Thursday, August 6, 2009 - 8:25 AM

SYMP 18-2: Two millennia of biogeochemical and vegetation responses to disturbance and climate in a Colorado watershed

Bryan N. Shuman1, M. F. Mechinech2, V. Stefanova2, A. K. Henderson2, and J. P. Donnelly3. (1) University of Wyoming, (2) University of Minnesota, (3) Woods Hole Oceanographic Institution


Climate changes in the western U.S. are causing drought-induced forest dieback and frequent stand-replacing fires.  These changes are likely to have important consequences for ecosystem services such as clean water and carbon storage, but the long-term ecosystem consequences are unclear. Paleoecological studies demonstrate the responsiveness of ecosystem composition to both climate and disturbance, but new studies are needed to demonstrate how climate and disturbance interact to jointly re-organize ecosystem structure and function. Here, we present a novel lake sediment record of climate, disturbance, vegetation composition, and bio/geochemical fluxes from a single watershed in Colorado over the past 2000 years. The data come from sediment cores collected from a lake at the base of a 94-ha watershed of lodgepole pine (Pinus contorta var. latifolia) forest, where we have obtained evidence of repeated stand-replacing fires and long-term droughts. We use these data with detailed x-ray fluorescence analyses of the cores to test two hypotheses: 1) that repeated episodes of post-fire succession led to nutrient retention within the watershed (and low inputs into the lake) similar to that documented by chronosequences studies, and 2) that elemental fluxes decreased during drought due to low rates of weathering and erosion. The lake has no surface outlets, and is assumed to retain most of the material transported out of the watershed.  


Our analyses show that sedimentary base cation concentrations are lowest during intervals that span from 10-40 years after fire episodes inferred from sedimentary charcoal peaks, and may be consistent with early-succession nutrient retention. Our data, however, are likely more sensitive to the fluxes of mineral than ionic phases, and could indicate periods of reduced mineral erosion and transport (due to root binding) during early succession rather than ionic retention within the increasing watershed biomass. In contrast to the apparent biogeochemical response to fires, our preliminary data also indicate no effect of drought on the elemental fluxes into the lake, which is consistent with other recent studies showing limited effects of climate on weathering rates. If so, the results challenge the current understanding of climate-weathering feedbacks.