Abstract
Natural and managed ecosystems provide a variety of ecological, economic, and cultural benefits; yet most have been altered by human activity such that they exhibit deficits in both biodiversity and functionality. Identifying factors accelerating the recovery of key species and associated functions in degraded systems is therefore a global priority. We tested the hypotheses that explicitly incorporating biodiversity into restoration design will lead to greater ecosystem function and that positive effects of diversity will strengthen over time due to an increase in the importance of complementarity relative to selection effects. We did this by manipulating salt marsh plant species richness across a tidal elevation gradient as part of a coastal wetland restoration project in southern California. Overall, diversity enhanced biomass accumulation in experimental plots, with the magnitude of the effect strengthening from one to three years post-restoration due to a combination of decreasing performance in monocultures and increasing performance in multispecies mixtures over time. Positive diversity effects were initially due exclusively to selection, as mixtures were dominated by species also exhibiting high performance in monoculture, although the identity of the highest performing species varied across tidal elevations and over time. By the end of the study, complementarity, indicative of niche partitioning and/or positive interactions among species, contributed to productivity at least as much as selection effects. Our study provides real-world support for a recent theoretical model predicting strong positive biodiversity effects when functionally different species coexist in a heterogeneous landscape. Incorporating biodiversity into restoration designs can result in net gains in ecosystem function especially in low diversity systems, yet shorter experiments lacking broad environmental and species trait variability may both underestimate the strength of and misidentify the mechanisms underlying positive diversity effects.