Converging forest community composition along an edaphic gradient threatens landscape-level diversity

Aim Plant communities across the temperate zone are changing in response to successional processes and human-induced disturbances. Here, we assess how upland forest under- and overstorey community composition has changed along an edaphic gradient. Location Northern Wisconsin, USA. Methods Forest sites initially sampled in the 1950s were resampled for overstorey composition and diversity, basal area, and understorey composition and diversity. We used clustering methods to identify groups of stands based on overstorey composition, and we used similarity indices, ordination and diversity indices to evaluate changes in species abundance and overall community structure. Results Sites clustered into four overstorey groups along the edaphic gradient: 'hemlock' sites dominated by hemlock in 1950, 'mesic' sites dominated by northern hardwoods, 'dry' sites with a significant pine inclusion in the canopy and diverse 'dry-mesic' sites in the middle. Collectively, forests gained maple, ash and cherry while losing pines, birches and red oaks. The hemlock forest sites gained hardwoods, while the dry-mesic sites shifted towards a more mesic hardwood composition. Only the driest sites have remained relatively stable in species composition. Main conclusions These trends reflect both 'mesification' and homogenization among northern forests. Highly diverse mid-gradient and mesic hemlock-dominated stands are transitioning to maple dominance. Fire suppression may be favouring invasions of more mesic plants into historically drier sites, while high deer abundance likely limits hemlock regeneration. If current trends continue, maples will dominate the majority of northern forests, with significant losses of local native species richness and substantial shifts in understorey composition.