Báo cáo khoa học: "Natural black pine (Pinus nigra subsp salzmannii) forests of the Iberian eastern mountains: development of the phytoecological basis for their site evaluation *"

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Original article Natural black pine (Pinus nigra subsp salzmannii) forests of the Iberian eastern mountains: development of the phytoecological basis for their site evaluation * R Elena-Rosselló P Regato-Pajares 1 Centro de lnvestigación Forestal, INIA, apartado 8111, Madrid; 2 Departamento de Silvopascicultura, Universidad Politecnica de Madrid, 28040 Madrid, Spain (Received 2 January 1994; accepted 2 January 1995) Summary — A phytoecological study of the Pinus nigra subsp salzmannii forests in the dolomite- limestone mountains of eastern Spain was undertaken. Starting from several floristic and ecological data collected from 355 relevés, classification and ordination numerical analysis were realized. A typifica- tion of the different pine forest communities was thus obtained and a series of floristic groups was defined, which can be used as a basis for the classification of distinct sites. Following the phytosoci- ological method, 2 main groups, which can be considered as climax vegetation of the high supra- and mountain-Mediterranean levels, have been defined: a continental group, Thalictro tuberosi-Pinetum salz- mannii, and a subcontinental group, Lonicero xylostei-Pinetum salzmannii, which represents the southern range limit of Pinus nigra forests in the eastern Pyrenees. nigra / numerical analysis / phytosociology / climax / floristic group Pinus Résumé —Typologie phytoécologique des stations forestières: les forêts naturelles de pin de Salzmann (Pinus nigra subsp salzmannii) des montagnes orientales ibériques. La présente étude concerne la caractérisation phytoécologique des forêts de Pinus nigra subsp salzmannü des mon- tagnes orientales de l’Espagne. Des analyses numériques de classification et ordination ont été réa- lisées avec 355 relevés comprenant des données floristiques et écologiques. La typologie des diffé- rents groupements silvatiques de Pinus salzmannii a permis d’établir plusieurs groupes floristiques, susceptibles d’être utilisés dans la caractérisation des stations forestières de cette essence. Selon la méthode phytosociologique, ont été distinguées 2 associations qui représentent sûrement la végé- tation climatique à l’horizon supérieur de l’étage supraméditerranéen et à l’étage montagnard-médi- terranéen : Thalictro tuberosi-Pinetum salzmannii dans la partie occidentale avec des conditions cli- * in Bolos et al The present work complies with the nomenclature given et al (1990), Castroviego or Tutin et al (1964-1980). (1986-1993)
matiques méditerranéo-continentales, et Lonicero xylostei-Pinetum salzmannii dans la partie orientale avec des conditions climatiques sub-continentales. Les forêts de pin de Salzmann qui appartiennent à la dernière association représentent la limite méridionale de ce groupement caractéristique des Pyrénées orientales. Pinus nigra / analyse numérique / phytosociologie / climat / groupe floristique INTRODUCTION Therefore, a more ecologically oriented site classification, based on phytosociological concepts and approaches, was developed in Pinus nigra subsp salzmannii has its cen- an attempt to solve some to these specific tral core of distribution in the dolomite-lime- problems. As a first attempt, Cajander’s stone mountain ranges of the eastern portion approach (1926) defines vegetation types of the Iberian peninsula (Sistema Ibérico) meaningful to forest productivity. After this (fig 1),the main forest region of Mediter- very early work, other vegetation-oriented ranean Spain. Exceptionally a relict popula- studies were conducted (Maycock, 1960; tion stand isolated in areas of the central- Pfister, 1977; Carleton, 1980; Jeglum et al, western granitic range, representing a special 1982; Jones, 1984; Kotar, 1984). All efforts paleogeographic and phytogenetic interest have been conducted to develop a better (Regato et al, 1992). The total natural pop- understanding of natural vegetation patterns ulations of this species extend over approx- in order to establish an ecological classifi- imately 380 000 hectares. cation of forest types. This is the basis for The black pine forests found in the Sis- carrying out site evaluation