Litter yield in shrubs of larrea in the andean piedmont of Mendoza, Argentina

Vegetatio 101: 21-33, 1992. © 1992 Kluwer Academic Publishers, Printed in Belgium. 21

Litter yield in shrubs of larrea in the andean piedmont of Mendoza, Argentina

E. Martinez Carretero & A. D. Dalmasso U.I.D. Botany and Plant Soc&logy, Argentine Institute of Arid Lands Research (IADIZA)-(CONICET), C. C. 507. C. Central -5500- Mendoza, Argentina

Accepted 12.11.1991

Keywords: Litter, Phenology, Rainfall, Statistic

Abstract

The production of litter was measured for the piedmont communities of Larrea divaricata ssp. dicaricata (1500m asl) and L. cuneifolia (1130 m asl) near Mendoza, Argentina. Litter was collected in traps randomly distributed in each stand: herbaceous, shrubby and uncovered soil. The monthly average weight of litter was recorded in each stand during one year and discontineously during other two years more.

The material was classified as: leaves and fruits of Larrea sp., fruits of Stipa sp. (by its relative abundance), gramineous and non grarnineous material. The litter and the soil in each sample were bromatologically analyzed and the N, Ca and P levels also determined.

The annual contribution of nutrients was calculated totally and layer by layer. The L. cuneifolia and the L. divaricata thickets produce 4,71 t.ha- tyear- 2 and 2,02 t.ha- ~.year- 2 of litter respectively, that mean 78,66 kg/ha of N, 4,24 kg/ha of P and 94,2 kg/ha of Ca in the first thicket and 31,14 kg/ha of N, 1,82 kg/ha of P and 34,17 kg/ha of Ca in the second one; in one year of study.

Zusammenfassung

In den Larrea divaricata ssp. divaricata und L. cuneifolia Geb~ischen des mendoziner Andenvorlandes (1500 m und 1130 m MeereshOhe) wurde die Streuproduktion untersucht. Die Streu wurde getrennt nach Strauchschit, Krauchschit und nacktem Boden in Fallen gesammelt, die stichprobenartig in den Be- st~nden verteilt waren. Ein Jahr lang wurde in jedem Bestand der monatliche Streuanfall ermittelt, wobei die Bl~iter und Fr~chte von Larrea von den Stipa -Frtlchten zwecks Berechnung der relativen Mengen-, grasartige und nicht grasartige, getrennt wurden. Streu und Boden der Best~tn wurden bromatologisch analysiert und die Stockstoff, Kalk und Phosphorgehalte bestimmt Derj~rliche Nahrstoffeintrag wurde schichtweise und als Gesamtmenge berechnet. D as Larrea cuneifolia GebOsch erzeuchte j ~lich 4,71 T/ha und das L. divaricata 2,20 T/ha Streu. Diese enthielt 78,66 kg/ha N, 4,24 kg/ha P und 94,20 T/ha Ca im L. cuneifolia- und 31,14 kg/ha N, 1,82 kg/ha P, 34,17 kg/ha Ca im L. divaricata- Gebt~sch.

22

Introduction

The shedding of plant parts and their accumula- tion on the soil as litter is ecologically very important in desert zones. Such zones are often characterized by high losses of organic matter through oxidation, and scarce microbial soil activity. On the other hand, litter may limit nutrient losses by recycling, especially under the more fully closed shrub canopies. Nevertheless, most of the existing literature on mineral cycles re- flects research performed in forested ecosystems. Gosz etal. (1972) found that the tree canopy of the Hubbard Brook, (New Hampshire) forests yielded 98 ~o of the nutrients passing through the litter layer and the remaining 2 ~o from the shrub level. Magg & Pearson (1977) analyzed the seasonal changes of litter and its mineral concentra- tions during one year in the coastal shrub near Sidney, Australia. They found that litter mineral concentration has seasonal variations, which in the case of K, is related to rainfall. Moreover, the mean turnover rate for the litter (dry weight) was 3.8 years, approximately one year for K and more than three years for other minerals (P, Ca, N). Litter is the primary source of energy for most of the microorganisms acting in the nutrient trans- formation and mineralization process (Reichle et al. 1980).

