Effects of long-term fire exclusion on tree species composition and stand structure in an old-growth Pinus palustris (Longleaf pine) forest

Frequent fire is an integral component of longleaf pine ecosystems, creatin g environmental conditions favoring survival and growth of juvenile pines. This study examined stand structure, species composition, and longleaf pine regeneration in an old-growth tract of longleaf pine forest (Boyd Tract) e xperiencing long-term (>80 yr) fire exclusion in the Sandhills of North Car olina. Sampling of woody stems (i.e., greater than or equal to 2.5 cm diame ter at breast height) and tallies of longleaf pine seedlings were carried o ut in plots established randomly on upland, mesic areas and lowland, xeric areas within the Boyd Tract. Dominant woody species in mesic plots were bla ck oak, hickories, and large, sparse longleaf pines. Xeric plots had high d ensities of turkey oak with the large longleaf pines, as well as higher fre quencies of smaller longleaf stems. These differences between areas were as sociated with higher clay content of upland soils and higher sand content o f lowland soils. Age-class frequency distributions for fire-suppressed long leaf pine following the last wildfire at the Boyd Tract approximately 80 yr ago contrasted sharply with data from an old-growth longleaf tract in sout hern Georgia (Wade Tract) that has been under a long-term frequent fire reg ime. Post-burn recruitment for the Boyd Tract wildfire appears to have been initially high on both site types. Longleaf pine recruitment diminished sh arply on the mesic site, but remained high for similar to 60 yr on the xeri c site. Currently, longleaf pine regeneration is minimal on both site types ; several plots contained no seedlings. Sharp contrasts in longleaf pine do minance and stand structure between the Boyd and Wade Tracts demonstrate th e importance of large-scale disturbance, especially hurricanes and fire, in shaping the structure and function of longleaf pine ecosystems of the sout heastern United States. In particular, long-term exclusion of fire on the B oyd Tract has altered stand structure dramatically by permitting hardwoods to occupy at high densities the characteristically large gaps between longl eaf stems that are maintained by fire and other disturbances.