Regeneration of northern red oak: Positive and negative effects of competitor removal

The decreased ability of northern red oak (Quercus rubra) to regenerate throughout its range in the eastern United States has important ecological and economic implications. We studied regeneration of northern red oak in oak and pine stands on moderately productive sites in northern Lower Michigan. Our objectives were (1) to investigate the hypothesis that regeneration of northern red oak is more successful in pine than in oak stands and (2) to test whether removal of potential overstory and understory competitors increases regeneration success on moderately productive sites. Northern red oak acorns and 2-yr-old nursery seedlings were planted in spring 1991 in three natural oak stands and three red pine (Pinus resinosa) plantations on comparable, moderately productive sites. Each stand contained four canopy cover treatments: clearcut, 25% cover (50% the first year), 75% cover, and uncut. Each canopy cover treatment contained four understory treatments: herb-layer removal, shrub-layer removal, litter removal, and control. Seedling survival, performance, and damage due to deer and late spring frosts were quantified along with vegetation characteristics, light, soil moisture, air temperature, soil temperature, and precipitation during the 1991 and 1992 growing seasons. Survival of seedlings was significantly lower in the pine than in the oak stands; the pine stands had a higher incidence of white-tailed deer browsing and lower levels of belowground resources than the oak stands. Canopy cover treatments produced a gradient from high stress (low light and soil moisture) and low disturbance (slight browsing and frost damage) in uncut plots to low stress (high light and soil moisture) and high disturbance (high browsing and frost damage) in clear-cut plots. Understory vegetation development and soil temperature were low in uncut plots, high in clear-cut plots, and intermediate in plots with partial overstory removal. Despite poor growth, the mortality of seedlings planted in uncut plots was low. In contrast, mortality was high and surviving seedlings exhibited good growth in clearcuts. Understory treatment effects were slight compared with overstory treatment effects. Results on our sites (1) do not support the hypothesis that regeneration success of northern red oak may be greater in pine than in oak stands and (2) indicate that the positive effects of removing potential competitors on seedling growth and physiological status may be compromised by simultaneous negative effects of browsing and frost damage