© 2002 Laser Pages Publishing Ltd., Jerusalem

Israel Journal of Plant Sciences Vol. 50 2002 pp. 217–228

*Author to whom correspondence should be addressed. E-mail:rgavilan@farm.ucm.es

Spatial structure and interspecific interactions in Mediterranean high mountainvegetation (Sistema Central, Spain)

ROSARIO G. GAVILÁN,a,* DANIEL SÁNCHEZ-MATA,a ADRIÁN ESCUDERO,b AND AGUSTÍN RUBIOc

aDepartamento de Biología Vegetal II, Facultad de Farmacia, Universidad Complutense,E-28040 Madrid, Spain

bÁrea de Conservación y Biodiversidad, Escuela Superior de Ciencias Experimentales y Tecnología,Universidad Rey Juan Carlos, E-28933 Móstoles (Madrid), Spain

cDepartamento de Silvopascicultura, E.T.S.I. Montes, Universidad Politécnica, E-28040 Madrid, Spain

(Received 12 June 2001 and in revised form 13 March 2002)

ABSTRACT

Interspecific interactions among species are generally accepted as important pro-cesses in communities. Positive interactions seem to be most common in environ-ments with high stress, but negative effects also appear simultaneously. The mecha-nisms of these interactions are diverse and include shade, improvement of soilcharacteristics, or protection from animals in the case of facilitation, or allelopathyor direct contact, in the case of competition. We studied vegetation patches in twohigh areas of the Sistema Central (Spain): Sierra de Béjar and Sierra de Guadarrama.This vegetation is very rich in cushion chamaephytes such as Armeria caespitosa,Silene elegans, or Jasione centralis. The aim of this study was to determine whetherinterspecific associations exist in this Mediterranean high mountain vegetation andif such associations are important in plant communities. We also wanted to know ifenvironmental features can explain spatial distribution of species on patches. Forthat purpose, we recorded species composition of patches and their cover, as well asthe highest and lowest diameter of patches, altitude, and exposition. Successionalchanges have also been checked through the total species richness and size ofpatches. Results showed that patches dominated by different cushion plants did notdiffer substantially in species composition, although species affinity for patches isdifferent depending on the number of species. Herbaceous species appear to act aslate colonizers, mostly restricted to species-rich patches, although some species,such us Agrostis truncatula, act as early colonizers in Sierra de Béjar. Flat cushionplants could be considered nurse plants, as they can modify micro-environmentalconditions and allow establishment and survival of associated species. However, insome cases they can also act in competition with other cushion plants or herbs for thesame space.

INTRODUCTION

Species interactions are generally accepted to be impor-tant processes that affect plant community developmentand dynamics. Facilitation and competition occur indifferent parts of the environmental gradient, and themore productive the environmental gradient, the more

important the competition between species, whereasfacilitation occurs when the environmental gradient isless productive (Bertness and Callaway, 1994). Positiveinteractions appear to be most common in environmentswith relatively high physical disturbance, stress, or pre-dation, where associated species can increase thegrowth and survival of other species unable to survive in

Abbreviations. DCA: Dentrended Correspondence Analysis;CCA: Canonical Correspondence Analysis; TVE: Total Vari-ance Explained.

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isolation (Callaway et al., 1996; Kikvidze andNakhutsrishvili, 1998; Núñez et al., 1999). Negative inter-actions have been described as a strong contest amongplant species for abiotic resources, where the success ofone species affects the performance of others (Goldberg,1990; Jones et al., 1997). However, facilitative and com-petitive mechanisms do not act in isolation from eachother in nature and they usually occur in combinationswithin the same community or between plant species,producing different effects (Bertness and Shumway,1993; Callaway and Walker, 1997; Callaway et al., 2000).

The mechanisms involved in facilitation are diverseand include shade and improvement in soil characteris-tics (moisture, nutrients, oxygenation, or mycorrhizae),in pollination, and in protection from herbivores(Callaway and Puignaire, 1999). Shade provided by thecanopies of large plants can protect seedlings or smallerplants from temperature extremes, reducing the loss ofwater from soils. These mechanisms have developed indifferent habitats such as deserts, savannas, saltmarshes, etc. (Parker and Muller, 1982; Valiente-Banuet and Ezcurra, 1991; Bertness and Hacker, 1994).

