ECOPHYSIOLOGY OF SEEDLINGS OF OAKS AND RED MAPLE ACROSS A TOPOGRAPHICGRADIENT IN EASTERN KENTUCKY

Throughout much of the eastern United States, oaks (Quercus spp.) are being replaced through natural succession by more shade-tolerant speci es. In Robinson Forest in eastern Kentucky, A. rubrum is numerically d ominant in the understory and appears to be successionally replacing f ive oaks that occur at different topographic positions in upland fores ts. To help predict future succession, we compared A. rubrum and the f ive oaks in terms of patterns of gas exchange and water relations in u nderstory seedlings subjected to artificial, saturating light levels. Physiological responses exhibited clear diurnal patterns, but were not strongly related to topography (mesic versus xeric aspects). Despite their different topographic distributions, oaks differed only minimall y in responses. The lack of oak species and topographic effects on phy siology may have stemmed from nonlimiting moisture conditions during t he study. In contrast, A. rubrum exhibited very low A and g(ww) and ve ry high psi(leaf) compared to the oaks. Furthermore, in the oaks, A wa s controlled mainly by g(wv) whereas in A. rubrum, psi(leaf), and T-le af were more strongly correlated with A than was g(wv), probably refle cting relatively low dehydration tolerance in A. rubrum leaves. Future light regimes should be enhanced by gap-phase dynamics. Under these c onditions, the higher photosynthetic capacity of oaks compared to A. r ubrum may contribute to higher leaf carbon gain. On the other hand, ab undant tall understory A. rubrum may continue preempting light from sm aller oak juveniles, promoting eventual replacement of oaks by A. rubr um at Robinson Forest.