From individual behavior to community structure: Competitive encounters alter plant root foraging strategies

Background/Question/Methods Plants are capable of complex responses to external stimuli, increasingly referred to as behaviors.  One of the most studied plant behaviors is the response of plant roots to soil nutrient heterogeneity.  In general, plants preferentially place roots inside high-quality soil patches compared to poorer-quality soil, and this confers a significant growth advantage.  Although this behavior is widespread among taxa, studies have focused on individual plants in the absence of competition. Here we describe a study designed test whether competitive encounters alter plant root foraging behavior, and if so, whether foraging has any consequence for community structure and function. We grew four species in mesocosms in a factorial design consisting of two levels of soil nutrient heterogeneity, and three levels of stem density. Plants were grown outdoors for 4 months, at which time we harvested roots and shoots. Because plant roots are typically visually indistinguishable, we used molecular techniques to identify the root foraging response of each species within each mesocosm. We also used the response of isolated plants to model a null community and compared community productivity, evenness and foraging precision of our actual communities to the null.

Results/Conclusions In general the individual foraging response of plants declined with increasing neighbor density. When plants grew individually, they proliferated roots into nutrient patches, and were significantly larger in heterogeneous compared to homogeneous soil. However, heterogeneous soils conferred little growth advantage when plants grew with neighbors, with only one species significantly larger in heterogeneous soils at high stem density. Surprisingly, this species was not the one with the highest foraging response when grown in isolation, suggesting performance of individuals does not necessarily predict the performance of the same species in a community. At the community level, total productivity was significantly enhanced by heterogeneous soil compared to homogeneous soil. However, actual productivity was lower than predicted by the null community. Evenness of our four target species was significantly lowered by both the presence of neighbors, and by heterogeneity. Belowground, the root foraging response of our actual community was lower by about half than was predicted by the null community. These results suggest that plant root foraging behavior has the potential to influence community structure and function but that this effect cannot be predicted from individual responses.