Prescribed burning in ponderosa pine: fuel reductions and redistributing fuels near boles to prevent injury

Authors: PROGAR, R - Forest Service (FS); HRINKEVICH, K - Oregon State University; CLARK, E - Oregon State University; Rinella, Matthew

Submitted to: Fire Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/1/2017
Publication Date: 4/11/2017
Citation: Progar, R.A., Hrinkevich, K., Clark, E., Rinella, M.J. 2017. Prescribed burning in ponderosa pine: fuel reductions and redistributing fuels near boles to prevent injury. Fire Ecology. 13(1):149-161. doi:10.4996/fireecology.1301149.

Interpretive Summary: Prescribed burning can be an effective tool for thinning forests and reducing fuels to lessen wildfire risks. However, prescribed burning sometimes fails to substantially reduce fuels and sometimes damages/kills valuable, large trees. This study compared fuel reductions between fall and spring prescribed burns and evaluated the effect of removing (i.e. raking) fuels within 1 m of boles on prescribed burning injury to ponderosa pine. In 2007 and 2008, raking was applied to alternating trees along 18 transects in central Oregon. Fuels surrounding 292 trees were burned fall 2010 and fuels surrounding 216 trees were burned spring 2012. Most fuel types were similarly affected by both seasons of burn, but fall burning much more effectively reduced litter and duff, some of the most abundant fuels. Where burning occurred fall, raking reduced the probability of bole scorch and a visual bole scorch rating. Where burning occurred spring, tree damage was milder, and raking did not did not significantly reduce damage. These results suggest raking and other means of redistributing fuels will have the most value when burn conditions are most conducive to reducing fuels. Follow-up measurements taken 3-4 years after burning indicated the little mortality that occurred was split nearly evenly between raked (25 trees) and not-raked (30 trees) trees, so raking did not translate into substantially increased survival in this study. However, the finding that raking reduced damage suggests it could help ponderosa pine survive hotter fires capable of killing trees. Also, raking may prevent the reduced growth that sometimes follows prescribed burn injury.

Technical Abstract: Prescribed burning can be an effective tool for thinning forests and reducing fuels to lessen wildfire risks. However, prescribed burning sometimes fails to substantially reduce fuels and sometimes damages/kills valuable, large trees. This study compared fuel reductions between fall and spring prescribed burns and evaluated the effect of removing (i.e. raking) fuels within 1 m of boles on prescribed burning injury to ponderosa pine (Pinus ponderosa Lawson & C. Lawson). In 2007 and 2008, raking was applied to alternating trees along 18 transects in central Oregon. Fuels surrounding 292 trees were burned fall 2010 and fuels surrounding 216 trees were burned spring 2012. Most fuel types were similarly affected by both seasons of burn, but fall burning much more effectively reduced litter and duff, some of the most abundant fuels. Where burning occurred fall, raking reduced the probability of bole scorch 0.37(0.16, 0.60) [point estimate(95% CI)] and raking also reduced a visual bole scorch rating. Where burning occurred spring, tree damage was milder, and raking did not did not significantly reduce damage. These results suggest raking and other means of redistributing fuels will have the most value when burn conditions are most conducive to reducing fuels. Follow-up measurements taken 3-4 years after burning indicated the little mortality that occurred was split nearly evenly between raked (25 trees) and not-raked (30 trees) trees, so raking did not translate into substantially increased survival in this study. However, the finding that raking reduced damage suggests it could help ponderosa pine survive hotter fires capable of killing trees. Also, raking may prevent the reduced growth that sometimes follows prescribed burn injury.