The Amelioration of Grazing through Physiological Integration by a Clonal Dune Plant

Abstract

Rhizomatous growth and associated physiological integration can allow a clonal dune species to potentially compensate for the selective removal of leaves associated with herbivory. Hydrocotyle bonariensis is a rhizomatous clonal plant species that is abundant in the coastal dune environments of the southeastern United States that are inhabited by large feral horse populations. H. bonariensis has been shown to integrate resources among ramets within extensive clones as an adaptation to resource heterogeneity in sandy soils. In this study, we hypothesized that clonal integration is a mechanism that promotes H. bonariensis persistence in these communities, despite high levels of herbivory by feral horses. In a field experiment, we used exclosures to test for herbivory in H. bonariensis over a four-month period. We found that feral horses utilized H. bonariensis as a food species, and that while grazing will suppress clonal biomass, H. bonariensis is able to maintain populations in a high grazing regime with and without competition present. We then conducted an experiment in which portions of H. bonariensis clones were clipped to simulate different levels of grazing. Half of the clones were severed to eliminate the possibility of integration. We found that after 12 weeks, the mean number of leaves and ramets increased as the grazing level increased, for integrated clones. Integrated clones had significantly increased biomass production compared to the severed equivalents. Our research suggests that rhizomatous growth and physiological integration are traits that allow clonal plant species to maintain populations and to tolerate grazing in coastal dune environments.

Halophytic Clonal Plant Species: Important Functional Aspects for Existence in Heterogeneous Saline Habitats

“…H. bonariensis form extremely large genets with thousands of connected ramets occupying over 40 m 2 of dune surface [103]. A high level of physiological integration can be seen with respect to resource acquisition and translocation [103,104] and it also gives an ability to avoid patches of grass [105] as well as promotes resilience to grazing [106]. Most importantly in the context of the present review, clonal integration allowed for undisturbed growth of H. bonariensis in conditions of spatially heterogeneous salinity, when ramets located in soil with increased salinity were supported by ramets located in non-saline spots, with no negative effects on growth [102].…”