Rainfall water can be retained in soils by plants,either due to the presence of an overstory that enhancesthe understory (Vetaas, 1992), or due to the presence ofan understory that improves soil moisture by differentmechanisms (Richards and Caldwell, 1987). Water istransported from deep to dry surface soils at night. Soilbeneath perennials is richer in nutrients than is bareground. This has been reported in many systems, al-though it appears to be more frequent in dry habitats andsavannas (Vetaas, 1992; Belsky, 1994). The enrichmentmay occur via nutrient pumping; deep roots of perenni-als take up nutrients unavailable to shallowly-rootedplants. Moreover, a perennial canopy can trap airborneparticles, or enrichment can occur indirectly via nitro-gen fixation (Puignaire et al., 1996; Whitford et al.,1997; Callaway and Puignaire, 1999). Some mycor-rhizal fungi can also facilitate the exchange of carbonand nutrients in soils, improving the richness of lesscompetitive species (Grime et al., 1987; Walter et al.,1996). Oxygenation of soils is very important for wet-land plants, and occurs from leaves to roots. Sometimessubmerged or merged roots create favorable conditionsfor other plants, which also increase the redox potential(Armstrong, 1979; Armstrong et al., 1992; Castellanoset al., 1994; Hacker and Bertness, 1996).

Pollination could be improved by certain species thatare more attractive to pollinators. It has been reportedthat some species receive more visits from pollinatorswhen mixed with other species than when alone (Thomp-son, 1978; Laverty and Plowright, 1988). Predation is,in some cases, a major factor limiting the establishing of

some species (Callaway et al., 1996). Some plants canprotect seedlings or small plants from predators, shad-ing them and also preventing animals from reachingthem (Callaway, 1992).

The aim of this study was to investigate whetherinterspecific associations exist in high mountain vegeta-tion patches and if such associations are important in theformation of plant communities. We have explored theroles of plant species in forming organized communitiesby comparing small-scale spatial associations amongspecies in order to see if facilitation is a more importantphenomenon than competition in these highly stressedalpine environments. Comparison of the species distri-butions of plants with some environmental variableswill help to elucidate the most important environmentalfeatures that explain the spatial distribution in thosecommunities. For this purpose we have studied vegeta-tion patches in two areas of central Spain.

Nomenclature used basically follows the standard-ized floras, Castroviejo et al. (1986–1999) and Tutin etal. (1969–1980, 2000).

MATERIALS AND METHODS

Study area and vegetation

Sistema Central is an east–west-running mountain rangesituated in central Spain (Fig. 1). Our study was carriedout in two areas of this range: Sierra de Guadarrama,close to Madrid, and Sierra de Béjar, situated west ofSierra de Guadarrama. The highest summits of thesemountains are Peñalara peak (2429 masl) in Sierra deGuadarrama and El Calvitero peak (2401 masl) in Sierrade Béjar (see Fig. 1). High mountain vegetation in theseareas includes different communities, depending on thehabitats. These range from rupicolous or stone field com-munities to Nardus-grasslands in sites where snow coverremains for extended periods. We focused our study onthose communities that represent potential vegetation.Two phytosociological associations have been describedas representing the Spanish Sistema Central: Hieraciomyriadeni–Festucetum curvifoliae in eastern territories(Sierra de Guadarrama and Ayllón) and Agrostiorupestris–Armerietum bigerrensis in western ones (Sierrade Gredos and Béjar). These communities are settled onsoils that receive water from rainfall and snow, but thiswater does not remain for a long time. The particulargeomorphology of these ranges and the natural erosionprocesses they have suffered have caused the top ofthese mountains to have flat surfaces that are usuallyexposed to strong winds. In these environments, plantcommunities are very rich in plants adapted to extremeconditions. There is an abundance of cushion plants when

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compared to other communities covered by snow in win-ter, such as Nardus-grasslands. Armeria caespitosa,Jasione centralis, Silene elegans, or Minuartia recurvaare very common among cushion plants found in theseareas together with other hemicryptophytic taxa such usFestuca curvifolia, Hieracium myriadenum, Avenellaiberica, Jurinea humilis, or Senecio boissieri (see taxafrequencies in Appendix).

