Simulating interactions of fire, climate, and management to characterize the future range of variability in dry forests of the East Cascades

Abstract

Changing climate and altered disturbance regimes highlight the need for understanding the past, present, and future range of variability of ecosystems and natural disturbances. For many interior dry forests of the western USA, frequent fire (occurring at multi-year to multi-decadal intervals) is an essential disturbance that drives heterogenous forest structure composition and patterns that foster resilience to future wildfire. More than a century of fire suppression and exclusion has increased density, fuel accumulation, and homogenous structure, leaving dry forests vulnerable to drought stress and uncharacteristic amounts of high severity wildfire. The reference range of variability (rRV), or an estimate of forest conditions before management- and land use-driven forest change, is a common guide for locating and quantifying the need for density and fuel reduction treatments in forests that have departed from their historical conditions. However, as compounding effects of climate and land use change alter fire regimes, forest managers need new guideposts for facilitating forest adaptation to future conditions. The future range of variability (FRV) can be a better tool for adaptive management by forecasting the effects of different drivers of forest change—wildfire, climate change, and management—on forest structure at long temporal and broad spatial scales Here, I use the Individual-based Forest Landscape and Disturbance (iLand) model to simulate the FRV of landscape-level forest structure patterns for interior dry forests in the Pacific Northwest under different future wildfire, climate, and management scenarios. We incorporate spatially explicit gridded vegetation, climate, and soils data to simulate emergent outcomes of forest structure, across a 46,000 ha dry forest landscape in the Washington East Cascades over a period of 75 years into the future. Wildfire decreased the cover and continuity of mid-seral closed canopy forests and increased the cover of late seral open canopy forests. Climate change increased the cover of closed canopy forest overall. When fire and climate change were enabled in simulations, adding management reduced the mean percent cover and continuity of mid-seral closed canopy forest. Although wildfire alone reduced the mean cover and continuity of mid-seral closed canopy structure in dry forests, a surplus of mid-seral closed canopy remained on the landscape (33% more than the rRV). Findings from this study demonstrate that wildfire is more effective than management at reducing the cover and continuity of mid-seral closed canopy forest, which is expected and aligns with existing studies of the effects of wildfire on current forest structure restoration need in eastern WA. Comparison of the FRV with the rRV shows that wildfire and management are insufficient for reducing mid-seral closed canopy cover closer to reference ranges in dry forests over the course of the century. Further study is needed to understand how alternative adaptive management strategies influence the FRV of forest structure.