Effects of site preparation and vegetation control on the plant communities, successional dynamics, and stand structure of a loblolly pine (Pinus taeda) plantation.

Abstract

Sustainability has become a focus for forest management all over the United States, but particularly for the South, where harvest intensity has increased in recent years. Considerable interest has focused on the effects of high intensity silvicultural practices and short rotations on ecological processes, biodiversity, and wildlife habitat. In order to better understand these effects, we conducted a comparative study of second and third rotation plant communities and the effects of silvicultural practices on succession, biodiversity and stand structure in a loblolly pine (Pinus taeda) plantation in the Piedmont of North Carolina.

Following harvest of the second rotation stand, a 2 x 2 x 2 split-split-plot experiment was established using two harvest (stem only versus complete tree), two site preparation (chop versus shear, pile, and disk) and two cultural (vegetation control versus no vegetation control) treatments. Presence/absence of vascular plant taxa was determined prior to harvest for ages 22 and 41 of the second rotation as well as for ages 1, 2, 6, 12, and 18 in the third rotation. We also obtained cover data for years 1, 2, 6 and 18 in the third rotation.

Since the harvest treatments did not differ significantly for any of the parameters throughout the third rotation, our analyses focused on the effects of site preparation and vegetation control. Results indicated a significant difference in species composition from the second to the third rotation, with disturbance-responsive species positively associated with vegetation control in the third-rotation plots. The addition of these species to the vegetation control plots largely accounted for the addition of approximately forty species from the second to the third rotation, although trace numbers of woodland herbaceous species were recruited across all treatments. The least intensively managed third-rotation stands were more similar to the unmanaged stands of the second rotation. We also found that early in the life of the third rotation stand, intensive site preparation influenced species composition and cover across the site. However, following canopy closure, individual treatments converged to similar composition and cover.

Despite the lack of long-term treatment effects on the species composition and cover of the plant communities, we found that vegetation control treatments significantly affected stand structure at all height strata, prevented the development of a midstory and promoted the persistence of an herbaceous layer. At the lower shrub and sapling levels, site preparation affected stand structure within the plots with no vegetation control, with chop plots contributing multiple hardwood sprouts and disk plots contributing more individual seed-germinated saplings.

Our results indicate that high intensity silvicultural practices, such as shearing, piling, and disking and vegetation control, reduced initial competition and thus permitted the persistence of early successional species, increasing overall diversity from the second to the third rotation. We also found evidence that this stand is evolving from its agricultural legacy to a forest through the recruitment of several late successional herbaceous species. Although intensive silvicultural practices play an important early role in determining species cover and composition and continue to affect forest stand structure after twenty years, they apparently do not disrupt successional processes. Interestingly, differences in species composition due to soil variation across the study site were seen through both rotations, while treatment differences diminished over time during the third rotation. Thus we find that intensive management in a secondary forest stand had little long-term impact on succession and plant species composition, but may have important ramifications for wildlife due to the alteration of stand structure.