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

Aim Wildfire is often considered more severe now than historically in dry forestsof the western United States. Tree-ring reconstructions, which suggest that historicaldry forests were park-like with large, old trees maintained by low-severity fires,are from small, scattered studies. To overcome this limitation, we developed spatiallycomprehensive reconstructions across 927,000 ha in four landscapes, using anew method based on land surveys from c. 1880.Location Dry forests of the western United States.Methods We reconstructed forest structure for four large dry-forest landscapesusing forest descriptions and tree data from historical land surveys. Using multipleelements of historical forest structure from this study along with corroboratinginformation 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-1 and some dense understoreys of shrubsand small trees. Park-like stands of large trees maintained by low-severity firepredominated only in parts of the study landscapes. Only 3, 12, 40 and 62% of thefour landscapes fit a low-severity fire model; 38–97% had evidence of higherseverity(mixed- and high-severity) fire. Some large modern wildfires (e.g. Rodeo-Chediski), perceived as catastrophic, had fire severity congruent with historicalvariability.Main conclusions Spatially extensive reconstructions from the late 1800s showthat these forests were structurally variable, including areas of dense forests andunderstorey trees and shrubs, and fires varied in severity, including 15–65% highseverityfire.A set of laws, policies and initiatives that aim to uniformly reduce fuelsand fire severity is likely tomove many of these forests outside their historical rangeof variability with adverse effects on biological diversity. Macroscale survey-basedreconstructions and palaeoecological studies reveal that higher-severity fires wereand are a part of the normal dynamics of dry forests.