Fire Severity Trends in the Western U.S.

Science Center Objects

How will increased drought affect forest fire severity? WERC’s Dr. Phil van Mantgem is testing the idea increased drought stress may affect forest fire severity independent of fire intensity. Drought stress prior to fire can affect tree health, potentially resulting in a higher sensitivity to fire-induced damage. Thus, with drought there may be ongoing increases in fire severity (the number of trees killed), even when there is no change in fire intensity (the amount of heat released during a fire). Dr. Phil van Mantgem and his collaborators are considering this question at a subcontinental scale by synthesizing existing information from plot-based prescribed fire monitoring databases across the western United States of America.

WERC prescribed fires maintains open forest conditions
Repeated prescribed fires maintains open forest conditions at Lava Beds National Monument, California(Credit: Phil van Mantgem, USGS. Public domain.)

Prescribed fire data are particularly well suited in exploring the relationship between changing drought stress and fire severity. This is because prescription burns are conducted over a relatively narrow range of fire weather but over a potentially wide range of inter-annual climatic conditions. Results of this study will determine the role of temperature increases on fire severity. Relationships developed here will also help managers predict changes in fire severity from large-scale climatic anomalies (e.g., ENSO, PDO) and from fluctuations in temperature and drought at a regional scale.

Historically, many forests across the western U.S. experienced frequent surface fires. However, there is a growing realization that increased temperatures may be linked to increasing forest fire size and frequency. Increased temperatures may increase tree stress by (1) water deficits and thus drought stress on trees (2) enhanced growth and reproduction of insects and pathogens that attack trees (3) a combination of both. Trees subject to chronic stress are more sensitive to subsequent fire damage, suggesting that recent temperature increases may lead to a de facto increase in fire severity, even when there is no change in fire intensity.

Specifically, we will consider two topics

  1. Quantify the contribution of temperature and drought, both preceding and following fire, on fire severity (as measured by post-fire tree mortality)
  2. Detect fluctuating trends in fire severity

We have collected data from numerous fire effects monitoring networks and have begun to manage these data for analyses. In addition to products and publications, all data will be downloaded, compiled, error checked, and formatted for analysis.