Biomass stability lower in diverse restored grasslands
There is increasing concern about the stability of ecosystem functioning as natural habitats are more frequently and severely disturbed by climate change and other global change factors. Theory and experiments strongly suggest that diverse ecosystems stabilize ecosystem functions such as productivity; however, these ideas have not been widely extended to natural systems, where diversity and composition, and abiotic conditions may vary among sites. Stability may be affected by species richness, evenness, or dominant species identity. The relative importance of diversity, versus abiotic conditions that drive ecosystem functioning, in determining stability remains uncertain. Finally, the application of these ideas to the restoration of disturbed ecosystems remains in its infancy. In 28 grassland restorations that ranged in initial richness from 3.7 to 23.6 species/m2, we measured aboveground biomass before, during, and after a drought year. We used a model comparison approach to test for the direct and relative effects of biotic (plant species richness, evenness, dominant species identity, and the change of each following drought) and abiotic (soil properties, fire history, site age and size) factors on three components of biomass stability: temporal stability, and resistance and resilience to drought.
Although the relative strength of biotic and abiotic effects varied across components of stability, pre-drought richness was consistently correlated with lower stability. Sites that maintained or increased richness following drought were, however, more resilient. Diversity generally predicted stability well when compared to models controlling for either abiotic factors or dominant species. By comparison, the strength and direction of dominant species effects varied by species. Pre-drought abundance of the most common dominant species, Andropogon gerardii, was correlated with greater stability. Sites that maintained or increased abundance of two other dominant species, Sorghastrum nutans and Poa pratensis, were less temporally stable. In general, dominant species effects covaried or interacted with the effects of abiotic predictors, in particular fire history. For example, the effect of A. gerardii on resistance depended on historical fire frequency. Model-averaging indicated different primary drivers across components of stability. Temporal stability was controlled most strongly by dominant species and fire history. Specifically, burning in the year before the drought reduced temporal stability. Pre-drought richness and evenness strongly reduced resistance while the maintenance of richness strongly supported resilience. The maintenance of diversity in the face of drought may be important for the resilience of ecosystem functioning following environmental perturbations.