COS 149-1 - Competition and climate interact to modify drought stress thresholds in mixed conifer forests of the Western United States

Abstract: Across the Western United States, climate warming and increasing forest densities have exacerbated the extent and/or severity of wildfires, bark beetle outbreaks and forest dieback events in mixed conifer forests. This situation has spurred a number of calls for fuels reduction treatments and prescribed fire to decrease stand density, basal area, fuel continuity and resistance to bark beetles and drought. These treatments are designed to mimic stand structures described in early forest inventories, dendrochronological reconstructions, and written records from the late 19^th and early 20^th centuries. If one goal for fuel reduction treatments includes producing more drought-resistant forests, then it will be important to quantify how competition has affected drought stress at forests across the Western United States. Toward that end we combined forest inventory data with tree-ring growth and carbon stable isotope data from mixed conifer forests dominated by ponderosa pine (Pinus ponderosa) located in Oregon and California.



A previous investigation at one location determined that drought stress increased in trees during the 20^th century when stand basal area, antecedent to organized fire suppression efforts, exceeded a 25 m² ha^-1 threshold. Our expanded analyses determined that mesic vs. xeric mixed conifer forests displayed basal area thresholds of 21 and 15 m² ha^-1, respectively. After a fuels reduction treatment this would represent a residual stand density of 187 and 122 trees ha^-1 (Stand Density Index, SDI of 44 and 39%) for a prescription that removed trees across all sizes and 86 trees and 63 trees ha^-1 (SDI of 42 and 37%) under a prescription in which small trees (< 30 cm DBH) are preferentially removed because of their tendency to act as ladder fuels. These data suggest that in stands treated to reduce fuel continuity and drought stress, the goals for residual basal area or stand density should be lower (i.e., 15 m² ha^-1 or 63 large trees ha^-1) at xeric sites and can be modestly greater (i.e., 21 m² ha^-1 or 86 large trees ha^-1) at mesic sites. Overall, this study demonstrates that mixed conifer forests will be more likely to avoid severe drought stress when stand basal area is less than 17 m² ha^-1 or stand density is less than 75 large trees ha^-1 (SDI < 40%) but that these thresholds shift lower for xeric sites and higher for mesic sites.