in well-estab- tema Ibérico account for two-thirds of the lished stands inside each forest type. total black pine formations in the Iberian In a first attempt to analyze the black pine peninsula. Together with Pinus sylvestris wood area of Spain, Elena-Rosselló and woods, they represent the most extensive Sánchez-Palomares (1991) found a good forests of the eastern mountains. While P relationship between yield and floristic sylvestris forests have been easily managed, groups. Given the encouraging results of resulting in good even-aged stands, P nigra that early evaluation, a more in-depth anal- forests actually have critical problems due ysis in the largest territorial area of P nigra in part to the lack of basic understanding (Sistema Ibérico) was conducted (Regato, about the regeneration biology of this long life 1992) in order to characterize the different species. Furthermore, disturbance processes habitat types of this species, an essential in the area (geomorphological dynamism, element to determine the potential produc- high frequency of storms, etc) generally tivity of the different sites. resulting in uneven-aged stands and the ran- dom exploitation of woods, carried out since the beginning of the century, contribute to Geobotanical background the present open-structured forests. Historically, major problems have been The most important geobotanic studies were encountered when trying to establish a site index for the different types of forests. In conducted by Willkomm (1844, 1852, 1896), particular, when stands are not even-aged, and they provided very accurate descrip- have mixed species compositions or have tions of the main forests of this species. received severe growth damage, problems When describing black pine woods along with site index are greater (Monserud, 1977). the Sistema Ibérico, he mentioned the exis-
cionis, which is characteristic of the Sis- tence ofpristine forests, which he described Ibérico, and located between the shady canopy of gigantic trees, includ- tema as a upper woods of Pinus sylvestris and the ing several specimens with an estimated mixed oak forests (Quercus faginea and Q age of than 1 000 years. As far as the more ilex subsp ballota). Nevertheless, such con- structure and degree of development are siderations were eventually invalidated, and concerned, he claimed these woods to be the sites occupied by the Pinus nigra woods perfectly comparable to the best preserved were considered to be either potential oak ones in Central Europe. Twenty years later, forests (Quercus faginea, Q pubescens and the same author regretted the serious degra- Q ilex subsp ballota) or potential Juniperus dation of these pine woods; today, it is diffi- thurifera steppic forests. cult to find mature formations with an aver- age age of more than 150 years. Under this prevailing theory, black pine is just an accessory species in such types of Since the begining of phytosociological forests, and its populations are considered studies in Spain, the role of Spanish Pinus as a consequence of anthropogenic expan- nigra forests has been undervalued, if not sion. Thus, a deep phytosociological and neglected. Gaussen (1945) originally ecological study of these pine woods was defined a potential vegetation series for the largely neglected. Pyrenees, headed by P nigra subsp salz- mannii, while Rivas-Goday (1946) Recently, all over western Europe, woods of Pinus nigra subsp salzmannii were reval- described a vegetation level, Pinetum lari-
ued and given greater ecological and phy- in the logical value of Pinus nigra Spanish tosociological importance in France (Quezel vegetation landscape. and Barbero, 1988) and in Spain (Gamisans and Gruber, 1988; Gamisans et al, 1991; Ecological features Elena-Rosselló and Sánchez-Palomares, 1991; Regato, 1992). Starting from a num- The Sistema Ibérico is a range of moun- ber of historical elements, as well as the tains with moderate high elevations often ecological, biogeographic and biological fea- over 2 000 m, surrounded by high plateaus tures of this species, it is thought that Pinus with an average height of 1 200 m. Most of nigra subsp salzmannii stands are an impor- the Pinus nigra forests are located in the tant element of the potential vegetation of supra- and mountain-Mediterranean levels, Spain, defining climatic forests which con- between 1 000 to 1 500 m, ranging from the stitute a special vegetation level. It seems lowest points at roughly 400 m, to the high- therefore appropriate to revive the initial pro- est ones in the oro-Mediterranean level (fig posals of Gaussen and Rivas-Goday, and to 2). Under particular conditions and in the determine with greater precision the eco-
