Direct litter interference and indirect soil competitive effects of two contrasting phenotypes of a spiny legume shrub drive the forb composition of an oromediterranean community

Contrasting phenotypes of foundation species are known to differentially affect understorey plants. However, there is little knowledge on both the mechanisms of competition (resource competition versus interference) of stress‐tolerant phenotypes and the importance of indirect interactions. In an oromediterranean community from Mount Lebanon we assessed the effects on understorey forbs of two contrasting phenotypes, a tight competitive from stressful habitat and a loose facilitative from more benign habitat. In a dry south and a wet north exposure we assessed short‐term resource effects removing shrub canopy and long‐term soil effects (including litter interference) with the comparison of forb performances in adjacent naturally open areas vs no shrub. Indirect effects were quantified through the removal of grasses. Abundance, richness and biomass of forbs were measured in all treatments after one year of experiment, together with litter depth and soil moisture. We found strong direct negative soil effects of the tight phenotype on all forb performances and in particular in south exposure. These effects were due to litter interference on water availability, but not to resource competition. They were likely explained by the high hydrophobicity of organic matter accumulating in the stressful habitat of the tight phenotype. We also found an indirect competition of the loose phenotype for forb richness, due to its direct positive soil effect on competitive grasses, and in particular in south exposure. Our results improve our knowledge on the importance of litter interference in dry nutrient‐poor habitats and the role of indirect interactions in phenotypic effects on understorey species.     

Invertebrate,not small vertebrate,herbivory interacts with nutrient availability to impact tallgrass prairie community composition and forb biomass

The effects of herbivores and their interactions with nutrient availability on primary production and plant community composition in grassland systems is expected to vary with herbivore type. We examined the effects of invertebrate and small vertebrate herbivores and their interactions with nutrient availability on grassland plant community composition and aboveground biomass in a tallgrass prairie ecosystem. The abundance of forbs relative to grasses increased with invertebrate herbivore removals. This increase in forb abundance led to a shift in community composition, where invertebrate removals resulted in greater plant species evenness as well as a divergence in composition among plots. In contrast, vertebrate herbivore removals did not affect plant community composition or aboveground biomass. Nutrient additions alone resulted in a decrease in plant species richness and an increase in the abundance of the dominant grass, but the dominant grass species did not greatly increase in abundance when nutrient additions were combined with invertebrate removals. Rather, several subdominant forbs came to dominate the plant community. Additionally, the combined nutrient addition and invertebrate herbivore removal treatment increased forb biomass, suggesting that invertebrate herbivores suppress the responses of forb species to chronic nutrient additions. Overall, the release of forbs from invertebrate herbivore pressure may result in large shifts in species composition, with consequences for aboveground biomass and forage quality due to altered grass:forb ratios in grassland systems.     

灌丛化对黄土高原草地植物群落结构和地上生物量的影响

中国北方草地普遍出现灌丛化现象,灌丛化改变植物群落结构、植物多样性和生产力,直接影响着草地生态保护与可持续利用.该研究以黄土高原灌丛化草地为研究对象,通过植被调查,分析比较不同坡向的灌丛斑块与禾草斑块植物群落结构(物种组成、优势种及物种多样性)和地上生物量的差异.结果发现:(1)灌丛化草地不同坡向对物种多样性及地上生物...     

Spatial patterns in a two-tiered semi-arid shrubland in southeastern Spain

Abstract. Single species and bivariate distribution patterns in a semi-arid shrubland in southeastern Spain, dominated by the tall leguminous shrub Retama sphaerocarpa, were investigated by second-order spatial analysis based on Ripley's K-function. Shrubs were significantly clumped because of a strong association of dwarf shrubs, mostly Artemisia barrelieri, under the canopy of Retama. Retama shrubs were randomly distributed, but when different size-classes were analysed separately, the pattern changed from significantly clumped to random and then to regular with increasing canopy diameter, suggesting increasing intraspecific competition with shrub size. Artemisia was significantly clumped at all scales because of aggregation under the canopy of large Retama shrubs. The association between the species became stronger with increasing canopy diameter of Retama shrubs, suggesting that facilitation prevailed over interspecific competition because of niche separation in different tiers, both above and below ground. Retama shrub size thus determined both the type of pattern for its own size class and tier, and the scale and intensity of the association with its understorey shrubs.     

