Microenvironmental heterogeneity and Quercus michauxii regeneration in experimental gaps

Abstract

Canopy gaps are foci for forest regeneration; the gap environment promotes establishment and tree growth into the canopy. In bottomland hardwood forests, microtopography and light are modified simultaneously in natural gaps. Their combined effects may influence seedling establishment and growth. Desirable red and white oaks are often regenerated in bottomland hardwood forests through group selection cutting that mimics the gap environment. We tested effects of canopy openness and microtopography on regeneration of a moderately shade-intolerant oak, Quercus michauxii, in experimental canopy gaps in a bottomland hardwood forest on the Atlantic Coastal Plain. Gap sizes were centered on the median opening size of natural gaps in an old-growth bottomland hardwood forest. Acorns were planted at 100 random level locations in and around each of two replicate 7, 10, 14, 20, 29, and 40 m radius gaps in early spring. Acorns were also planted on pit and mound microtopography complexes created in the center, at the edge, and under intact canopy surrounding two 29 m and three 40 m radius gaps. Over all gaps, 21–37% of the randomly located acorns emerged and survived one summer; 8–26% survived two summers. Survival was influenced more by seedling size than by gap size or distance from gap center. Seedlings on level locations that survived two seasons were taller in the first year, had greater maximum leaf area, and tended to be farther from the center of large gaps than seedlings that died after 1 year. Height of second-year seedlings was related more to first-year plant size than to distance from gap center or gap size. Seedling emergence from acorns planted on microtopography complexes was greater on mounds than in pits at gap center and beneath intact canopy; no seedlings emerged in pits in the gap center. Once established, seedling survival and growth on the microtopography complexes was higher at the gap center. Management of bottomland hardwoods for Q. michauxii regeneration from acorns may be most successful if practices that create conditions favorable for formation of a seedling bank, i.e. raised topography beneath a thin canopy, are followed by light harvests that promote seedling growth and ascent into open canopy.