Forests, through photosynthetic fixation of carbon dioxide, serve as natural carbon (C) sinks, thereby offsetting a substantial fraction of anthropogenic greenhouse gas emissions. However, forests of the upper Great Lakes region are on average getting older, prompting uncertainty in the region's future capacity to sequester C. Several studies demonstrate higher-than-expected rates of C sequestration in biomass and soils, or net ecosystem production (NEP), of older forests, with declining C losses from soils a proposed mechanism sustaining NEP as forests age. Forest plots spanning nearly 200 years of ecosystem development and including three different old-growth forest types were used to examine changes with age in growing season soil respiration (Rs) - the C emitted from soils by roots and soil microbes. How heterotrophic soil respiration (Rh) - the C emitted from soils by microbes only - varies among old-growth forest types and with soil environment was also examined. Mean growing season Rs and soil temperature were significantly lower in old-growth forests, with the former declining as forests aged. Laboratory incubations and scaled estimates of Rh suggest significantly higher rates in more tree species diverse mixed canopy old-growth forests than in less diverse conifer and deciduous old-growth forest stands. Soil moisture was an important driver of Rh in all old-growth forest types, particularly in the top, carbon-rich organic horizon. Based on the results of this study, it can be concluded that declining C emissions from the soils of old-growth forests may contribute to unexpectedly high rates of forest C sequestration as forests age.
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Vol. 88 • No. 3