Strojan et al. (1979) studied the individual litter yield of six shrub species in the Mojave Desert. There, the leaf fraction accounted for most of the litter, and for Larrea divaricata ssp. tridentata the quantity of leaves accumulated in the litter layers grew significantly with increases in rainfall. Mo- rello (1955 & 1958) mentions that the leaves of L. divaricata are retained on the plant for one year and the leaves ofL. cuneifolia for two years. In the Mendoza piedmonts, Morales (1989) analyzed the organic matter and organic nitrogen variation of the litter under the main shrub species for both shady and sunny slopes in the L. cuneifolia belt. He found that there was a larger content of organic matter and organic nitrogen in the litter of the shady slopes during most of the decay cycle. Taxonomic studies of invertebrates (Formicidae) involved in these decay processes are only now

beginning in the piedmont areas, and there are no quantitative studies about their occurrence and importance.

Except for some isolated recordings of litter production which were done for a few species, the literature for desert areas is scarce and papers from a quantitative standpoint are even fewer. Good quantitative measurements of litter yield are almost non-existent for the arid zones of Ar- gentina. Because of the phytogeographic importance of Larrea shrubs in the arid Argentine zones, it is interesting and essential to study the litter yield and the resulting distribution of nutrients in the soil where each shrub species grow. In this study we consider litter those plant parts (leaves, stems, flowers and fruits) which are nat- urally shed and that may accumulate on the soil surface.

The study area

The two communities studied were located west of the Department of Lujan, Mendoza (Argen- tina) at 33°05 ' south, 69°00 ' west; one located at the piedmont of Cerro Cacheuta and the other in the place named Pampa del Cebo. The first community corresponds to Larrea divaricata ssp. divaricata belt with alluvial soils, clasts and gravel in a fine sandy matrix and a calcareous layer 40- 50 cm deep. The general downward slope is 7 ~o with an easterly trend. The second community is a Larrea cuneifolia community and lies over the E1 Zampal Formation (Polanski 1962), with deep soils, loamy-clay texture and a 2-3 ~o slope.

These areas were chosen because the communities are almost unaltered and in semi-confine- ment. In contrast with the remaining piedmont, the area has had virtually no human alterations. Moreover, it is representative of zones with high agricultural potential, especially for horticultural activities.

Both communities belong to the Monte phytogeographic province and they are each situated in the Larrea belt. The lower one belongs to L. cuneifolia between 750-1250 m asl, and the upper one, of L. divaricata, between 1250-2200 m asl, de-

scribed by Roig (1976). For both communities, rainfall is sporadic, but intense when it occurs, with 72 ~o occurring in the summer season.

Materials and methods

Monthly litter yield was measured in both communities at three places: under the shrub stratum, in the herbaceous stratum, and on the uncovered soil. Cylindric traps (15.5 cm diameter and 20 cm deep) were used with a sieve at the bottom to retain the litter yet allowing the water flow. Fif- teen of these traps were utilized in each community, randomly distributed and buried, leaving 2 cm above the ground to avoid material entering the traps by lateral surface flow. Five repetitions were carried out in each stand. The material was collected every thirty days during the first year (1983-84), and discontinuously during an addi- tional two years. Samples were dried at 70 °C until they reached a constant weight. Foreign material such as soil, insects, etc. were then elimi- nated and the samples were reweighed.

Collected materials were classified into four fractions: Larrea leaves, Stipa fruits, gramineous and non-gramineous material (stems, fruits, etc.). The relative importance of each of these fractions represented the basis for the classification proce- dure. The amount of K and P (chlorhidric envi- ronment), N (Kjeldahl method) and of Ca (EDTA titration method) was determined in the material accumulated at each of the three locations and in each community during the first year. The soils of each stand were also analyzed for pH, CEA, organic matter ((Cr207K2) method in sulfuric en- vironment), K (Prats method), P (by carbonic extraction, Arizona method), N (by Kjeldahl), and texture (International method), up to a depth of 20 cm. At the time of each litter collection (every 30 days in 1982) during three years (1982- 84), phenologic observations were conducted fol- lowing the procedures of Newman and Beard (1962). The phases represented by the start and end of budding, blossoming, fruiting and/or spike bearing were observed. These were obtained by randomly selecting plants and estimating in par-

23

cels the occurrence percentage of each phase. The beginning of each one of the phases was regarded as such when at least 30~o of the individuals of the same species experienced the change.

