The foundation species influence of eastern hemlock (Tsuga canadensis) on biodiversity and ecosystem function on the Unglaciated Allegheny Plateau

Abstract

Foundation species are defined as abundant species that regulate ecosystem processes of a community through a small number of strong interactions, and eastern hemlock (Tsuga canadensis) is often cited as an example foundation species. Much of the understanding of its influence on ecosystem dynamics comes from investigations into the impact of hemlock mortality following invasion by an invasive pest, hemlock woolly adelgid (Adelges tsugae). While this information is invaluable, uncertainty remains about how much of these changes are due to disturbance, and are thus temporary, and how much is attributed to the role of hemlock, which are more permanent. To clarify the role of hemlock in intact forests, we investigated community composition and resource availability in eight hemlock dominated riparian forests on the Unglaciated Allegheny Plateau of Ohio, USA. In transects at 10, 30 and 50m from headwater streams, we measured all vegetation strata and physiographic context. Light availability was quantified using hemispherical photography during the growing season and the deciduous leaf-off period. We also measured soil chemistry and leaf litter biomass and chemistry as metrics of nutrient cycling and productivity, and determined the relative decomposition rate using cellulose paper.Comparisons across transects indicated a high degree of similarity in community composition and function due to the direct and indirect effects of hemlock dominance. Species richness was low, with slight increases moving upslope from the streams. Productivity (leaf litter biomass), light availability (canopy openness) in the growing season and deciduous leaf-off period, and nutrient cycling (decomposition and leaf litter chemistry) was also similar across transects. Non-metric multidimensional scaling analyses indicated differences in species composition 10m from streams compared to 30 and 50m away from the streams. Structural equation modeling (SEM) indicated hemlock dominance has a strong negative influence on vegetation species richness, as well as light availability and productivity. This indicates that as a foundation species, hemlock exerts a stronger biotic control than dampens the abiotic influence of environmental differences, overwhelming subtle but predictable patterns in species composition. Whether other foundation species, particularly conifers, structure ecosystems through similar mechanisms merits further investigation.