Chapter 11 · 2 Pedro Antonio Tiscar and Juan Carlos Linares or uneven-aged and multistrata, including very large and old individual trees. They would also include a better representation

In: Pine Forests: Types, Threats and Management ISBN: 978-1-61324-493-7 Editor: Chris Thomas Frisiras ©2011 Nova Science Publishers, Inc.

Chapter 11

PINUS NIGRA SUBSP. SALZMANNII FORESTS FROM SOUTHEAST SPAIN: USING STRUCTURE AND

PROCESS INFORMATION TO GUIDE MANAGEMENT

Pedro Antonio Tiscar*1 and Juan Carlos Linares2 1 Centro de Capacitación y Experimentación Forestal, Cazorla, Spain

2Departamento de Sistemas Físicos, Químicos y Naturales, Universidad Pablo de Olavide, Sevilla, Spain

SUMMARY Forests of Pinus nigra subsp. salzmannii (Pinus nigra hereafter) cover about half a

million hectares of pure stands in Eastern Spain and Southern France. They have been felled for timber during centuries, for example their stems were used in naval construction. Yet, these forests are part of the European Union endangered habitats listing of natural habitats requiring specific conservation measures, due to their biological values. As a consequence, Pinus nigra forests represent a suitable area where implementing sustainable forestry.

How to manage forests in a sustainable way is a current topic in forestry research. In this respect, most published papers deal with the link between ecosystem structure and function, and management.

Mediterranean ecosystems have experienced major changes in ecological structure and process because of long term human activities. Therefore, a clear reconstruction of their evolutionary environment and reference conditions, as done for some North-American pine forests (Pinus ponderosa for example), is not possible. Nevertheless, our aim in this study was to define a preliminary reference conditions for Pinus nigra forests from Southeast Spain. To do this, we used data from an old-growth stand and historical records to describe the natural structure of the mentioned forests.

Results proved that Pinus nigra natural stand structure is more complex that the structure found in current stands exploited for timber. Stands with a natural structure would be all-sized

* Corresponding author:E-mail: [email protected]

Pedro Antonio Tiscar and Juan Carlos Linares 2

or uneven-aged and multistrata, including very large and old individual trees. They would also include a better representation of Quercus species (Quercus ilex and Q. faginea), along with other broadleaf species such as Sorbus aria and Acer opalus subsp. granatensis. Nevertheless, Pinus nigra would be clearly the dominant species with over a 90% of the overall standing biomass.

Since structure is the consequence of a demographic process, we confirmed the consistency of the above results inferring the disturbances natural regimen from both dendroecological records and information about the species natural history. Additionally, we considered the recruitment dynamic of the species in the study area. Disturbances and regeneration are two key processes of forest dynamic.

Fire is a principal disturbance in Mediterranean forests at present, because most of them are human ignited. Present experience has proved that forests of Pinus nigra hardly regenerates after wild fires, as also shown by palinological studies. It has been shown that Pinus nigra and P. Sylvester’s populations sharply declined 6000 years ago as a consequence of the extensive use of fire by humans. Compared to other Mediterranean-pine species more exposed to fire-proned environments, such as Pinus halepensis and P. pinaster, P. nigra presents poorer adaptations to survive crown fires and seems unlike that fire has been an important condition in the evolutionary environment of this species. On the other hand, fire turnover has been established in 229 years in the study area, so that more frequent and less intensive disturbances than fire: heavy rain, snowfalls, insect outbreaks would be more determinant in modeling the natural structure of Pinus nigra forests.

The life cycle of Pinus nigra was studied as a conjunction of consecutive stages (pre-dispersed seed, dispersed seed, seedling and sapling) connected by processes with specific transition probabilities. Thus, we could explore Pinus nigra recruitment dynamic analyzing the relative contribution of the different potential limiting factors. The effects of masting, ageing of mother plants, post-dispersal seed predation, litter, drought and light availability were also explored. Results showed that the recruitment of new Pinus nigra individuals is mainly limited by summer drought. This situation, along with the long species longevity and irregular production of pine seeds, give rise to a slow rate of colonisation that generates uneven-aged and multistrata stands.

1. INTRODUCTION The science of forestry has long considered forests to be systems at equilibrium,

characterized by relative constancy in structural and compositional features and by a predictable successional trend towards a climax state. Under this paradigm, forests could easily be organized following the normal forest model, i.e. an idealized forest composed of fully stocked stands with a balanced age-class distribution and preferably even-aged. This point of view has changed over the last decades, after the realization that disturbances are an inherent component of ecosystem development rather than events hindering it (Kuuluvainen, 2002). As a consequence, forests are now interpreted as complex ecosystems that show multiscale heterogeneity and non-equilibrium dynamics, and traditional forestry as a practice that reduces the structural and compositional diversity of natural forests or, in other words, a practice that reduces forest biodiversity (Hunter, 1999).

Pinus Nigra Subsp. Salzmannii Forests from Southeast Spain: … 3

Biodiversity is one of the main topics in the new management strategies proposed under the paradigm of sustainable forestry that arose following the 1992 Earth Summit held in Río de Janeiro (Tíscar, 2006). Thus, foresters are currently challenged to produce wood and other resources whilst maintaining forest biodiversity. This is a premise easy to endorse, but how can foresters achieve this goal? The concepts of evolutionary environment and reference conditions might provide the necessary background to do so.

Forest-dwelling species have evolved life-history strategies which make use of the spatiotemporal distribution of habitats available in natural forests. This availability is dependent on the characteristics of natural disturbances, so that the concept of evolutionary environment assumes that forest-dwelling organisms will be better able to cope with logging disturbances if they are designed to imitate natural disturbances (Seymour and Hunter, 1999). The concept of reference conditions is closely related to the evolutionary environment concept, and refers to the variability of composition, structure and function found in natural forests (Moore et al., 1999; Kuuluvainen, 2002). We suggest that reference conditions might be used as an alternative to the normal forest model and be used as a point of reference to design sustainable forestry.

In this article, we review relevant literature and show new results about the structure and process of Pinus nigra forests from southeast Spain in order to propose a set a preliminary Reference Conditions for the area. We also address what could constitute the natural state and process for this ecosystem (the evolutionary environment concept).

2. STUDY SPECIES AND SITE European black pine (Pinus nigra Arnold) is a circum-Mediterranean pine species. Its

natural range extends from Spain and north Morocco to Austria, Turkey and Cyprus. Most forests occur in mountainous areas between 1000 and 1500 m a.s.l. As a result, its populations are fragmented and exhibit high morphological, physiological and ecological variability. Numerous subspecies, varieties, and forms have been named following that diversity; five subspecies are currently recognized: nigra, salzmannii, dalmatica, pallasiana and laricio (Alejano and Martínez-Montes, 1996).

