Water availability, functional trait diversity, and competitive interactions in Sonoran Desert winter annuals

Background/Question/Methods

Competition is a major driving force in structuring plant communities.  Since traits of individual species in communities are important in determining the outcome of competitive interactions, a critical challenge in the field of ecology is to understand how functional traits determine population dynamics and community structure.  Further, competitive interactions are expected to change with environmental conditions, making it important to investigate how functional traits influence species interactions under different environmental settings.  Previous work with Sonoran Desert winter annuals identified a trade-off between relative growth rate (RGR) and water-use efficiency (WUE) that relates to species differences in population dynamics over time, and contributes to long-term species coexistence.  Functional traits that underlie this RGR-WUE tradeoff explain differences among species in their sensitivity to weather patterns, particularly precipitation.  Here, we ask whether species with different functional traits respond differently to competition, and whether environmental conditions mediate those relationships.  We conducted competition trials for three species that occupy different positions along the RGR-WUE tradeoff axis.  Because water is the major limiting resource in the Sonoran Desert, we compared the effects of competition at both high and low water availability. 

Results/Conclusions

The tradeoff between relative growth rate (RGR) and water-use efficiency (WUE) led to shifts in competitive responses among species at different soil moistures.  The high RGR species was most strongly impacted by competition at low water availability, and showed the greatest decrease in performance in response to drought.  In contrast, the low RGR species (with high WUE) had the best response to drought conditions and was proportionally more affected by competition at high water. Surprisingly, competitive interactions did not scale directly with functional trait differences.  Instead, the species with intermediate RGR and WUE tended to have the strongest competitive effects on all species.  Such condition- dependent species interactions can have strong impacts on community dynamics, and can promote species coexistence in variable environments.  Thus, this study provides an important step in mechanistically understanding the link between functional traits, environmental conditions, and the outcome of competitive interactions.