This type of potential vegetation occurs between(2000) 2100–2592 masl, mainly in the so-called cryoro-submediterranean belt (Rivas-Martínez et al., 1991,1999; Rivas-Martínez and Loidi, 1999). There are nometeorological stations at these altitudes; the highest issituated at 1890 masl, but it has been estimated thatmean annual temperatures are between 2 and 4 °C, witha mean maximum temperature and mean minimum tem-perature of the coldest month of –3 to 1 °C and –9 to–6 °C, respectively. The estimated annual rainfall isabout 1400 mm (humid ombrotype). In some specialtopographic situations (i.e., northern slopes), these com-munities can reach lower altitudes in the orosubmedi-terranean belt.

Sampling

We sampled about 200 patches in each area, coveringapproximately 200 m2. Patches were identified asisolated structures surrounded by bare ground. Forevery patch we recorded total number and plant cover(in percentage) of species, the highest and lowest diam-

eter, altitude, and exposition. Patches with single spe-cies were considered “solitary plants”; species associa-tions were tested in patches with more than onespecies. From 200 sampled patches in each area, 102 inSierra de Guadarrama, and 98 in Sierra de Béjar werecomposed of only one species. The rest contain fromtwo to nine species. In these multi-species patches wehave studied the occurrence, the optimum of their altitu-dinal range, and life form for all species (Appendix).

Data analysis

Positive or negative pairwise species associations weretested using Fisher’s exact test (Sokal and Rohlf, 1995).Analyses were done using STATGRAPHICS Plus 4.0(1999). In order to evaluate the fidelity of species fordifferent stage patches, we used two approaches. Thefirst consisted of the percentage of occurrence of speciesfor patches. Following other approaches in similar envi-ronments (Kikvidze, 1993), patches were divided intothree categories that could fit three stages of patch devel-opment based on their floristic composition: (1) six spe-cies or more, (2) five species, and (3) four species or less,which represent developed, medium-developed, and low-developed patches, respectively.

To complement the weakness of this first approach,we carried out a second one based on constrained ordi-nations. Two matrices were built, a floristic matrix and asecond matrix, called “spatial”, which included thenumber of species in every patch, the diameter of

Fig. 1. Location of study area (Sistema Central). Vegetation sampling was focused on Sierra de Guadarrama and Sierra de Béjar.

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patches, the product of both, and the altitude of patches.First, the floristic matrix was analyzed by DCA to seehow large the ecological gradients were. Then, using thespatial matrix as the constraining data matrix we con-ducted a CCA analysis. The total variation explained bythis data set was calculated as the sum of all canonicalaxes extracted (Verdonschot and ter Braak, 1994;Legendre and Anderson, 1999). It accounts for the frac-tion of floristic variation explained by the spatial matrix(Borcard et al., 1992). To establish the accuracy of therelationship between the two matrices, a Monte Carlopermutation test was performed (1000 randomizations)and the F-ratio calculated from the sum of all constrainedeigenvalues. In this type of multivariate approach, normal-ity or transformation of variables is not necessary since theprocedure is weighted averaging (ter Braak, 1986). All themultivariate analyses were done with CANOCO (terBraak and Smilauer, 1998).

RESULTS

Patch composition shows a majority presence of peren-nial plants, mainly hemicryptophytes and chamaephytes(Appendix). Among chamaephytes, cushion plants suchas Armeria caespitosa, Silene elegans, Jasione centra-lis, or Plantago penyalarensis are common in patches,and are restricted to the highest altitudinal level (COMbelt, see Appendix for abbreviations), except forArmeria caespitosa, which also appears at lower alti-tudes. Saxifraga willkommiana, only recorded in Sierrade Guadarrama, is rare due to its preference for rockcrevices, and also grows at lower altitudes in the sameenvironments. Thymus penyalarensis, more common inSierra de Guadarrama, is a small shrub with a veryrestricted distribution in high mountain communities(COM belt). Other chamaephytes such as Cytisusoromediterraneus or Juniperus alpina have their opti-mum in other alpine communities at lower altitudes,appearing in patches as small shrubs. The number ofhemicryptophytic species is rather larger than chamae-phytes (see Appendix), and most of them show a wideraltitudinal range of distribution than chamaephytes;these are grasses such as Festuca curvifolia, F. iberica,Nardus stricta, Avenella iberica, Koeleria crassipes, orAgrostis truncatula that are very frequent in patches.Only a few species of Compositae, such as Leu-canthemopsis alpina or Hieracium myriadenum, show amore restricted distribution in the cryorosubmed-iterranean belt (COM, see Appendix), although otherspecies of the same family, such as Jurinea humilis, alsoshow a wider altitudinal distribution.

