Print PageCommunity composition is influenced by the interaction between functional traits of individual species and environmental conditions. In a Sonoran Desert winter annual plant community, we have identified a trade-off between relative growth rate (RGR) and water use efficiency (WUE), such that species with high RGR have low WUE and vice versa. The RGR-WUE tradeoff relates to patterns of demographic variability and species coexistence. Here we investigate patterns of selection on WUE, RGR, and related traits within populations of four species that are at different points along the RGR-WUE tradeoff. We measured RGR, WUE, specific leaf area, root mass ratio, and leaf N content on hundreds of individuals of each species at two study sites. Using biomass as our proxy for fitness, we used multiple regression to investigate patterns of selection on traits.
Results/Conclusions
The tradeoff between RGR and WUE that exists among species was not found within species. In fact, for the two high-WUE species included in our study, individuals with high RGR also had high WUE. The strength of selection varied depending on the species and the study site, but all species experienced positive directional selection for WUE, RGR, and leaf N content and negative selection for root mass ratio. Selection tended to be stronger on WUE at the warmer and drier study site, and selection on RGR tended to be stronger at the cooler and wetter site. These results indicate that the tradeoff that exists among species does not represent a fundamental phenotypic constraint, but instead may represent assembly rules for what species can coexist. Although all species experienced selection on all traits in the same direction, their unique positions along the RGR-WUE tradeoff indicate specialization on different traits.
