A SPATIALLY EXPLICIT TEST OF FORAGING BEHAVIOR IN A CLONAL PLANT

Through plasticity in traits controlling clonal morphology, clonal pla nts pos sess the potential to selectively place ramets within a hetero geneous environment or ''forage.'' Although many studies document plan t responses that are consistent with foraging, few studies test direct ly whether plants can preferentially locate ''good'' patches or avoid ''bad'' patches when grown in a heterogeneous environment. We conducte d such a test for Hydrocotyle bonariensis, a clonal dune species that inhabits soils known to be extremely patchy in both time and space. We subjected Hydrocotyle to a temporally constant but spatially variable soil resource environment created by clipped patches of grass. Clonal morphology in the patch treatment was compared to that in two homogen eous treatments: no grass and full grass. in order to predict the effe ct of our treatments on the long-term expansion of clones, we calibrat ed diffusion models of clonal growth with data on the morphological re sponse of Hydrocotyle to our treatments. Clones of Hydrocotyle bonarie nsis were able to respond to the presence of patches by selectively pl acing ramets outside of grass patches, thus providing direct evidence of an effective foraging response. For each of the three traits identi fied as potentially contributing to an overall foraging response (bran ching, internode distance, and direction of rhizome growth), the respo nse in and out of grass patches was substantially different from the d egree of plasticity manifested by the two homogeneous treatments. For example, no difference in main rhizome internode lengths was found bet ween the two homogeneous treatments. In contrast, ramets in the hetero geneous (patchy grass) treatment responded to their local environment by increasing internode lengths when in the unfavorable (grass patch) portion of the environment. Empirically calculated diffusion models in dicate that habitat complexity has considerable impact on the long-ter m expansion of clones: only in the heterogeneous treatment did clones expand less rapidly in favorable habitat than they did in unfavorable habitat. In the heterogeneous treatment, Hydrocotyle rhizomes exhibite d a previously undocumented behavior: they appeared to veer away from patches of grass. Finally, we discuss how the foraging response of Hyd rocotyle ramets may be enhanced by its previously documented capacity for resource integration.