Where forest may not return in the western United States

Abstract

Droughts that are hotter, more frequent, and last longer; pest outbreaks that are more extensive and morecommon; and fires that are more frequent, more extensive, and perhaps more severe have raised concern thatforests in the western United States may not return once disturbed by one or more of these agents. Numerousfield-based studies have been undertaken to better understand forest response to these changing disturbanceregimes. Meta-analyses of these studies provide broad guidelines on the biotic and abiotic factors that hinderforest recovery, but study-to-study differences in methods and objectives do not support estimation of the totalextent of potentially impaired forest succession. In this research, we provide an estimate of the area of potentiallyimpaired forest succession. The estimate was derived from modeling of an 18-year land cover and NormalizedDifference Vegetation Index (NDVI) time series supported by an extensive ancillary dataset. We estimate anupper bound of approximately 3470 km2 of disturbed forest that may not return or reattain prior compositionand structure. Based on the data used, fire appears to be the main disturbance agent of impaired forest succession,although climatic factors cannot be discounted. The numerous field studies routinely cite distal seedsources as a factor that hinders forest recovery, and we estimate that 20 % of the upper bound estimate has noforest cover within a 4.4-ha neighborhood. Our upper bound estimate is about 0.5 % of the 2001 mapped extentof western United States forests. The estimate is cognizant of measurement and modeling uncertainties (i.e.,upper bound) and uncertainties related to successional rates and trajectories (i.e., potential).

Citation

Wickham, J.; Neale, A.; Riitters, K.; Nash, M.; Dewitz, J.; Jin, S.; van Fossen, M.; Rosenbaum, D. 2023. Where forest may not return in the western United States. Ecological Indicators. 146: 109756-. https://doi.org/10.1016/j.ecolind.2022.109756.