Impact and recovery of forest cover following wildfire in the Northern Rocky Mountains of the United States

Abstract

Anthropogenic climate change is expected to catalyze forest conversion to grass and shrublands due to more extreme fire behavior and hotter and drier post-fire conditions. However, field surveys in the Northern Rocky Mountains of the United States show robust conifer regeneration on burned sites. This study utilizes a machine learning (GBM) approach to monitor canopy cover systematically on a census of burned areas in two large wilderness areas from 1985 to 2021, to contextualize these recent field surveys and create a monitoring baseline for future change. A predictive model was developed from coincident LiDAR and Landsat observations and used to create time series of canopy cover on 352 burned sites (individual wildfires subset by number of times burned), which were then summarized using fire impact and recovery metrics. Fire impact, defined as canopy cover loss relative to pre-fire condition, was highly correlated with burn severity (Spearman’s R = 0.70). Recovery was characterized by the following: (1) whether a burned area began gaining canopy cover and (2) how long would it take to reach pre-fire cover given observed rates of gain. Eighty-five percent of the land area studied showed evidence of recovery. Areas that are failing to recover are burning more recently than their recovering counterparts, with 60% of non-recovering sites burning for the first time after 2003. However, the 5-year probability of recovery is similar among recent burns and for those that burned earlier in the record, suggesting that they may recover with more time. Once sites begin recovering, median time to reach pre-fire cover is 40 years. Seven sites have expected recovery times greater than 200 years, six of which burned for the first time after 2006. Overall, burned sites in wilderness areas of the Northern Rocky Mountains are broadly recovering from wildfire. However, anthropogenic climate change adds a layer of uncertainty to the future prognosis of conifer recovery. This work provides a framework for systematic monitoring into the future and establishes a baseline of impact and recovery in the mountains of western Montana and northern Idaho.