Old‐Growth Forest Area Projected to Increase on United States Federal Lands Under Alternative Future Scenarios

Abstract Changing climate conditions, wildfires, and tree harvests could affect the area of mature and old‐growth forests in the United States. We used a stochastic modeling system to project future areal extents of mature and old‐growth forests on National Forest System (NFS) and Bureau of Land Management (BLM) land across the conterminous United States, and to assess threats to these forests under a variety of socioeconomic and climate futures, to 2070. The area of old‐growth forest is expected to increase 19%–27% on NFS and BLM lands while mature forest trends (0.8% decrease to 2.5% increase) depend on the socioeconomic and climate future. Trends vary regionally, with old‐growth area increasing everywhere but the South, where it is projected to remain approximately the same, while the area of mature forest remains relatively flat everywhere but the Pacific Northwest, where an increase is expected. Old growth is projected to increase for most major forest types while mature forest increases for some and decreases for others. The volume of trees killed by fire annually is projected to increase by as much as 48% for old‐growth and 22%–100% for mature forests, while harvested volume could increase by 367%–441% in old‐growth and 125%–150% in mature forests under some scenarios. Fire and cutting would cause only a small proportional decrease in overall tree volume, however. By identifying the ecosystems susceptible to loss of older forest, these results can inform management and conservation efforts to retain such forests and help them adapt to future change.