Size structure of a tree population not only reflects its past history but also how it is likely to change in the future. Three species, all located in a wetland forest in northern Kentucky near the Ohio River, were examined. This forest is part of a network of newly established permanent plots in North America, and the acquisition of baseline data for this network is critical. The Fagus grandifolia (American beech) population was the dominant species in a mature stand of approximately 1 ha, while the Liriodendron tulipifera (tulip-poplar) and Celtis occidentalis (hackberry) populations were located in an adjacent second-growth stand (also ca. 1 ha) that has developed over the past 50 years. F. grandifolia had a close fit to a power law function, but the deviations from the function suggest that the population is not currently reproducing fast enough to replace itself. L. tulipifera, on the other hand, had a bell-shaped size distribution, indicating that it is no longer reproducing and will decrease in importance over time. C. occidentalis had a very close fit to a power law function, suggesting it will maintain itself at current levels in the future. Current F. grandifolia levels in the younger secondary stand suggest that it will increase its importance in the future and will come to resemble the population in the mature stand.