Homogenization of a temperate old-growth forest remnant in central Illinois following the introduction of Dutch elm disease (Ophiostoma novo-ulmi) and emerald ash borer (Agrilus planipennis)

Forests in the eastern United States are undergoing wide-scale transformations driven by the combined effects of biological and anthropogenic factors. Understanding how these drivers interact to affect forest dynamics is important for predicting the resilience of forest ecosystem services, but few long-term records exist for old-growth forests that have not been directly altered by humans. In this study, we utilized historical datasets covering a 96-year period to investigate temporal changes in forest structure and composition at Trelease Woods, an old-growth temperate forest remnant in central Illinois. Our objectives were to (1) characterize shifts in forest composition and structure, and (2) determine how tree mortality due to biotic invasions may have affected the dynamics. Our findings documented substantial compositional changes across the different census periods. Species richness and diversity both declined over time, with saplings exhibiting markedly lower diversity values. The relative densities and basal areas of sugar maple ( Acer saccharum L.) and Ohio buckeye ( Aesculus glabra ) increased across the sapling (< 10 cm) and mid-sized (10–50.7 cm) strata, while the overstory (> 50.7 cm) has become heavily dominated by sugar maple since 1936. These observed temporal trends suggest that Trelease Woods is experiencing species homogenization likely driven by tree mortality associated with pest and pathogen outbreaks, rising deer populations, and mesophication due to fire suppression. These findings provide insights into the historical compositional and structural shifts that temperate forests in the eastern U.S. have undergone over the past century and the potential implications for future changes in dynamics of old-growth forests in eastern North America. • Analyzed 96 years of temporal changes in structure and composition. • Evidence of species homogenization, especially in the sapling layer. • Species richness and diversity declined throughout all strata. • Sugar maple and Ohio buckeye dominance increased across all size classes. • Elm and ash density, basal area, and importance declined rapidly.