Presentation from the 2020 annual meeting of the
Ecological Society of America (ESA), which took place online August 3-6, 2020.
Click the link above to view the recording of this presentation.
Managing
for Subsequent Fires: Considering How Re-Burns Will Impact Forest Regeneration
Andrea Thode, Northern
Arizona University
Larissa Yocom, Utah State
University
Jose Iniguez, USDA Forest
Service
Rachel Loehman, U.S.
Geological Survey
Background/Questions/Methods
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.
Our results suggest that reburn fires that occur with uncharacteristic
severity can initiate transitions to new forest types.
