier-textured soils, where the soil has had no opportunity to attenuate them is potentially hazardous (Dowdy and Volk, 1984).

Upper critical levels (adequacy~toxicity Very little factual information has been published on the concentrations of

Zn associated with toxicity in plants. Considering forest trees, a study by Van Lear and Smith ( 1973 ) put an upper critical level > 300 mg Zn kg- 1, for slash pine (Pinus elliottii) but this was possibly flawed by a coincidental gross deficiency of phosphorus in the seedlings. A recent glasshouse-based study by Burton et al. ( 1983 ) has set "an upper critical tissue concentration" for Sitka spruce (Picea sitchensis (Bong.) Carr. at 226 mg Zn kg- ~ in laboratory trials. De Vries and Tiller (1978) however, issue a caution about using this approach. They grew lettuce (Lactuca sativa) in the same soil in small pots in the glasshouse, in large containers in the open air, and in a field experiment.


At the highest rate of sewage sludge amendment of the soil, the uptake into the tops was 300 mg Zn kg-1 in the glasshouse, 60 mg Zn kg-~ in the large containers and only 40 mg Zn kg- 1 in the open field. Despite these problems (and of interest to forest wildlife managers) limits have been suggested such that the 'tolerance' in agronomic crops should be 300 mg Zn kg- ~ (as Zn 2 ÷; Chumbley, 1971; Knezek and Miller, 1976 ). Chaney ( 1983 ) stated that maximum levels of Zn in forages permissable for continuous long-term feeding were 300-1000 mg Zn kg- 1.

The upper criticality levels which have been quoted for grasses and cereals, wheat and maize at 100 mg Zn kg- 1 (Collins, 1981 ) and spring barley at 290 mg Zn kg-1 Davis and Beckett (1978 ) need cautious interpretation. Just as there is a marked difference in the susceptibility of plant species and varieties to Zn deficiency, the same is true for tolerance of Zn toxicity. The most frus- trating aspect of current information upon toxicity is the lack of data available linking descriptions of symptoms with tissue concentrations.

Physiology of zinc tolerance~resistance The physiology of heavy-metal resistance in plants has been studied by Ernst

(1975) and his co-workers in a field that has received little attention. Al- though Antonovics ( 1975 ) had claimed that a number of species had evolved metal tolerance, Ernst suggested that it was not geneticically based since no resistant enzymes specific for tolerant populations of mosses and higher plants have been observed. They found that specific metal resistance was due to specific pathways. Zinc tolerance was accompanied by an accumulation of malic acid which was able to transport the Zn innocuously through the cell plasm and deposit the excess metal in the vacuolar system. No differences in metal uptake or exclusion mechanisms were found by Ernst's group, nor were there differences in the capacity of cell walls to bind metals. Antonovics suggested that, if there were a genetic base to tolerance, it was likely to be polygenetic and nucleic, i.e., unlikely to show any maternal effects.

The claim of Wu et al. (1975) that there was potential for evolution of heavy-metal tolerance in plants, citing the rapid development of copper tolerance by Agrostis spp., was not borne out by the facts presented. Their data can be adequately explained as selection within a broad gene pool. Most bi- ologists would be likely to intepret their analysis as development of an eco- type, or land-race, under severe stress from an environmental chemical. Col- onization was clonally based, strategy similar to that of bracken-fern, Pteridium spp. (a genus in which speciation is controversial) and supports conventional interpretation.

Lepp (1981 ) pointed out that lower plants have an interesting interrela- tionship with trace metals. The 'infinite' capacity of some diatoms, bryophytes and lichens to absorb trace nutrients (Bortels, 1927 ) is probably wor- thy of considerably more investigation.


Remedial treatment for toxic and noxious zinc in forest conditions Remedial treatment for Zn toxicity and excess mobility in the root-zone

has advocated the same mechanisms as for Zn deficiency but used in the reverse direction. However, the similarity has ended there, for there appear to be few relatively simple choices such as were found with augmenting Zn supply. Control, to date, has been sought primarily through the control of soil reaction, pH, and particularly the reaction of the soil solution itself.

The concentration of Zn found in the soil solution, in contrast to some transition metals, is highly dependent on soil reaction (Loneragan, 1975; Jeffery and Uren, 1983). The concentration at a given pH has been found to be dependent on the total amount of metal present (Kuo and Baker, 1980).

