Bark Beetle Effects on Fire Regimes Depend on Underlying Fuel Modifications in Semiarid Systems

Although natural disturbances such as wildfire, extreme weather events, and insect outbreaks play a key role in structuring ecosystems and watersheds worldwide, climate change has intensified many disturbance regimes, which can have compounding negative effects on ecosystem processes and services. Recent studies have highlighted the need to understand whether wildfire increases or decreases after large-scale beetle outbreaks. However, observational studies have produced mixed results. To address this, we applied a coupled ecohydrologic-fire regime-beetle effects model (RHESSys-WMFire-Beetle) in a semiarid watershed in the western US. We found that in the red phase (0-5 years post-outbreak), surface fire extent, burn probability, and surface and crown fire severity all decreased. In the gray phase (6-15 years post-outbreak), both surface fire extent and surface and crown fire severity increased with increasing mortality. However, fire probability reached a plateau during high mortality levels (>50% in terms of carbon removed). In the old phase (one to several decades post-outbreak), fire extent and severity still increased in all mortality levels. However, fire probability increased during low to medium mortality (=50%) but decreased during high mortality levels (>50%). Wildfire responses also depended on the fire regime. In fuel-limited locations, fire probability increased with increasing fuel loads, whereas in fuel-abundant (flammability-limited) systems, fire probability decreased due to decreases in fuel aridity from reduced plant water demand. This modeling framework can improve our understanding of the mechanisms driving wildfire responses and aid managers in predicting when and where fire hazards will increase.

Automated summary

Although natural disturbances play a key role in structuring ecosystems and watersheds worldwide, climate change has intensified many disturbance regimes, which can have negative effects on ecosystem processes and services. This study used a modelling framework to investigate the effects of beetle outbreaks on wildfire dynamics in a semiarid watershed. The results showed that wildfire extent and severity varied across different phases of beetle outbreaks and were influenced by fire regime and fuel availability. This framework can aid in predicting fire hazards and improving our understanding of wildfire responses.