The floristic composition of both communities as well as each species cover were determined through a the Point Quadrat method (Daget & Poissonet 1971) with three chance repetitions 15 m long in each shrub and with regardings each 10 cm.

Rainfall was recorded in situ using 10 gages each with a 78.5 cm 2 surface, buried 5 cm, and known levels of evaporation retarding oils. The gages were read monthly. This method has been shown to produce reliable results (Horno, unpublished).

The amount of litter yield was transformed through Log n function. The variance homogene- ity was verified through Bartlett's test. The litter yield in each Larrea community was analyzed by month and by stratum using a factorial ANOVA and compared through Tukey's test (~ = 0.05).

Results

Floristic composition of each community

Phenologic observations in both communities

Figures 1 and 2 show the phenologic spectrum (three years recording) of species which compose the studied communities. Most of the evergreen species have a greater activity in the summer season. Terophytes of the upper belt have mainly spring and summer, flowering while species of the lower belt flower in winter and spring. Both Lar- rea species show small variations in budding, blooming and fruiting. L. cuneifolia blooms from the beginning of September until the end of De- cember; this phase is repeated in April. L. divaricata blooms at the beginning of October and con- tinues to bloom fairly regularly until the end of April. Blooming and fruiting coincide for both species between middle-October and mid-Janu- ary. Similar records have been obtained for the Monte by Morello (1955). In both Larrea com-

24

Life forms/species

Shrub stratum

Nanophanerophyte

Larrea cuneifolia

Atriplex la~pa

Lyciu3n tenuispinosum

Atriplex argentina

~rbaceous ~rat~

Geophyte

Hoffmanseggia glauca

Therophyte

Bromus brevis

Conyza sp.

Eragrostis pilosa

Erodium cicutarium

Descurainia canescens

Hordeum sp.

Lappula redowskii

Sa~sola kali

Sisymbrium sp.

Hemicr~tophyte

Diplachne dubia

Pappophorum caespitosum

Scleropogon brevifolius

Trichloris crinita

Stipa tenuis

Baccharis pingraea

Sphaeralcea mendocina

Trixis papillosa

Als o N 01 Fi/

I

J

I

I

I

Flowerin~-Fruiting I I Budding

Fig. 1. Phenologic observations in the Larrea

munities, differences in number of species are great, but, the perennial and shrubby species per- centages are very similar: in L. divaricata community 58 ~o are perennial species of which 45 ~o are shrubs, and in L. cuneifolia community 55~o are perennial and 44~o of them are shrubs.

Rainfall

There are no meteorological stations in the studied zone, the nearest is 15 km away (Lujan de Cuyo) and at the belt of L. cuneifolia. Values recorded in situ and at the station are shown in Table 2 for the first year.

According to the data, (1984) was an uncom- monly rainy year. The record of 31 years (1917-

cuneifolia commumty (Period: 1983-86).

1947) for the luj~m de Cuyo station, with an annual average amount of 165 mm shows 1984 to be unusually wet. For the years of 1985 and 1986 respectively only 112 and 55mm respectively were received. The difference was higher in the upper belt and temperatures were lower in 1984. Cavagnaro (1988) did not, however, find a significant linear correlation between ele- vation and rainfall for this piedmont. Minetti (1989) established two climatic levels, arid and semiarid, either under or above the 1120 m asl limit. For the upper and lower belts respectively, total fungal cover is 28.64~ and 8.95 ~o- The fungal cover indicates a larger water supply in the arid zone, but it comes, mainly from dew, because of lower temperature in the L. divaricata upper belt.

Life forms/ species

Shrub stratum

Chamaephyte

Acantholippia seriphioides

Stipa pl~iosa

Nanophanerophyte

La~rea divaricata

Ephedra triandra

Bougainvillea spinosa

Cercidium praecox ssp. g~aucum

Grabowskia obtusa

Lycium chilense

Prosopis flexuosa vat. depressa

Salvia gilliesii

Verbena aspera

Herbaceous strat~

Geophyte

Habranthus j~esonii

He~iotropi~m me~Zocinwn

Rhodophiala mendocina

Su~ulents

Cereus aethiops

Op~ntia sulphurea

Trichocereus candicans

Therophyte

Aristida adscencionis

Eragrostis cilianensis

Gamochaeta sp.

