Drought, wildfire and forest transformation: characterizing trailing edge forests in the eastern Cascade Range, Washington, USA

Climate change and the compounding effects of drought and wildfire are catalyzing rapid ecosystem changes throughout the world. Relatively dry, trailing edge (TE) forests are especially vulnerable to ecological transformation when tree regeneration is moisture-limited following high-severity fire. Here, we illustrate the potential landscape-scale impacts of changing disturbance regimes by focusing on TE forests in the eastern Cascades of Washington, USA. Our specific objectives were to: (1) map TE forests based on climatic water deficit and forest cover; (2) characterize the composition, structure, and ownership of TE and non-TE forests; (3) quantify recent fire activity in TE and non-TE forests; (4) identify locations of potential forest loss where recent fires have burned severely in TE forests. Across the study area, TE forests encompassed 387 000 ha, representing a substantial portion (21 per cent) of the total forested landscape. TE forests generally were characterized by dry, mixed-conifer forest types with more open structure and less biomass than non-TE forests. The structural and compositional conditions within TE forests make them ideal locations for management strategies designed to enhance landscape resilience and sustain fire-resistant trees. TE forestland ownership is diverse (35 per cent federal, 19 per cent Tribal, 16 per cent Washington State, 14 per cent private non-industrial and 13 per cent private industrial), indicating that successful land management will require collaboration among numerous partners. Recent wildfires (1984-2020) cumulatively covered 84 300 ha (22 per cent) of TE forests and 363 500 ha (25 per cent) of non-TE forests. TE forests experienced less high-severity fire than non-TE forests (39 per cent vs. 46 per cent, respectively). Recent high-severity fire effects in TE forests occurred primarily in the northern portion of the study region, reflecting the distribution of individual large fires. By quantifying the variability of TE forests and their recent fire activity, this study supports adaptive management strategies for landscape restoration, post-disturbance reforestation and climate adaptation.