COS 56-1 - Understanding Drivers of Post-Fire Delayed Tree Mortality (PFDTM) in Temperate Forests in the Pacific Northwestern region in the US

Tuesday, August 8, 2023

8:00 AM – 8:15 AM PDT

Location: B113

Presenting Author(s)

AH

Andres Holz

Associate Professor
Portland State University
Portland, Oregon, United States

Coauthor(s)

SB

Sebastian Busby

Postdoctoral Associate
USFS, United States
CE

Cody Evers

Research Associate
Portland State University
Portland, Oregon, United States

Abstract: Increasingly frequent, severe, and large wildfires across the globe, compounded with warming conditions and increasingly frequent droughts, are threatening long-term forest resilience. Remnant patches of live trees post-fire, otherwise known as fire refugia, are critical for successful forest regeneration following wildfire, especially in obligate-seeding conifer forests. Postfire delayed tree mortality (PFDTM; defined here as tree mortality occurring after one-year postfire) can substantially reduce the extent of fire refugia over time, limiting natural tree regeneration that in turn can promote forest conversions to alternative or non-forest states. Additionally, and in the absence of fire, studies on drought-mediated mortality report that tree height is a good predictor of tree vulnerability and mortality to drought, which in turn could explain some of the PFDTM findings. While PFDTM at the individual tree level is relatively well understood in dry forests in the western United States (US), little is known about spatial and temporal patterns and mechanisms behind proximate and ultimate causes of PFDTM, especially in productive ecosystems, such as temperate forests in the western Cascades in the Pacific Northwest in the US. The goals of this study are twofold: to identify spatial patterns and predict drivers of landscape-scale PFDTM in the western US, and assess pre- & post-fire climate effects on PFDM mediated by tree height & tree-scale competition in Oregon’s Cascade forests. Our approach to addressing our goals was to conduct a cross-scalar analysis of PFDTM by mapping and quantifying large-scale spatial patterns of PFDTM between 1-5 years post-fire in 30 wildfires in three large ecoregions in the western US during the 2008-2014 period using high-resolution satellite imagery, and to examine the effects of climate, tree height, and tree-scale competition on pre- and post-fire radial growth during the 1970-2019 period using dendroecological techniques. Here we show that at landscape scales, 5-yr PFDTM can increase the original (1-yr postfire) burned fire perimeter up to 5-25% and that the burn severity of the immediate fire effects, topography, and climate are strong predictors of PFDTM in the western US. Our preliminary treering results indicate that PFDTM was related to fire-caused injuries and patterns in growth rate and patterns in long-term growth. In conclusion, these studies suggest that conifer tree mortality related to fire effects occurs at an ecologically important rate beyond 1yr post-fire at large spatial scales, and that pre-fire conditions affecting tree vigor may influence post-fire tree mortality probabilities.