Effects of Wild Pigs on Beech Growth in Great Smoky Mountains National Park

Abstract

Growth of trees in 9 high-elevation American beech (Fagus grandifolia) gaps in Great Smoky Mountains National Park (GSMNP) was examined from June to August 1982, for impact from rooting by wild pigs (Sus scrofa). Beech trees exhibited significantly greater shoot elongation (F2,6 = 7.71, P < 0.05) with increased exposure to rooting by wild pigs; however, tests for changes in radial increment (F,, = 5.42, P < 0.10) were not as significant due to confounding effects from elevation and stand age. Annular-ring widths of trees from young stands demonstrated greater variation in growth for years with pigs present compared to years before pigs inhabited the sites. Comparable changes were not observed for older stands. The growth responses of trees observed probably resulted from an enhanced nutrient mobilization in soils disturbed by pigs. J. WILDL. MANAGE. 50(4):655-659 Distribution of the wild pig has expanded to all continents as a direct consequence of introduction by man (Tisdell 1982). Wild pigs are abundant in several regions of the United States (Wood and Barrett 1979) and now occupy 13 areas managed by the U.S. National Park Service (Singer 1981). Impacts caused by the rooting of wild pigs are varied and, depending on the local situation, have been viewed as either detrimental or beneficial (Tisdell 1982). A management objective of national parks is to conserve native species in their natural environments. The presence of exotic wild pigs threatens the achievement of such an objective for GSMNP. Wild pigs in GSMNP have caused changes in herbaceous vegetation (Bratton 1974, 1975; Howe and Bratton 1976) and soils (Lacki and Lancia 1983, Singer et al. 1984). Rooting by wild pigs is known to damage tree roots and increase the amount of sprouting or root suckering (Huff 1977), but the responses by overstory components in forests of GSMNP have not been examined. To fully assess the implications of a resident population of wild pigs in GSMNP, the impact of these animals on all ecosystem components needs to be quantified. The objective of our study was to determine the effect of rooting by wild pigs on tree growth in the most disturbed forests of GSMNP. Our research was funded by the Natl. Park Serv. (NPS) through Uplands Res. Lab., GSMNP (NPS Contract X5460-0-2592) and, in part, by a Theodore Roosevelt Mem. Fund Grant to the senior author. This is Contrib. 10249 from the N. C. Agric. Res. Serv. journal series. We are indebted to R. J. Fredrickson and W. M. Whiteside for field assistance and to L. A. Nelson, K. H. Pollock, and W. L. Hafley of the Deps. Stat. and For. at North Carolina State Univ. for statistical advice. We are grateful to N. H. Shrimpton for measuring increment cores. We thank Uplands Res. Lab. and the Resour. Manage. Sect. GSMNP, and the Deps. For. and Zool. at North Carolina State Univ. for logistic support. Comments and suggestions were provided by C. L. Campbell, C. B. Davey, P. D. Doerr, J. D. Hair, R. Lea, and P. M. Vitousek.