Pinus nigra subsp. salzmannii Dunal (Franco) (Pinus nigra or Black pine hereafter) is native to the calcareous mountains of southeast France, eastern Spain and northern Morocco. French and Moroccan populations cover no more than 4000 ha, but Pinus nigra is the dominant tree species in 544.286 ha of Spanish forests (Alejano and Martínez-Montes, 1996; Figure 1). Different historical documents prove that Black pine timber was already used for building construction in Spain 1100 years ago. During the eighteenth century, the Spanish Navy used the species for ship building, although trees had to be harvested hundreds of miles away from the coast. Later on, these pineforests suffered pressure from railway companies that used black pine timber to make sleepers. Nowadays, timber is still a valuable resource, and trees are felled despite economic or environmental constraints. In this respect, it is worth remembering that the price of Spanish timber has collapsed during the last two decades and that 21% of the forests occupied by black pine are currently within nature reserves (100% in southeast Spain). In fact, these pinewoods have been recognized as a priority habitat under the European Union Habitats Directive, due to their biological values.


Figure 1. (left) Geographical distribution of Pinus nigra subsp. salzmannii in Spain (dark areas). Cazorla-Segura mountain range is located within the rectangle in the south east of the country. (right) Detail of the distribution of Pinus nigra subsp. salzmannii in the area of Cazorla-Segura mountain range. The area where most of the studies reported here were carried out appears within a rectangle.

In southeast Spain (our study area), Pinus nigra is found in ten separate mountain ranges (Cazorla-Segura-Castril, Mágina, Sagra, Huétor, Baza, Nevada, Almijara, María, Lúcar and Filabres) all of which are part of the Béticas Mountain Range (Figure 1). The ecology and management of Pinus nigra forests from the Cazorla-Segura area (ca. 60.000 ha) have been extensively studied during the last decade; all the studies and results we report here were carried out in this mountain range, unless otherwise stated.

Cazorla-Segura mountain range has a Mediterranean type climate. Snowfalls and frost are common during the winter, but summers are dry and hot. At the core of Cazorla-Segura forests, the average rainfall is 1075 mm, of which 55 mm occur during the summer, and mean monthly temperatures range from 5.1ºC in February to 22.3ºC in August.

Pinus pinaster, P. halepensis, Juniperus oxycedrus, Quercus ilex and Q. faginea are possible companions of Black pine in those localities where it mixes with other tree species, but Pinus nigra stands are mostly monospecific in the study area. Shrub species such as Juniperus communis, Crataegus monogyna, Rosa sp., Amelanchier ovalis, Berberis hispanica and Satureja montana are frequent across the natural distribution area of Pinus nigra in the Béticas Mountain Range.

Most forests from Cazorla-Segura have been harvested following management plans since the late 19th Century. At that time, Pinus nigra forests were put under the shelterwood method, with a shelter-phase of 20 years and a rotation period of 120 years. This silvicultural method changed to an uneven-aged system some decades later, due to the difficulties in achieving successful natural regeneration. An ideal reverse-J diameter distribution was then defined as the management target. The cutting cycle was established as 15 years and the maximum residual diameter at breast height (DBH) as 50 cm. The ratio between the number of trees in one diameter class to the number of trees in the next larger class (q) was set at 1.7 for 10 cm width diameter classes.


Since 1986, Cazorla-Segura mountain range forms part of a Natural Park, well known for its biological richness which includes an important number of endemic species.

3. EVOLUTIONARY ENVIRONMENT FOR SPANISH BLACK PINE

The term evolutionary environment refers to the environment in which a species evolved

(Moore et al., 1999). Although we are basically interested in the evolutionary environment of the species Pinus nigra, it is important to realize that trees are main structural elements of forest ecosystems and that they directly or indirectly modify, maintain and create habitats for other organisms (Jones et al., 1994). Therefore, forests of Pinus nigra themselves constitute an essential part of the evolutionary environment of the species dwelling in them. The loss of natural habitats has been quoted as one of the greatest threats to biological diversity (Primack and Ros, 2002). For these reasons, it seems essential to have some information about the evolutionary environment of Pinus nigra to design silvicultural methods able to maintain the biodiversity of these forests whilst producing resources that society needs and consumes.

Pinus nigra is one of the six pine species native to the Iberian peninsula. Native Spanish pines exhibit different life-histories which may represent adaptations to fire types and frequencies. Thus, Pinus nigra, Pinus sylvestris and Pinus uncinata thrive in mountainous areas and they all share some evolutionary traits, such as late flowering and absence of serotinous cones, which indicate that their natural forests did not evolve under an environment of frequent crown fires (Tapias et al., 2004). Pinus nigra also shows other evolutionary traits: height at maturity (up to 50 m), longevity (> 600 years), dispersal season (late winter) and bark thickness (relatively thick), all of which can be interpreted as adaptations to low-intensity surface fires.

Similarly, the populations of Maritime pine (Pinus pinaster) located close to Pinus nigra forests within Cazorla-Segura mountain range are thought to have evolved under a low-intensity fire regime, because individuals are thick-barked with a low degree of serotiny compared to other populations of this species (Tapias et al., 2004). Further evidence of the absence or low frequency of stand replacement fires in the study area is the existence of an old-growth patch with an abundance of trees over 600 years old (see section 4.1). Indeed, the site has been extensively sampled to carry out different studies on dendroclimatology and no fire scars have been identified in the cores extracted from the pinetrees (E. Gutiérrez, personal communication). However, naturally ignited fires do occur in the area with an estimated recurrence of 229 years (López-Soria and Castell, 1992). This frequency is low enough to make minor disturbances (individual or small groups of black pines may be uprooted by wind or heavy snowfalls, or killed by pathogens) prevail over major or stand-replacing ones (Oliver and Larson, 1996). It is also known that post-fire regeneration of Pinus nigra forests is almost nil after wildfires (Espelta et al., 2003; Rodrigo et al., 2004).

The continuous presence of Pinus nigra as the main tree species in the study area has been confirmed by paleoecological indicators. Thus, Pinus nigra-type pollen predominates in a stratigraphy elaborated from sediment cores that goes down to year 11500 BP. Microcharcoal particles are also visible along the whole stratigraphy, indicating the occurrence of fires over the mentioned period of time. The presence of microcharcoal particles is more persistent during the last 5000 years, as climatic conditions have become


drier and human activities have increased in the area. Nevertheless, there is no evidence concerning intensification of agriculture and grazing before Roman times, 2000 years ago (Carrión, 2002).

