A new approach for modeling delayed fire-induced tree mortality

Global change is expanding the ecological niche of mixed-severity fire regimes into ecosystems that have not usually been associated with wildfires, such as temperate- and rainforests. In contrast to stand replacing fires, mixed-severity fires may result in delayed tree mortality driven by secondary factors such as post-fire environmental conditions. As these effects vary as a function of time post-fire, their study using commonly applied logistic regression models is challenging. Here we propose overcoming this problem through the application of time-explicit survival models such as the Kaplan-Meier (KM-) estimator and the Cox-Proportional Hazards (PH-) model. We use data on tree mortality after mixed-severity fires in beech (Fagus sylvatica L.) forests to (i) illustrate temporal trends in the survival probabilities and the mortality hazard of beech, (ii) estimate annual survival probabilities for different burn severities, and (iii) consider driving factors with possible time-dependent effects. Based on our results we argue that the combination of KM-estimator and Cox-PH models have the potential of substantially improve the analysis of delayed post-disturbance tree mortality by answering ‘when’ and ‘why’ tree mortality occurs. The results provide more specific information for implementing post-fire management measures.