A multi-proxy record of hydroclimate, vegetation, fire, and post-settlement impacts for a subalpine plateau, central Rocky Mountains, U.S.A

Abstract

Apparent changes in vegetation distribution, fire, and other disturbance regimes throughout western North America have prompted investigations of the relative importance of human activities and climate change as potential causal mechanisms. Assessing the effects of Euro-American settlement is difficult because climate changes occur on multi-decadal to centennial time scales and require longer time perspectives than historic observations can provide. Here, we report vegetation and environmental changes over the past ~13,000 years as recorded in a sediment record from Bison Lake, a subalpine lake on a high plateau in northwestern Colorado. Results are based on multiple independent proxies, which include pollen, charcoal, and elemental geochemistry, and are compared with previously reported interpretations of hydroclimatic changes from oxygen isotope ratios. The pollen data indicate a slowly changing vegetation sequence from sagebrush steppe during the late glacial to coniferous forest through the late Holocene. The most dramatic vegetation changes of the Holocene occurred during the 'Medieval Climate Anomaly' (MCA) and 'Little Ice Age' (LIA) with rapid replacement of conifer forest by grassland followed by an equally rapid return to conifer forest. Late Holocene vegetation responses are mirrored by changes in fire, lake biological productivity, and watershed erosion. These combined records indicate that subsequent disturbance related to Euro-American settlement, although perhaps significant, had acted upon a landscape that was already responding to MCA-LIA hydroclimatic change. Results document both rapid and long-term subalpine grassland ecosystem dynamics driven by agents of change that can be anticipated in the future and simulated by ecosystem models.

Vegetation Response to Wildfire and Climate Forcing in a Rocky Mountain Lodgepole Pine Forest Over the Past 2500 Years

“…During the early and middle Holocene, climate inferred from paleoecological studies in the Rocky Mountain region was characterized as warm with increased precipitation. This period of warming was followed by cooling with increases in winter precipitation during the late Holocene (Anderson, 2011;Anderson et al, 2015Anderson et al, , 2016Calder et al, 2019;Jiménez-Moreno et al, 2011;Jiménez-Moreno and Anderson, 2013). These findings were consistent with other reconstructions in the Rocky Mountain region (Table 1.2), and these studies attribute the combined effects of orbital oscillations and the El Niño Southern Oscillation (ENSO) as the main driver of changes in temperature and precipitation balance throughout the Holocene (Anderson et al, 2016;Jiménez-Moreno et al, 2011).…”

“…Some reconstructions are focused on capturing post glacial vegetation establishment with paleoecological datasets that span the entire Holocene (~ 11,000 years) (e.g. Anderson et al, 2015;Carter et al, 2013;Jiménez-Moreno et al, 2011; Jiménez-Moreno and Anderson, 2013;Minckley et al, 2012), while other datasets are centered around smaller timeframes to capture specific events like the Medieval Climate Anomaly, Little Ice Age, and other brief periods of climate variation during the late Holocene from about 7000 calibrated years before 1950 AD (cal yr BP) to present (e.g. Caffrey and Doerner, 2012;Calder and Shuman, 2017;Dunnette et al, 2014;Leys et al, 2016).…”

“…Climate throughout the Holocene was variable which had a substantial influence on fire activity and vegetation composition (Anderson et al, 2015;Carter et al, 2013;Jiménez-Moreno et al, 2011;Jiménez-Moreno and Anderson, 2013). During the early and middle Holocene, climate inferred from paleoecological studies in the Rocky Mountain region was characterized as warm with increased precipitation.…”

“…Reconstructing this temporal range of focus has been aided by new developments in paleoecological data analysis that allow for higher resolution reconstructions of fire, climate, and (Anderson et al, 2015;Leys et al, 2016) and the development of CHARAnalysis (Higuera et al, 2009) allows for the identification of local fire events by identifying peaks in charcoal against background data. Multiproxy studies can aid in improved interpretations of past fire activity, an example of which is when the results of CHARAnalysis are paired with Magnetic Susceptibility data, a lacustrine core-based proxy for catchment erosion pulses (Dunnette et al, 2014;Morris et al, 2015;.…”

High sensitivity of gross primary production in the Rocky Mountains to summer rain

“…Proxy and instrumental records of cold and warm season precipitation amounts show that the two generally vary independently of one another [ Crawford et al , ; Griffin et al , ; Dannenberg and Wise , ; Anderson et al , ] (supporting information Figure S6). However, an analysis of seasonal precipitation trends over the last three decades shows declines in both snowpack and summer rain that has resulted in region‐wide stability in their relative contributions to annual precipitation (supporting information Figure S7).…”