Forests from Iberian mountains tend to be dominated by one single species due to ecological and paleobiogeographical reasons (Blanco et al., 1998), although other tree species are present. For example, Pinus pinaster, Taxus bacata, Acer opalus and Sorbus aria in Pinus nigra-dominated forests from the study area. Deciduous (Quercus faginea) and evergreen oaks (Q. ilex) are also present in today’s forests, but, in this case, pallinological studies confirm their presence and relative abundance for the last 7200 years (Carrión, 2002).

4. STRUCTURE OF PINUS NIGRA FORESTS Rather than describe the current structure of Pinus nigra forests, we will focus on two

structural topics which are important for the establishment of Reference Conditions. Firstly, we will describe the structure of a forest site relatively undisturbed by human activities. Forests undisturbed by human activities provide an opportunity to obtain information about their natural composition, structure and process, i.e. the three attributes that determine the biodiversity of an area (Noss, 1993). This information can be used as a baseline to design sustainable silvicultural systems.

Secondly, we will analyze the changes caused by selviculture in the structural diversity of managed forests. Forest management normally implies the felling of trees in order to control the establishment, growth and structure of forest stands. By cutting trees down, foresters influence some ecosystem processes at various spatio-temporal scales (e.g. mortality rates of tree species or the frequency of disturbances in given sites) and modify the structure of forest stands (e.g. grouping harvesting operations in some stands). So, the monitoring of temporal changes occurring in the structure of exploited forests seems a proper method for analyzing the long-term influence of forestry practices on forest biodiversity.

4.1. Structure of an Old-Growth Pinus Nigra Forest Old-growth forests can be used as templates to delineate nature-based forest management

with the aim of producing timber whilst maintaining forest biodiversity. Much of the existing information about the composition, structure and processes of old-growth forests come from boreal and wet-temperate regions, where relatively large old-growth forests still persist (Foster et al., 1996). Studies from the circum-Mediterranean Regions are comparatively scarce and normally refer to small patches located within larger human-disturbed forest areas (Piovesan, 2005; Peterken, 1996). Obviously, this situation reflects the historical use of fire, grazing and other forms of anthropogenic disturbances that have transformed Mediterranean forests over several millennia of civilization, so that forests free of human activity are virtually unknown in the region. Nevertheless, a forest does not need to be virgin in order to be classified as old-growth (Peterken, 1996).

Definitions for old-growth forest include criteria relating to the degree of human disturbance and to the age, size and successional status of the overstorey trees (Foster et al.,


1996). In this article, we will consider as old-growth a patch of forest located in a remote site from Cazorla-Segura mountain range. As said earlier, various pollen stratigraphys indicate that the area has been continuously wooded by Pinus nigra forests since the beginning of the Holocene. Two thousand years ago, under climatic conditions similar to present and immediately before people became a significant ecological factor, Pinus nigra was the main tree species in areas above 1200 m a.s.l. (Carrión, 2002). Accordingly, forests of Pinus nigra are seen as the endpoint in the vegetation successional development of the oromediterranean belt (Valle et al., 1989).

The study patch (ca. 60-ha) is located at 1800 m a.s.l. in a site that remained quite inaccessible until 1955. This circumstance prevented the felling of trees until the early 1970’s. Then, one harvesting operation decreased the patch naturalness, although the previous forest structure could be reconstructed, because the harvest was quantified and the data recorded in management files, and many stumps are still visible in the area. No other trees have been logged afterwards. Grazing by domestic livestock has not been allowed since 1893, although wild ungulates are present. Among this guild of animals, there are two exotic species introduced in the 1950’s: wild sheep (Ovis musimon) and fallow deer (Dama dama), that exert a small amount of pressure on Pinus nigra saplings (Cuartas and García-González, 1992). The patch was discovered whilst carrying out a study on dedroclimatology, which led to the identification of an unusually high number of very old trees and large snags (Creus, 1998).

In summary, the forest patch we describe here is composed of native species that represent the late successional condition in the area. Trees regenerate naturally, all age-classes are represented and many trees are very old and large. The site exhibits a small degree of human disturbance and it is located within an area that has been continually wooded since immemorial time. Despite obvious limitations, these characteristics make the population a good template for the purpose of defining a set of Reference Conditions for Pinus nigra forests in southeast Spain.

The current tree structure of the old-growth patch shows a mean density of 115.88 ind·ha-1 for stems with DBH ≥ 10 cm, and a mean density of 324.65 ind·ha-1 with DBH < 10 cm. Mean basal area is 25 m2·ha-1 and mean volume 176.44 m3·ha-1 (Table 1). These mean values were calculated from data collected in nine circular plots placed at random in the site, each of 20 m radius. The two most widely separated plots were 800 m apart, and the two closest ones 50 m apart. Stumps from the harvest operation carried out in the early 1970’s were not found within the plots.

The observed among-plots variability (expressed by the coefficient of variation, CV) indicates that the site structure is highly heterogeneous, varying at relatively short distances. Similarly, the range of diameters at breast height and the presence of regenerates in all the plots would indicate an elevated degree of vertical complexity, since DBH and height were positively correlated (RPEARSON= 0.82, n = 133, p-valor< 0.01; Table 1).


Table 1. Structural variables of the nine plots sampled.

Plot Ind·ha-1 (dbh


larger diameter class, is low in the old-growth study patch. On the other hand, the old-growth patch contains a much higher density of old trees (some of the trees in the old-growth patch are over 1000 years old (Creus, 1998), Figure 2).

Perhaps more than a fifth of the fauna of woodland depends on dead and decaying trees (Fuller and Peterken, 1995). Current standing volume of dead trees (snags) is approximately 15 m3/ha, which represents an 8.5% of the overall standing volume.

4.2. Changes in Forest Structure Over 60 Years of Management Most forests from Cazorla-Segura mountain range are divided in comparments to aid the

planning of tactical considerations, i.e. to answer the questions where and when silvicultural treatments should be applied. In areas where Pinus nigra is the dominant tree species, regularly harvested comparments usually exhibit the diameter distributions plotted in Figure 2. As can be observed, one effect of management is that large trees (DBH > 60 cm) are very scarce in logged forests compared to those forests with minor human disturbances.

Data for Figure 2 came from 1996 forest inventory. Several forest inventories have been carried out in the study area over the last century, as part of the planned management implemented in Pinus nigra forests after 1893. Until the 1980’s, inventories were made independently for each compartment by tallying all trees with a DBH equal to or bigger than 20 cm in classes of 10 cm.

