Influence of selection systems and shelterwood methods on understory plant communities of longleaf pine forests in flatwoods and uplands
Abstract
Although longleaf pine (Pinus palustris Mill.) forests have mostly been managed with even-aged methods, interest has been rising in uneven-aged systems, as a means of achieving a broader range of stewardship objectives. Selection silviculture has been practiced on a limited scale in longleaf pine, but difficulty of using traditional approaches and absence of an evaluation across a range of site types has left managers in doubt about its suitability. This study was conducted to quantify the effects on understory plant communities of applying single-tree selection, group selection, irregular shelterwood and uniform shelterwood in longleaf pine forests on flatwoods and uplands of the southeastern United States. Wiregrass (Aristida beyrichiana Trin. & Rupr.) and numerous other graminoids are highly desirable understory species, because they facilitate the essential ecological process of recurrent surface fire that sustains longleaf pine ecosystems. Forbs such as partridge pea (Chamaecrista fasciculate (Michx.) Greene) and low shrubs such as gopherapple (Licania michauxii Prance), blueberries (Vaccinium spp. L.) and huckleberries (Gaylussacia spp. Kunth) are also desirable as components of good wildlife habitat. Selection treatments reduced stand basal area to ~11.5 m2 ha-1 and shelterwood treatments left a basal area of ~5.8 m2 ha-1. While higher levels of logging traffic from shelterwood treatment caused a significant decline in sawpalmetto (Serenoa repens W. Bartram) cover and increases in wiregrass at the flatwoods site, on the upland site it resulted in a sharp decline in wiregrass and silverthread goldaster (Pityopsis graminifolia (Michx.) Nutt.). Absence of prescribed fire during the post-treatment years led to progressive increases for shrub cover broadly across the flatwoods. Group selection caused modest understory change in flatwoods (temporary decrease in shrubs and increase in wiregrass), but resulted in a doubling of understory plant cover on uplands, with significant increases for hardwood tree seedlings, shrubs, vines, wiregrass, forbs and ferns. Single-tree selection caused no lasting impact on saw-palmetto, a decline in gallberry (Ilex glabra (L.) A. Gray) and increase in wiregrass in flatwoods and was related on uplands to increases in oak (Quercus spp. L.), dangleberry (Gaylusaccia frondosa (L.) Torr. & A. Gray ex. Torr.), broomsedge bluestem (Andropogon virginicus L.) and several forbs. Single-tree selection produced less change in the forest than group selection, which caused less alteration than shelterwood treatment. Selection silviculture appears to be a lower risk option for guiding longleaf pine forests along a trajectory of gradual improvement, with adjustments provided by frequent surface fires and periodic tree harvest. Long-term observation is needed to verify that selection can sustain diverse plant communities on sites characterized by differing environments.