Can climate change increase fire severity independent of fire intensity

We tested the idea that climate may affect forest fire severity independent of fire intensity. Pervasive warming can lead to chronic stress on forest trees (McDowell et al. 2008; Raffa et al. 2008), resulting in higher sensitivity to fire-induced damage (van Mantgem et al. 2003). Thus, 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). We examined this question at a subcontinental scale by synthesizing existing information from plot-based prescribed fire monitoring databases across the western United States of America (USA). Prescribed fire data are particularly well suited to exploring the relationship between climate and fire severity because prescribed burns are conducted over a relatively narrow range of fire weather but over a potentially wide range of inter-annual climatic conditions. Specifically, we considered two topics, (i) quantifying the contribution of climate to fire severity (as measured by post-fire tree mortality), and (ii) detecting any secular trends in fire in the climate/fire severity relationship. Statistical models based on data from >330 forest plots showed that across regions and major taxa, probabilities of fire-caused tree mortality were strongly sensitive to pre-fire changes in climatic water deficit, an index of drought. Our downscaled climate data indicated that changes in the climatic water deficit were due to increasing temperatures, without detectable trends in precipitation. These climatic trends were correlated with increasing probabilities of fire-caused mortality over time. Results from this study demonstrate that incorporating measures of pre-fire climatic stress and/or tree health into models of post-fire mortality used by prescribed fire managers may substantially improve their predictive capabilities. The relationships developed here will help managers predict changes in fire severity from large-scale climatic anomalies (e.g., ENSO, PDO) and from secular trends in climate. Background and purpose There is a growing realization that current warming trends may be linked to increasing forest fire size, frequency, and severity (the number of trees killed) across the western United States (Westerling et al. 2006; Miller et al. 2009). The mechanism whereby fire severity might increase in response to warming is presumed to be increasing probabilities of severe fire weather (higher air temperature and lower relative humidity resulting in lower fuel moisture) (Fried et al. 2008). While likely true, this singular view discounts the biological context of the fire event. It has been suggested that trees subject to chronic stress are more sensitive to subsequent fire damage (van Mantgem et al. 2003; Nesmith et al. in review), implying that recent climatic trends may lead to a de facto increase 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). Current evidence implies that regional warming may already be contributing to increasing tree stress. From the late 1980s, mean annual temperature of the western United States increased at a rate of 0.3 to 0.4° C decade, even approaching 0.5° C decade at the higher elevations typically occupied by forests (Diaz & Eischeid 2007). Warming may increase tree stress by (i) increasing water deficits and thus drought stress on trees (McDowell et al. 2008), (ii) enhancing the growth and reproduction of insects and pathogens that attack trees (Raffa et al. 2008), or (iii) both. A contribution from warming to tree stress is consistent with the apparent role of warming in episodes of recent forest die-back in western North America (Breshears et al. 2005; Raffa et al. 2008; Allen et al. 2010), and the positive correlation between JFSP 09-3-01-68, page 3 background tree mortality rates and warming observed across the western United States (van Mantgem & Stephenson 2007; van Mantgem et al. 2009) and boreal forests in Canada (Peng et al. 2011). A consequence of these trends may be that forests in these landscapes are becoming increasingly sensitive to fire. Testing this climate-fire relationship is needed to better understand the nature of threats faced by temperate forests under likely future climate scenarios. While disturbance is an integral part of natural systems in forests of the western USA (e.g., Agee 1993), increasing tree mortality from fires would increase long-term carbon emissions, representing a positive feedback to climatically forced warming trends (Adams et al. 2010). The warming experienced so far in the western US is small compared to projected future conditions (Salathe et al. 2008; Overpeck & Udall 2010); even small contributions of the current climate to fire severity would therefore have profound implications for forest conservation and management. Mounting climatic stress and subsequent fire-caused mortality may lead forest managers to inadvertently increase the severity of prescribed fires under expected future climatic conditions. We approached this problem from two complimentary directions. First, we analyzed patterns of fire-induced tree mortality across the western United States by synthesizing existing fire-effects monitoring data (Topic 1. Large-scale analysis of fire effects data). Second, we conducted an in-depth analysis of this question using tree-ring records for a species of special concern in the Sierra Nevada, sugar pine (Pinus lambertiana Douglas). Here, we measured individual tree mortality probabilities using traditional measures of fire-caused damage (e.g., crown scorch, bark char height), supplemented with measures of pre-fire tree vigor, as determined from annual growth rings (Topic 2. Growth rate and fire damage as predictors of mortality of sugar pine). Study description and location Topic 1. Large-scale analysis of fire effects data