A field test of the stress‐gradient hypothesis along an aridity gradient

Aims: The stress‐gradient hypothesis (SGH) predicts how plant interactions change along environmental stress gradients. We tested the SGH in an aridity gradient, where support for the hypothesis and the specific shape of its response curve is controversial. Location: Almería, Cáceres and Coimbra, three sites in the Iberian Peninsula that encompass the most arid and wet habitats in the distribution range of a nurse shrub species –Retama sphaerocarpa L. (Boiss) – in Europe. Methods: We analysed the effect of Retama on its understorey plant community and measured the biomass and species richness beneath Retama and in gaps. We estimated the frequency (changes in species richness), importance and intensity of the Retama effects, and derived the severity–interaction relationship pattern, analysing how these interaction indices changed along this aridity gradient. Results and conclusions: The intensity and frequency of facilitation by Retama increased monotonically with increasing environmental severity, and the importance tended to have a similar pattern, overall supporting the SGH. Our data did not support predictions from recent revisions of the SGH, which may not apply to whole plant communities like those studied here or when interactions are highly asymmetrical. Facilitation by Retama influenced community composition and species richness to the point that a significant fraction of species found at the most arid end of the gradient were only able to survive beneath the nurse shrub, whereas some of these species were able to thrive in gaps at more mesic sites, highlighting the ecological relevance of facilitation by nurse species in mediterranean environments, especially in the driest sites.     

Interactive effects of grazing and shrubs on the annual plant community in semi‐arid Mediterranean shrublands

Question: We studied the interactive effects of grazing and dwarf shrub cover on the structure of a highly diverse annual plant community. Location: Mediterranean, semi‐arid shrubland in the Northern Negev desert, Israel. Methods: Variation in the biomass and plant density of annual species in the shrub and open patches was monitored during four years, inside and outside exclosures protected from sheep grazing, in two contrasting topographic sites: north and south‐facing slopes that differed in their dominant dwarf shrubs species: Sarcopoterium spinosus and Corydothymus capitatus, respectively. Results: Above‐ground biomass, density and richness of annual species were lower under the canopy of both shrub species compared to the adjacent open patches in the absence of grazing. Grazing reduced the biomass of annuals in open patches of both topographic sites, but not in the shrub patches. On the north‐facing slope, grazing also reduced plant density and richness in the open patches, but increased plant density in the shrub patches. At the species level, various response patterns to the combined effects of grazing and patch type were exhibited by different annuals. Protection against the direct impacts of grazing by shrub cover as well as species‐specific interactions between shrubs and annuals were observed. A conceptual mechanistic model explaining these interactions is proposed. Conclusion: In semi‐arid Mediterranean shrublands grazing and dwarf shrub cover interact in shaping the structure of the annual plant community through (1) direct impacts of grazing restricted to the open patches, (2) species‐specific facilitation/ interference occurring in the shrub patches and (3) subsequent further processes occurring among the interconnected shrub and open patches mediated through variation in seed flows between patches.     

Fire-mediated effects of overstorey on plant species diversity and abundance in an eastern Australian heath

Overstorey shrub species are known to influence the composition of theunderstorey in Southern Hemisphere heathlands. Overstorey densities aresusceptible to variations in fire frequency; thus, fire regimes may influenceoverstorey/understorey interactions and overall floristic composition. Wecompared patches of Banksia heath which had supported anoverstorey during a fire interval of about 30 y with patches wherethe overstorey was absent during the same period, and tested for differences inspecies composition as a function of overstorey presence. Floristic compositionvaried significantly between overstorey patches and open patches. Most specieswere less abundant in overstorey patches, however some were more abundant. Therelative abundance of species in relation to overstorey was unrelated to theirfire response, propagule longevity or propagule storage location. There wassignificantly less biomass in overstorey patches compared with open patches.Theeffect of the overstorey varied with soil moisture. In a dry area, the numberof species was lower in overstorey patches, with fewer herb and shrub speciespresent compared with open patches. Fewer species were recorded in a wetterarea, but overstorey had no effect on the number of species recorded. Reducedintensity of competition among understorey species in overstorey patches couldbe responsible for the higher abundance of some species in these patches. Wepostulate that full diversity will be maintained when the density of overstoreyshrubs fluctuates widely over a relatively short period of time. This is mostlikely when fire frequency is highly variable.     

