The Amelioration of Grazing and Sand Burial Through Physiological Integration by a Clonal Dune Plant

     Clonal plant species possess complex life histories and are able to move horizontally through the production of rhizomes. This allows clones to dominate a range of spatially variable environmental conditions and... [ view full abstract ]

     Clonal plant species possess complex life histories and are able to move horizontally through the production of rhizomes. This allows clones to dominate a range of spatially variable environmental conditions and exploit new environments through the translocation of resources. We examined the ability of a clonal dune plant, Hydrocotyle bonariensis (dune pennywort) to overcome the negative effects of grazing and sand burial through resource integration. Clonal plants reproduce vegetatively by producing copies of themselves (ramets) through the extension of underground stems called rhizomes. Ramets that remain interconnected via rhizomes have the potential to share resources and in doing so, allow a clone to become physiologically integrated.  Rhizomatous growth and associated physiological integration can allow a clonal dune species to expand into areas of active sand deposition and potentially compensate for the selective removal of leaves associated with herbivory. H. bonariensis is extremely abundant on the barrier islands of North Carolina that have large feral horse populations.  Horses consume dune plants, as well as promote sand burial through their trampling and grazing activities.  Grazing and burial can be detrimental to a plant’s ability to survive, decreasing photosynthetic area and plant biomass. Through two separate experiments, we showed that resource integration within clones allowed H. bonariensis to successfully expand into areas of high herbivory and high levels of sand deposition. In response to the grazing treatments, clones showed a higher degree of ramet production when grazed. This was accomplished through integration of resources, which allowed the clone to increase the number of ramets produced. However, this increased ramet production occurred at a cost to total clone biomass. H. bonariensis responded to burial by growing vertically up through 45 cm of sand, placing new ramets near the sand surface. This was achieved through resource integration upward from buried ramets.  Our research suggests that rhizomatous growth and physiological integration are traits that provide a clear adaptive advantage to plant species growing in heterogeneous environments such as coastal dunes.