The Effects of Crown Scorch on Post-fire Delayed Mortality Are Modified by Drought Exposure in California (USA)

Abstract

Accurately predicting the mortality of trees that initially survive a fire event is important for management, such as planning post-fire salvage, planting, and prescribed fires. Although crown scorch has been successfully used to predict post-fire mortality (greater than one-year post-fire), it remains unclear whether other first-order fire effect metrics (e.g., stem char) and information on growing conditions can improve such predictions. Droughts can also elevate mortality and may interact, synergistically, with fire effects to influence post-fire tree survival. We used logistic regression to test whether drought exposure, as indicated by summarized monthly Palmer Drought Severity Index (PDSI) over ten-years could improve predictions of delayed mortality (4–9 years post-fire) at the individual tree level in fire-affected forest inventory and analysis (FIA) plots in California (USA). We included crown scorch, bark thickness, stem char, soil char, slope, and aspect in the model as predictors. We selected the six most prevalent species to include in the model: canyon live oak, Douglas-fir, Jeffrey pine, incense-cedar, ponderosa pine, and white fir. Mean delayed mortality, based on tree count, across all FIA plots across all tree species and plots was 17%, and overall accuracy was good (AUC = 79%). Our model performed well, correctly predicting survivor trees (sensitivity of 0.98) but had difficulty correctly predicting the smaller number of mortality trees (specificity of 0.27) at the standard probability=0.5 mortality threshold. Crown scorch was the most influential predictor of tree mortality. Increasing crown scorch was associated with greater risk of delayed mortality for all six species, with trees exhibiting over 75% crown scorch having a probability of dying that exceeded 0.5. Increasing levels of stem char and soil char (first order indicators) were associated with increasing mortality risk but to less effect than crown scorch. We expected that greater drought exposure would increase delayed post-fire mortality, but we found that increasing drought exposure (median and minimum PDSI) was associated with a modest decrease in post-fire mortality. However, we did find that trees with high levels of crown scorch were less likely to survive with increasing drought exposure (median PDSI). Delayed mortality risk decreased as terrain slope increased. Taken together, our results suggest that trees with substantial crown damage may be more vulnerable to delayed mortality if exposed to drought and that crown scorch is an effective post-fire mortality predictor up to 10 years post-fire.

Effect of Socioeconomic Variables in Predicting Global Fire Ignition Occurrence

“…The baseline model (BL) captured the general ignition patterns of GFA, particularly in the high ignition regions (Figure 1a,b). However, it consistently overpredicted the average number of monthly ignition occurrences and overpredicted fire ignitions across most regions (Figure 1c), a limitation previously identified in regression models [54,55]. The areas with largest overpredictions were Central America, Northeastern Brazil, sub-Saharan Africa, Eastern Europe, India, East and Southeast Asia, and the coastal regions of Australia.…”

Wildfires Improve Forest Growth Resilience to Drought

“…Indeed, in southern France, crown scorch volume was the variable that best explained the growth decline and mortality of P. pinea after wildfires [44]. The higher bark thickness observed in the H plot, compared with the L and U plots, suggested that fire could improve the resilience to new events by the inversion of the tree in bark tissues [20] or that individual trees with higher bark thickness before the fire increased their survival rate [25] In Californian conifers, increasing crown scorch was also associated with a greater risk of post-fire mortality, but trees with substantial crown damage were more vulnerable to delayed mortality if also exposed to drought [45]. These findings suggested an interaction between fire damage and vulnerability to drought stress, which acts on the tree and stand scales and should be investigated while considering the long (decadal) temporal scales.…”

Predictive accuracy of post‐fire conifer death declines over time in models based on crown and bole injury

“…Drought or competitive stress before fire also have an effect on mortality, exacerbating the fire-caused stresses caused by crown and bole injury (Furniss et al, 2020(Furniss et al, , 2022Hood et al, 2018;Nesmith et al, 2015;van Mantgem et al, 2013van Mantgem et al, , 2018. Similarly, post-fire climate (duration and severity of drought) may be an important predictor in later post-fire years (Agee, 2003;Barker et al, 2022;Furniss et al, 2019;Knapp et al, 2021). Root injury from smoldering fires has also been suggested as mortality agents for trees with deep, dry, duff layers (Harrington, 2013;Hood, 2010;Ryan & Frandsen, 1991;Swezy & Agee, 1991;Varner et al, 2007).…”