Hysterionica jasionoides

Partheni~m hysterophorus

Plantago patagonica

Portulaca grandif~ora

Solanum atriplici~liw~

Schis~s barbaras

Hemicryptophyte

Aristida me~ocina

Digitaria caii~rnica

Elym~s erianthus

Setaria mendocina

Sporobolus crypta~s

Stipa eriostachya

Stipa vaginata

Cucurbitella asperata

OxybaSws ovat~s

Lecanophora heterophylla

Thymophylla belenidium

Verbena merZocina

Epiphyte

Tristerix verticillatus

F M A M d d

t J

L I

mlm i r

t i

mll L I

L

! I

r • . ~ l

I

I t - -

I

I

[-

L J

I

1

L

I I

1

J

I J

25

Fig. 2. Phenologic observations in the Larrea divaricata community (Period: 1983-86).

26

Table I. Floristic composition and relative specific cover of the most conspicous species of shrubs of Larrea divaricata ssp. divaricata and L. cuneifolia.

Specific relative cover

1500masl 1130masl

Shrub stratum:

Larrea divaricata 17.16 Verbena aspera 15.86 Acantholippia seriphioides 8.66 Cercidium praecox ssp. glaucum 2.02 L ycium chilense 1.71 Salvia gilliesii O. 16 Larrea cuneifolia Atriplex lampa 1.87 Lycium tenuispinosum Atriplex argentina

14.95 18.02 9.10 2.45

Herbaceous stratum:

Diplachne dubia 5.79 Aristida mendocina 5.42 Cardionema kurtzii 2.77 Thymophylla belenidium 1.66 Stipa plumosa 1.46 Echinocactus sp. 0.63 Stipa tenuis O. 18 Trichloris crinita 10.85 Pappophorum caespitosum 2.56 Setaria mendocina 0.59 Cereus aethiops 0.25

Fungous stratum between plants 6.00 3.40 Fungous stratum under plants 23.80 2.80 Litter between plants 18.40 20.40 Litter under plants 44.60 45.40

Total vegetal cover 65.40 58.80 Shrub stratum cover 72.53 75.71 Herbaceous stratum cover 27.47 24.29 Uncovered soil 10.20 17.40

Other species which presence is scarce and which cover has not been determined in the Larrea divaricata shrub are: Habranthus jamesonii, Heliotropium mendocium, Rhodophiala mendocia, Opuntia sulphurea, Trichocereus candicans, Aristida adsencionis, Eragrostis cilianensis, Histerionica jasionoides, Partheniurn hysterophorus, Plantago patagonica Solanum atri- plicifolium, Portulaca grandiflora, Schismus barbatus, Digitaria californica, Elymus erianthus, Sporobolus cryptandrus, Stipa eriostachya, Stipa vaginata, CucurbiteUa asperata, Oxybaphus ova- tus, Lecanophora heterophylla, Verbena mendocina, Ephedra triandra, Bougainvillea spinosa, Grabowskyua obtusa, Prosopis flexuosa var. depressa and Tristerix verticillatus; and in the Larrea cuneifolia shrub: Hoffmmansegia glauca, Bromus brevis, Eragrostis pilosa, Erodium cicutarium, Descurainia canescens, Lappula redowsky, Salsola kali, Scleropogon brevifolius, Bac- charis pingraea, Sphaeralcea mendocina and TriMs papillosa.

Sot'/s

The soils texture is different in the shrub communities. Soils belonging to the L. cuneifolia community are clay loam, deep, with a larger amount of silt in the first 30-40 cm. The L. divaricata community soils are fine sand, with gravel and an increasing concentration of fine materials due to illuviation, between 40-50 cm depth. Analytic results (Table 3) show a larger N content in the L. cuneifolia community the amount of organic matter is less by almost half as much. There are no significant differences between the values of each stand for each community.

Litter amount yielded by each community

Table 4 shows the amount of litter yielded by each community studied and by stratum (mean of 5 repetitions). For each stratum cover, the litter yield per hectare of each is shown in Table 5.