Patches can also contain some annual plants, but theyappear as pioneer colonizers, as is the case of Sedum

candollei, a typical plant of high mountain vegetationwhose behavior has been related to an early colonizationof places largely covered by snow (Fernández-González,1991). Other annuals, such as Cerastium ramosissimum,probably have the widest distribution among the set ofplants, from the foothills to the top of the range.

Geographical distribution of species is similar inboth territories (Appendix), however, some specieshave a particular territorial distribution. This is the casefor Festuca curvifolia and Armeria caespitosa, whichare distributed in eastern territories, or Dianthusgredensis in western ones. Regarding the life form,there are also differences in both territories. Cushionplants are more prevalent in Sierra de Béjar than in Sierrade Guadarrama, except for Minuartia recurva (seeAppendix). This fact could be related to the stage ofpatches, as those rich in cushion plants are at earlierstages. In other cases, the presence of some herbs, suchas Agrostis truncatula, could also be related to pioneerpatches (see Table 5).

The number of species in patches is different in thetwo territories studied (Figs. 2 and 3). In Sierra deGuadarrama, the majority of patches have five speciesand the rest have more than five. In Sierra de Béjar, thesituation is different and most patches have four or fewerspecies. Following similar studies (Kikvidze, 1993), wecan also establish five as the minimum number of speciesthat compose a climax patch. In this case, patches fromSierra de Béjar indicate a younger stage of vegetation.

Interspecific associations between species are higherin Sierra de Guadarrama than in Béjar (93 vs. 53,respectively), probably due to the higher number of totalspecies (32 vs. 23) and also to the different stage ofpatches. The number of positive interactions is also higher(Tables 1 and 2; 83 vs. 10 in Sierra de Guadarrama and 37vs. 16 in Sierra de Béjar), but in Sierra de Béjar negativeassociations are more common, which could also be re-lated to the stage of patches. The rest of the most frequentspecies are cushion plants such as Jasione centralis,Silene ciliata, Armeria caespitosa, A. bigerrensis,Minuartia recurva, Thymus penyalarensis, etc., whichshow significant positive associations with the rest of thespecies, more in Sierra de Guadarrama than in Sierra deBéjar.

Armeria caespitosa, a cushion plant, shows negativevalues of association with another cushion plant, Sileneelegans, or with some perennial herbs such as Festucacurvifolia or Hieracium myriadenum, the two speciesthat give their name to the community in Sierra deGuadarrama. Minuartia recurva also shows negativeassociations with Thymus penyalarensis, due to itsspreading creeping growth habit that is not compatiblewith sharing space with a cushion life form.

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Sierra de Guadarrama

Number of species in patches

Fre

quency

2 3 4 5 6 7 800

5

10

15

20

25

30

9

Fig. 2. Total frequency of species in patches in Sierra deGuadarrama.

Sierra de Béjar

Number of species in patches

Fre

qu

en

cy

2 3 4 5 6 7 8 900

5

10

15

20

25

30

Fig. 3. Total frequency of species in patches in Sierra deBéjar.

Table 3Distribution of plant species (as percentage) in different stagesof Sierra de Guadarrama’s vegetation patches: 1, pioneer stage;

2, seral stage; 3, climax stage. No. of species in parentheses

Species 1 2 3(3–4) (5) (>6)

Agrostis truncatula 0 23 10Armeria caespitosa 25 39 40Avenella iberica 0 8 25Biscutella pyrenaica 0 15 5Cerastium ramosissimum 13 8 0Coincya montana 0 0 5Cytisus oromediterraneus 13 0 5Festuca curvifolia 75 31 60Festuca iberica 2 16 35Hieracium myriadenum 0 23 35Jasione centralis 0 0 15Jasione carpetana 0 46 50Juniperus alpina 0 0 30Jurinea humilis 25 15 5Koeleria crassipes 25 15 15Leucanthemopsis alpina 0 23 25Luzula hispanica 13 0 50Minuartia recurva 25 16 55Sedum candollei 0 15 10Nardus stricta 0 8 10Paronychia polygonifolia 0 8 10Phyteuma hemisphaericum 0 0 5Plantago penyalarensis 0 0 5Ranunculus alpinus 13 0 0Rumex angiocarpus 0 8 0Saxifraga willkommiana 0 8 5Sedum brevifolium 38 46 50Senecio boissieri 13 0 5Senecio carpetanus 0 0 25Silene elegans 13 8 55Thymus penyalarensis 38 8 45Veronica cantabrica 13 16 25