Data from a sample of 12 compartments were used in a repeated-measurements analysis of variance, in order to follow changes in forest structure between 1920 and 1979. Records from earlier surveys were not considered, because they were thought to be insufficiently accurate. Data from forest inventories carried out after 1979 were not considered either, because they used sampling methods affected by an unknown error at the compartment level. In the analysis, compartments were considered as sample units, because their boundaries have never changed since they were first delineated.

Results from the analysis of variance, summarized in Table 2, indicate that the number of trees up to 40 cm DBH increased steadily over the period considered, although a diminution was recorded between 1967 and 1979. The density of thin (and young) trees increased, probably due to the effective control of some grazing practices during the 20th century, which favoured Pinus nigra regeneration. No significant changes were noted in the abundance of trees of other sizes, although the density of large trees (DBH > 60 cm) was reduced by half between 1920 and 1979.

Old black pines show a distinctive arquitecture that might provide biodiversity with habitats different from those provided by younger trees, but little is known in this respect. As far as we are aware, the only existing information refers to the importance of large black pines as primary sources of deadwood, because the survival of some endangered saproxilic beetles depends on the availability of large snags and logs (Molino, 1996). Therefore, the observed reduction of large tree density might represent a decrease in biodiversity associated with Pinus nigra forests, although this reduction was not significant in the repeated-measurements analysis of variance.

Similarly, large trees contribute to stand structural diversity, which can indicate overall species diversity, as shown in studies concerning plant, avian and insect diversity (Jonsson


and Jonsell, 1999; Tellería et al., 1992; González-Esteban et al., 1997; DeGraff et al., 1998; Kirby, 1992).

Table 2. Diameter distribution of the mean number of trees per compartment and forest

inventory.

Year of forest inventory Diameter class (cm) 1920 1944 1959 1967 1979

25 1619.92(a) 2126.00(b) 2304.83(b) 2885.25(c) 3154.17(c) 35 1070.92(a) 1298.25(a) 1374.25(a) 1501.67(b) 1544.58(b) 45 763.75 860.75 841.75 884.83 753.00 55 479.08 448.42 386.08 386.17 280.42 65 197.33 196.42 151.67 162.08 99.42 75 84.33 67.08 52.42 53.25 29.00 85 31.00 28.17 20.83 22.00 9.25 95 14.08 10.92 6.75 7.83 3.08 >100 5.42 6.42 3.92 3.75 3.58 > 40 cm 1574.99 1618.18 1463.42 1519.91 1177.75 > 60 cm 332.16 309 235.59 248.91 144.33 TOTAL 4265.83 5042.43 5142.50 5906.83 5876.50 n = 12 comparments, with a mean surface of 48 ha. Different letters indicate significant differences in a

repeated-measurements ANOVA (p< 0.05). Among the variety of indices available for expressing stand structural diversity, species

diversity indices have gained wide acceptance in forestry and, perhaps, Shannon’s index is the most commonly used (Staudhammer and LeMay, 2001). It is defined as follows:

∑=

−=S

ipipiH

1ln' [1]

where pi is the proportion of individuals in the ith species, and S is the number of species. Here, ‘species’ is a convenient term for the categories into which we place individuals, i.e. either tree species or diameter classes can be thought of as ‘species’.

We computed equation [1] using the diameter distribution of the same 12 comparments mentioned above. Diameter classes were considered as species. Results show that a significant decrease occurred in the stand structural diversity over the period of time considered (Kruskall-Wallis test; K = 10.67; p-value = 0.03; Figure 3). Little is known about likely relationships between the existing biological diversity and the observed structural diversity in any given site from the study area, but a positive relationship has been found in other areas (Jonsson and Jonsell, 1999; Tellería et al., 1992; González-Esteban et al., 1997; DeGraff et al., 1998; Kirby, 1992).

The sampling of relatively undisturbed sites and the remeasuring of permanent plots, or comparments, may be suitable methods for establishing reference conditions (Moore et al., 1999). As found in other studies on the impact of forestry in natural forests (Sendak et al., 2003), the silvicultural removal of trees changed the size distribution in the sampled compartments, and reduced their structural diversity (Linares et al., 2010). When these results


are compared with the current structure of the old-growth patch, the most obvious effect of forest management on Pinus nigra natural forests is a reduction of large tree density.

Shannon's indices

0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

Yea

r of f

ores

t inv

ento

ry

1920

1944

1959

1967

1979

Figure 3. Changes in the values of Shannon’s index over 59 years of forest management. Error bars represent the 5th/95th percentiles, boxes represent the standard errors, solid lines represent the mean values and points are outliers. N = 12 comparments.

Forestry can affect species composition by the silvicultural removal of less desirable species. Management plans from the study area have continuously considered Pinus nigra the principal forest species. This explicitly justified the felling of Quercus species for charcoal production with the ultimate aim of substituting oaks by pinetrees, at least during the first half of the 20th century. Consequently, oaks only survived in specific sites which management plans would eventually accommodate coppice stands.

5. THE REGENERATION PROCESS IN BLACK PINE FORESTS The structure of a forest stand depends on the nature of the disturbances which occur, i.e.

their frequency, amount of overstory removed, size and shape of the area disturbed (Olive and Larson, 1996), and on how new individuals and species appear after disturbance. Thus, disturbance and regeneration patterns are essential in the development of nature-based forest management.

The identification of factors that affect recruitment rates is important at both the population and the community level (Harper, 1977; Clark et al., 1999). It is also a major goal for foresters, who have long appreciated a quick stand regeneration following harvest operations (Davis et al., 1987). Factors affecting forest regeneration might be biotic or abiotic, and may act limiting seed production, seed dispersal, seed germination or seedling


establishment (Tíscar, 2007). When recruitment is difficult because the number of seeds is small, dispersal success is low or the number of seeds available for germination decreases dramatically due to post-dispersal seed predation, plant recruitment is said to be seed-limited. On the other hand, microsite-limitation occurs when recruitment is confined to a few favorable micro-environments or patches (Eriksoson y Ehrlén, 1992; Rey et al., 2006).

The regeneration process of Pinus nigra has long concerned Spanish foresters. In this section, we revise literature and present new results which are relevant for the regeneration of this species in the Southeast mountain ranges. A number of factors that limit Pinus nigra recruitment have been quoted over the last decades (Tíscar, 2005). They can be grouped as seed-limiting factors: (i) the irregular production of pineseeds, (ii) the ageing of mother plants and (iii) the predation of seeds, or microsite-limiting factors: (iv) the presence of litter, (v) drought and (vi) the availability of light.

