Climate change and fire alter biotic interactions and tree growth in high‐elevation forests

Abstract Climate change and shifting fire regimes have the potential to alter forest structure and tree growth dynamics in high‐elevation forests; however, the relative role of competitive and facilitative interactions in shaping postfire responses remains unclear. To investigate these dynamics, we studied tree growth across an elevational gradient in the subalpine and treeline forests of the Selway‐Bitterroot Wilderness (Idaho and Montana, USA), where fire atlas data and a history of limited fire suppression enabled the implementation of a 2 × 2 factorial study design. We sampled burned and unburned stands in two elevation strata: within the transition from closed canopy subalpine forest to subalpine parkland (forest–parkland transition; FPT) and near the upper limit of single‐stem upright tree growth (alpine treeline ecotone; ATE), which represent low environmental stress and high environmental stress conditions, respectively. We measured radial growth and tree neighborhood composition for focal trees (≥20 cm diameter at breast height dbh) of two functional groups informed by their ecological roles: stress‐tolerant, early successional “benefactor” species (whitebark pine, Pinus albicaulis and subalpine larch, Larix lyallii ), and shade‐tolerant, late successional “beneficiary” species (subalpine fir, Abies lasiocarpa and Engelmann spruce, Picea engelmannii ). Patterns of neighborhood stem density across our sites indicated a shift from competition to facilitation with increasing elevation, while fire served to reduce neighborhood stem density in focal tree plots. Growth responses to fire were group‐specific: spruce/fir showed a clear fire‐induced growth release, particularly at lower elevations, while whitebark pine/larch exhibited a more limited response. Although fire did not alter the sensitivity of tree growth to climate, relationships with winter precipitation shifted from uniformly negative (pre‐1979) to neutral or positive (post‐1979) across all species and elevations. Ultimately, our results suggest that while responses to disturbance are likely to vary across species and elevations, directional climate change is uniformly reducing abiotic limitations on tree growth in the high‐elevation forests of the northern Rocky Mountains.