Table 4Distribution of plant species (as percentage) in different stagesof Sierra de Béjar’s vegetation patches: 1, pioneer stage; 2,

seral stage; 3, climax stage. No. of species in parentheses

Species 1 2 3(3–4) (5) (>6)

Agrostis rupestris 3 0 0Agrostis truncatula 71 50 80Armeria bigerrensis 0 13 0Avenella iberica 7 25 0Carex furva 7 13 0Cerastium ramosissimum 0 13 0Crocus carpetanus 0 0 20Dianthus gredensis 23 13 80Hyeracium myriadenum 0 13 0Holcus gayanus 0 13 0Jasione carpetana 0 0 20Jasione centralis 61 75 20Juncus squarrosus 3 0 0Jurinea humilis 3 0 20Leucanthemopsis alpina 19 38 40Minuartia recurva 10 0 0Narduus stricta 3 38 60Paronychia polygonifolia 0 25 40Plantago penyalarensis 32 0 40Rumex angiocarpus 7 38 60Sedum brevifolium 10 38 100Sedum candollei 3 13 20Silene elegans 65 63 100

Distribution of species per patch type (Tables 3 and 4)shows similarities between the territories but also differ-ences. Some species such as Silene elegans, Leucan-themopsis alpina, Nardus stricta, or Paronychiapolygonifolia show similar behavior in both territories,with a greater presence in later stages (>5 plants). Dif-ferences appear in plants such as Jasione centralis,Jurinea humilis, or Avenella iberica, which in Sierra deBéjar have a greater presence in pioneer stages, while inGuadarrama they are more common in older stages.

Multivariate analysis supports the results explainedabove. DCA results show that the length of the extractedgradients of the floristic data set was large enough(l1 = 4.26, l2 = 4.93 in Sierra de Guadarrama; l1 = 5.72; l2= 3.73 in Sierra de Béjar) to carry out a CCA-type

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222

Tabl

e 1

Sign

ifica

nt in

ters

peci

fic a

ssoc

iatio

ns d

etec

ted

in S

ierra

de

Gua

darra

ma

by χ

2 an

alys

is: +

= p

ositi

ve; –

= n

egat

ive;

+ =

p <

0.0

5; +

+ =

p <

0.01

; +++

= p

< 0

.001

Gavilán et al. / Spatial structure and interspecific interactions in Sistema Central vegetation

223

Tabl

e 2

Sign

ifica

nt in

ters

peci

fic a

ssoc

iatio

ns d

etec

ted

in S

ierra

de

Béja

r by χ2

anal

ysis;

+ =

pos

itive

; – =

neg

ativ

e. +

= p

< 0

.05;

++

= p

< 0.

01; +

++ =

p <

0.0

01

Israel Journal of Plant Sciences 50 2002

224

Fig. 4. Biplot from the CCA applied to the Sierra de Guadarrama data sets. G.D. is the greatest diameter of patches; N.sp. is thenumber of species; G.D.*N.sp. is the product of both. Species abbreviations are: Agtr, Agrostis truncatula; Arca, Armeriacaespitosa; Avib, Avenella flexuosa subsp. iberica; Bipy, Biscutella valentina subsp. pyrenaica; Cera, Cerastiumramosissimum; Como, Coincya cheiranthos subsp. montana; Cyor, Cytisus oromediterraneus; Fecu, Festuca curvifolia; Himy,Hieracium vahlii subsp. myriadenum; Jaca, Jasione laevis subsp. carpetana; Jace, Jasione crispa subsp. centralis; Jual,Juniperus communis subsp. alpina; Juhu, Jurinea humilis; Kocr, Koeleria crassipes; Leal, Leucanthemopsis pallida subsp.alpina; Luhi, Luzula hispanica; Nast, Nardus stricta; Mire, Minuartia recurva; Papo, Paronychia polygonifolia; Phhe,Phyteuma hemisphaericum; Plpe, Plantago alpina subsp. penyalarensis; Raal, Ranunculus ollissiponensis subsp. alpinus; Sawi,Saxifraga willkommiana; Sebr, Sedum brevifolium; Seca, Sedum candollei; Sebo, Senecio boissieri; Sepy, Senecio pyrenaicussubsp. carpetanus; Siel, Silene ciliata subsp. elegans; Thpe, Thymus praecox subsp. penyalarensis.

