Understory vegetation, resource availability, and litterfall responses to pine thinning and woody vegetation control in longleaf pine plantations

In six 8- to 11-year-old plantations of longleaf pine (Pinus palustris Mill.) near Aiken, S.C., responses of understory vegetation, light, and soil water availability and litterfall were studied in relation to pine thinning (May 1994), herbicidal treatment of nonpine woody vegetation (1995-1996), or the combined treatments (treatment responses described below are in absolute units). Treatment differences in fifth-year (1998) herbaceous species density were as follows: pine thinning > woody control = combined treatments > untreated (33, 30, 30, and 25 species per 40 m2, respectively). Forb and grass covers were 13 and 8% greater, respectively, after pine thinning and 7 and 9% greater after woody control. Pine thinning stimulated a large increase in third-year gap fraction (0.26), short-term increases in soil water content (1%), and a reduction in pine litterfall by half (-120 g·m-2 per year). Woody control had no effect on gap fraction, decreased litterfall of nonpine woody vegetation (-32 g·m-2 per year), and stimulated season-long increases in soil water content (1-2%). The ranking of factors affecting herbaceous vegetation responses was as follows: light > soil water > herbicides > litterfall. Herbaceous species density and cover can be promoted in longleaf pine plantations by intensive thinning of pines and herbicidal control of non-pine woody vegetation.