Exotic species have become common and even dominant in some grasslands forming novel ecosystems because the species in them have no common evolutionary history.  Recent work on these novel ecosystems suggest that exotic species contribute to diversity declines.  In order to identify the plant traits that contribute to this decline, we sampled plots in 2008 that were part of the maintenance of exotic vs. native diversity (MEND) experiment in the Blackland Prairie region of Texas.  This experiment, a randomized block design comprised of two blocks (representing two planting times) uses 18 exotic and 18 native species paired for phylogeny, growth form, and mode of photosynthesis planted in plots that were either all exotic or all native as either monocultures or random draws of 9 species planted with realistic relative abundances.  Draws were done so that exotic-native pairs were always selelcted together. We compared leaf traits (leaf area, specific leaf area (SLA), and C:N ratio), whole plant traits  (species heights, biomass, and light capture) and community-level traits  (relative yield in mixture) by functional group.  These traits were tested in both ambient and climate change conditions.

Results/Conclusions

Native communities had significantly higher functional diversity than exotic communities.  In exotic communities, C₄ grass dominance appeared to cause functional diversity decline.  With an initial relative yield of 56% in mixture at planting, exotic C₄ grasses grew to 88% of the total yield at harvest resulting in the abundance decline of C₃ grasses by 8%, forbs by 17%, and legumes by 7%.  Exotic C₄ grasses had greater leaf area, peak light capture, and SLA than native C₄ grasses suggesting a greater ability to capture resources, which led to exotic C₄ grasses producing the most biomass of all groups tested.

Stability and functional diversity in native communities was maintained by C₃ grasses and forbs.  Native C₄ grasses decreased in abundance by 8%, while C₃ grasses increased by 3% and forbs by 12%.  The native C₃ grasses were taller and had six times greater biomass than their exotic counterparts, which led to greater peak light capture.  Native forbs were also taller and had greater biomass than exotic forbs.  Native C₃ grasses and forbs were more productive than exotics despite having less leaf area and SLA suggesting greater resource use efficiency.