Submitted to: Plant Ecology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/13/2012
Publication Date: 3/1/2013
Citation: Daneshgar, P.P., Wilsey, B.J., Polley, H.W. 2013. Simple plant traits explain functional group diversity decline in novel grassland communities of Texas. Plant Ecology. 214:231-241. Interpretive Summary: Many native grassland ecosystems have been converted to communities of exotic or introduced plant species. Species typically exhibit greater disparities in relative abundances (evenness) on exotic than native grasslands, but it is not know whether evenness is inherently lower in exotic than native plant communities or results from greater soil fertility or other factors often associated with exotic invasions. In order to determine why evenness often is low in exotic communities, we measured changes in the relative abundances of groupings of plants that are thought to function similarly (functional groups). Changes in evenness were studied in experimental plant communities planted to either all exotic or all native perennial plants common to the Blackland Prairie region of central Texas. Evenness of functional groups declined more rapidly in exotic than native plant communities. During the year following planting, the relative abundance of warm-season grasses remained near 50% in native communities but increased to 88% in exotic communities. Broad-leaf herbaceous species (forbs) comprised more than one-third of native communities, but just 8% of exotic mixtures. Evenness declined more rapidly in exotic than native communities because warm-season grasses quickly dominated exotic communities. Exotic grasses were taller and had greater leaf area than their native counterparts. Perhaps because they were selected to produce forage for cattle, exotic warm-season grasses grow rapidly, depress forbs and other neighboring plants, and dominate communities of introduced plants.
Technical Abstract: Recent work on novel ecosystems suggests that exotic species contribute to functional group diversity decline as exotic systems replace native ones. We experimentally compared 18 exotic and 18 native prairie species paired for phylogeny, growth form, and mode of photosynthesis grown both in monoculture and in 9-species mixtures of exclusively exotic or native species to identify which traits lead to decline. We compared community-level traits (yielding behavior and relative abundance in mixture), whole plant traits (height, biomass and light capture) and leaf traits (area, specific leaf area, and C:N ratio) by functional group under both ambient and climate change conditions (summer irrigation). Exotic communities had less functional group diversity than native communities after the first growing season. High relative abundance and overyielding of exotic C4 grasses were correlated with functional group diversity decline. Exotic C4 grasses had greater leaf area, specific leaf area, height, biomass and light capture than native C4 grasses, suggesting that they were better at capturing resources. In native communities, functional group diversity was maintained by overyielding by C3 species. Native C3 grasses were taller and had greater biomass in monoculture than comparable exotic C3 grasses despite having smaller leaf area and specific leaf area. This suggests that native C3 plants may have high resource use efficiency. We observed no significant differences in how irrigation impacted species traits between exotic and native species. Our results suggest that plant traits in dominant species (in this case, C4 grasses) drive functional group diversity decline in exotic communities, whereas lower dominance by C4 grasses and/or greater resource use efficiency in C3 species helps to maintain native diversity.