COS 237-6 - CANCELLED - Regional variation in the strength of fire–vegetation feedbacks and resistance to reburning across the North American boreal forest

Background/Question/Methods

The climate is warming across the North American boreal forest at roughly twice the mean global rate. With such rapid warming, we might expect a marked increase in wildfire activity. However, the vegetation that develops after fire in the boreal region tends to resist reburning due to slow fuel accumulation and abundant deciduous trees in some sites. This resistance could partially counteract the increase in wildfire activity otherwise expected in a warming climate. Here, we quantify variation in the strength of resistance to reburning across the North American boreal forest over recent decades (1986–2018). We divide the region into 27 Fire Regime Units (FRUs), and for each FRU, we generate a null model of the area expected to have burned more than once (i.e., reburned) under the null hypothesis that the annual probability of burning is independent of previous fires. Then, we conduct a spatial simulation to generate confidence intervals around this value by accounting for stochastic variation in annual fire patterns. Finally, we use these confidence intervals to quantify the strength of the departure of the observed reburn extent within each FRU from that expected under the null model.

Results/Conclusions

The observed reburn area was significantly less than the null value in 16 of the 27 FRUs, representing a 5 million-ha reduction in reburn extent, or an average of 150,000 ha less reburn area per year than would be expected in a system with no feedbacks. The observed reburn area was farthest below the null value in the FRUs with the shortest fire rotations, and the difference between observed and null values generally decreased as fire rotations became longer. The FRUs where the observed reburn area was not significantly different from the null model had fire rotations >280 years, and the analysis window may have been too short to detect a difference. Although a warming climate is likely to drive increases in both the annual area burned and the extent of short-interval reburns, the magnitude of increase is likely to be much smaller than expected in the absence of fire–vegetation feedbacks. The strong resistance to reburning in the most fire-prone FRUs (fire rotations < 100 years) under current climate suggests that resistance to reburning will continue to dampen climate change-driven increases in wildfire activity for the foreseeable future.