The results of the measurements made in the first year, in which sampling was more stricter, the L. cuneifolia community yielded over twice as much as the litter as the L. divaricata community.

Figures 3 and 4 show the total vegetal litter yielded monthly and by stratum for both communities (during the 1983-84 period).

Histograms of the total litter yield show two seasons of yield: spring-summer and autumn. L. cuneifolia is displaced by approximately two months in its autumn increase with respect to the L. divaricata community. The general shape of the curves is similar for both shrubs. Yield decreases during the winter months (July, August, Septem- ber).

Three peaks of higher litter yield can be iden- tified in both communities. In the community of L. divaricata, the February increase represents 20,3 ~o of the annual total; the November increase is 11.67~o. In the L. cuneifolia community the April increase is 10.7 ~o, whereas at the beginning of summer (similar increases for October and No- vember) it is 21.6~o.

It is interesting to point out that in both communities there is a significant increase in July

27

Table 2. Monthly total precipitation in the study area (a and b) and at the Lujan de Cuyo (c) meteorological station (our data and F.G.S.M. data) for 1984.

D/83 J/84 F M A M J J A S O N Total

(a) L. divaricata shrub: 32.2 71.4 73.6 63.4 62 .4 0 .00 37 .6 44.0 12.6 31 .8 20 .4 16.0 433.2 (b) L. cuneifolia shrub: 32.2 71.4 66.2 57.8 89 .7 0 .00 73 .2 36.7 14.8 36.2 17.2 25 .0 486.8 (c) Lujan de Cuyo Station: 5.0 86.0 34.0 206 12.0 0.00 12.0 4.0 15.0 36.0 4.0 6.0 432.0

Table 3. Analysis of soils per stratum, in the L. cuneifolia and L. divaricata communities.

o/ Community N(/o ) P(~o ) K(ppm) M.O. CEA pH (%) (mS/era)

L. cuneifolia:

Uncovered soil 0.112 0.0386 983 2.59 776 7.02 Shrub stratum 0.184 0.0989 991 2.89 1353 6.99 Herbaceous stratum 0.182 0.0947 1064 2.63 430 7.03

L. divaricata

Uncovered soil 0.0756 0.0820 874 0.060 325 7.07 Shrub stratum 0.0980 0.0935 886 1.12 349 7.04 Herbaceous stratum 0.1008 0.1003 872 1.01 281 7.04

(winter max imum) which is 8.63 ~o and 8.57 ~o for the L. divaricata and L. cuneifolia communit ies , respectively. Annual variability in the yield can be seen by the variat ion coefficient (Fig. 3), in which values of the L. divaricata communi ty have a greater variat ion coefficient. The total amount of litter yielded by these arid shrubs is well within the upper and lower annual values repor ted for other arid regions (Table 6).

Characterization of the litter

The weight o f the material collected every 30 days was recorded and was simultaneously classified: e.g. Larrea sp. leaves, Stipa sp. fruits, gramineous

and non-gramineous material. F r o m this, we can see quantitatively the significance of litter yielded by shrubs dominat ing each communi ty . Figures 5 and 6 show the monthly percentage variat ion in litter composi t ion for the first recorded year.

Shedding of Larrea sp. leaves occurs through- out the year in both communit ies , but increases in a period at the end of summer to the beginning of au tumn (February to April). According to Passera (1983) the annual total litter yield by L. cuneifolia consists of 67~o leaves and fruits, and 33 ~o f rom ligneous parts . The fruit product ion of Stipa (mainly Stipa tenuis) is always concentra ted in the summer season, with a larger yield being recorded in the L. cuneifolia shrub.

Table 4. Mean values of annual litter yield by litter stratum (g/m2/year) for both communities, CV: variation coefficient.

