influences of bioclimatic factors and grazing. At the four reduced sample size levels, the positive effect held true for most cases in 400 observations (> 92%).

Conclusions

Contrary to observations in previous studies in natural grasslands, our data showed a positive effect of species richness on above-ground biomass. This suggests that, as a short-term effect of diversity on productivity, niche complementarity among coexisting species tends to be an important process in arid and semi-arid natural grasslands. We hold that biodiversity conservation is among the fundamental approaches required to maintain productivity of grasslands in arid and semi-arid areas.