Oak regeneration in southeastern bottomland hardwood forest

Abstract

Oaks ( Quercus spp.), such as cherrybark oak ( Quercus pagoda Raf.), are valuable timber species in bottomland hardwood (BLH) forests of the southeastern U.S. Oak regeneration is affected by plant flooding tolerance; wind disturbances or forest harvests; and interactions with seed predators and herbivores. We investigated oak regeneration (seedling establishment and early growth, composition of seedling and sapling banks) in experimental openings, from 7 to 40 m radius, created by group-selection harvest in a BLH forest. Seed dispersal, seed predation, and microsite conditions on the forest floor, rather than canopy openness, were the first filters on oak regeneration. In the first 3 years after gap creation, naturally established oak seedlings were most abundant in relatively ‘undisturbed’ microsites with litter cover and around gap edges. Swamp chestnut oak ( Quercus michauxii Nutt.) germination was inhibited in pits, and seedling establishment was greatest under edge or less open canopy. Cherrybark oak seedling establishment and growth were reduced by seed predation and shade. Six years after acorns were planted, swamp chestnut oak seedling height and maximum root depth were greatest in gap centers. Ten years after gap creation, oaks were more abundant beneath the surrounding forest canopy, but were tallest in gap centers. Thus, in contrast to its lack of influence on acorn germination and initial establishment, canopy openness appears to filter oak persistence and height growth in the seedling bank. Forest management that provides level or raised microsites beneath thin canopy and protection from seed predation can facilitate oak seedling establishment in southeastern BLH forests. Maintaining a thin to open canopy can promote oak persistence in the seedling bank; growth into the sapling bank may require reopening the canopy if gap centers have grown up over time. Although longer study time is needed to capture regeneration from seed germination through ascent from the sapling layer into the subcanopy and canopy strata, this large field experiment has revealed there is no optimal gap size for oak regeneration. Rather, biotic interactions, canopy openness, and particularly flood duration at the microsite scale create complex patterns of oak abundance and heights in the seedling and sapling layers following disturbance in bottomland hardwood forests.