Factors that control Zn in labile and bound forms within the soil have to be considered together. The situation is complicated by differences between soils in texture, clay content, organic-matter, organic-matter constituents (especially chelates) and mineral oxides, carbonates and sulphides, all of which may interact (Emmerich et al., 1982a,b,c). In addition to this general set of variables, spatial variations within the forest stand due to the influence of individual trees will also need to be taken into consideration. Not least, at a much finer scale, the localized reduction in pH around feeding roots, that arises directly from zinc assimilation, should not be forgotten. The contrary behaviour of various constituents of organic matter, either antagonistic or beneficial, adds further complexity.

Problems are likely to arise in controlling Zn because most Zn of anthropogenic origin is specifically adsorbed onto the surface of oxides and humus (Hani, 1984; Hani and Gupta, 1985 ). Valdares et al. (1983 ) found that Zn added in sludge did not behave as Zn originating from inorganic-Zn fertilizer such as Zn sulphate.

Adjustment of soil reaction to pH 6.5-7 was recommended by Herms and Brummer (1980) when heavy Zn loadings occured. This range, however, is too high for the optimal growth and nutrition of many forest tree species, other than calcicoles. Most forest trees of the temperate zone and many in the tropics have evolved in soils with reaction in the range pH 5-5.5 and have tolerated more-acid conditions imposed through disturbance. This has been particularly true for coniferous species and broadleaf species that favour an ammonium-based nitrogen economy (McFee and Stone, 1969; Ingestad, 1979). A number of potentially harmful side-effects exist from other ions, especially soluble aluminium, when control of both the total amount of Zn received by forest sites and the concentration of soluble Zn in soils of moderate acidity has to be managed. The reaction of the solution itself has been demonstrated to be 0.5 pH units lower than the bulk soil.

Coarse-textured soils, common in temperate-forest areas, present special difficulties. Cation exchange capacity has been found to depend almost en- tirely on organic-matter content. Zinc, in these circumstances, has correlated


highly with the organic-matter content and the small amount of clay (Mac- Lean and Langille, 1983 ).

Zinc, because of its usually predominant contribution to noxious inputs and its relatively high mobility, is considered to be one of the prime transition metals to be controlled in the suite of these metals reaching sites in one way or another (Kiekens and Cottenie, 1985). Sources of Zn and flow into sites will have be considered. Remedial practices, above all, will need to control mobility of Zn in labile forms partioned so that the Zn can be absorbed into sinks, where it is either beneficial or can be rendered harmless (Richenderfer et al., 1975; Seaker and Sopper, 1984). Doelman and Haanstra (1984) and Tyler ( 1974, 1976) have demonstrated how increased Cu and Zn concentrations have almost linear negative effects upon soil microbial respiration rates, dehydrogenase activity, the activity of hydrolytic soil enzymes (urease, acid phosphatase and beta-glucosidase) at rates associated with the metal-pollution rates found near industrial installations. A similar negative effect of increased Zn concentration was found by Tyler (1975a,b) to inhibit the rate of nitrogen mineralization.

SUMMARY

The abundance of zinc in the forest environment has, with few exceptions, not been considered to be of much significance in forest nutrition and management until recent times; exceptions invariably involved deficiency. Natu- ral control of abundance, marginally modified by human activities, predom- inated until mid-Century. In the latter half of the 20th Century a variety of anthropogenous sources have been found to exceed, or have the potential to exceed in the short term, natural sources of Zn and reach noxious levels in forest ecosystems. Some unsuspected occurrences of Zn dependence have been found in forests as monitoring a far greater number of forest sites has taken place. Once Zn deficiency has been diagnosed in forests it is simple to remedy. Remedial treatments for Zn deficiency have been found to be easy and cheap to apply.

In natural ecosystems, Zn is least abundant in the parent materials of soils developed on the resistate group of sedimentary materials, in areas remote from the influences of glaciation for > 100 million years. Aeolian sedimenta- tion of resistate materials, quartzitic and calcareous, in dunes or sand sheets, are likely to have the lowest Zn reserves among forest soils.

