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.资源竞争理论通常将资源供给速率视为独立变量，但自然生态系统中养分供给往往存在强烈的协同变异，而这种协变关系会改变模型的预测结果。本研究通过构建模型验证了这一普遍现象：土壤水分介导了植物对氮素的竞争，该过程可引发物种共存、交替稳定状态（alternative stable states）以及竞争排除（competitive exclusion）三种结果。在该模型中，土壤水分通过调控微生物过程、养分淋溶与植物吸收过程，进而影响氮素的可获得性。植物通过改变水分可获得性，会间接影响自身及其竞争者的氮素可获得性，进而改变竞争结局。 来自地中海的外来一年生植物已在加利福尼亚地区取代了大量本土多年生草本植物。已有研究表明，氮素与水分均是该生态系统中的潜在限制性因子。我们针对加利福尼亚草原对模型进行了参数化，并证实：由土壤水分介导的氮素竞争，可解释干旱地区外来一年生植物占据优势的现象，而湿润区域则更易出现物种共存的局面。 上述研究结果与美国太平洋沿岸各州草原入侵的宏观生物地理格局相符：加利福尼亚州多数炎热干旱的草原已被外来一年生植物入侵，而北加州湿润的海岸草原以及俄勒冈州与华盛顿州的草原，则仍以外来多年生草本植物为主导。