Rich Mesic Forests: Edaphic and Physiographic Drivers of Community Variation in Western Massachusetts

Rich Mesic Forest, a Northeastern variant of the species-rich Mixed Mesophytic Forest association of eastern North America, is an Acer saccharumdominated forest type typically associated with calcareous bedrock and nutrient-rich, mull soils. Rich Mesic Forest (RMF) is a priority for conservation in the Northeast due to its limited areal extent, high plant species richness, and numerous rare taxa, yet the community characteristics and environmental correlates of this forest type are incompletely understood. This study undertook a quantitative classification of RMF of the northeastern edge of the Berkshire Plateau in western Massachusetts. Cluster analysis of data from ten sites identified two vegetation types within the RMF community, the Acer saccharum–Allium tricoccum–Caulophyllum thalictroides Type (AAC) and the Acer saccharum–Dicentra cucullaria–Polystichum acrostichoides Type (ADP); in addition, two subtypes were distinguished within ADP. Ordination of vegetation data using detrended correspondence analysis identified increased soil pH and calcium concentration as key environmental factors associated with variation within RMF and differentiation of RMF from the typical northern and transition hardwoods vegetation of the region. On sites with high soil pH and nutrients, vegetation variation was primarily associated with physiography, particularly the presence of exposed bedrock and solar insolation levels as controlled by slope and aspect. Occurrences of RMF on steep sites with frequent bedrock exposures and high insolation provide habitat for uncommon plant species reaching their northeastern range limits in the Northeast, and are of particular interest for biodiversity conservation. Comparisons between the results of this study in western Massachusetts and prior research on Mesophytic Forests in other regions of eastern North America illustrate a striking constancy in species composition, with over 70% of common species in Mesophytic Forests of the Southeast and Midwest also being recorded in RMF in our study region.