Seed Bank and Understorey Species Composition in a Semi-arid Environment: The Effect of Shrub Age and Rainfall

Understorey vegetation in patches of Retama sphaerocarpa shrubsin semi-arid environments is dependent on the overstorey shrublife history. Community structure changes with shrub age asa result of physical amelioration of environmental conditionsby the canopy and organic matter accumulation in the soil. Weinvestigated the effect of the canopy on understorey speciesdiversity in the field and its relationships with the soil seedbank under 50 shrubs from 5 to 25+ years old, and compared speciescomposition in the field in a wet and a dry year. Species compositionof the soil seed bank under R. sphaerocarpa shrubs did not differsignificantly with shrub age, but seed density increased asthe shrubs aged. In the field, community composition changedwith shrub age, increasing species richness in a process thatdepended on the amount of spring rainfall. Our results suggestthat the soil seed bank is rather uniform and that the shrubcanopy strongly selects which species appear in the understorey.There were seeds of many species present under both young andold shrubs but which only established under old shrubs. Thisshowed dispersal was not limiting species abundance and suggestedthat the canopy was an important sorting factor for speciespresent in the understorey. Less frequent species contributedthe most to patch diversity, and rainfall effectively controlledspecies emergence. Understorey community composition dependedon multiple interspecific interactions, such as facilitationby the shrub and competition from neighbours, as well as ondispersal processes. Facilitation in this environment is a keyfeature in the structuring of plant communities and in governingecosystem functioning. Copyright 2000 Annals of Botany Company Community structure, competition, dispersal, facilitation, species composition, rainfall variability, Retama sphaerocarpa, seed bank, semi-arid environments     

Shrubs as ecosystem engineers in a coastal dune: influences on plant populations,communities and ecosystems

Question: How do two shrubs with contrasting life‐history characteristics influence abundance of dominant plant taxa, species richness and aboveground biomass of grasses and forbs, litter accumulation, nitrogen pools and mineralization rates? How are these shrubs – and thus their effects on populations, communities and ecosystems – distributed spatially across the landscape? Location: Coastal hind‐dune system, Bodega Head, northern California. Methods: In each of 4 years, we compared vegetation, leaf litter and soil nitrogen under canopies of two native shrubs –Ericameria ericoides and the nitrogen‐fixing Lupinus chamissonis– with those in adjacent open dunes. Results: At the population level, density and cover of the native forb Claytonia perfoliata and the exotic grass Bromus diandrus were higher under shrubs than in shrub‐free areas, whereas they were lower under shrubs for the exotic grass Vulpia bromoides. In contrast, cover of three native moss species was highest under Ericameria and equally low under Lupinus and shrub‐free areas. At community level, species richness and aboveground biomass of herbaceous dicots was lower beneath shrubs, whereas no pattern emerged for grasses. At ecosystem level, areas beneath shrubs accumulated more leaf litter and had larger pools of soil ammonium and nitrate. Rates of nitrate mineralization were higher under Lupinus, followed by Ericameria and then open dune. At landscape level, the two shrubs – and their distinctive vegetation and soils – frequently had uniform spatial distributions, and the distance separating neighbouring shrubs increased as their combined sizes increased. Conclusions: Collectively, these data suggest that both shrubs serve as ecosystem engineers in this coastal dune, having influences at multiple levels of biological organization. Our data also suggest that intraspecific competition influenced the spatial distributions of these shrubs and thus altered the distribution of their effects throughout the landscape.     

Effects of Soil Nutrient Heterogeneity and Earthworms on Aboveground Biomass of Experimental Plant Communities

Soil nutrients are commonly heterogeneously distributed and earthworms are one of the most common soil organisms. While effects of both soil nutrient heterogeneity and earthworms have been well studied, their interactive effect on plant community productivity has rarely been tested. In a greenhouse experiment, we constructed experimental plant communities by sowing seed mixtures of four grasses, two legumes and two forbs in either a heterogeneous soil consisting of low and high nutrient soil patches or a homogeneous soil where the low and high nutrient soil patches were evenly mixed. The earthworm Eisenia fetida was either added to these soils or not. Aboveground biomass of the whole communities, grasses and legumes did not differ between the homogeneous and heterogeneous soils or between the soils with and without earthworms. However, soil nutrient heterogeneity reduced aboveground biomass of forbs, and such an effect did not interact with earthworms. In response to soil heterogeneity and earthworms, biomass ratio of the three functional groups showed similar patterns as that of their biomass. At the patch level, aboveground biomass of the whole community, grasses and legumes were greater in the high than in the low nutrient soil patches within the heterogeneous soil. A similar pattern was found for the forbs, but this was only true in the absence of earthworms. Our results suggest that soil nutrient heterogeneity and earthworms may not influence aboveground biomass of plant communities, despite the fact that they may modify the growth of certain plant functional groups within the community.     

