Stable or seral? Fire-driven alternative states in aspen forests of western North America.

As important centres for biological diversity, aspen forests are essential to the function and aesthetics of montane ecosystems in western North America. Aspen stands are maintained by a nuanced relationship with wildfire, although in recent decades aspen mortality has increased. The need to understand the baseline environmental conditions that favour aspen is clear; however, long-term fire history reconstructions are rare due to the scarcity of natural archives in dry montane settings. Here, we analyse a high-resolution lake sediment record from southwestern, Utah, USA to quantify the compositional and burning conditions that promote stable (or seral) aspen forests. Our results show that aspen presence is negatively correlated with subalpine fir and that severe fires tend to promote persistent and diverse aspen ecosystems over centennial timescales. This information improves our understanding of aspen disturbance ecology and identifies the circumstances where critical transitions in montane forests may occur.