Zinc sufficiency is particularly important to forestry from its vital role on securing the regeneration of the trees and plants. It has been demonstrated that Zn plays a crucial role in gene replication on the one hand, and in its regulation of tryptophan synthesis as a precursor of auxin, on the other. One ensures the replication of the species and new growth as an increase in cell numbers, whilst the other ensures that extension and expansion growth can


proceed. Toxicity, by poisoning the several enzymes important to cell metabolism, has been shown to disrupt vitality by restricting steps ancillary to photosynthesis. In either case, tree health and forest productivity are likely to decline to unacceptable levels.

Forests in remote rural locations receive most inputs of Zn from atmospheric sources and a small amount from weathering of minerals. The quantity received aerially has been found to be related to distance from emitter sources. Several decades ago, remote rural forests could be distinguished from those near conurbations and industrial areas by markedly different inputs of Zn in sedimentary cycles. Distinctions between them are now lessening as the dis- persal and accumulation of industrial wastes has intensified and persisted. In remote forests, the amount of Zn available for nutrition has been largely a function of geologic formation, weathering and the amount of rainfall. Trees in forest and woodland concentrate the catch of Zn from 4 to 8-fold per hec- tare which eventually reaches the forest floor in a non-uniform pattern. Zinc appears to accumulate in the surface of undisturbed virtually wilderness forests, or in forests with a hiatus in litter breakdown.

Concentration of Zn in the rooting zone, in comparison with the contents at greater depths, appears to be greatly affected by the amount of disturbance to litter and topsoil by natural causes, such as bushfire, wind and water ero- sion. Human interference has been slight but extensive; historically it may extend back for tens of thousands of years in the middle and lower latitudes. Disturbance has intensified since the mechanization of agriculture and forestry. It may be greatly dissipated by arable farming and by afforestation or reafforestation practices. Afforestation in southern Australia has made a natural poverty of Zn worse, and has led to acute Zn deficiency in nutrition on 'virgin' sites.

In industrial regions, atmospheric sources of Zn have increased as emitter sources have intensified in number and activity. Some have arisen from direct use of Zn and its compounds by society. Others have contributed Zn as a by-product. In both cases, wastes from from industry have added substan- tially to more extensive urban sources of a domest ic /municipal kind.

Wastes not emitted to the atmosphere have become subject to land or water disposal. Disposal methods which leak noxious materials into groundwater are now publicly found objectionable. Similar considerations have been applied to wastes from intensive animal husbandry. Consequently, extensive disposal of sewage and solid wastes in forests is being widely encouraged. Sewage treatment and composting solids have both concentrated potentially noxious elements, of which Zn is usually the most abundant in sludges and composts, through dewatering, to levels liable to raise Zn contents of plant and animal food sources to toxic levels. This may also apply to wild animals.

Applying sludges or composts to land as a means of waste disposal has increased Zn concentrations in topsoil up to ten times the normal range of nat-


ural levels found in remote forests of similar composition and stand structure. Heavy atmospheric loadings have increased the supply to the forest floor several-fold in the course of a decade.

Management of forests in developed industrial regions will need to find ways of controlling the amount and intensity of Zn mobility, to prevent deep leaching at noxious levels on the one hand, and maintain a vital forest stand to serve as an innocuous sink accumulating Zn in the perennial biomass on the other. Above all, each site is likely to require a specific investigation of soil chemistry physics and hydrology if a satisfactory, enduring healthy forest ecosystem is to be sustained. Whilst environmental-impact statements are likely to be mandatory for operations involving waste-disposal in forests, the insidious increase in Zn from aerial deposition increasing background concentration of topsoil is less likely to be monitored.

Remedial treatments to control of excessive amounts of Zn in mobile from are likely to prove difficult to achieve. Recent and current research has identified factors which may be amenable to control although many gaps in knowledge exist before they can be depended upon. A 'package' of practices to achieve a desirable end seems likely. At the heart of these will be control of soil reaction, particularly acidity, because the mobility of Zn is highly dependent on reaction. A solution is likely to be demanded by the public which will not harm or change the composition or structure of the forest stands or plantations with which it is familiar.

Finally, Zn has to recognised as being the most abundant transition metal found in wastes emanating from modern industrialised societies, and though relatively of low toxicity, it has the potential to predominate in quantity. The temptation to dispose of wastes close to their source is ever present, as ferry- ing costs are so high. Forests near centres of population are likely to be seen as places in which to dispose of excess Zn wherein the Zn can be safely accumulated into the standing trees.

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Documents

The role of zinc in forestry. I. Zinc in forest environments, ecosystems and tree nutrition