No precipitation legacy effects on above-ground net primary production and species diversity in grazed Mediterranean grassland: a 21-year experiment

Question: What are the controls on biomass production and species diversity in Mediterranean grasslands? Do medium-term (i.e. inter-annual) precipitation legacy effects on biomass production and diversity occur in these grasslands?. Location: Karei Deshe Experimental Range Station, Upper Galilee, northern Israel. Methods: We used a long-term (21-r) cattle-grazing experiment in a Mediterranean grassland to search for evidence of precipitation legacies on herbaceous above-ground biomass production and species diversity. Biomass production was measured in two seasonal late grazing treatments, in the winter and at peak vegetative production in the spring, while species diversity was determined at peak production. We used ANCOVA to test the effects of residual biomass, previous year's stocking density and previous year's biomass on current biomass production. We ran forward selection and ANCOVA to test for precipitation legacies, using precipitation variables relating to both previous and current year's rainfall. In addition we examined linear regressions of biomass and species diversity on current year's precipitation. Results: Biomass production and species diversity showed weak to negligible relationships with current year's precipitation. Biomass production, and to a lesser extent, diversity, were affected by inter-annual fluctuations in growing conditions. However, evidence of inter-annual precipitation legacies were found only for early season biomass production, and not for peak biomass production or species diversity. We found no effect of grazing on biomass production at the onset of the grazing season, regardless of important differences in previous year's stocking density. Additionally, we found no effect of previous year's biomass production or stocking density on the following year's biomass or diversity, but peak biomass production was strongly related to previous year's residual biomass. Conclusions: The lack of an inter-annual precipitation legacy on peak biomass production contributes to a growing body of evidence showing that Mediterranean ecosystems are unique among semi-arid and sub-humid ecosystems, where precipitation legacies play an integral role. Mediterranean ecosystems are unique in their combined long-term high variability in rainfall and long grazing history. This lack of precipitation legacy led to the development of vegetation that is highly resistant to inter-annual fluctuations in growing conditions. We propose that the high resistance contributes to the lack of inter-annual, or medium-term, precipitation legacies on the studied grassland, and may contribute to its persistence under future projected climate change in the region.