5.1. Masting as a Impediment for the Regeneration of Harvested Pinus Nigra Stands

When the principles of forestry began to be applied in Spain at the end of the XIX

century, most Pinus nigra forests were intended to regenerate following prescriptions of the uniform shelterwood method, with a shelter-phase of 20 years and a rotation period of 100 or 120 years. Pinus nigra stands often failed to regenerate under these prescriptions, so some forests eventually adopted more flexible silvicultural methods, such as the selection method, or extended the shelter-phase up to 40 years. Where the uniform shelterwood method has maintained, pine-seeds might be added to the ground in order to increase the density of seedlings.

Foresters resort to direct seeding because Pinus nigra shows considerable fluctuation in the number of seeds produced from one year to another, there being years in which the standing trees exhibit a very low crop-size. As a consequence, regeneration might be delayed within the regeneration period fixed by silvicultural methods, and foresters tend to look at crop-size variation as an impediment to establish new individuals after the realization of harvest operations. However, from a biological point of view, Pinus nigra is a very long-lived species, and can well pay the price of not reproducing during some years, if other benefits are gained by doing so. This means that the irregular production of seeds may have evolutionary implications that should be acknowledged, if more ecological-based silvicultural methods are to be implemented.

The species producing intermittent crops of seeds are said to show “mast seeding” or “masting”. More specifically, masting is “[the production of] seed crops synchronously at irregular intervals but with an average periodicity characteristics of the species” (Silvertown 1980). Among Spanish foresters, Pinus nigra is considered as a species that shows masting, because it produces large seed outputs every 3-4 years and small crops in between (Ruiz de la Torre, 1979).

Nevertheless, long data sets are needed to make accurate calculations of intermast intervals (Houle and Filion, 1993), and no such a long-term record exists for the most recent years in the study area. However, there are records of annual crop-sizes from 1906 to 1925, registered by Enrique Mackay, a forest engineer who worked in the area. He recorded


annually the amount of pinecones according to an ordinal scale: abundant, medium, scarce and nil, where an abundant outcrop was two-fold a medium one (Mackay, 1926).

Table 3 shows the mentioned records, besides available data of spring and September rainfall. Mackay (1926) suggested that a lack of water during those moments of the year would significantly reduce current crop size. The possibility that climate determines seed output is the most parsimonious explanation for masting: seed production varies simply because it mirrors variation in the environment. Indeed, all plants are expected to be affected by environmental variability, but there may also be an adaptive value in masting so selective forces may act to favour variation in crop-size.

Table 3. Crop sizes and meteorological variables over 20 years of observations.

Year Crop size Spring rainfall September rainfall 1906 Abundant - - 1907 Nil - - 1908 Nil - - 1909 Abundant - - 1910 Medium - - 1911 Nil - - 1912 Nil 402 77 1913 Nil 241 118 1914 Scarce 285 0 1915 Scarce or Nil ¿? 465 0 1916 Scarce or Nil ¿? 193 104 1917 Scarce or Nil ¿? - - 1918 Medium - - 1919 Medium - - 1920 Abundant 371 31 1921 Medium 337 109 1922 Abundant 226 38 1923 Medium 342 35 1924 Scarce or Nil ¿? 171 0 1925 Scarce or Nil ¿? 342 13 ¿? It was not possible to determine whether crop size was scarce or nil.

Kelly (1994) proposed different types of masting ranging from strict through normal to

putative masting, and ways to recognize them. For instance, normal masting can be confirmed (i) by finding that annual seed output does not fluctuate around an average value, but exhibits a marked trend toward bimodality, (ii) or by the presence of switching: in years of large crops, resources are diverted from vegetative growth or reserves. Pinus species seem to display normal masting.

Our personal observations are that most trees in a Pinus nigra population may produce no cones at all during one or more consecutive years, although there are always some trees producing either few or many pine cones. Table 3 exhibits this same pattern. In a study on dendroclimatology, we also found a significant reduction in vegetative growth coinciding with known high seed years (Linares and Tíscar, 2010). Additionally, we used data from a 4-


year study to calculate a coefficient of variation for seed output, which resulted in a value of 1.57 (if CV>1.6, switching is highly likely to be present (Kelly, 1994)). Together, this evidence indicates that Pinus nigra shows normal masting.

Studies on masting have concluded that mechanisms related to economy of scale (i.e. a synchronization of the reproductive effort improves pollination efficiency and/or more seeds survive seed predation) should favour the evolution of occasional large reproductive efforts rather than regular smaller ones (Norton and Kelly, 1988; Kelly, 1994; Herrera et al., 1998). In this respect, the wind pollination hypothesis states that wind-pollinated plants obtain reproductive benefits by synchronizing large flowering efforts, because it increases the probability of pollination (Smith et al., 1990). It is been observed that Pinus nigra produces higher percentages of empty seeds (unpollinated) in low flowering years (Tíscar, 2007). Similarly, the predator satiation hypothesis states that large seed crops are likely to satiate seed predators, which thus destroy a lower percentage of crop (Kelly, 1994). Most Pinus nigra dispersed-seeds are predated by rodents and birds in low seed years, while a higher percentage survives predation in high seed years (Tíscar, 2003).

Whether masting has an adaptive value and even whether Pinus nigra actually shows masting or not, might be of little interest for foresters aiming to regenerate Pinus nigra stands in harvested forests. However, it might be convenient to admit that the irregular production of pine-seeds is a natural phenomenon, when designing nature-based silvicultural methods.

5.2. Fertility of Senescent Trees of Pinus Nigra Harper (1977, p.686) depicted an idealized life cycle according to which polycarpic

plants would increase their reproductive output over the years to a plateau and then gradually decline during a phase of senescence. This variation in the lifetime fecundity of individual plants is expected to influence patterns of recruitment dynamics, with important consequences for the demographic structure and persistence of populations. This ecological fact has been interpreted by Spanish foresters from a practical point of view: the presence of many old trees is a seed-limiting factor that might prevent forest stand regeneration (Madrigal, 1994). Yet new forestry strategies, developed under the paradigm of sustainability, recommend maintaining old trees to favour biodiversity (Ferris and Pritchard, 2000). Thus, it is worth recalling that most Pinus nigra forests from the study area are currently included in natural parks, where management plans require foresters to consider both timber products and biodiversity conservation.

The relationship between fertility and mother plant age has been studied to some extent for the Pinus nigra populations from Cazorla-Segura mountain range. It has been found that individuals over 200 years old show reduced fertility compared to that of younger trees, fertility being defined as the capacity to produce sound seeds able to germinate (Tíscar, 2002). Two-hundred years must be seen as an arbitrary threshold to distinguish old trees; the rotation age in the study forests is 120 years, so 200 years well represents the upper limit of the time a black pine will remain in a managed stand.