nemoro-Mediterranean humid (VI(IV)2) and southernmost mountains, Sierra de nemoral (VI(VII)) phytoclimatic Javalambre, the species reaches the tim- substeppic types. The most xeric nemoro-Mediter- berline at 1 700-1 800 m. ranean type (VI(IV)1) would roughly corre- While most Spanish ranges have a west spond with the semi-arid bioclimate typical orientation, the Iberic Mountains to east of the lower and most continental areas. cross the eastern part of the peninsula from north to south, representing a barrier to the Continentality is remarkable, with winter main northwestern rain fronts. As a conse- minima temperature as low as -7°C mean quence, the climate becomes highly conti- and absolute minima reaching values of nental to the core of this mountainous region - 25°C. The frost-free season can be as and results in different characteristics of the short as 1 mid-summer month, which also water regime between the Mediterranean- tends to be characterized by a more or less and the inner face of these mountains. acute hydric deficiency. Under such extreme conditions, the vegetative period is consid- The physiography of these mountains is erably short and, as stated by Walter (1968), particularly affected by the alternance of dif- evergreen coniferous species take the place ferent lithological types. Karstic elevations of broad-leaf marcescent species. prevail, and doline fields, lapiaces and river canyons are frequent. Gravity slopes, upland rocky plains and ridges are mainly made of MATERIALS AND METHODS more or less pure dolomites, while slopes and the floor of the valley are of different lithologic types (limestone, dolomites, marls, Data from 355 forest sites were collected over sandstone and gypsum), which influence the full geographic range of Pinus nigra in the Sistema Ibérico (Regato, 1992). The sampling the slope profile. method used, that is, preferential sampling Soils are poorly developed and mostly (Gauch, 1982), subjectively selects sample sites superficial, with a prevalence of the rendz- that appear to be homogeneous and distributes ina-type (Sánchez-Palomares et al, 1990). them equitably throughout the black pine study area according to the altitudinal range and to the According to these authors, in spite of the geomorphological variability. The phytosocio- degree of soil evolution of the black pine logical relevés were made using the Braun-Blan- woods area, these should be considered as quet method (1951).Each relevé represented a mainly mature, as they represent the comparatively homogeneous area, generally edaphic potentiality of such mountains. The from 200-400 m Species’ cover-abundance . 2 abundance of dolomites, which typically values were transformed according to Van der have a difficult chemical weathering, makes Maarel (1981). Elevation, slope, aspect and pro- portion of rocks in the surface were calculated soil evolution even more difficult. for each relevé. Potential solar radiation was cal- From the climatic point of view (Regato, culated using latitude, aspect and slope (Gan- 1992), the areas where these pine woods dullo, 1974). are mainly found have humid and subhu- Polythetic divisive classification was conduced mid types of bioclimates, in their "cold" and with TWINSPAN (Hill, 1979) on a data matrix "very cold" variations (according to Emberg- comprising 355 sites x 550 species (Regato, 1992). Subsequently, all final TWINSPAN er’s classification in Daget, 1977) (fig 3). dichotomies were explored by detrended corre- Exceptionally, they can also be found in a spondence analysis (DCA) (Hill and Gauch, 1980) semi-arid superior cold bioclimate, corre- and canonical correspondence analysis (CCA) sponding to the lower and more continental (Ter Braak, 1988) to determine to which extent areas of its distribution range. According to the dichotomies reflected a discontinuity in the Allue-Andrade’s classification (1990), black site floristic data and their relations with certain pine woods are to be found mainly in the variables (Regato, 1992).
