Thursday, August 7, 2008 - 4:00 PM

COS 97-8: An experimental test of seed limitation of recruitment, biomass, and species richness of herbaceous communities by livestock grazing in semi-arid shrubland

Bertrand Boeken, Ben-Gurion University of the Negev

Livestock grazing is very common in semiarid shrubland in the world. Sheep, while grazing herbaceous vegetation, remove plant biomass and simultaneously trample the surface. Trampling of shrub patches may reduce heterogeneity. increase resource loss and decrease productivity and diversity. The question is how livestock herbivory affects productivity and diversity of the intershrub herbaceous vegetation. We tested the hypotheses that 1) simulated herbivory (mowing) causes seed limitation of recruitment and biomass due to failed seed production of common species, and 2) presence of ungrazed species-rich shrub patches compensates for seed limitation in the intershrub.
A two-year field experiment was done in the Negev desert of Israel with 150 mm annual rainfall. Three blocks were used, each with 9 plots of 6 x 6 m, with all combinations of landscape state (0, 50 and 100% shrub patches removed) and mowing (none, intershrub, and also under shrubs). Shrub patches were removed in spring 2004. The vegetation was mowed in spring 2005 and 2006. The intershrub vegetation was sampled in spring 2006 and 2007, in three 20 x 30 cm quadrats per plot to determine abundance and biomass per species. Four seed traps were placed per plot until first rainfall.
In both years mowing of the intershrub alone significantly reduced intershrub plant density by 40% and biomass by 30%, due to ca. 50% reduction of the dominant grass S. capensis. Mowing also increased species richness by 20-30%. Shrub removal had no significant effect, but interacted with mowing on density of the subordinates. Their density and biomass correlated negatively with S. capensis density. Seed trap data showed that mowing decreased seed availability of S. capensis by more than 50%, but not of subordinates.
The experiment confirmed that mowing reduced intershrub plant density and productivity due to seed-limited recruitment of the dominant. Mowing released competitive suppression of other species by the dominant, indicating site limitation. The hypothesis that shrub patches compensate for seed limitation was rejected, as S. capensis is sparse there. Repeated livestock grazing may cause drastic declines in productivity due to reduced S. capensis populations. Intermittent grazing may mitigate the effect, and help in conserving biodiversity.