The study of traits has been proposed as a method of linking such disparate fields as physiology, population, community, and ecosystem ecology. However, a number of assumptions underlie “trait-based ecology” (TBE). While phenotypic plasticity has long been recognized as an important feature of how organisms interact with their environment, to date TBE has tended to downplay the extent to which traits of species shift in response to environmental changes. More importantly, TBE tends to assume that even if species vary in their traits in response to environmental variation, such changes are predictable and relatively constant across species. Finally, TBE places great importance on the existence of trade-offs between traits. These trade-offs have been shown to exist at large scales, but how strong they may be within a local flora is less certain. We grew 23 wetland species in a greenhouse experiment under three nitrogen treatments and three water table depths and measured a variety of leaf-level (SLA, photosynthesis, %N, C:N) and organismal (above- and below-ground biomass) to address three questions. 1) Does environmental variation lead to predictable changes in traits across species? 2) Are species consistent in their responses to environmental variability? And 3) do trait trade-offs exist within a local flora?
Results/Conclusions The effects of increasing nitrogen had significant and predictable effects on mean trait values across species. The effects of varying water table were less predictable, including significant effects on % N and C:N that were not predicted and, contrary to expectations no significant effects on above-ground biomass. In general, although there were significant species by treatment interactions for some traits, the correlation in trait values across species between different treatments were relatively strong, particularly for whole-organism traits (ranging from r=0.17 between photosynthesis in 0 cm and 15 cm water table treatments and r=0.96 between above-ground biomass in low and medium nitrogen additions). Ordination of species in trait space demonstrates that while species largely respond similarly to N treatments, there is considerable variability in species responses. Contrary to predictions, we found no relationship between Leaf %N and photosynthesis, Leaf % N and SLA, or SLA and photosynthesis. We found strong relationships between above-and below-ground biomass that were consistent across treatments. Together these results suggest that predictions based on traits should take into account variation in phenotypic plasticity across species in response to different environmental drivers.
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