Reducing Fire Severity and Extent Bolsters Subalpine Forest Resilience to Global Change Through Key Demographic Pathways

Abstract

High-elevation subalpine forests are experiencing rapid changes in climatic conditions, biological disturbances, and wildfire regimes. Despite this, evidence is mixed as to whether they will undergo major ecological transformation or be resilient to a confluence of global change drivers. Here we use subalpine fir (Abies lasiocarpa) and Englemann spruce (Picea engelmannii), which form co-dominant forests through much of the western United States, to investigate how species' demographic responses to global change influence forest community-wide resilience. We do this by adapting and building on an existing framework for post-disturbance ecological reorganization. With forest inventory data from the United States Forest Service Forest Inventory and Analysis (FIA) program, we quantify population trends for subalpine fir and Engelmann spruce across their joint distribution and organize them in a new conceptual framework for categorizing forest community trajectories. We then build hierarchical Bayesian demographic models of subalpine fir and Engelmann spruce mortality, regeneration, and recruitment as functions of climate, disturbance extent and severity, and forest structural predictors. We bring demographic predictions together in a multinomial classification model to quantify how combinations of demographic rates influence overall forest community trajectories. Finally, we apply future climate and disturbance scenarios to our demographic models to explore how subalpine forest resilience may change in the future. We found strong negative relationships between the demography of both species and disturbance extent and severity, and climatic responses in line with an energy-limited forest system. Future scenario model predictions indicate that reducing wildfire extent and severity can greatly bolster overall subalpine forest resilience; the preferred way to do this will vary according to fire history, forest type, biophysical setting, and land tenure. Opportunities for high-impact management interventions are concentrated in the northern Rocky Mountains, with centers of ongoing resilience in parts of the Oregon and Washington Cascades.