Determinants of temperature sensitivity of soil respiration with the decline of a foundation species

The eastern hemlock (Tsuga canadensis) is an important foundation species that is currently declining throughout eastern U.S. forests due to the exotic pests hemlock woolly adelgid (Adelges tsugae) and elongate hemlock scale (Fiorinia externa). Hemlock is often replaced by deciduous tree species, such as black birch (Betula lenta), and has been shown to have large consequences for carbon dynamics due to a substantial loss of soil organic layer carbon storage in hemlock forests when replaced by birch and higher decomposition found in black birch stands. Soil carbon is one of the most important components of the global carbon cycle and has high potential to feedback to climate change when large portions of stored carbon are lost to the atmosphere. There is a general consensus that soil respiration increases with temperature, but there has yet to be a consensus on how temperature sensitivity of soil respiration is affected by various biotic and abiotic factors, such as soil moisture and substrate quality. In this study, the effects of soil temperature and soil moisture on soil respiration (R(s)), the temperature sensitivity of soil respiration (Q(10)), and soil basal respiration (R(10)) were investigated for hemlock, young birch, and mature birch forest types annually for three years. The R(s) values of the three forest types were primarily driven by soil temperature rather than by soil moisture across all years. Soil respiration data collected from hemlock, young birch, and mature birch stands were used to determine annual Q(10) and R(10) values. The Q(10) and R(10) values were not significantly different between forest stands, but they were significantly different over the three years. Determinants of Q(10) and R(10) differed between forest type, with soil moisture primarily influencing Q(10) in hemlock and mature birch stands and soil temperature primarily influencing R(10) in mature birch stands. The results suggest a complex interaction of soil moisture and soil temperature, and potentially substrate quality and quantity, as determinants of temperature sensitivities in eastern U.S. forests that have transitioned from hemlock-dominated to black birch-dominated forests.