Canopy effects of the invasive shrub Pyracantha angustifolia on seed bank composition,richness and density in a montane shrubland (Córdoba,Argentina)

Abstract Invasive woody species frequently change the composition of the established vegetation and the properties of the soil under their canopies. Accordingly, invasion may well affect regenerative phases of the community, especially at the seed bank level, likely influencing community restoration. Pyracantha angustifolia (Rosaceae) is an invasive shrub in central Argentina that affects woody recruitment, particularly enhancing the recruitment of other exotic woody species. There is though no information regarding its effect on the soil seed bank within the invaded community. The present study was set up to gain further insight into the canopy effects of P. angustifolia. We aimed to assess whether the invasive shrub affects seed bank composition, richness and seed density as compared with the dominant native shrub Condalia montana (Rhamnaceae), and to relate the observed seed bank patterns with those of the established vegetation. We evaluated the composition of the germinable seed bank and the established vegetation under the canopy of 16 shrubs of P. angustifolia, 16 shrubs of C. montana, and in 16 control plots (10 m²) without shrub cover. The floristic composition of the seed bank differed among canopy treatments. However, seed bank richness did not differ significantly. There was an overall high seed density of exotic species throughout the study site, though exotic forbs showed significantly lower seed densities under the invasive shrub. Pyracantha angustifolia would not promote the incorporation of new species into the seed bank of the invaded community but rather favour the establishment of woody species that do not depend on seed banks. The absence of dominant woody species in the seed bank, the dominance of exotic forbs, and the high similarity between established exotic species and those present in the seed bank may surely affect community restoration following the main disturbances events observed in the region.     

Trampling enhances the dominance of graminoids over forbs in flooded grassland mesocosms

Questions: What are the interactive effects of flooding and cattle trampling upon the structural attributes and the floristic composition of a plant community? Do the effects on the plant community persist over an extended recovery period? Location: Flooding Pampa grasslands, Argentina (36°30′ S, 58°30′ W). Methods: We assessed the effects of 40‐d of flooding, trampling and the combination thereof on plant cover and biomass, vertical distribution of foliage and floristic composition in lowland grassland mesocosms. We considered a 120‐d recovery period to evaluate the persistence of flooding and trampling effects on the plant community. Results: Flooding, with or without trampling, increased cover and biomass of the graminoid species, especially marsh grasses, which developed a taller canopy, whereas most of the forb species were negatively affected. This was enhanced by trampling, as the aerial biomass of the dominant legume Lotus tenuis decreased by 90%, while three major forb species disappeared. Trampling under flooding conditions did not reduce the total above‐ground biomass production, as the growth enhancement of graminoids was enough to compensate for the breakdown of the forbs. Below‐ground biomass was lower when both perturbations occurred simultaneously. After 120‐d of recovery, graminoids continued to be dominant while the remaining forbs (including L. tenuis) recovered only partially. Below‐ground biomass recovered fully at the end of the growing season. Conclusions: The combination of flooding and trampling shifts the community co‐dominance of graminoids and forbs towards a persistent dominance of graminoid species. When both perturbations are combined, the above‐ground production of the grassland is unaffected and root biomass is rapidly recovered. However, the loss of the legume L. tenuis deserves attention because this is the unique nitrogen‐fixing species of the ecosystem, which improves the forage quality for livestock production.     

Plant responses to selective grazing by bison: interactions between light,herbivory and water stress

Two abundant tallgrass prairie forb species, Ambrosia psilostachya and Vernonia baldwinii, are commonly found intact in patches where the grasses have been selectively grazed by bison. Microclimatic patterns and physiological responses of these forbs were measured in grazed and ungrazed patches. These experiments demonstrated that bison herbivory indirectly enhanced water availability and productivity of forbs growing in grazed patches. This was due primarily to the reduction in transpiring grass leaf area in grazed patches and an increase in light availability. In grazed patches, incident light at forb mid-canopy height was 53% greater than ungrazed sites at midseason and soil temperatures were always warmer (e.g., 10°C at 5 cm), perhaps enabling forbs to initiate growth earlier in the spring. Enhanced leaf xylem pressure potential and stomatal conductance in plants in grazed areas were most evident when water availability was low (i.e., late in the growing season and over short-term dry periods characteristic of the tallgrass prairie environment). Relative to individuals in ungrazed areas, end-of-season biomass of A. psilostachya was 40% greater and reproductive biomass and head number of V. baldwinii was 45% and 40% greater, respectively, in plants in grazed patches. A favorable growing environment maintained in grazed patches during periods of water limitation enhances carbon gain in forbs leading to increased biomass and potential fitness.     

