Wildfire reburns do not currently make up a large percentage of southwestern firescapes, yet they are areas where an understanding of the post-fire regeneration environment and recruitment success can be critical, as uncharacteristic repeat fires can act as punctuated disturbances, persistently and abruptly changing landscapes. Post-fire regeneration is critical to our understanding of ecosystem resilience, especially in the context of ongoing climate changes and land use/management activities. These can lead to increased high-severity fire and delayed or failed post-fire recovery in southwestern US forests and woodlands, yet there is little research on the severity of reburn areas and the effects of reburns on regeneration. We summarize what research there is on reburns in the Southwest and develop a conceptual model of post-reburn fire regeneration trajectories. To better understand reburn severity, we examined burn severity records of large fires that burned on Forest Service lands in Arizona and New Mexico from 1984 through 2013 to characterize the extent and severity of reburns. Using GIS, we randomly placed points in areas that burned 1, 2, 3, and 4 times and extracted values of fire severity (RdNBR) as well as remotely-sensed biophysical characteristics such as vegetation type and elevation. To develop the conceptual model of reburn effects on regeneration, reburned sites around the Southwest were visited. Finally, we contextualize our findings via comparison with reburn impacts from other forest ecosystems around the world.
Results/Conclusions
198,000 hectares reburned in Arizona and New Mexico between 1984 and 2013, out of about 1.83 million hectares in the database. We found that the subset of points that reburned had, on average, lower first-fire severity than all points on average. Fire severity of the second fire was higher than first-fire severity for about half the points that reburned once. The more times a point burned, however, the more likely that severity decreased with each subsequent fire. When a reburn occurred within six years of the previous fire, fire severity was likely to be lower; however, after six years reburn severity was unpredictable. Six conceptual pathways for pine forest and pine-oak forest generation were identified with five end trajectories, three of which are type conversions away from historical, pine-dominated forest. Reburns have initiated type conversions in other, diverse forests – for example, in boreal forests of Alaska, short-interval repeat fires that burned at high severity favored deciduous over coniferous tree establishment.