Abstract
Abstract. Windthrow-driven changes in carbon (C) allocation and soil microclimate can affect soil carbon dioxide (CO2) efflux (Fsoil) from forest ecosystems. Although Fsoil is the dominant C flux following stand-replacing disturbance, the effects of catastrophic windthrow on Fsoil are still poorly understood. We measured Fsoil at a montane mixed-forest site and at a subalpine spruce forest site from 2009 until 2012. Each site consisted of an undisturbed forest stand and two adjacent partially cleared (stem-fraction-harvested) windthrow areas, which differed with regard to the time since disturbance. The combination of chronosequence and direct time-series approaches enabled us to investigate Fsoil dynamics over 12 years post-disturbance. At both sites Fsoil rates did not differ significantly from those of the undisturbed stands in the initial phase after disturbance (1–6 years). In the later phase after disturbance (9–12 years), Fsoil rates were significantly higher than in the corresponding undisturbed stand. Soil temperature increased significantly following windthrow (by 2.9–4.8 °C), especially in the initial phase post-disturbance when vegetation cover was sparse. A significant part (15–31%) of Fsoil from the windthrow areas was attributed to the increase in soil temperature. According to our estimates, ~500–700 g C m−2 year−1 are released via Fsoil from south-facing forest sites in the Austrian Calcareous Alps in the initial 6 years after windthrow. With a high browsing pressure suppressing tree regeneration, post-disturbance net loss of ecosystem C to the atmosphere is likely to be substantial unless forest management is proactive in regenerating such sites. An increase in the frequency of forest disturbance by windthrow could therefore decrease soil C stocks and feed back positively on rising atmospheric CO2 concentrations.
Highlights
The global carbon dioxide (CO2) efflux from soil (Fsoil) was recently estimated at 98 ± 12 Pg carbon (C) year−1 (BondLamberty and Thomson, 2010), representing the major pathway by which terrestrial ecosystems release CO2 into the atmosphere (Schlesinger and Andrews, 2000)
Soil temperature was significantly higher at Höllengebirge windthrow 2009” (HW09) than at Höllengebirge windthrow 2007” (HW07), whereas soil temperatures in the windthrow areas at Rax did not differ significantly
The hypothesized initial decrease in post-disturbance Fsoil was not confirmed at both sites
Summary
The global carbon dioxide (CO2) efflux from soil (Fsoil) was recently estimated at 98 ± 12 Pg carbon (C) year−1 (BondLamberty and Thomson, 2010), representing the major pathway by which terrestrial ecosystems release CO2 into the atmosphere (Schlesinger and Andrews, 2000). As natural forest disturbance regimes are likely to be altered by climate change (Dale et al, 2001), a detailed understanding of disturbance impacts on Fsoil is essential if the forest’s role in the global C cycle, and the climate system, is to be evaluated correctly. Considering the windthrow is stand-replacing (all trees killed), Fsoil is likely the main C flux before and during the primary phase of forest recovery (Knohl et al, 2002) and determines the magnitude of initial net ecosystem emission/uptake of CO2. In cases of increased Fsoil after disturbance, large amounts of ecosystem C can be lost to the atmosphere (Kurz et al, 2008; Covington, 1981). Quantifying post-disturbance changes in Fsoil is, crucial in improving our understanding of disturbance impacts on ecosystem C dynamics and the potential risk of ecosystem C loss
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