The influence of Prosopis canopies on understorey vegetation: Effects of landscape position

Abstract. The influence of canopy trees and shrubs on under‐storey plants is complex and context‐dependent. Canopy plants can exert positive, negative or neutral effects on production, composition and diversity of understorey plant communities, depending on local environmental conditions and position in the landscape. We studied the influence of Prosopis velutina (mesquite) on soil moisture and nitrogen availability, and understorey vegetation along a topographic gradient in the Sonoran Desert. We found significant increases in both soil moisture and N along the gradient from desert to riparian zone. In addition, P. velutina canopies had positive effects, relative to open areas, on soil moisture in the desert, and soil N in both desert and intermediate terrace. Biomass of understorey vegetation was highest and species richness was lowest in the riparian zone. Canopies had a positive effect on biomass in both desert and terrace, and a negative effect on species richness in the terrace. The effect of the canopy depended on landscape position, with desert canopies more strongly influencing soil moisture and biomass and terrace canopies more strongly influencing soil N and species richness. Individual species distributions suggested interspecific variation in response to water‐ vs. N‐availability; they strongly influence species composition at both patch and landscape position levels.     

Species Identity and Water Availability Determine Establishment Success Under the Canopy of Retama sphaerocarpa Shrubs in a Dry Environment

Seedling establishment in harsh environments is often enhanced by the proximity of adult shrubs. This information has been used in restoration work by placing seedlings of species being restored under the canopy of some shrubs. However, monitoring this process is often restricted to a single species, and comparisons with practices that protect planted seedlings against harsh conditions are scant. Similarly, few studies have supplied seedlings with water in the summer to observe the effects of water availability on the interaction. We compared sapling survival of three woody species (Olea europaea, Pistacia lentiscus, and Ziziphus lotus) under the leguminous shrub Retama sphaerocarpa and in gaps covered with piled branches that mimicked a shrub canopy. After 3 years, survival of saplings planted under Retama differed depending on species identity and water supply. Survival of Olea saplings placed under Retama shrubs was twice that under piled branches if not watered (35 ± 8 vs. 17 ± 2 %, respectively), whereas survival of saplings under Retama, if watered, was less (48 ± 11 vs. 68 ± 8%, respectively). Retama shrubs had a negative effect on Ziziphus; most saplings died under its canopy, whereas survival in piled branches ranged 10–54%. Pistacia was neither facilitated nor outcompeted by Retama. Facilitation of Olea by Retama shrubs was more apparent under dry conditions where watering increased competition and decreased facilitation. Overall, we conclude that Retama shrubs can help dry land restoration to a greater extent than artificial shade for Olea when not watered. The critical role played by water supply in determining nursing success rates warrants further study.     

The influence of canopy cover on understorey development in forests of the western Cascade Range,Oregon, USA

Natural disturbances, especially fire and treefalls, influence tree canopy composition in the Pseudotsuga menziesii forests of the western Cascade Range, Oregon. The composition of tree, shrub, and herb assemblages in the understorey of stands with different canopy types, such as maturing Pseudotsuga, Tsuga heterophylla, or mixed species stands, also differs.Differences in both canopy type and the prevalence of canopy openings correlated with different degrees of understorey development in stands of similar ages. This suggests that understorey assemblages also reflect disturbance history. Before understorey assemblages can be used to relate community samples to community or habitat types, the extent to which their composition reflects long term influences of stand history vs. differences in site potential must be determined.     

Influence of N2-fixing Trifolium on plant species composition and biomass production in alpine tundra

Alpine Trifolium species have high rates of symbiotic N₂-fixation which may influence the abundance and growth of plant species growing near them. The potential for facilitative effects on plant abundance and growth in dry meadow alpine tundra of Niwot Ridge, Colo., characterized by low resource availability, was investigated by measuring soil N, aboveground biomass production, and plant species composition in patches of Trifolium dasyphyllum and surrounding tundra. Extractable inorganic N was more than twofold greater and extractable P was 27% lower in Trifolium patches than in surrounding tundra. Aboveground production was twofold greater in Trifolium patches than in surrounding tundra. However, the difference was largely due to the production of T. dasyphyllum relative to the non-Trifolium component of biomass, which was not different between the Trifolium patches and surrounding tundra. In the Trifolium patches, the proportion of graminoid biomass was lower while the proportion of forb biomass was higher relative to surrounding tundra. Although the abundance of some species was positively associated with the presence of Trifolium, other species were less abundant, possibly due to increased competition for P and differential abilities of alpine species to respond to increased N availability. Trifolium may exert both facilitative and inhibitive effects on dry meadow alpine species and, in the process, substantially influence the spatial heterogeneity in community structure and primary production. Received: 14 October 1997 / Accepted: 2 February 1998     

