High diversity can buffer ecosystem productivity from negative perturbations caused by disturbances such as droughts. A common mechanism identified that causes increased ecosystem stability with increased diversity is asynchronies in how species respond to variation in climate and resources. Hence decreases in diversity can lead, either because fewer species are present, or because a decrease in species asynchrony to lower stability of productivity. Eutrophication especially, has been identified as a major factor decreasing stability, because eutrophication increases productivity, and through asymmetric light competition, lowers diversity. However, in dry grasslands such as Cedar Point Biological Station in western Nebraska nitrogen additions also induce decreases in stability even though plant diversity does not decline. Here we use a NUTNET site with 60 plots to examine the mechanism driving decreased stability with eutrophication in dry grasslands, i.e. where water is the primary limiting resource.
Results/Conclusions
Rainfall during the growing season at Cedar Point varies fourfold from 50 mm to 200mm, and productivity varies linearly with rainfall from 100 to 300 g/m2. Adding nitrogen does not increase productivity in dry years, but increases productivity to 550 g/m in wet years. This increase in productivity is not in perennials, but in annuals and increases over years, likely because an increase in soil annual seedbank. Thus, similarly to nitrogen limited grasslands, an increase in productivity decreases stability because of increased species asynchrony. However, in contrast to nitrogen limited grasslands, this increased species asynchrony is not because of a diversity decline, but because of an increase in annual species in wet years. In other words, adding nitrogen in a water limited grassland increases the vegetation responsiveness to rainfall variability. More general, adding a primarily limiting resource, like nitrogen in a nitrogen limited grassland causes permanent species composition changes and diversity decreases, because of increased biomass and litter leading to increased light limitation. In contrast, adding a secondarily limiting resource, nitrogen, in a water limiting grassland makes the plant community more responsive to a positive perturbation in the primarily limiting resource, water.