A spatially explicit test of foraging behavior in a clonal plant

Through plasticity in traits controlling clonal morphology, clonal plants possess the potential to selectively place ramets within a heterogeneous environment or @'forage.@' Although many studies document plant responses that are consistent with foraging, few studies test directly whether plants can preferentially locate @'good@' patches or avoid @'bad @' patches when grown in a heterogeneous environment. We conducted such a test for Hydrocotyle bonariensis, a clonal dune species that inhabits soils known to be extremely patchy on 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 on the patch treatment was compared to that in two homogeneous treatments: no grass and full grass. In order to predict the effect of our treatments on the long-term expansion of clones, we calibrated diffusion models of clonal growth with data on the morphological response of Hydrocotyle to our treatments. Clones of Hydrocotyle bonariensis were able to respond to the presence of patches by selectively placing ramets outside of grass patches, thus providing direct evidence of an effective foraging response. For each of the three traits identified as potentially contributing to an overall foraging response (branching, internode distance, and direction of rhizome growth), the response in and out of grass patches was substantially different from the degree of plasticity manifested by the two homogeneous treatments. For example, no difference in main rhizome internode lengths was found between the two homogeneous treatments. In contrast, ramets in the heterogeneous (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 indicate that habitat complexity has considerable impact on the long-term 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 exhibited a previously undocumented behavior: they appeared to veer away from patches of grass. Finally, we discuss how the foraging response of Hydrocotyle ramets may be enhanced by its previously documented capacity for resource integration.