Realistic Variation in Species Composition Affects Grassland Production,Resource Use and Invasion Resistance

We investigated the effects of realistic variation in plant species and functional group composition, with species occurring at realistic abundances, on ecosystem processes in exotic-dominated California grassland communities. Progressive species removals from microcosm communities, designed to mimic nested variation in diversity observed in the field, reduced grassland production, resistance to intentional invasions, and resistance to natural colonization by new species. Three lines of evidence point to the particular importance of intensified competition within a single functional group—late-active forbs—in explaining the observed effects of realistic species loss order on community resistance. First, reduced success of naturally colonizing species in more diverse assemblages was dominated by declining colonization by late-active forbs. Second, increasing late-active forb biomass appeared to reduce the biomass of intentionally introduced yellow starthistle (Centaurea solstitialis, a late-season forb) both within and across diversity levels. Finally, starthistle addition reduced biomass of resident late-season forbs but not of any other functional group. Increasing diversity increased light levels and soil moisture availability in spring and summer, providing a proximate mechanism linking our realistic species loss order to decreased community resistance. Starthistle addition reduced light and soil moisture availability but not N across richness levels, mirroring the apparent effects of the additional late-active forb species present in higher diversity treatments. Species losses that entail the early loss of whole or key functional groups could, through mechanisms like those we explore, have greater ecosystem consequences than those suggested by randomized-loss experiments.     

Soil fertility and herb facilitation mediated by Retama sphaerocarpa

Abstract. Woody legumes growing in dry climates can increase soil nutrient content and facilitate plant growth in their understorey. We investigated differences in soil fertility and herbaceous community in relation to the presence and absence of the legume Retama sphaerocarpa in a shrubland in a mediterranean type climate. The results showed a higher content of limiting nutrients for plant growth, such as phosphorus and nitrogen, under the R. sphaerocarpa canopy. Herbaceous biomass, cover and nitrogen content increased below the canopy. However, species richness and diversity were diminished in the presence of a dense canopy of R. sphaerocarpa. Nitrogen isotopic analysis did not indicate a clear relationship between symbiotic fixation in R. sphaerocarpa and nitrogen content of soil and plants under its canopy. Nevertheless, herbs growing in the understorey showed a linear correlation between foliar N content and ¹⁵N values. The existence of a dense shrub canopy induced a smaller monthly variation in herb richness, diversity and biomass, suggesting that it provides a stable microhabitat facilitating herbaceous vegetation establishment and growth. The ‘shelter’ effect was more important when environmental conditions became stressful for herbaceous plants.     

Spatial segregation of plant species caused by Retama sphaerocarpa influence in a Mediterranean pasture: a perspective from the soil seed bank

Retama sphaerocarpa is a leguminous shrub whose important role in the semi-arid regions of south-eastern Spain has already been assessed: shrub canopies reduce light intensity, and thus evapotranspiration; also their extensive radical system take water and nutrients from great volumes of soil, concentrating them in the understorey. Consequently, subcanopy vegetation benefits from these facilitation processes, increasing its productivity. However, these shrublands have been rarely studied at a broader scale, i.e. as a savannah-like system composed of a variable number of shrubs scattered in a herbaceous matrix. As the microenvironmental conditions associated to the understorey are rather different from those of the open spaces among shrubs, species composition of the herbaceous matrix is expected to change accordingly. Thus, R. sphaerocarpa would be an important and still unknown source of spatial heterogeneity to the system. Our main purpose was to evaluate, through the soil seed bank, the heterogeneity in the herbaceous community induced by this shrub species. Soil samples were collected around adult shrubs from three positions relative to the canopy: near the centre of the shrubs, at the edge of the understorey, and completely outside the canopy. Floristic composition was evaluated by germination under greenhouse conditions. The results show that each position has a different floristic composition, characterised by a group of different species. The herbaceous species associated with the external position have functional traits which enable them to resist water stress and herbivore pressure, such as hairs, CAM metabolism, early flowering, horizontal growth or tiny stature. The species associated with the central position lack those traits, and are more competitive in more mesic environments, rich in nitrogen. The lowest number of seedlings and species was found in the internal position, suggesting that in our study the facilitation process may have less importance for community dynamics due to less stressful environmental conditions.