COS 59-7
Plasticity in tree root architecture indirectly affects coexistence in an understory plant community
Interactions between plants can have strong effects on community structure and function. Variability in the morphological, developmental, physiological and biochemical traits of plants can influence the outcome of plant interactions and thus have important ecological consequences. However, the ecological ramifications of trait variability in plants are poorly understood and have rarely been tested in the field. We experimentally tested the effects of morphological variation in root architecture of Quercus douglasii trees on interactions between understory plants and community composition. Stipa pulchra seedlings were transplanted under mature Q. douglasii trees with either a deep-rooted or shallow-rooted morphology. Under each tree, S. pulchra seedlings were planted either directly into the matrix of existing vegetation or in the center of a 20 cm diameter removal treatment in which all aboveground biomass of neighbors was removed. After 20 weeks of growth, we harvested, counted the total number of seeds on each plant, dried, and weighed the aboveground biomass of all individuals. To explore whether variation in the root morphology of Q. douglasii corresponded with differences in the abundance of S. pulchra, we counted the number of S. pulchra individuals in three randomly located 60-cm diameter circular plots under each tree.
Results/Conclusions
We found that morphological variation in Quercus douglasii root architecture had strong direct and indirect effects on the performance of the native perennial bunchgrass, Stipa pulchra in the understory plant community. Trees with a deep-rooted morphology facilitated exotic annual grasses and these annual grasses in turn competitively excluded S. pulchra. In contrast, Q. douglasii trees with shallow-rooted morphologies directly suppressed the growth of exotic annual grasses and indirectly released S. pulchra individuals from competition with these annual grasses. Variability in Q. douglasii tree root systems created substantial conditionality in the outcomes of competition among understory species and enhanced the potential for indirect interactions to sustain coexistence and community diversity.