Climate variability and fire effects on quaking aspen in the central Rocky Mountains, USA

Aim: Our understanding of how climate and fire have impacted quaking aspen (Populus tremuloides Michx.) communities prior to the 20th century is fairly limited. This study analysed the period between 4500 and 2000 cal. yr bp to assess the pre-historic role of climate and fire on an aspen community during an aspen-dominated period. Location: Long Lake, south-eastern Wyoming, central Rocky Mountains, USA. Methods: Sedimentary pollen and charcoal were analysed to reconstruct the vegetation and fire history for a subalpine catchment currently dominated by lodgepole pine. Modern pollen-climate relationships were applied to the fossil pollen spectra to interpret past climate variability. Nonparametric ANOVA and Tukey HSD tests were used to determine whether the reconstructed climate and fire parameters were different throughout the study period. Results: The modern pollen-climate data suggest a c. 150-year long drought centred on 4200 cal. yr bp, which caused the aspen ecotone to shift upslope. Between 3950 and 3450 cal. yr bp, an anomalous period of abundant quaking aspen pollen (Populus) occurred at the study site. Optimal climatic conditions coupled with frequent fires facilitated local quaking aspen dominance for roughly 500 years. After 3450 cal. yr bp, Populus pollen declined coincident with a return to less frequent fires and conifer dominance. Reconstructed climate variables from 550 cal. yr bp to present suggest conditions were not favourable for quaking aspen establishment at Long Lake. The Tukey HSD test confirms that the period of abundant Populus pollen was significantly different than any other period during this study. Main conclusions: Quaking aspen shifted upslope in response to warmer temperatures, and persisted for roughly 500 years as a result of optimal climatic conditions and frequent fire events.