EFFECTS OF HISTORIC LAND USE ON THE WHITE RIVER PLATEAU, COLORADO EXCEEDS LANDSCAPE RESPONSE TO HOLOCENE CLIMATE VARIABILITY

Apparent changes in beetle kill and forest fires regimes throughout western North America, in terms of size and severity, has prompted investigations of the relative importance of human activities and climate change as potential causal mechanisms. Each contributes to changes in vegetation and biogeochemistry, but it is difficult to assess their individual roles versus their combined effects as both have changed substantially since Euro-American settlement in the early 19^th century. Historic observations of western North American landscapes cannot provide adequate context for changes that occur on multi-decadal to centennial time scales. Here we evaluate an alpine area in northwestern Colorado known as the White River Plateau (WRP) using lake sediment components to compare climate and landscape change during the past ~12,000 years with those of the past 150 years. Several proxies are used, including stable isotopes, pollen, charcoal, and elemental geochemistry. The WRP is considered remote and pristine at >2500 m in elevation, but logging, hunting, and grazing activities are documented since the turn of the 20^th century.

Holocene oxygen isotope ratios reflect hydroclimatic change on multi-decadal time scales and indicate a shift in precipitation during the mid-to-late Holocene from a rain to snow dominated seasonal balance that is typical of modern climate. Elemental data indicate greater watershed erosion concurrent with greater snowfall whereas pollen percentages are typical of the central Rockies and major taxa accumulation rates are nearly constant. The isotopes indicate that the most rapid and abrupt climate changes of the Holocene occurred in the past ~3500 years and are reflected by changes in watershed erosion, pollen and charcoal accumulation rates. However, the effects of these climate shifts are subsequently eclipsed by those after ~AD1850, including the typical decline in charcoal accumulation since ~AD 1900. We hypothesize that these changes reflect Euro-American elk hunting, fire suppression, grazing, and logging. Continuous high-resolution sampling excludes the possibility that the results are a function of sampling density. Our results indicate that the present-day WRP ecosystem is unprecedented during the Holocene, and appears to be largely a legacy of Euro-American land use.