Grazing of Panicum amarum in a Louisiana barrier island dune plant community: Management implications for dune restoration projects

Abstract

Panicum amarum (bitter panicum), an economically important dune grass used in dune building and stabilization projects throughout the southeastern United States, was planted in protected (exclosed) and unprotected (unexclosed) plots to assess the importance of vertebrate herbivory on P. amarum transplant survival on Timbalier Island, Louisiana. After one year, vegetative cover of P. amarum in exclosed plots increased to 33% with an accompanying stem density increase of 67%, whereas herbivory in the unexclosed plots resulted in 0% vegetative cover of P. amarum with no surviving stems. Total plant cover ( P. amarum plus invading plant species) was also significantly greater within exclosures. Based on field observations of tracks and scat in the study area, nutria (Myocastor coypus) is believed to have been responsible for most, if not all, of the grazing. One of the invading plant species, Heterotheca subaxillaris, had significantly greater vegetation cover in exclosed plots, whereas the other invading species, Spartina patens, had similar percentage cover values within exclosed and unexclosed plots. Percentage N, crude fiber, moisture, and phenolic compounds varied significatly among the three plant species. The concentration of total phenols in Heterotheca was four to five times greater than those in Panicum and S. patens. We hypothesize that Panicum may be the heaviest grazed of these species because its leaf tissue is characterized by a combination of relatively high moisture content and low total phenols. Since the impact of herbivory varied among the three plant species, herbivores may significantly alter dune plant community structure by differentially consuming palatable plant species in the dune environment on some barrier islands. This may have serious management implications for Louisiana's barrier islands if plant species with the most desirable sand-accumulating and sediment-binding characteristics are also the most severely grazed by herbivores.