|HOME SCHEDULE AUTHOR INDEX SUBJECT INDEX|
Species diversity in annual plant communities: small-scale interactions between productivity, grazing and plant size.
Kigel, Jaime*,1, Osem, Yagil1, Perevolotski, Avi2, 1 Faculty of Agriculture, Rehovot, Israel2 Faculty of Agriculture, Rehovot, Israel
ABSTRACT- In the Mediterranean semi-arid region annual species are the main biomass producers in herbaceous communities with a long history of livestock grazing (ca 8000 years).In this region, the large heterogeneity of rainfall and habitat topography results in highly patchy distribution of resources for plant growth and in large variation of productivity in small scales of space and time. The interactive effects of productivity, sheep grazing and plant size on species diversity were studied in a semi-arid region of Israel (300 mm/year)with high annuals diversity- 130 species. Protection from grazing reduced species richness in high productivity sites (Wadi-dry river terraces, 200-700 g/m2), but this effect was reversed in low productivity sites (Hilltop, South- and North-facing slopes, 10-200 g/m2). Annual species that behaved as protection increasers had relatively large plants (>0.10 g/plant), while small species (<0.05 g/plant)were usually protection decreasers. Responses of intermediate size species were site-dependent: increasers in the poorer sites and decreasers in the high productivity sites. A conceptual model is proposed in which the responses to grazing are explained by interactions between soil resources availability and plant size.In high productivity sites, the larger and more palatable species became more abundant in the absence of grazing and the resulting closed canopy constraints the smaller and less competitive species, thus reducing species richness. In the low-productivity sites, where species were smaller and the vegetation less dense, competition for canopy resources was lower and lack of grazing increased plant density and species richness. The observed patterns of response to grazing and productivity at the small spatial scale, are comparable to those reported at the ecosystem scale for semiarid and semi-humid perennial grasslands, with a long evolutionary history of grazing.
KEY WORDS: Annuals, Grazing, Mediterranean, Semi-arid