RESULTS sites of mesophilous conditions (cooler and wetter), and generally located at the highest altitude (ranging between 1 100 to 1 500 The TWINSPAN classification analysis m), and black pine forest associated with resulted in 27 different floristic groups. Sub- more xerothermic sites (ranging between sequently, all final TWINSPAN dichotomies 900 to 1 100 m). were explored using DCA and CCA. On the basis of these ordination analyses, 13 floris- Some typical species of the bushy for- tic groups were definitively established. The mations of the area, Thymus vulgaris, reduction from the initial 27 group classifi- Lavandula latifolia and Koeleria vallesiana, cation to the final 13 group classification is appear as nonindicative of the 2 groups that represented in figure 4. result in the first division (fig 4). This sug- gests a certain degradation of the under- The resulting 13 groups are ranked in story in most black pine woods, particularly the dendrogram according to a xerothermic gradient. The first dichotomy in TWINSPAN those that are subject to heavy timber classification hierarchy distinguishes exploitation. Furthermore, the subrupicu- between black pine forests associated with lous nature of many of these woods also
contributes to the presence of these species val- phases appear towards the negative characteristic of open scrub communities. of the axis 1, while those forests which ues have a more sparse structure appear In the second division level of the classi- towards the positive values of the axis (fig 5), fication, both mesophytic and xerothermic being typical of lowest xerothermic areas, sites are divided into 2 groups: a more con- where P nigra is found at the limits of its tinental group typical of the inner mountains distribution, or of degraded areas where (western sector), and another group with more xerophytic species colonize the sub- subcontinental character typical of the ranges closer to the Mediterranean Sea canopy. (eastern sector) (fig 4). In the CCA ordination analysis, groups resulting from subsequent divisions of the These 4 groups resulting from the second TWINSPAN classification analysis are the tier are separately located in the 4 quarters best defined. Such groups are associated of the DCA diagram, defined by the first 2 to sites with a high proportion of rocky sub- Axis 1 represents a xerothermic gra- axes. strates and steep slopes, both factors dient, while axis 2 represents a continen- strongly associated with axis 2. An altitudi- tality gradient. Therefore, those black pine nal gradient becomes apparent along the forests which have good mesophyllous con- ditions and are typical of the most advanced axis 1 (fig 6).
dolomite-limestone mountains of the west- Mesophytic black pine woods ern Iberian system (Puertos de Beceite, of the eastern sector: groups 1-3 Maestrazgo and western stations of Gudar and Javalambre sierras). A group of sub- This grouping includes 40 sites associated Mediterranean and eurosiberian species to the highest altitude zones of the eastern characterizes both the scrub and the herba- mountains characterized by the lowest con- layers, belonging to Quercetalia ceous tinentality. Frequently, its sites are located in pubescentis, or in a wider scope, to Querco- the ubacs, where the comparatively higher Fagetea. Indicator and preferential species air relative moisture attenuates their ther- are Primula veris subsp columnae, Hepatica mic continentality. Their phytoclimatic type, nobilis, Brachypodium sylvaticum, Fragaria located between 1 000 and 1 700 m of alti- vesca, Pteridium aquilinum, Acer opalus tude, is humid nemoro-Mediterranean subsp granatense, Sorbus aria, Buxus sem- (VI(IV)2) or substeppic nemoral (VI(VII)). pervirens, Ilex aquifolium, among others. Dolomite substrates are predominant. Sites Mixed forest formations with Pinus sylvestris, in groups 1-3 are located mostly in the lower characteristic of the upper forest level, are left quarter of the DCA diagram. very often defined. Due to floristic similarities Group 1: includes forest formations well ver- of black pine woods in this zone with the tical-structured and developed, with nemoral woods described in the Pyrenees understory, that can be considered as cli- (Gamisans and Gruber, 1988), it can be max vegetation of the high supra- and low considered that both belong to the same association, Lonicero xylostei-Pinetum salz- mountain-Mediterranean level of the
and xeric continental climate. These sites mannii (table I). Therefore, black pine woods in this group may be a southern expansion are located in the transitional zone from the forest of the more continental western sec- from the Pyrenees formations, and repre- sent a transition from these to the more con- tor to the eastern sector, and therefore their tinental ones. Furthermore, some typically characterization is sometimes difficult. Fur- Pyrenean species are found in the under- thermore, the lack of floristic elements in story, and they are representative of their the understory makes it difficult to deter- southern limit (Lavandula angustifolia and mine their phytosociology. Indicator and dif- Teucrium pyrenaicum). ferential species show the orophylous char- acter of such forest formations: Juniperus forest formations with Group 2: comprises sabina, Astragalus granatensis, Thymus open structure that define the timberline an leptophyllus, etc. High mountain pastures, of the western slopes of Javalambre and Camarena sierras, towards Teruel, with cold favored by human intervention, clearly have
Mesophytic black pine woods contributed to the open-structure charac- of the western sector: groups 4-7 teristic of forests of this group. Group 3: forests located on steepy sites with unstable substrates, and superficial rocks This grouping comprises 145 sites associ- and boulder fields. These conditions favor ated with the most mesophilous conditions of the establishment of certain subrupiculous the supra-Mediterranean and Mediterranean taxa, with the subsequent impoverishment of mountain belts, between 900 and 1 500 m. the more sciophilous species. Preferential The main phytoclimatic type is humid species are Festuca gauthieri, Amelanchier nemoro-Mediterranean (VI(IV)2), with high ovalis, Thalictrum tuberosum, Sorbus values of thermic continentality (seasonal domestica, Paeonia officinalis and Lonicera extremes of temperature). The characteris- tic substrate is dolomite-limestone, with an xylosteum. This community has been abundant appearance of massive dolomite defined as festucetosum gauthieri (Regato, covering the surface of a high plateau or flat- 1992) subassociation of the climax type topped mountain. In the DCA diagram, sites Lonicero-Pinetum (table II).
