Canopy Openness, Understory Light Environments, and Oak Regeneration

Understory light environments were evaluated in four mixed-oak forests in southern Ohio using hemispherical photography. Within each forest, plots were divided into nine treatment combinations based on three pretreatment fire categories and three Integrated Moisture Index (IMI) categories. For each of 108 photographs we determined the percentage of open sky, direct beam, and diffuse radiation; the latter two variables were combined to produce an estimate of the percentage of global radiation reaching the regeneration layer (1.5 m). Global light ranged from less than 1 to 23 percent and averaged 5.5 to 8.3 percent across treatment combinations; it did not differ significantly as a function of study area, treatment unit, or IMI. Xeric sites had a significantly greater percentage of open sky, but this influenced primarily the amount of diffuse radiation rather than direct beam radiation. Nearly 75 percent of the variance in global radiation could be predicted from open sky. The relationship between in situ light availability and IMI was evaluated with respect to oak regeneration in the seedling, pole, and sapling size classes. There was no strong relationship between the amount of ambient light or IMI and the amount of oak regeneration. Under ambient conditions in mature mixed oak forest, light and moisture may not be the primary determinants of establishment and early survival.