Low light improves ability of Appalachian oak seedlings to compete with mesophytes in nitrogen-rich soils

Historically, oaks have dominated forests and woodlands in the eastern US, but oaks are being replaced by mesophytic tree species. Mesophication was likely triggered by moist, shaded conditions related to fire exclusion, and thus, moisture and fire have been the focus of prior research. However, forests undergoing mesophication also experience high soil nitrogen (N) availability, and timber harvests often create high-light conditions. Oaks may compete poorly with mesophytes in these resource-rich conditions due to their distinct strategies for growth and nutrient acquisition. Here, we examined how light and N influence competition among one of three oak species ( Quercus alba , Quercus montana , and Quercus rubra ) and their common mesophytic competitors ( Liriodendron tulipifera ). In a greenhouse, we grew seedlings for 12 weeks along a gradient of light and two levels of N, both alone and sharing a pot with two competitors. When grown individually, mesophytes consistently reduced their root:shoot biomass ratio relative to oaks under high N, which may allow mesophytes to overtop oaks in N-rich conditions. When grown in competition, all oaks were competitively viable with mesophytes at 40 % light transmittance and below when grown in low N conditions. However, under high N conditions, white, chestnut, and northern red oak were only competitive with mesophytes at 20 %, 40 % and 10 % light levels, respectively. Our findings suggest potential management strategies for creating conditions that favor oaks over mesophytes. The intensity of timber extraction could be adjusted based on soil N richness, while regular prescribed fire and/or limiting the dominance of early-successional N fixers may help reduce soil N richness. • Mesophytes more consistently reduce root:shoot ratio under N fertilization. • N fertilization reduces light levels where oaks remain competitive with mesophytes. • Management practices can help create conditions that favor oaks over mesophytes.