Specifically, the quoted study found trees over 200 years old to have a fertility of about 81% (i.e. 19% of the sampled pine-seeds were empty or unable to germinate), 7.5 points less than the fertility observed in mature trees with ages up to 120 years (Table 4). The possibility that the overall seed output could be smaller in old trees was not tested, but that is not


probably the case. Individual size is a surrogate for reproductive output and older pinetrees normally have larger crowns (Dodd and Silvertown, 2000). Additionally, our personal observations are that old trees, even up to 600 years old or more (and therefore far beyond the 200 years threshold), still produce abundant crops in masting years.

Table 4. Mean values ± standard errors of fertility and seedling performance estimates

for the three groups of age considered.

VARIABLE Fertility

(%) Root length (mm)

Stem length (mm)

Above-ground biomass (mgr)

Bellow-ground biomass (mgr)

Age 201 yr

89.21 ±1.04(a) n= 66 82.59 ±1.87(ab) n= 54 81.74 ±1.25(b) n= 78

248.55 ±4.20(a) n= 120 258.51 ±5.72(a) n= 80 249.81 ±5.37(a) n= 100

34.98 ±0.63(a) n= 120 37.25 ±0.86(a) n= 80 34.28 ±0.81(a) n= 100

101.16 ±2.82(a) n= 120 103.86 ±3.85(a) n= 79 90.16 ±3.13(b) n= 124

65.82 ±6.31(a) n= 12 69.44 ±8.59(a) n= 8 50.93 ±6.45(a) n= 13

Different letters indicate significant differences (Scheffé pos hoc test, p


On the other hand, the accumulation of plant litter is another factor acknowledged by foresters and ecologists for obstructing tree regeneration (Xiong and Nilsson, 1999). Thus, it is generally accepted that Pinus nigra establishes better on soils disturbed by harvesting operations (Tíscar et al., 2010).

Here, we report results from an experiment designed to account for both predation and litter effects. In an area ca. 10 ha, we identified ten sites (gaps of similar size) where Pinus nigra saplings had recently established, and randomly selected six of these sites (blocks). At each site, we sowed 176 pineseeds distributed in 24 square plots (12.5-cm x 12.5-cm) according to a three-factor design: predators (allowed versus excluded by wire cages), soil disturbance (undisturbed versus disturbed by removing litter and vegetation until the ground was bare), and density (three sowing densities: 4, 6 and 12 seeds per plot). Each treatment combination was replicated twice within each site. 1056 pineseeds were sown for the whole experiment. We examined the effects of predation and disturbance on seedling emergence (percentage of seedlings emerged from the sown seeds) by a randomized block design, where blocks were considered random, and predation and disturbance fixed factors.

A total of 169 seedlings emerged by the end of the experiment. Mean probability of emergence per plot was 0.159 ± 0.021 seedlings emerged / seeds sown (range 0-1). 45 plots in the predator-exclusion treatment contained at least one seedling by the end of the emergence period, while only 8 did in the predator-allowed treatment (Table 5). Thus, the factor predation explained satisfactorily the probability of emergence, i.e. the probability of finding emerged seedlings in a plot (χ2 = 45.44; p-value < 0.001), and its interaction with the factor disturbance (χ2 = 4.60; p-value < 0.05).

Table 5. Results of ANOVA on the probability of emergence (seedlings emerged / seeds

sown).

Source d.f. MS F P Block 5 442.40 2.748 0.3581 Predation 1 13116.6 107.709 0.0001 Density 2 169.71 0.614 0.5602 Disturbance 1 5306.21 70.310 0.0004 Block x Predation 5 121.78 0.571 0.7217 Block x Density 10 276.25 1.151 0.4140 Predation x Density 2 341.28 1.641 0.2419 Block x Disturbance 5 75.47 0.722 0.6387 Predation x Disturbance 1 5678.74 78.233 0.0003 Density x Disturbance 2 392.02 3.951 0.0544 Block x Predation x Density 10 207.97 3.092 0.0447 Block x Predation x Disturbance 5 72.59 1.079 0.4275 Block x Density x Disturbance 10 99.22 1.475 0.2751 Predation x Density x Disturbance 2 303.78 4.516 0.0401 Blo. x Pred. x Density x Perturbac. 10 67.27 0.835 0.5971 Error 72 80.60 Angular transformed data.


Woodmice consumed most of the pineseeds deposited on sowing plots. These rodents usually search for food under the shelter of scrub, groups of saplings, or dense forest stands (Alcántara et al., 1999), which happened to be the vegetation structure around the six study sites in the sowing experiment we are reporting here. Accordingly, other studies have also found that seeds of Pinus nigra are more intensively predated in the proximity of shrubs (Tíscar, 2003, see section 5.7).

5.4. The Effect of Plant Litter on Pinus Nigra Recruitment As with predation, plant litter also had a negative effect on seedling emergence. Litter

covering the ground of Pinus nigra stands is normally composed of forbs and pine-needles, and its removal favours seedling emergence as shown by results from the sowing experiment reported earlier (Table 5). However, another study found a positive correlation between litterfall depth and density of seedlings in Pinus nigra stands from the study area (Alejano et al., 1997). Since we have experimentally shown that emergence of new seedlings is higher in disturbed soils, i.e. soils where litter has been completely removed (Table 5, see section 5.3), results reported by Alejano et al. (1997) were likely to be reflecting a higher probability of survival in sites with abundant litterfall, rather than higher emergence. Litter normally has a stronger effect on plant germination than on establishment (Xiong and Nilsson, 1999). As a consequence, there must be a balance between the negative and the positive interactions of litter with Pinus nigra recruitment which eventually determines stand regeneration (see section 5.6).

5.5. Drought as a Limiting Factor for Black Pine Regeneration Summer drought is considered to be the main factor impeding the successful regeneration

of Pinus nigra stands in the study area. For instance, Tíscar (2007) observed less than 6% of post-summer survival in a cohort of seedlings emerged in spring of that same year. Water stress was reported as the major cause of mortality. This result and cause of mortality coincide with other populations of Pinus nigra (Cerro et al., 2009), as well as with other plant species in the area (Herrera et al., 1994; Rey and Alcántara, 2000; Gómez-Aparicio, 2008). The irregular occurrence of late summer storms has consequently been quoted as a key event for the regeneration of Pinus nigra stands (Mackay, 1926; Alejano et al., 1997). However, temperature might be equally important, through its effect on water balance. Thus, most saplings (height < 1.30 m) surveyed in a stand from the study area were found to have established in years with an intensity of drought far below the observed mean value (684 mm), as shown in Figure 4. Drought intensity was estimated as the sum of monthly differences between precipitation data and Thornthwaite potential evapotranspiration for June, July, August and September. Correlation between drought intensity and establishment of new trees might seem unclear in Figure 4, as some years with low drought intensity do not recruit saplings, but it must be remembered that Pinus nigra produces intermittent crops of pineseeds (see section 5.1).


