More than a decade ago in the midst of a string of severe drought years, the Pumpkin and Rodeo-Chediski fires whipped through a combined 484,000 acres of Arizona’s ponderosa pine forests. Large areas experienced complete or almost complete tree mortality.
But now, a team of Flagstaff and Colorado researchers has found some encouraging signs of ponderosa pine regrowth in those high-severity burns. Their paper, set to publish in December, recorded regeneration in all burned areas and also found evidence that the new trees may come back in lower densities than before and intermixed with more drought-resistant species, making the entire forest more resilient to climate change and wildfires in the future.
“I don’t think we can prevent all (high severity fires) so if they do occur I do think there is hope,” the paper’s lead author Suzanne Owen said in an interview.
The study focused on six 4-hectare plots per fire, all located in high severity burn patches that had experienced 100 percent tree mortality. Half the plots were located in the interior of the fire scar, and the other half were adjacent to live forest on the edges of the burn area. Owen recorded the location of every regenerating tree and collected data on its species type and height.
In the end, she said, the study resulted in several surprising findings.
Ponderosa pine seeds spread mostly via wind, so seedlings usually establish within 100 meters of their parent trees, Owen said. In her fieldwork however, she found seedlings sprouting more than 300 meters from the nearest potential parent tree. That means regeneration was occurring not only near the edges of the burn area, but also in severely burned interior parts. How the seedlings got to where they are requires more research, but Owen said certain types of scatter-hoarding birds and really strong winds may be possible explanations.
GROUPS AND CLUMPS
The paper’s authors expected new ponderosa pine seedlings in the interior of the burn area to be rare and randomly distributed but Owen said she instead found a mix of single trees and groups of different-aged trees. If the trees survive, which is still far from guaranteed, that initial observation suggests they will reestablish in a pattern that emulates the forests’ diverse and less-dense historical structure, Owen said.
That structure would make the forest more resilient to increasing wildfires and a warmer, drier future climate compared to densely packed forests that preceded the wildfire, she said.
The fact that Owen also recorded different-aged trees regrowing in the burn areas indicates they are continuing to establish for several years following the fire.
That’s encouraging because there has been an assumption that there is a window of opportunity after a wildfire, but this and other studies show it might be more episodic, said Carolyn Sieg, a research ecologist at the Rocky Mountain Research Station. Seig is also Owen’s advisor and a co-author on the paper.
In Owen’s Rodeo-Chediski burn area study plots, new ponderosa seedlings were poking up in dense sections of resprouting oak and juniper trees just as easily as they were growing in places without competition from those species.
That suggests those sprouting species might not be limiting ponderosa pine regeneration, which is contrary to what a lot of people think, Owen and Sieg said.
The mixing of ponderosas with more drought-tolerant species like oak and juniper also could help the forest be more resilient to climate change than it was before the wildfire, the paper stated.
Sieg and Owen said they are hopeful their research will help land managers better approach landscapes affected by fire. The study shows, for example, that abundant oak and juniper regrowth doesn’t preclude ponderosa pine regeneration in the same area. It also suggests that it could be worth it for land managers to hold off on post-fire replanting to see what natural regeneration develops, they said.
“We think it might be interesting for managers to use a ‘wait and see’ approach,” Owen said.