Compositional dynamics and light regimes in a midwestern old-growth forest preserve1

Abstract

Forest structure, composition, and understory light patterns were measured at Allee Memorial Woods (AMW) near the forest–prairie ecoregional boundary in west-central Indiana to determine (1) compositional trends over five decades, (2) how different disturbance histories may have affected the dynamics, (3) current understory light regimes and their relationship to sapling densities, and (4) implications for the future of the forest preserve. Permanent sampling grids were re-established during 2011–12 in an uncut stand and two others that were selectively or heavily cut during 1890–1920. All trees > 1 m tall were censused, and those > 7.5 cm in diameter were mapped and measured, to compare structure (density, mean diameter, basal area) and species composition (richness, diversity, similarity) within and across size classes. Photosynthetic photon flux (PPF) was measured in three ways and correlated with sapling densities. Substantial compositional changes occurred in both the uncut and selectively cut stands during 1959–60 to 2011. They have declined in species richness and diversity, especially in smaller size classes, and the relative densities of Acer saccharum L., Fagus grandifolia Ehrh., or both have increased in sapling (1 m tall to 7.5 cm diameter) and mid-sized (7.6–50.7 cm) strata while the overstory (> 50.7 cm) has become more heavily dominated by Quercus spp. and Liriodendron tulipifera L. Light levels under intact canopy are very low in general and sapling densities are correlated with light, especially with longer-term direct PPF measurements at 0.5 m. All three stands show patterns consistent with widespread understory shade tolerance shifts in eastern North America, and the uncut and selectively cut stands have converged in overall size class structure and composition through different pathways related to their disturbance histories. Increased dominance of the overstory by Acer and Fagus, further declines in tree species diversity, and inhibition of regeneration by less shade-tolerant species are predicted for AMW unless there are substantial changes in climate and/or disturbance.