constrained ordination. The CCA model for Sierra deGuadarrama, using the spatial data set as the constrain-ing matrix (Fig. 4), was highly significant (see Table 5).The CCA ordination shows species distributed along thetwo first axes that are related to patch size and richnessfor the first axis and altitude for the second. Cushionplants such as Armeria caespitosa, Silene ciliata, andJasione centralis remain in the central position, exceptfor Plantago penyalarensis, which appears at higheraltitudes in Guadarrama (near Peñalara peak, see

Fig. 1). Cytisus and Juniperus appear close to the edgesof the axes because their life form produces the biggestpatches. In the case of Cytisus, the altitude at which itappears is the lowest of the sampling. Agrostistruncatula remains in small or poor patches, but notvery far from other plants. Nevertheless, in Sierra deBéjar the spatial variables considered are not enough toexplain the floristic variability (see Table 5), probablydue to the greater heterogeneity in patch size and greaterdynamism of patches in this territory.

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DISCUSSION

The balance between positive and negative interactionsamong plant species in alpine environments has beenshown to be very important for plant community devel-opment (Choler et al., 2001; Kikvidze et al., 2001). Incentral Spain, pairwise comparisons have shown thatinterspecific positive interactions are more commonthan negative ones (Chapin et al., 1994). Cushion shrubsare not the only species that act as nurse plants forherbaceous or other cushion species (Kikvidze andNakhutsrishvili, 1998; Núñez et al., 1999); species withother life forms, such as hemicryptophytic species, alsofavor the establishment of other herbs.

Some of the most frequent species do not show statis-tically significant associations, as has been demon-strated in similar studies (Kikvidze, 1993). For instance,Festuca curvifolia does not facilitate the growing ofother species, probably due to its particular clonalgrowth (Körner, 1999), being able to colonize newenvironments by forming monospecific bands of veg-etation in some special situations. Similarly, other spe-cies, mostly cushion but also other hemicryptophyticones such as Leucanthemopsis alpina or Agrostistruncatula, can also act as pioneers. These species formmonospecific patches in pioneer stages that allow theestablishment of other species, contributing to thedevelopment of the plant community.

Differences in the number of species on patchesbetween both territories could be due to the pressurefrom grazing on these communities: In the high altitudesof Sierra de Guadarrama, the mountain goat (Caprapyrenaica) is absent or uncommon, in contrast to Sierrade Béjar. In this Sierra there is greater pressure from

grazing by cattle (sheep and cows). In Sierra deGuadarrama there is no sheep grazing, and the presenceof Festuca curvifolia in patches makes this grasslandnot very palatable for cows, due to the hard leaves of thisplant. Cows prefer Festuca iberica or Nardus strictapastures, which usually develop in meadows or concavesites where snow remains longer, except in very drysummers. In our field trips we have verified that theflowers of a cushion plant, Silene elegans, are usuallyeaten by sheep in Sierra de Béjar. This fact could haveconsequences on the reproduction of this species andalso on plant community development. On the otherhand, the higher altitudes of the western mountains andalso the greater influence of westerly winds bringingmore winter precipitation in the form of snow or ice(Gavilán and Fernández-González, 1997; Gavilán et al.,1998) could also produce a different plant communitydynamism.

Although nurse plants can improve some environ-mental conditions, they can also have negative effectson other factors, not allowing seedling emergence orlimiting the potential growth of a newly establishedplant. These negative effects have also been observedand occur simultaneously with positive interactions, in-volving the same species. In some cases negative inter-actions occur between cushion plants or chamaephytes.They probably compete for the same space and tend tolimit the growth of neighboring cushion chamaephytes,where the mechanisms involved are allelopathy, directcontact, etc. (Connell, 1990). On other occasions, cush-ion plants can compete with other hemicryptophyticherbs. This interaction could be related to species thatbelong to different succession stages (Kikvidze, 1993)and is supported throughout the constrained ordination

Table 5Results of the CCA (Monte Carlo test) and stepwise CCA forward selection, including the “spatial” variables and the order inwhich they were selected. F is the F-ratio statistic, p is the level of significance of the reduced model (1000 randomizations), VEis the variance explained, and TVE is the total variance explained (in percentage). Note how the spatial variables are not

enough to explain the floristic variability in Sierra de Béjar, in contrast to Sierra de Guadarrama

