Spatially extensive reconstructions show variable‐severity fire and heterogeneous structure in historical western United States dry forests

Abstract

ABSTRACTAim Wildfire is often considered more severe now than historically in dry forests of the western United States. Tree‐ring reconstructions, which suggest that historical dry forests were park‐like with large, old trees maintained by low‐severity fires, are from small, scattered studies. To overcome this limitation, we developed spatially comprehensive reconstructions across 927,000 ha in four landscapes, using a new method based on land surveys fromc. 1880.Location Dry forests of the western United States.Methods We reconstructed forest structure for four large dry‐forest landscapes using forest descriptions and tree data from historical land surveys. Using multiple elements of historical forest structure from this study along with corroborating information from tree‐ring studies, we were able to interpret past forest dynamics. Hypotheses concerning historical structure and dynamics were then tested.Results These reconstructions show that dry forests were structurally variable, containing from 20 to over 1000 trees ha−1and some dense understoreys of shrubs and small trees. Park‐like stands of large trees maintained by low‐severity fire predominated only in parts of the study landscapes. Only 3, 12, 40 and 62% of the four landscapes fit a low‐severity fire model; 38–97% had evidence ofhigher‐severity (mixed‐ and high‐severity) fire. Some large modern wildfires (e.g. Rodeo‐Chediski), perceived as catastrophic, had fire severity congruent with historical variability.Main conclusions Spatially extensive reconstructions from the late 1800s show that these forests were structurally variable, including areas of dense forests and understorey trees and shrubs, and fires varied in severity, including 15–65% high‐severity fire. A set of laws, policies and initiatives that aim to uniformly reduce fuels and fire severity is likely to move many of these forests outside their historical range of variability with adverse effects on biological diversity. Macroscale survey‐based reconstructions and palaeoecological studies reveal thathigher‐severity fires were and are a part of the normal dynamics of dry forests.