1983/84 1985 1986 CV 1983/84 1985 1986 CV

Uncovered soil 243.0 141.25 1898.39 15 98.57 49.28 285.12 52 Herbaceous strat. 264.0 107.31 155.56 26 129.31 62.39 175.63 32 Shrub stratum 642.6 265.3 319.08 27 306.2 105.04 282.89 29

28 Table 5. Values of annual yield and by litter stratum (Kg/ha) in both communities.

L. cuneifolia (Kg/ha) L. divaricata (Kg/ha)

1983/84 1985 1986 1983/84 1985 1986

Uncovered soil 422.8 245.8 3303.2 110.4 55.2 319.3 Herbaceous strat. 499.8 203.1 294.5 267.4 129.0 363.2 Shrub stratum 3787.5 1828.9 1880.6 1644.3 564.5 1519.1 Total community 2277.8 5478.3 2022.1 2022.1 748.3 2201.6

g/m2/m/stratum

5-

' i

, ' ~ x o 9 r.k x

~ J 250

g/m2/m/ t o t a l

Fig. 3. Monthly litter yield (g/m2/m), total and by stratum, in the Larrea cuneifolia community.

g/ m2/m/ stratvm 70.I

29

I i

' i I

' I I I

, I

J

o~-~- Y : ~o v:'~

i00

g/m2/m/t otal

Fig. 4. Monthly litter yield (g/m2/m), total and by stratum, in the Larrea divaricata community.

Distribution of fitter in community

According to Bartlett's test, variances are homogeneous ( > 0.05). In the factorial ANOVA, F val-

ues are significant for all interactions: community, month, stratum.

For the Larrea cuneifofia community, significant differences were found (<0 .001) for the

Table 6. Litter yield (t/ha) in different arid regions.

Community Location Litter Source (t/ha)

Chaparral SW of California 8.126 Coastal shrub 5.98 Desert shrub 0.53 Xerophytic shrub 3.71 Artemisia community 1.28

Piedmont shrub:

L. cuneifolia 4.71 L. divaricata 2.02

Mojave, Nevada, USA Espafia USSR

Mendoza, Argentina Mendoza, Argentina

Gray & Schlesinger (in M. Ortega 1983). Maggs & Pearson 1977. Strojan et al. 1979. Merino Ortega 1983. Kurochina, Borovskaya 1976.

present study present study

30

N D d F M A M d d A S O N

Larrea (Jarilla) leaves non-gramineous material

Gramineous material I--] Stipa fruits

Fig. 5. Monthly percentual variation of the litter composition the Larrea divaricata community.

month factor (F (),t= 11, ~,e= 132)= 17.70); the stratum factor (F (Tt = 2, 7e= 132)= 77.9); and for the month-stratum interaction (F (Tt = 22, ),e = 132) = 3.69). In the shrubby stratum, month-

to-month and stratum litter differences are significant between the months of August, September and October and April and May (Tukey's test, 0~= 0.05). In the first case the non-gramineous

N D d F M A M J d A S O

Fig. 6. Monthly percentual variation of the litter composition in the Larrea cuneifolia community.

N

material dominates the litter, being principally leaves and fruits of Lycium tenuispinosum which originated in the previous vegetative period as well as flowers of Erodium cicutarium, fruits of Lappula redowsky and Sysimbrium irio and leaves of Baccharispingraea. In April and May increases in L. cuneifolia leaves and the first senscent leaves ofL. tenuispinosum may be found besides the contribution by the summer terophytes, such as E. c- icutarium. In autumn the non-gramineous materials acquire importance through species that reach the end of their annual cycle, e.g. Erioneuron pilosum and Scleropogon brevifolius.

In the herbaceous stratum, October and May differ significatively while the rest of the months are homogeneous. In October the contribution of fruits of Stipa tenuis, together with the winter terophytes (Lappula redowsky, Descurainia canescens, etc.) increased. The contribution by leaves and fruits of L. tenuispinosum is also important then, as are the additions by grasses such as Pappo- phorum caespitosum and S. brevifolius.

On the uncovered soil, the litter layer in general shows much variation through the year, with great variation in each measurement repetition. This is most likely caused by factors which may include wind or annual plants that occasionally occupy these open areas.

In the Larrea divaricata community significant differences (~<0.001) exist between the month factor - (F (Tt = 11, 7e= 130)= 9.7) - and stratum - (F (7t=2, 7e= 130)=33.9) - . In the shrubby stratum (Tuckey's test, ~ = 0.05) October differs significantly with respect to March and May. In September the litter contribution is very low, consisting mainly of L. divaricata leaves and winter-spring species as Schismus barbatus, which thrives densely under the shrubs, and Stipa vaginata growing between the shrubby plants. In March and in May the litterfall consists mostly L. divaricata leaves and annuals herbs (e.g. Parthenium hyterosphorus, Hysterionica jassionoides and leaves of Prosopisflexuosa).