included within these groups are located in variation of group 4 climatic wood, which has both higher and lower left quarters. been defined as the subassociation astra- galetosum granatensis (Regato, 1992) of the Group 4: includes all those black pine forests climax type Thalictro-Pinetum (table IV). in the western sector which have the best structure and development and can thus be 6: includes mesophytic black pine Group considered the climax or mature vegetation forests adapted to steppic conditions. These under these specific ecological conditions. are typical of the highest areas of plateaus These are mainly located in ubacs, although and hilly uplands, which share with the step- it should be considered that this might pic Juniperus thurifera forests. Pine woods depend on the fact that adrets tend to be are mainly placed on dolomite substrates, managed by humans for cattle-raising and while Juniper formations tend to develop in agricultural purposes. The understory is char- limestone-marl areas. Juniperus thurifera is acterized by the abundance of sub-Mediter- quite common in the pine wood subcanopy ranean and central-European scrub and tree layer, where the sub-Mediterranean herbaceous species. Among indicator and bushy element becomes rare. Among their preferential species are Viburnum lantana, indicator and preferential species are Brachy- Ligustrum vulgare, Buxum sempervirens, podium sylvaticum, Geum sylvaticum, Lath- Rosa pimpinellifolia, Thalictrum tuberosum, yrus filiformis, Prunus spinosa, Rosa Lathyrus filiformis, Geranium sanguineum pimpinellifolia, Hepatica nobilis, Berberis vul- and Phyteuma orbiculare. Such pine forests garis, Buxus sempervirens, Thymus bractea- have been described as a new association, tus, etc. This be considered can as a geo- Thalictro tuberosi - Pinetum salzmannii morphological variation of the typical mature (Regato, 1992), which is considered as the pine woods of group 4, to more extreme cli- potential vegetation type of the high supra- matic conditions in the upland plains and and low mountain-Mediterrranean levels on flat-topped mountains. This formation has the mountains of the western Iberian Range been defined as juniperetosum thuriferae (Serrania de Cuenca, Montes Universales (Regato, 1992) subassociation of the climax and western side of Sierra de Gúdar), (table type Thalictro-Pinetum (table V). III). The main phytoclimatic type is the Group 7: includes subrupicolous black pine nemoro-Mediterranean humid, VI(IV)2. forests of dolomitic gravity slopes and rocky Group 5: includes those mesophytic black plains, with abundant dolomite-limestone pine forests of a more steppic nature, which indicator taxa. This is clearly differentiated in are typical of the transitional mountains the CCA diagram. The black pine has a very between the western and eastern sectors. irregular development, and hardly ever con- These are situated around the very cold and stitutes a proper canopy. Indicator and pref- xeric depression of Teruel. Substeppic erential species are Jasonia glutinosa, nemoral, VI(VII), is the main phytoclimatic Juniperus phoenicea, Stipa offneri, Fumana type and limestone-marl substrates prevail. ericoides, Alyssum lapeyrousianum, etc. In several sites, the sparse structure of the forest is due to intensive human manage- ment. The understory is poorer in sub- Xerophytic pine forests Mediterranean species, while species of the of the western groups 8-10 area: bushy formations are more frequently found. Among indicator and preferential species are Astragalus granatensis, Avenula pratensis, This grouping comprises 140 sites found in Festuca rubra, Scabiosa turolensis, Brachy- the lowest altitude ranges of the southern podium phoenicoides and Avenula bro- and western portion of the Serranía de moides. This can well be considered as a Cuenca, where the xeric nemoro-Mediter-
