Climate change will exacerbate the degree of abiotic stress experienced by semi-arid ecosystems. While abiotic stress profoundly affects biotic interactions, their potential role as modulators of ecosystem responses to climate change is largely unknown. Using plants and biological soil crusts, we tested the relative importance of facilitative -competitive interactions and other community attributes (cover, species richness and species evenness) as drivers of ecosystem functioning along stress gradients in semi-arid Mediterranean ecosystems. Biotic interactions shifted from facilitation to competition along stress gradients driven by water availability and temperature. These changes were, however, dependent on the spatial scale and the community considered. We found little evidence to suggest that biotic interactions are a major direct influence upon indicators of ecosystem functioning (soil respiration, organic carbon, water-holding capacity, compaction and the activity of enzymes related to the carbon, nitrogen and phosphorus cycles) along stress gradients. However, attributes such as cover and species richness showed a direct effect on ecosystem functioning. Our results do not agree with predictions emphasizing that the importance of plant -plant interactions will be increased under climate change in dry environments, and indicate that reductions in the cover of plant and biological soil crust communities will negatively impact ecosystems under future climatic conditions.

The encroachment of woody plants into grasslands, and the conversion of savannas and open woodlands into shrublands, has been widely reported during the past decade. Encroachment has generated considerable interest among ecologists worldwide, but there have been few quantitative syntheses of the effects of encroachment on ecosystem processes across large areas of the globe. Here we summarize the results of observations of the effects of encroachment by woody plants on 111 ecosystem response variables using data obtained from 1722 encroached-unencroached pairs, reported in 160 studies from North and South America, Africa, Europe, Australia, and Asia. We used an extensive review of the literature, including both published and unpublished data, to summarize available data on the effects of a change from open woodland or grassland to shrubland or closed woodland, on richness of plant and animal taxa, soil chemistry, and the status of the soil surface. Our database is restricted to arid, semi-arid, and dry sub-humid environments (drylands) receiving average annual rainfall between 850 and 200 mm. An analysis of the impacts of shrub encroachment on ecosystem structure and function has already been reported using a large subset of these data. This updated data set can provide an opportunity to test further hypotheses about the effects of encroachment on plant and animal communities and on soil processes related to ecosystem functioning.

This study aimed to address which factors, other than topography, contribute to the floristic variation of forested slopes. The natural forest studied is located in the Sudoeste Alentejano e Costa Vicentina Nature Park (southwestern Portugal). We sampled topographic, edaphic, floristic and community structure variables along three bottom-top hillside transects. Multivariate analyses of soil variables (by PCA), and of woody species composition and floristic-environmental relationships (by CCA and pCCA) were performed. Environmental-floristic trends strongly associated with the elevation gradient were identified. At lower altitudes, the lowest species richness, the highest soil fertility, and the tallest and most dense (least available light) canopy occurred. The spatial variation in woody species composition and abundance was closely associated with Zn availability in the soil and litter groundcover, but these varibles had significant spatial structure in the studied forest. The non-spatially structured species variance was better predicted by soil NO 3 À and NH 4 + . The spatial variation of species data not shared with environmental variables was also calculated. We suggest that the influence of topographic gradient on the variation of edaphic variables and on the distribution and abundance of woody species was mediated by overstorey tree composition. Locally dominant tree species, in particular Quercus faginea and Quercus suber, may function as ''ecosystem engineers'' promoting environmental changes (i.e., Zn availability in soil, litter accumulation and light availability) that influence overall floristic variation.

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