Canopy structure of woody landscape modulators determines herbaceous species richness along a rainfall gradient

Landscape modulators are ecosystem engineers that create distinct patches that differ from their surroundings in abiotic conditions and species composition. Woody plants are principal landscape modulators in many terrestrial ecosystems. The effects of woody landscape modulators on herbaceous species richness have been studied at the site level in Israel. Here we examine the effect of woody landscape modulators on the general pattern of herbaceous species richness over a large spatial scale along a rainfall gradient, and develop a novel quantitative model to explain that pattern. A uniform experimental design was implemented at five sites along a rainfall gradient in Israel. Herbaceous species richness was determined in woody (modulated) patches and in their surrounding (unmodulated) areas, both in undisturbed (control) plots and in plots in which the canopies of the woody plants had been removed. Herbaceous species richness increased asymptotically with increasing rainfall in the unmodulated areas, but not in the modulated patches of the no-removal plots. Removal of woody plant canopies resulted in a similar pattern of herbaceous species richness to that of the unmodulated areas. Woody landscape modulators modify the pattern of herbaceous species richness along rainfall gradients by creating a biotically induced gradient of abiotic conditions, mainly via their canopy structure. The proportion of species in the site-level species pool that are filtered into the modulated patches declines in strong negative correlation with increasing LAI of the landscape modulator. The combination of climatic and biotically induced gradients determines the overall pattern of herbaceous species richness in two-phase landscape mosaics.