and Quercus ilex subsp ballota are fre- ranean type (VI(IV)1) is the main phytocli- mate. Under such climatic conditions, Pinus quently present in the tree layer, as the zone nigra finds its ecological limit and gives ori- is ecotonal with the woods of such oak gin to moderately developed formations. species. Among indicator and preferential These tend to have a sparse structure, species, only xerothermic taxa of bushy for- mainly due to anthropic action as well as to mations, such as Rosmarinus officinalis, the subrupicolous features of several sites. Brachypodium retusum, Juniperus Such structure favors a xerothermic nature phoenicea, Salvia lavandulifolia, Satureja of the wood understory. Quercus faginea intricata and Erinacea anthyllis can be found.
The almost complete absence of nemoral Xeromesophytic pine forests species in the understory and the frequent of the eastern section: groups11-13 appearance of Quercus species make it very difficult to characterize these ecotonal This grouping includes 33 sites found in the sites, where Pinus/Quercus mixed forest is lower elevation areas of the dolomitic ranges most likely their foreseeable forest type.
in the proximity of the coast. Escarpments Sorbus aria and canyons are common, producing very Hepatica nobilis heterogeneous site conditions. Group 11 Rosa pimpinellifolia has the most nemoral conditions, and can Lathyrus filiformis be considered as a xerothermic variation of Juniperus communis the mature black pine woods of the eastern Thalictrum tuberosum sector, Lonicero-Pinetum subassociation Lonicera xylosteum genistetosum patentis (Regato, 1992) (table Primula veris ssp columnae I). There is a considerable amount of mes- Viburnum lantana ophytic taxa in the understory, but with a Sanicula europaea lower abundance index. The presence of Amelanchier ovalis species such as Juniperus oxycedrus, Geum sylvaticum Juniperus phoenicea, Bupleurum fru- Buxus sempervirens ticescens and Brachypodium retusum indi- Avenula pratensis cates their xeromesophytic character. Acer opalus ssp granatensis Brachypodium sylvaticum Groups 12 and 13 are clearly differenti- ated in the CCA diagram. The former These woods, included in Cl Querco- includes the most thermic sites of black pine Fagetea (or, Quercetalia pubescentis), rep- formations in the Sistema Ibérico, and it resent the ecological optimum (with real should be considered as azonal open com- nemoral understory conditions and well ver- munities with the worst growth potential. tical-structured canopy) of extensive areas The latter group comprises the subrupi- that were previously established as potential colous sites, where the canopy hardly exists, sites of more xerophytic vegetation (Junipe- and where trees have an irregular distribu- rus thurifera cold steppic woods and Quer- tion over the rocky slopes. cus ilex subsp ballota thermic woods). In these climax communities, dis- we can DISCUSSION AND CONCLUSION tinguish 2 site types: 1) Those stands associated with the hilly Black pine forests have their ecological opti- uplands, where the subcanopy is dominated between the supra- and mountain- mum by the herbaceous layer. The arbustive layer Mediterranean levels of these dolomite-lime- is poor and integrated by the most conti- stone ranges, under a very cold humid nental species (Juniperus communis, Rosa nemoro-Mediterranean continental phyto- pimpinellifolia and Berberis vulgaris subsp climate. Under these conditions, the poten- seroi). This type has the best site quality, tial for growth of Pinus nigra is better than particularly over convex reliefs or plains. In that of other species. In the Sistema Ibérico, the floor of some doline fields, the growth there are 2 climax communities, the more rate of Pinus nigra is very high. Neverthe- continental one, Thalictro-Pinetum salz- less, soil conditions in these sites have an mannii, located in the western part (groups unstable equilibrium, often broken by over- 4-6) and the less continental one, grazing and clear-cutting practices. Conse- Lonicero-Pinetum salzmannii, located in quently, important soil losses and problems the eastern part (groups 1, 3 and 11),sim- in tree regeneration will arise. The abun- ilar to the black pine woods of the Pyrenees. dance of Juniperus thurifera in the tree layer The indicator species group of the best can be considered a good index for deter- sites is a combination of sub-Mediterranean mining the worst conditions of these kind of and central-European taxa. Some of the sites. Such bad conditions are frequently related to the concave reliefs. most common characteristics are:
mine its site quality. Highest degradation is Those stands are associated with the 2) revealed with the appearance of Festuco- steepy sites on karstic valleys and canyons. poetalia species (eg Festuca hystrix, Poa The understory is dominated by the arbustive ligulata, Arenaria erinacea and Globularia layer. They have a good site quality despite repens), which show strong soil denudation their usually uneven-aged structure. This (groups 7 and 10). The black pine usually depends on the heterogeneous conditions shows special growth limits with a charac- of the substrate (rocks, boulder fields, steep teristic table-shaped crown. slopes). Although the growth potential of black pine is good, the canopy structure may The presence of taxa typical of more not be uniform. The proportion of subrupi- xerothermic bush communities (Rosmarino- colous taxa can be used as an indicator ericion) (eg Rosmarinus officinalis, value of the potential heterogeneous canopy. Helianthemum hirtum, Coris monspeliensis and Brachypodium retusum) is considered The characteritics landform in the Sis- as being evidence of the lowest site qual- tema Ibérico is the "cantil-talud" (gravity ity. This generally corresponds to sites slope-pediment) system, where intense geo- where Pinus nigra has its ecological limit morphologic dynamics occur (Calvo, 1987). on the lowest xerothermic slopes (groups The slope retreat maintains the verticality 8 and 9). These species are also typical of of the cliff. The mixed pine/oak woods grow- sites which correspond to rocky or eroded ing in pediments with best edaphic condi- adrets at a higher elevational level. tions are modified by rock avalanches. These dolomitic blocks remove the soil, In the eastern subcontinental sector, the increasing dolomites and rock surface pro- worst site quality corresponds to the lowest portion. Under such conditions, black pine sites, where azonal black pine communities plays an important role in stabilizing and are defined, having an open structure and a restoring the site conditions. predominant Quercetea ilicis species under- story (group 12). At the oro-Mediterranean level of the southern mountains (Sierra Javalambre) This phytoecological classification has (group 2), the characteristic cold climate made it possible to recognize Pinus nigra becomes more xeric, tending to steppic con- climax communities, representing the poten- ditions. The indicator taxa are dwarf scrubs tial vegetation for this mountain region. Once (eg Juniperus sabina, J hemisphaerica, the potential area and ecological optima for Prunus prostrata and Astragalus Pinus nigra are established, a precise basis granatense), revealing an open structure of for determining the quality of its different the wood. Nevertheless, these scrubs offer site types is available. protection to the black pine saplings and to When analyzing the TWINSPAN dendro- the few nemoral species that only grow gram, several interesting conclusions were below them. Pines have a medium growth obtained. At the first division level, climax potential and, according to Elena-Rosselló Pinus nigra sites were separated from the and Sánchez-Palomares (1991), their site azonal ones. In its second level, both site quality appears to be average. types were divided into 2 groups according to A high proportion of characteristic species regional climate reasons: the subcontinen- of Ononido-Rosmarinetea bush communi- tal types, located in the eastern sector, and the continental types in the western sector. ties reveal a somewhat extensive under- story degradation. In the mountain-Mediter- Lower divisions can only be understood when taken into account physiographic factors, ranean level, this usually reveals anthropical showing the landform patterns of the regional degradation (overgrazing; cleaning and thin- ning processes) and it is difficult to deter- geomorphological typical structure.
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