Abstract
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 (Rs), the temperature sensitivity of soil respiration (Q10), and soil basal respiration (R10) were investigated for hemlock, young birch, and mature birch forest types annually for three years. The Rs 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 Q10 and R10 values. The Q10 and R10 values were not significantly different between forest stands, but they were significantly different over the three years. Determinants of Q10 and R10 differed between forest type, with soil moisture primarily influencing Q10 in hemlock and mature birch stands and soil temperature primarily influencing R10 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.
Highlights
The world is witnessing an unprecedented acceleration in warming trends, which may have devastating implications for ecological process [1]
Soil temperature was highest in the mature birch, followed by young birch, followed by hemlock plots
The determinants of R10 and Q10 varied between forest types, with the Q10 of mature hemlock and birch stands influenced by soil moisture and the R10 of the mature birch stands influenced by soil temperature
Summary
The world is witnessing an unprecedented acceleration in warming trends, which may have devastating implications for ecological process [1]. Despite numerous studies that have devoted efforts to addressing this problem there is currently no consensus on projecting future shifts in soil respiration rates with a warming climate. Soil respiration (Rs, flux of carbon dioxide (CO2) from the soil to the atmosphere) represents one of the largest fluxes of the global carbon cycle [2] and it has been estimated that soil respiration (plant root and microbial) contributes ~90 Pg C yr-1 [3]. Some studies show the global soil-to-atmosphere (total soil respiration) carbon dioxide flux is increasing [3,5]. Contrary to single-site studies, global synthesis data has not found evidence of acclimation of soil respiration to warming [9]. Due to the complex nature of the interaction of other factors influencing soil respiration, including soil moisture, carbon (C) substrate levels, quality of carbon substrate, and nutrient availability [10,11,12], deciphering how rising temperatures will affect soil respiration rates remains a difficult task
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