In the herbaceous stratum significant differences occurred between the months of September and December. In December, grasses such as Aristida mendocina, Digitaria californica, Setaria

31

mendocina, form the bulk of the litterfall. There are no significant differences for the uncovered soil during the different months.

The soil litter cover appears quite uniform in each community. Total cover and cover under and between plants do not differ statistically among both shrubs. In the L. divaricata belt, litter covers 63~o of the soil: 39.2~o under plants and 27.8~o between plants; and 24~o between them. The amount of litter accumulated in the shrub stratum of both communities is significantly larger (e = 0.05) than that accumulated in the herbaceous stratum or on the uncovered soil. This coincides with what was observed by Strojan et aL (1979) in the Mojave Desert, where L. divaricata ssp. tridentata litter occurs mainly under the shrubs crown.

Content of nutrients

The total accumulated litter in each community for the 1983-84 period, and the annual mineral content of the litter (kg/ha) is shown in Table 7.

These data are very similar to that found by Maggs and Pearson (op. cit.) in a coastal shrub (44 kg/ha/yr of N and 1 kg/ha/yr of P), this re- veals the importance of litter in the nutrient supply of the soils of arid ecosystems. Litter also constitutes a significant nitrogen source for the microorganisms taking part in the nutrients mineralization of soils.

Conclusions

Results show a clear variation in annual litter yield over one year, as occurs in other arid regions

Table 7. Litter contribution of nutrients, by community in the 1983-84 period.

Nutrient L. cuneifolia shrub L. divaricata shrub 1983/84 1983/84

N 78.66 31.14 P 4.24 1.82 K 94.20 34.17

32

(Rapp 1969, Strojan et aL 1979). Due to complex interactions involving soil moisture, temperature, and the seasonal phenologic patterns of the var- ious species involved, (including the activity of shrubs, occurrence of annual species), among others, seem to be very important. The observations made during one year, they provide important information for these arid ecosystems which can be compared with other arid systems. Much more needs to be done.

The figures of the monthly yield and of the phenologic spectra show that we can see that in both shrub communities the autumn increase in litter yield corresponds to the greater summer activity of the evergreen species, while the spring increase, in the L. cuneifolia shrub, can be related to the winter terophytes. In the L. divaricata shrub, the spring-summer increase in related to winter-spring terophytes.

When the rate of litter production is compared with only one factor, such as rainfall, the former appears to be independent of the latter, both in monthly values as in annual totals. Similar results have been obtained by Brasell etal. (1980) in Australia, and Bray and Gorham (1964) in forests systems, etc.

There are no systematic observations known on litter yield for the arid zones of Argentina, by species or by community, except those by Passera (op. cit.) who recorded 582.6 Kg/ha of litter for the piedmont shrub community ofL. cuneifolia in Mendoza. Results of the present work, at community level, are higher in L. cuneifolia (4.71 t/ha/ year) than for L. divaricata (2.02 t/ha/year). These values are generally similar to values obtained in other desert communities.

These results do not consider the possible in- fluences of invertebrate fauna (Formicidae) which could digest litter between observation periods. During the field observations there was no evi- dence clues of an important action by this fauna; however it is possible termites were present in this area. Thus it is necessary to carry out taxonomic an quantitative studies of the invertebrate fauna and their decomposition processes in the Men- doza piedmont.

Soils cover with litter is uniform under each

shrub community; there are no significant differences when considering total cover, cover under plants, and between plants. The amount (in weight) accumulated in each stratum in 1984 shows signicantly higher amounts in the shrub stratum in both communities. This difference did not become as apparent in the other years due to less frequent sampling and the lower number of repetitions. The quali-quantitative significance of Larrea species in the composition of litter yielded in each community is evident. Further- more the role of litter in the recycling of nutrient minerals in this arid zone, as shows in Table 7 is also clear.

Acknowledgements

To Engs. Fidel Roig and Lic. A. Diblassi (IADIZA), to Dr. U. Eskuche and to Prof. J. Neiff (CECOAL) for their valuable suggestions and reading of the text, to Dr. Meentemeyer (Dept. of Geography, the University of Georgia) for his critical revision of the text, and M. E. Soler for the translation.