Regeneration year

1980 1985 1990 1995 2000

Dro

ught

inte

nsity

(mm

)

0

20

40

60

80

100

120

140

160

180

No.

of e

stab

lishe

d sa

plin

gs

0

20

40

60

80

100

120

140

160

180

Drought intensitySaplings

Figure 4. Relationship between summer drought and establishment of new individuals of Pinus nigra. The horizontal line represents the mean value for drought intensity from June to September.

Over the last 40 years, the mean June temperature has increased significantly in the study area (0.92ºC/decade), while mean precipitation has decreased 6.41 mm/decade. On the contrary, July, August and September do not show any significant trend in mean temperature or precipitation values (Linares and Tíscar, 2010). This suggests that wet, cold weather in early summer could be more important for Pinus nigra recruitment than the irregular occurrence of storms later on. Unfortunately, Climatic Change projections and observational data indicate that Pinus nigra recruitment may be very difficult in the future.

5.6. Light and Water Balance Interactions Foresters manipulate canopy cover to allow light to reach the ground and trigger the

establishment of new tree individuals. Indeed, the manipulation of canopy cover is at the core of silvicultural methods implemented to regenerate stands. Thus, the control of light appears as a key aspect of a forester’s work. However, canopy cover also modulates soil water balance (e.g. offering protection from excessive evapotranspiration, ameliorating extreme temperatures, improving soil properties), which can be of more importance under Mediterranean conditions (Gómez-Aparicio et al., 2009). Thus, different authors have addressed the importance of facilitative interactions in which seedlings of Pinus nigra benefit from the special abiotic conditions present beneath the overstory (Aunós et al., 2009; Trabaud and Campant, 1991). Cover is normally expressed as a percentage, although stand basal area measured in m2/ha is also used. Stand basal area is easy to measure and works as a surrogate of cover, so foresters frequently use stand basal area as a control variable in silvicultural


prescriptions. For Pinus nigra, recommendations are 18-30 m2/ha of stand basal area to successfully regenerate the species (Cerro et al., 2009; Alejano et al., 1997).

That range of stand basal area supposes a canopy cover of 60-70%, which may represent a compromise between light availability and facilitation benefits (improved water balance) in Pinus nigra stands. Thus, there must be an upper limit of canopy cover above which Pinus nigra regeneration is impeded, due to physiological constraints (Gómez-Aparicio et al., 2006). On the other hand, despite the fact that Pinus nigra withstands high radiation, water supply is the main limiting factor that controls regeneration in open spaces (see section 5.5), so some cover below an upper limit is necessary to prevent excessive evapotranspiration and improve water balance (Gómez-Aparicio et al., 2006 and 2009).

5.7. Recruitment Dynamics So far, we have considered separately to the role of masting, mother plant age, post-

dispersal seed predation, soil conditions and summer drought in the recruitment dynamics of Pinus nigra. However, those factors are just parts of a whole, i.e. recruitment is a multiphase process involving several sequential life-history stages (i.e. seeds, seedling, saplings) connected by transitional processes (i.e. dispersal, emergence, survival) which are affected by factors, such as those already analyzed (Gómez-Aparicio, 2008; Jordano and Herrera, 1995; Rey and Alcántara, 2000).

Figure 5 summarizes the stages (rectangles) that were considered in a previous study on Pinus nigra recruitment dynamics (Tíscar, 2007). In the same figure, ovals represent processes, i.e. any event that affects the probability of a propagule moving from one stage to the next, and factors influencing processes appear on the right hand side.

Four stages are included in Figure 5: pre-dispersed seed, dispersed seed, seedling and sapling. Pre-dispersed seed refers to seeds still contained in either closed or open cones. Some insects (Pisodes sp.), birds (Loxia curvirostra, Sitta europaea, Parus sp.) and squirrels (Sciurus vulgaris) are potential predators of pine-seed before they are removed by wind from the open cones and dispersed onto the ground. Here, woodmouse and some ant and bird species consume seeds as it was said earlier. If seeds survive post-dispersal predation and overcome soil conditions preventing germination, they reach the stage of seedling. The seedling needs to survive water stress, herbivory and other difficulties before it becomes a sapling.

For every transitional process, there is an associated transition probability (TP) that represents the probability of moving forward between consecutive stages. This probability is measured as the number of propagules completing a stage divided by the number of propagules entering the stage. Additionally, an overall probability of recruitment (OPR) can be calculated as the product of every TP.

TPs and OPRs permit comparisons to be made of recruitment dynamics between microhabitats. Thus, Table 6 shows the spatial dynamics of recruitment for the four microhabitats most frequently found in Pinus nigra stands. They are: dense forest (cover > 80%), gap (openings in the canopy between 1000 and 1400 m2 in surface (Tíscar and Ruiz, 2005)), border (the dense forest-gap ecotone) and clear forest (cover < 50%). For a given microhabitat, the probability of dispersal was defined as the ratio of mean seed density in this microhabitat divided by the sum of mean seed densities in all the microhabitats. Therefore,


probability of dispersion provides a measure of the relative likelihood of seeds being deposited in each microhabitat. In the study we are reporting here, mean seed densities were calculated from the number of seeds collected in seed traps (0.0744 m2 aluminium trays), that had been place at 48 sampling locations per microhabitat.

Insects, birds and squirrels

Wind

Ants, birds and woodmice

Litter and other factors preventing germination

Physical damage, herbivory, Pests and diseases, water stress

As above

Pre-dispersed seed

Dispersed seed(sound + empty)

Saplings

Seedlings

Escape from pre-dispersal predation

Escape from post-dispersal predation

Survival

Establishment

Dispersal

Germination and emergence


Figure 5. diagram of recruitment to show stages (rectangles), proceses (ovals) and factor influencing proceses (right hand side).

Probability of dispersion was rather similar between microhabitats (between 0.24 and 0.27, Table 6), i.e. all the microhabitats received a similar amount of pine-seeds which, on the other hand, were equally viable (filled seeds able to germinate). However, the probability of a pine-seed escaping seed predators was lower in clear forests, where the abundance of shrubs (Juniperus communis, Berberis hispanica, Rosa sp.) favoured the presence of woodmice. Once pineseeds had escaped mice and other seed predators, a seedling emerged from nearly every sound seed deposited in the microhabitats border and clear forest (TPs ≈ 1), whilst the probability of emergence decreased for seeds dispersed either to the gap or to the dense forest microhabitats (TPs equaled 0.65 and 0.77 respectively). In any case, the main “bottleneck” in the recruitment dynamics of Pinus nigra was survival to summer drought. Thus, OPRs ranged from 0 to 0.0027, i.e. from none to 27 emerged seedlings out of 10000 were recruited after the first hot and dry season in seedlings life.

