General agreement has emerged over the past three decades that biodiversity can significantly affect a variety of ecosystem processes. However, the mechanisms underlying such effects can vary over time and in different ecosystem types. In particular, complementary resource use among different plant species can affect primary production and nutrient uptake, but its strength and prevalence depend in part on patterns of community assembly. To what extent do mechanisms allowing species coexistence also contribute to complementary resource use that may influence primary production? Which plant functional traits help predict both complementarity and coexistence? From 1992-2017, we conducted a suite of experiments and observational studies in California serpentine grasslands aimed at assessing, by four independent pathways, potential for complementarity: 1. Experimental studies of diversity effects on productivity and nutrient use; 2. Experimental studies of invasion success based on functional composition; 3. Patterns of species and functional composition in intact grasslands; and 4. Observational studies of invasion success and community assembly in plots differing in initial composition. We hypothesized that functional traits related to complementary resource use and primary production in experimental plots would also predict experimental and observational success of invaders, and functional composition in intact grasslands.
Results/Conclusions
Multiple independent lines of evidence support complementarity among plants that differ in functional traits related to resource acquisition. However, we also found that effects of complementarity on community assembly and productivity have limits. First, rooting depth and phenology (timing of maximum resource use) predicted multifunctional complementarity in resource use. However, such complementarity did not necessarily lead to greater total primary production. Second, initial invasion success – experimental and observational – was best predicted by differences in resource acquisition traits between invaders and the existing community. Over time, however, disturbance and propagule pressure overwhelmed complementarity as determinants of invader success. Third, similar functional composition developed over time in experimental communities that started from different initial functional compositions. This finding contrasts with predictions from other studies that suggested competitive dominance of late season annuals over early season annuals. Finally, functional composition across resource gradients in both experimental plots and intact grassland was consistent with that developing over time in experimental community assembly. However, in both cases, dominance within functional groups in different resource environments depended on traits related to the leaf economic spectrum. Bottom line: complementarity is real, but other ecological forces can limit its effects on community assembly and ecosystem functioning.