References

Brassell, H. M., Unwin, J. L. & Strocker, G.C. 1980. The quantity temporal distribution and mineral-element content of litter fall in two forest types at two sites in tropical Australia. Journal of Ecology 68: 13-169.

Bray, J. R. & Gorman, E. 1964. Litter production in forest of the world. Adv. Ecol. Research 2: 101-157.

Cavagnaro, J. B. 1980. Distribution of C3 and C4 grasses at different altitudes in a temperate arid region of Argentina. Oecologia 76: 273-277.

Daget, Ph. & Poissonet, J. 1971. Une methode d'analyse phytologique des praires. Criteres d'aplication. Ext. Ann. Agron. 22(1): 5-41.

De Fina, A., Giannetto, F., Richard, A. & SabeUa, L. 1963. Difusion geografica de los cultivos indices de la provincia de Mendoza y sus causas. Publ. Instituto de Suelos y Agro- tecnia, Bs. As.

Gosz, J. R., Likens, G. E. & Bormann, F. H. 1972. Nutrient content of litter fall on the Hubbard Brook Experimental Forest, New Hampshire. Ecology 53(5): 769-784.

Gray, J. T. & Schlesinger, W. 1981. Biomass production and litter fall in the coastal shrub of southern California. Amer- ican Journal of Botany 68(1): 24-33.

Horno, M. E. 1989. Rainfall interception by Prosopis flexuosa, Atriplex lampa and Larrea divaricata, unpublished.

Kurochina & Borovskaya 1976. In: L. E. Rodin ed., Produc- tivity of Arid Zone Vegetation. Nauka, Leningrad, U SSR.

Maggs, J. & Pearson, C.J. 1977. Litter fall and litter decay in coastal shrub and grassland at Sidney, Australia. Oeco- logia 31: 239-250.

Merino Ortega, F. 1983. Producci6n de hojarasca en el ma- torral de la Reserva Biol6gica de Dofiana. Tesis de Licen- ciatura, 73 p.

Minetti, J. 1989. Clima y Desertificaci6n y Control de la De- sertificaci6n. Apuntes de clases del Curso Latinoam6ricano, Mendoza 1-25 de octubre de 1987.

Morales, R. 1989.Variaciones estacionales de Nitr6geno del suelo en el flrido mendocino, en Detecci6n y Control de la desertificati6n. Conferencias, trabajos y resultados del Curso Latinoam6ricano. UNEP-IADIZA: 236-241.

Morello, J. 1955. Estudios botfinicos en las regiones firidas de la Argentina. Rev. Agr. NO Arg. 1(3): 301-370, Tucumfin, Argentina.

Morello, J. 1958. La provincia fitogeogfifica del Monte. Opera Lilloana II, 155 pag. Tucum/m, Argentina.

33

Newman, J. E. & Beard, J. B. 1962. Phenological observations: the dependent variable in bioclimatic and agromete- orological studies. Agronomy Journal 54(5): 399-403.

Passera, C. 1983. Productividad primaria neta en el piedemonte ~rido de Mendoza. Deserta 7: 156-171. IADIZA.

Polansky, J. 1962. Estratigrafia, Neotect6nica y Geomorfo- logia del Pleistoceno Pedemontano entre los rios Diamante y Mendoza. Rev. As. Geol. Arg. XVII (3-4): 127-349.

Rapp, M. 1969. Production de Litiere et apport au sol d'elements mineraux dans deux ecosystemes mediterra- neens: la foret de Quercus ilex L. et la garrige de Quercus coccifera L. Oecologia Plantarum IV: 377-410.

Reichle, D. R., O'Neill, R. V. & Harris, W. F. 1980. Princip- ios de intercambio de energia en los ecosistemas, en W. Van Dobben y R. H. Lowe-McConnell (eds.)

Roig, F. A. 1976. Las comunidades vegetales del Piedemonte de la Precordillera Mendocina. ECOSUR 3(5): 1-45.

Strojan, C., Turner, F. & Castetter, R. 1979. Litter fall from shrubs in the northern Mojave Desert. Ecology 60(5): 891- 900.

Documents

Litter yield in shrubs of larrea in the andean piedmont of Mendoza, Argentina