More research indicates some forests are not growing back after wildfires

Rim Fire, August 21, 2013.
Rim Fire, August 21, 2013. Photo by Robert Martinez.

Recent research in the Rocky Mountains has found what others also determined in a 2013 study in Oregon — significant decreases in post-fire tree regeneration. In a paper titled “Evidence for declining forest resilience to wildfires under climate change”, eight researchers noted reductions in tree regeneration in the 21st century.

Below are some excerpts:

Annual moisture deficits were significantly greater from 2000 to 2015 as compared to 1985–1999, suggesting increasingly unfavourable post-fire growing conditions, corresponding to significantly lower seedling densities and increased regeneration failure. Dry forests that already occur at the edge of their climatic tolerance are most prone to conversion to non-forests after wildfires. Major climate-induced reduction in forest density and extent has important consequences for a myriad of ecosystem services now and in the future.

Climate change is already affecting multiple ecosystem properties, leading to shifts in species composition and state changes (Walther et al. 2002; Donato et al. 2016). In the US Rocky Mountains, we documented a significant trend of reduced post-fire tree regeneration, even over the relatively short period of 23 years covered in this analysis. Our findings are consistent with the expectation of reduced resilience of forest ecosystems to the combined impacts of climate warming and wildfire activity. Our results suggest that predicted shifts from forest to non-forested vegetation (e.g. Bell et al. 2014) may be underway, expedited by fire disturbances (Kemp 2015; Donato et al. 2016; Harvey et al. 2016; Johnstone et al. 2016; Rother & Veblen 2016).

Regeneration failures, as measured by both seedling presence/absence and regeneration thresholds, occurred across all forest types (Figs 3 and 4d). Low-elevation forests, dominated by tree species near the warm, dry edge of their climatic tolerance may be particularly vulnerable to shifts to non-forest vegetation, because of the absence of any tree species that could reestablish under warmer, drier conditions (Harvey et al. 2016). Meanwhile, moist forest types may experience a shift in species dominance and a decrease in tree density. And while only 15% of the moist forest sites we studied lacked seedling after 21st-century fires, 35% of these sites did not meet the recruitment threshold. This represents a substantial increase (300%) relative to the 1985–1999 period, highlighting the impacts of warming in moist forests as well.

Research: climate change may reduce conifer regeneration after wildfire

regeneration after wildfire
A decade after a stand-replacement forest fire on the Metolius watershed in Central Oregon, almost no trees have begun to regenerate on one of the dry sites at lower elevation. (Photo courtesy of Oregon State University)

Government employees working for Oregon State University have determined that predicted increases in temperature and drought in the coming century may make it more difficult for conifers such as ponderosa pine to regenerate after major forest fires on dry, low-elevation sites, in some cases leading to conversion of forests to grass or shrub lands, a report suggests.

But even though you paid for it already by funding the research as a taxpayer, it will cost you $35.95 to purchase a copy of their findings, written by Erich Kyle Dodson and Heather Taylor Root of the University’s Department of Forest Ecosystems and Society in Corvallis, Oregon. The for-profit Elsevier corporation headquartered in the Netherlands published the paper. Wildfire Today supports open access to the results of taxpayer-funded scientific research. (UPDATE April 4, 2018: the research is now available for no additional charge.)

Dodson and Root concluded that moisture stress is a key limitation for conifer regeneration following stand-replacing wildfire, which will likely increase with climate change. This will make post-fire recovery on dry sites slow and uncertain. If forests are desired in these locations, more aggressive attempts at reforestation may be needed, they said.

The study, published in Forest Ecology and Management, was done in a portion of the Metolius River watershed in the eastern Cascade Range of Oregon, which prior to a 2002 fire was mostly ponderosa pine with some Douglas-fir and other tree species. The research area was not salvage-logged or replanted following the severe, stand-replacing fire.

“A decade after this fire, there was almost no tree regeneration at lower, drier sites,” said Erich Dodson, a researcher with the OSU Department of Forest Ecosystems and Society. “There was some regeneration at higher sites with more moisture. But at the low elevations, it will be a long time before a forest comes back, if it ever does.”

Similar situations may be found in many areas of the American West in coming decades, the researchers say, and recruitment of new forests may be delayed or prevented – even in climate conditions that might have been able to maintain an existing forest. While mature trees can use their roots to tap water deeper in the soil, competition with dense understory vegetation can make it difficult for seedlings to survive.

Openings in ponderosa pine forests created by wildfire have persisted for more than a century on harsh, south-facing slopes in Colorado, the researchers noted in their report. And fire severity is already increasing in many forests due to climate change – what is now thought of as a drought in some locations may be considered average by the end of the next century.

If trees do fail to regenerate, it could further reduce ecosystem carbon storage and amplify the greenhouse effect, the study said.

Restoration treatment including thinning and prescribed burning may help reduce fire severity and increase tree survival after wildfire, as well as provide a seed source for future trees, Dodson said. These dry sites with less resilience to stand-replacing fire should be priorities for treatment, if maintaining a forest is a management objective, the study concluded.

Higher-elevation, mixed conifer forests in less moisture-limited sites may be able to recover from stand-replacing wildfire without treatment, the researchers said.