PS 40-91 - Soil nutrients and seedling survival in an eastern hemlock forest under stress

Abstract: Rapid changes to forest composition caused by non-native pests can disrupt ecological processes, including soil nutrient cycling. The hemlock woolly adelgid (HWA) has caused major stress and mortality for Tsuga canadensis (eastern hemlock), a foundation species for northeastern U.S. temperate forests. T. canadensis plays a critical role in maintaining a characteristically species-poor and nutrient-poor understory environment by chemically weathering the bedrock, managing slow decomposition rates of low-nutrient needles, and accumulating organic matter. The objective of this study is to examine potential differences in soil composition patterns beneath T. canadensis stands versus Quercus rubra (northern red oak) stands. We also investigated the potential relationship between soil nutrients and survival rates of T. canadensis and Q. rubra seedlings from long-term surveys (2017-2022) at Harvard Forest in Petersham, MA. Previous research found that T. canadensis and Q. rubra seedlings had lower survival rates beneath conspecific mature trees. However, soil nutrients might also play a role in seedling growth and survival. To characterize the potential role of neighborhood adult tree composition on soil chemistry, we compared soil data collected from plots that were either from areas dominated by T. canadensis or Q. rubra. We analyzed mineral A-horizons of soil samples by measuring pH, total carbon and nitrogen content, and elemental XRF profiles.


We found differences in soil nutrient and elemental composition between T. canadensis and Q. rubra soils. Nutrient availability was consistently lower in T. canadensis soils than Q. rubra soils, which had less phosphorus (4.57% in T. canadensis, 9.88% in Q. rubra) and less potassium (1.02% in T. canadensis, 1.41% in Q. rubra). While there was no significant difference in total nitrogen content between T. canadensis and Q. rubra soils, total carbon content was higher in T. canadensis (9.11% versus 5.32% in Q. rubra). Differences in carbon content led to a higher C:N ratio in T. canadensis soils (39.2 in T. canadensis, 24.5 in Q. rubra). Lower overall nutrient content in T. canadensis soils could potentially impede new seedling survival and recruitment in the understory. This work can help elucidate the differences between soil composition beneath T. canadensis and will help to inform how these processes might change with HWA presence. With ongoing research to examine relationships among forest-floor soils and seedling survival, this study provides the foundations for understanding future forest dynamics and the long-term impacts of foundational-species loss on ecosystem function.