On the contrary, survival expectations seem to be much higher for saplings, i.e. any tree between 10 and 130 cm tall (Table 6).

Table 6. Spatial dynamics for Pinus nigra recruitment. Values shown are process-

specific transition probabilities (TPs), with overall probabilities of recruitment (OPR) given at the base of each column (shown in bold type).

Habitat Stage Process clear forest border gap dense forest (1) Pre-dispersed seed.......................... Reproduction 0.2366 0.2651 0.2446 0.2537 (2) Dispersed seed ...............……………

Dispersal (sound + unviable seeds) 0.8738 0.9400 0.8858 0.9197

(3) Dispersed seed ................................

Escape from post-dispersal predation 0.6979 0.8437 0.9270 0.9271

(4) Dispersed seed ..........................

Germination and emergence ~1 ~1 0.6533 0.7741

(5) Seedlings.......... Establishment 0.019 0.00 0.00 0.011 OPR 0.0027 0.00 0.00 0.0018 (6) Sapling survival....................................... 0.9704

6. REFERENCE CONDITIONS AND MANAGEMENT IMPLICATIONS In theory, reference conditions for a particular type of ecosystem should consider all its

components, including organisms, structures, biogeochemical cycles, processes of natural disturbances, etc. (Moore et al., 1999). In practice, many components are difficult to understand or quantify. For example, the number of organisms present in any patch of forest is enormous, so it is impossible to include all of them in management considerations. Cazorla-Segura mountain range is well known for its biological diversity, yet new invertebrate species, and possibly plants, are still to be discovered as the territory is better explored (Alonso et al., 2004). Similarly, it is now that the diversity of micro-organisms and


its role in ecosystem process are being devised (Herrera and Pozo, 2010). Considering such known and unknown biological richness in forest management plans seems an overwhelming task. However, management “shortcuts” are possible, because composition, structure and function are intimately related (Noss, 1993). For instance, forest structure is the result of processes, such as disturbance and regeneration, and both structure and process create habitats for forest-dwelling species, i.e. forest composition.

Because of that intimate relationship between composition, structure and function, the aim of sustainable forestry (producing resources whilst maintaining biodiversity) can be achieved by the application of general management principles that refer to structure and process in forests (Lindenmayer et al., 2006). The use of natural disturbance regimens to guide human disturbance regimes (silvicultural systems) seems particularly important (Seymour and Hunter, 1999). Other approaches simply emphasize creating structural diversity at the stand level (O’Hara, 1996).

From the body of available information, we will present a set of preliminary reference conditions which provide a baseline to: (i) determine the key structural variables of Pinus nigra natural forests, and (ii) characterize the key natural disturbance regime operating within them.

We infer that natural Pinus nigra forests were relatively open (canopy cover < 70%) and multi-aged, with an important presence of large trees. Density of large trees constitutes our first key structural variable. Based on a methodology explained in Emborg et al. (2000), we elaborated a model of forest cycle in order to compare current structure with that expected in a forest free of human intervention (Table 7). This model forest cycle offers a quantitative estimation of the density of large trees in terms of occupied surface (up to 62.5%). Other sources of data, such as the current structure of the old-growth study patch, offer information on diameter distributions and patterns (Figure 2).

Table 7. Duration of each phase of the estimated forest cycle and the actual area covered

by each phase in two comparments from the study area.

Model duration Current value Phase of the forest cycle years % surface % surface Innovation (Height < 2 m) 20 5 3 Early aggradation ( DBH < 50 cm) 130 32.5 84.9 Late aggradation (Heightmax = 27 m; 50 < DBH < 80)

200 50 12.1

Biostatic (DBH ≥ 80 cm) >50 12.5 0 Degradation Variable - 0

The exact composition of natural Pinus nigra stands can not be inferred, although it

would be expected that this species accounted for more than 90% of the overall standing biomass (Quercus, Acer, Fraxinus, Taxus, and Ulmus, among others, would account for the remaining 10% (Carrión, 2002)). Considering the abiotic environment of the study area, the life-history strategies of the species involved and the stochastic component of natural disturbances, we will suggest that species tend to segregate themselves in the study area, and would form mosaics with patches of single-species, although mixed-stands could also be possible.


On the other hand, we consider the natural disturbance regime operating within Pinus nigra forests in the study area to be characterized by minor disturbances, which would clearly prevail over stand-replacing ones. The death of individual or small groups of black pines, either uprooted by wind or heavy snowfalls or killed by pathogens, breaks the canopy, opening gaps in which Pinus nigra regenerates. As a result, natural structure would tend to show both vertical complexity and horizontal heterogeneity. Such a structure can develop under a surface fire regime (Moore et al., 1999).

In this article, we have suggested that reference conditions might be used as an alternative to the normal forest model, and be used as a point of reference to design sustainable forestry. We have also mentioned the convenience of using natural disturbance regimens to guide silvicultural systems. Considering these and what has been said about the evolutionary environment of Pinus nigra and about the structural attributes that one would expect to find in Pinus nigra natural forests, we understand that close-to-nature management may fit adequately within the aim of producing forest resources while maintaining biodiversity in this type of pinewoods. Close-to-nature management is based on the treatment of individual trees, taking into account the economic and ecological functions performed by trees at the stand level (Martín-Fernández and García-Abril, 2005). In close-to-nature management, harvest operations are restricted to trees of sufficient diameter that have already attained their maximum economic value (timber of good quality) for the species and site. Thus, a tree will remain in the stand, if it continues to increase its market price and/or it is favouring the accumulation of value in other trees (productive function) or benefiting biodiversity. Trees can develop productive functions on their own, or by the improvement of adjacent trees (e.g. favouring natural pruning and shelf-thinning processes). Benefits for biodiversity involve the conservation of rare tree species, and the maintenance of dead and decaying trees, and of trees bearing holes for nesting birds, etc.

A remarkable aspect of close-to-nature management is that the felling of trees is not affected by the desire to achieve a balance age-class distribution (a normal forest), rather trees are felled according to their own vitality. This offers a better opportunity to resemble the evolutionary environment of Pinus nigra forests while harvesting their resources.

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Chapter 11 · 2 Pedro Antonio Tiscar and Juan Carlos Linares or uneven-aged and multistrata, including very large and old individual trees. They would also include a better representation