Fire-climate-vegetation linkages in quaking aspen forests during the late Holocene

Abstract

Abstract Background - Fire plays an important role in shaping forested systems around the globe. In the western US, fires are a ubiquitous process that affects virtually all forest types. Fires exhibit a wide range of frequencies and severity, which are often associated with forest type. Despite displaying a wide range of traits that would suggest they have evolved with fire, quaking aspen (Populus tremuloides Michx.) has been paradoxically considered resistant to fire (asbestos forests). Unlike many other fire-driven forest types, our understanding of pre-settlement fire behavior in quaking aspen systems is limited. To better understand the frequency and severity of fires in a putatively stable quaking aspen forest, we carefully selected a small, key watershed for sediment coring to reconstruct fire history. Results - For the past ~ 4,000 years this basin has been dominated by quaking aspen but also subalpine fir (Abies lasiocarpa (Hook.) Nutt), and their relative composition has shifted inversely over this time period. Large, stand-replacing fires occurred, on average, every 170 years, with individual fire-free intervals ranging from 132–323 years. With such a long fire return interval it is tempting to suggest that the fire regime was controlled from the top down, limited by climate. However, we found that the occurrence of fire was not predictably related to climatic conditions as characterized by cool-season, or warm-season, moisture availability (drought proxies). Rather, fire occurrence was most strongly related to fuel accumulation associated with the predictable successional shift in species dominance from quaking aspen to subalpine fir. Conclusions - The characterization of this aspen system as fuel-limited removes some barriers to contemporary fuels management. Unlike in climate-limited systems where managers have little control over fire occurrence due to climatic conditions (e.g., drought), fuel-limited systems are controlled from the bottom up, where the explicit reduction or redistribution of long-term fuel buildup is an effective approach to reducing the likelihood and/or effects of fire in the short-term. In the long-term, managers can expect stand-replacing fires in aspen forests, regardless of their characterization as asbestos forests.