Functional trait approaches are a valuable tool to understand the mechanisms responsible for the maintenance of species diversity and outcome of competition in ecological communities. However, the ability of simple plant functional traits to predict other interactions, such as facilitative interactions, as well as their predictive capacity relative to more labor intensive eco-physiological measurements remain poorly understood. Here, we explored how simply-measured plant functional traits, as well as more eco-physiological traits related to phosphorus (P) mobilization predict the two components of competitive fitness - species’ intrinsic growth rates and their sensitivity to neighbors. Our study system included eight plant species coexisting in a P poor mountain grassland. We performed a pairwise field competition experiment in which each focal species was planted into a background of conspecific and heterospecific competitors at different densities including zero, to estimate their sensitivity to neighbors and their intrinsic growth rates. We recorded focal species growth over two growing seasons and used maximum-likelihood methods to estimate species interaction coefficients in a nested set of models. In the field, we measured numerous functional traits for all focal species and, in the greenhouse, we measured species patterns of mobilization and depletion of different chemical forms of P.
Results/Conclusions
We found that species interacted weakly in the field, but interactions ranged from positive to negative, suggesting competitive as well as facilitative effects of neighbors. Species differences in functional traits suggested differences in their resource acquisition strategies, and together with species differential mobilization of different P forms, almost perfectly explained variation among species’ in their growth rates in the field. Finally, we found that species differences in P mobilization and depletion were related to their sensitivity to neighbors. Species that tended to be facilitated by neighbors were generally more P limited than species that tended to experience competition. Furthermore, these facilitated species tended to have negative growth rates with no neighbors, suggesting that their persistence in the natural community is dependent on beneficial effects of neighbors. The simpler to measure functional traits did not predict species’ sensitivity to neighbors, reinforcing the importance of system-specific eco-physiological traits for accurately predicting species interactions.