PARENT SESSION
Oral Session #77: Biodiversity and Ecosystem Function II.
Presiding: G. Chong
Thursday, August 8. 9:00 AM to 11:30 AM. Coconino Meeting Room, TCC.
Competition intensity and species evenness: effects on expression of non-complementary interactions in plant species mixtures.
POLLEY, WAYNE*,1, WILSEY, BRIAN2, DERNER, JUSTIN1, 1 USDA / ARS, Temple, Texas2 Iowa State University, Ames, Iowa
ABSTRACT- Diversity is a function of species richness and evenness. Plant species richness is thought to influence primary productivity and other ecosystem processes via mechanisms that (1) favor species with particular traits (non-complementarity) and (2) promote niche differentiation or facilitation between species (complementarity). Influences of evenness on productivity are poorly defined, but may depend on which of these two effects dominate in species mixtures. We grew three species of annuals that co-occur in grazed grasslands in central Texas (Gaillardia pulchella , Lolium perenne , and Monarda citriodora ) in monocultures and in 3-species mixtures. Evenness and identity of the dominant species were varied in replicated mixtures at each of 3 density levels to assess effects of species relative abundances and competitive intensity (density) on expression of the dominant mechanism determining biomass production of plant mixtures. Non-complementary or antagonistic interactions clearly governed production of these assemblages. Biomass of most mixtures was smaller than the average biomass of species monocultures, and mixtures consistently produced less biomass than expected from monocultures (Relative Yield Total or RYT <1). These trends reflected an inverse correlation among species between competitive ability and size at maturity. Contrary to expectation, RYT of mixtures was insensitive to both competitive intensity and species evenness. The grass Lolium performed relatively better in mixtures than in monocultures at high density, but the large disparity in size between forbs and Lolium masked expression of this trend in mixtures. These results demonstrate that non-complementary or antagonistic interactions may be insensitive to species evenness, and illustrate the central role that size differences among plants may play in mediating the influence of species diversity on productivity.
KEY WORDS: annuals, biodiversity, grassland, productivity