Major changes in tree community composition and structure over 86 years in an old-growth beech-maple-hemlock forest1

Abstract

Old-growth forests dominated by shade-tolerant tree species are among the plant communities most likely to be in equilibrium. However, the assumption that these forests show long-term stability has rarely been tested. In this study, we resurveyed plots established 86 yr before in an old-growth beech-maple-hemlock forest in northeast Ohio to examine changes in tree community composition, diversity, and structure. Species richness and diversity remained constant from 1932 to 2018, but we found dramatic shifts in the relative abundances of species and the size structure of populations. Fagus grandifolia remained the most frequent and abundant tree species in the forest, while changing dramatically in size structure; larger Fagus grandifolia trees died and were replaced by large numbers of root suckers, increasing the overall stem density of the forest by 50%. Tsuga canadensis declined in frequency and stem density, shifted away from smaller size classes, and had almost no saplings. Plots dominated by Tsuga canadensis in 1932 showed the greatest magnitude of compositional change over time, shifting toward dominance by Acer saccharum, which along with Acer rubrum, increased greatly in frequency, stem density, and basal area. Eight species including Castanea dentata and Sassafras albidum were lost from the plots. In sum, Fagus grandifolia and Acer saccharum are displacing Tsuga canadensis in this stand. Tree diseases, pests, and deer herbivory likely contributed to these compositional shifts. Given the time scale of the directional changes we observed, even relatively undisturbed communities such as old-growth forests might be best viewed as not in equilibrium.