Data from: Plant water use affects competition for nitrogen: why drought favors invasive species in California

Resource competition theory typically treats resource supply rates as independent, however nutrient supplies covary strongly in natural systems and this covariance can alter model predictions. We demonstrate this general phenomena using model in which competition for nitrogen is mediated by soil moisture and leads to coexistence or alternative stable states as well as competitive exclusion. In the model, nitrogen availability is regulated by soil moisture as microbial processes, leaching and plant uptake are dependent on soil moisture. By affecting water availability, plants also indirectly affect nitrogen availability for themselves and for their competitors, and may therefore alter the competitive outcome. Exotic annual species from the Mediterranean have displaced much of the native perennial grasses in California. Both nitrogen and water have been shown to be potentially limiting in this system. We parameterise the model for a Californian grassland and show that soil moisture mediated competition for nitrogen can explain the dominance of exotic annual species in drier areas with coexistence expected in wetter regions. These results are concordant with larger biogeographic patterns of grassland invasion in the Pacific states of the USA, in which most of the hot and dry grasslands in California have been invaded by exotic annuals, whilst the moister coastal prairies in northern California and the prairies of Oregon and Washington are dominated by exotic perennial grasses.