High species diversity often enhances ecosystem function in ways that can be attributed to complementarity and selection effects. Several recent studies have shown that environmental variation can alter diversity-ecosystem relationships. However, how complementarity and selection effects change in response to natural ecosystem variation has been less explored. In intermountain grassland, Pinus ponderosa canopies and litter reduce the diversity of native species and increase the abundance of Bromus tectorum, an invasive species. This creates a good system for exploring context- dependency in the effects of diversity on ecosystem functions. We established native plant assemblages that varied in species richness and crossed species richness level with Pinus litter addition treatments. All plots were invaded with Bromus tectorum, which allowed us to explore context-dependency in the complementarity and selection effects on community productivity and resistance to exotic invasion.
Results/Conclusions
Polycultures had higher native plant cover, a surrogate for productivity, and stronger resistance to Bromus invasion relative to monocultures, suggesting a significant diversity effect. After partitioning the diversity effect into complementarity and selection effects, we found that the species-independent complementarity effect was the main contributor to the overall diversity effect on native plant cover, which increased as species richness in polycultures increased. Species-dependent complementarity/selection effects on native cover were negligible, and neither complementarity nor selection effects on resistance to Bromus invasion varied with native richness. Pinus litter had no effect on native cover or Bromus performance, but increased the positive effects of high native diversity on the resistance to Bromus invasion. Thus we found evidence for context-dependency, and that low species richness under Pinus canopies might facilitate Bromus invasion. However, the diversity-dependent increase in native plant cover was not correlated with increases in resistance to invasion, suggesting different mechanisms for how diversity enhances productivity versus resistance.