Burn Severity and Post-Fire Weather Are Key to Predicting Time-To-Recover From Australian Forest Fires

Climate change has accelerated the frequency of catastrophic wildfires; however, the drivers that control the time-to-recover of forests are poorly understood. We integrated remotely sensed data, climate records, and landscape features to identify the causes of variability in the time-to-recover of canopy leaf area in southeast Australian eucalypt forests. Approximately 97% of all observed burns between 2001 and 2014 recovered to a pre-fire leaf area index (+/- 0.25 sd) within six years. Time-to-recover was highly variable within individual wildfires (ranging between <= 1 and >= 5 years), across burn seasons (90% longer January to September), and year of fire (median time-to-recover varying four-fold across fire years). We used the logistic growth function to estimate the leaf area recovery rate, burn severity, and the long-term carrying capacity of leaf area. Time-to-recover was most correlated with the leaf area recovery rate. The leaf area recovery rate was largest in areas that experienced high burn severity, and smallest in areas of intermediate to low burn severity. The leaf area recovery rate was also strongly accelerated by anomalously high post-fire precipitation, and delayed by post-fire drought. Finally we developed a predictive machine-learning model of time-to-recover (R2: 0.68). Despite the exceptionally high burn severity of the 2019-2020 Australian megafires, we forecast the time-to-recover to be only 15% longer than the average of previous fire years. Australian eucalypt forests have evolved different strategies to recover from fire. While the meteorological drivers of bushfire are reasonably well understood, the various processes explaining how long a forest takes to recover from fire are not. We investigated a range of static (landscape) and dynamic (vegetation condition or meteorological) factors that could influence how long a forest's canopy leaf area would take to recover from fire. "Time-to-recover" after fire is highly variable, ranging from less than 1 year to more than 5 years even within an individual burn location. More intense fires cause greater forest canopy damage and generally (but not always) lead to longer recovery times, whereas wetter conditions after the fire can accelerate recovery. Using these factors and others, we developed a model capable of predicting the time-to-recover and applied it to the 2019-2020 Australian megafires. Our analysis suggests the canopy damage caused by these fires was far more severe than fires in years prior. This would normally lead to a prolonged time-to-recover, however we predict that anomalously high rainfall in the year following the fires will shorten recovery time, compensating for the high burn severity. Ultimately we predict the time-to-recover will be only slightly longer than average. Pre-fire leaf area, burn severity, and post-fire meteorological conditions combine to determine time-to-recover after fire Large geographic variation in time-to-recover can be explained by mean climate and landscape differences Time-to-recover can be predicted with high accuracy using information limited to the first year following fire