Variable F p VE

Sierra de Guadarrama CCA (Monte Carlo test) 2.698 0.0010 TVE: 23.06 Stepwise CCA f. selec. Step 1 Greatest Diameter (GD) 4.07 0.001 32.8

Step 2 Altitude 3.47 0.001 27.3Step 3 GD*Richness 2.23 0.006 16.79Step 4 Richness 2.09 0.004 15.38Final 3.124 0.001 TVE: 21.36

Sierra de Béjar CCA(Monte Carlo test) 3.88 0.047 TVE: 12.53 Stepwise CCA f. selec. Step 1 Altitude 2.42 0.0310 8.54

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diagram (Fig. 4), where the group of cushion plantsappears in a central position indicating an earlier stageof the community, while hemicryptophytic species ap-pear in the positive part of the diagram, indicating thatthey form part of older patches.

ACKOWLEDGMENT

We want to thank Prudence Brooke-Turner for revisingthe English.

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APPENDIXTaxa recorded in the studied territory. Abbreviations are: L.f.: Life form; Hm: Hemicryptophyte; PCh: Cushion chamaephyte; G:Geophyte; A: Annual plant. Fr1 & Fr2: Frequency of species in Sierra de Guadarrama and Sierra de Béjar patches. Alt. r.: Altitudinal

range (optimum). SM: Supramediterranean; OM: Oro-; COM: Cryorosubmediterranean; the “s” indicates the upper subbelt

Taxa Family L.f. Fr1 Fr2 Alt. r.

Agrostis rupestris Gramineae Hm . 1 OMs-COMAgrostis truncatula Gramineae Hm 12 70 SM-COMArmeria bigerrensis Plumbaginaceae PCh . 3 OMs-COMArmeria caespitosa Plumbaginaceae PCh 36 . SMs-COMAvenella flexuosa subsp. iberica Gramineae Hm 14 10 SM-COMBiscutella valentina subsp. pyrenaica Cruciferae Hm 8 . OM-COMCarex furva Cyperaceae Hm . 38 OMs-COMCerastium ramosissimum Caryophyllaceae A 5 2 MM-COMCoincya cheiranthos subsp. montana Cruciferae Hm 1 . OM-COMCrocus carpetanus Liliaceae G . 3 SM-COMCytisus oromediterraneus Leguminosae Ch 5 . SM-COMDianthus langeanus subp. gredensis Caryophyllaceae Hm . 30 (OMs)COMFestuca curvifolia Gramineae Hm 62 . SM-COMFestuca iberica Gramineae Hm 31 . SM-COMHieracium vahlii subsp. myriadenum Compositae Hm 24 3 (OMs)COMHolcus gayanus Gramineae Hm . 1 (OMs)COMJasione crispa subsp. centralis Campanulaceae PCh 38 65 (OM)COMJasione laevis subsp. carpetana Campanulaceae Hm 8 2 OM-COMJuncus squarrosus Juncaceae Hm . 1 SM-COMJuniperus communis subsp. alpina Cupressaceae Ch 14 . OM-COMJurinea humilis Compositae Hm 12 5 SM-COMKoeleria crassipes Gramineae Hm 17 . SM-COMLeucanthemopsis pallida subsp. alpina Compositae Hm 19 28 OM-COMLuzula hispanica Cyperaceae Hm 26 . COMMinuartia recurva s.l. Caryophyllaceae PCh 41 8 COMNardus stricta Gramineae Hm 8 18 SM-COMParonychia polygonifolia Caryophyllaceae Hm 8 10 OM-COMPhyteuma hemisphaericum Campanulaceae Hm 1 . COMPlantago alpina subsp. penyalarensis Plantaginaceae PCh 2 31 COMRanunculus ollissiponensis subsp. alpinus Ranunculaceae Hm 1 . SMs-COMRumex angiocarpus Polygonaceae A 2 20 SM-COMSaxifraga willkommiana Saxifragaceae PCh 5 . SMs-COMSedum brevifolium Crassulaceae Hm 45 28 SM-COMSedum candollei Crassulaceae A 10 8 COMSenecio boissieri Compositae Hm 5 . COMSenecio pyrenaicus subsp. carpetanus Compositae Hm 12 . OM-COMSilene ciliata subsp. elegans Caryophyllaceae PCh 31 77 OMs-COMThymus praecox subsp. penyalarensis Labiatae Ch 30 . COMVeronica fruticans subsp. cantabrica Scrophulariaceae Hm 24 . COM