GAP-PHASE REGENERATION IN LONGLEAF PINE WIREGRASS ECOSYSTEMS

Naturally regenerated seedlings of longleaf pine are typically observe d to cluster in the center of tree fall canopy gaps and be encircled b y a wide zone from which they are generally excluded. Twelve represent ative canopy gaps distributed across 600 ha of a naturally regenerated uneven-aged longleaf pine forest in the sandhills of north central Fl orida were examined to determine which aboveground and belowground fac tors are responsible for development of this seedling exclusionary zon e. Within 12 m of adult trees growing along the gap edge, significantl y fewer longleaf pine seedlings were present, The canopy of overstory trees, however, extended only 4-5 m into the gap. The relatively open structure of the longleaf pine canopy (57% cover) allowed photosynthet ically active radiation (PAR) to be evenly distributed upon the forest floor across each canopy gap. Thus, light avail-ability was not relat ed to pine seedling clustering near gap centers, Significantly greater forest litter mass beneath adult trees (5 Mg ha(-1)) could result in fires more intense than would be supported by the litter mass near gap centers (2.5 Mg ha(-1)). However, litter mass was significantly eleva ted only within 4 m of the gap edge. The fine root biomass within 12 m of the gap edge (3-4.5 Mg ha(-1)) was two to six times that measured near the gap center and most closely coincided with the width of the s eedling exclusionary zone along the margin of each gap. Thus, while th e canopy of adjacent adult trees may indirectly influence seedling mor tality through deposition of needle litter and greater fire intensity within 4 m of the gap edge, the root systems of these adults also appe ar to directly compete with seedlings within 12-16 m of the gap edge f or limited site resources, To effectively regenerate and sustain longl eaf pine wiregrass ecosystems, caution should be used in prescribing s ingle-tree selection harvest methods so as not to create gap openings so small (< 30 m diameter) that intraspecific competition from adult t oes totally excludes seedlings. Group selection methods may prove more effective in creating canopy gaps of sizes suitable (greater than or equal to 40 m diameter) for ensuring that intraspecific competition fr om adults is maintained at a level which allows for survival and growt h of seedlings. Shelterwood and deferment methods may also prove usefu l where more open canopy conditions are desired. However, the pine see dlings' need for light cannot be used as an appropriate rationale for application of clearcutting methods. (C) 1998 Elsevier Science B.V.