Patch Sizes, Area, and Spatial Distribution of Nonforests Are Vital Ingredients to Fire-Adapted Western US Forests

Abstract:  In prior published work with reconstructions of early and late 20^th century forest landscapes, we were surprised by the large amount of historical meadows, shrubfields, sparse woodlands, and bare ground (hereafter, nonforests) we observed on forest-capable biophysical settings. We also noted a trend of forest encroachment and densification in the late 20^th-century. Here, using LANDFIRE remotely sensed, existing vegetation height and cover, and environmental site potential map layers for seven western provinces--rescaled to match the grain of photogrammetric data--we quantitatively compare the area and patch size distributions of early- (E20^th), late 20^th-century (L20^th), and early 21^st-century (E21^st) nonforest conditions. Our results showed a trend of increasing nonforest area from the E20^th to E21^st-century and declining forested area in most provinces, with increases occurring primarily in the larger patch sizes. Our results coupled with other reburn modeling research suggest that extensive nonforest patchworks are intimately linked to forest landscape resilience, which is changing in uncharacteristic ways in some provinces. For example, in the Northern and Southern Cascade, and Blue Mountains provinces, we see an uncharacteristic coarsening of the grain by recent fires, while in the Upper Klamath province, we see a return to a large fire event-driven system. In a physical science sense, our results suggest that fire-prone forests -- in the largest context -- function as stored potential energy, and there is an ongoing tug-of-war waged over space and time between factors growing and removing forests. Nonforests on forest capable sites represent areas where stored potential energy has been reduced. Modern changes we observe in forested area foreshadow changes we can expect with climate warming.