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New study shows how ‘green islands’ help forests regenerate after fire

Portland State Study Shows How 'Green Islands' Help Regenerate Forests After Fire

A remote sensing aerial image used to quantify fire refuges from the 2012 Shadow Lake Fire in Oregon. Credit: National Agricultural Imagery Program

Due to climate change, high-altitude forests in the Central Cascade Mountains of the Pacific Northwest are burning more frequently and extensively than in the recent past, leaving researchers and fire managers questioning whether forests will be able to recover from these emerging fire patterns and whether they will need human assistance to do so.

A new study by Portland State University researchers characterizes the role of fire refugia — the green islands of living trees left over from wildfires — in forest regeneration after large and severe fires in the High Cascade mountains of Oregon and Washington.

The results of this study may help determine when human intervention in the form of tree replanting is warranted, when not, where replanting efforts should be directed, and which species should be prioritized. This is important to note as replanting can be expensive and overcrowded forests with abnormally high tree density and/or with tree species available in nurseries can reduce habitat quality or pose a greater future fire hazard.

Sebastian Busby, former PSU Ph.D. student and current postdoctoral researcher at the US Forest Service, and Andrés Holz, associate professor of geography, conducted the study, which was published in Borders in forests and global change

“To a large extent, these fire patterns occur in high-altitude forests as a result of climate change,” says Holz. “After the fires, these green islands we call refugia may be the last bit of hope we have for the forests to recover naturally.”

Portland State Study Shows How 'Green Islands' Help Regenerate Forests After Fire

Close-up example of sampled fire refugium (spot of tall trees) and post-fire regeneration in the fire area (small tree seedlings). Credit: Sebastian Busby

Forests in the High Cascades have historically been burned only once a century (or even less often) due to a wet and cold climate. As the climate continues to warm, some areas have burned two or more times in the past twenty years, with individual and/or repeated fires severe enough to wipe out the vast majority of trees in large expanses of forest.

Once burned, forests in the Cascades regenerate primarily through the dispersal of seeds from living trees in fire refuges. If there are only a few living trees in an area, it may be difficult for the forest to regenerate naturally and these areas may transition into a different type of ecosystem, such as grassland, scrub or another type of forest.

In this study, Busby and Holz used a combination of satellite imagery and fieldwork to determine how different features of fire refuges affected their ability to regenerate the surrounding forest area.

Portland State Study Shows How 'Green Islands' Help Regenerate Forests After Fire

Aftermath of the 2012 Shadow Lake Fire in Oregon. Fire refugia patch is visible to Mount Washington. Credit: Sebastian Busby

The researchers found that certain features of refugia led to more tree renewal. For example, refugia with taller and presumably older and larger trees increased tree regeneration and certain species, such as berghemlock, were able to disperse a greater amount of seeds further into burned areas, leading to much higher tree regeneration compared to others. species, such as subalpine and Pacific firs. “This tells us that the fine-grained composition and structure of refugia is important and can stimulate very different responses to forest regeneration after fire,” said Busby.

Busby and Holz also used their data to create statistics for seed resource availability.

“They give us an idea, for a particular spot in the landscape, roughly how many seeds it makes there and how much tree regeneration is likely to take place,” said Busby.

In predicting the natural regeneration of trees in burned landscapes, the researchers found that both the proximity and density of fire refugia were extremely important, as well as how those attributes interacted with local environmental and climate conditions. In particular, cooler and wetter (mesic) conditions after fires improved tree regeneration, while warmer and drier (dry) conditions prevented regeneration.

“This means that under particularly difficult conditions, such as those seen in many areas of the High Cascades, artificially replanting trees after a fire could not only be an unnecessary waste of resources, but could lead to denser forests at a higher risk.” on mortality from drought, insects and fire, especially in a warming climate,” said Busby.

Portland State Study Shows How 'Green Islands' Help Regenerate Forests After Fire

Big Lake, Oregon (same lake seen in remote sensing photo) after 2012 Shadow Lake Fire. Mount Washington can be seen in the background. Credit: Sebastian Busby

In fact, the results of this study suggest that some previous replanting efforts may not have been necessary, such as a 2010 replanting at the site of the 2008 Gnarl Ridge Fire on Mount Hood. “That probably wasn’t necessary,” Busby says. “What we saw in the field and modeling work suggested that the natural regeneration in that area was sufficient, and it is possible that the replanting may have exacerbated tree density in a negative way for the forest’s resistance to fire, drought, insects and pathogens.” .”

The results and techniques of this research can now be used by forest managers as a new tool to determine where natural tree regeneration after fires is likely or unlikely. This prevents overplanting, which can be unnecessarily expensive and can actually increase the risk of future forest fires.

“If you don’t think about that, it can lead to negative unintended consequences,” says Busby. “The forest can become super dense and if a fire starts, it’s more likely to spread quickly and burn badly.”


Climate driven, more frequent, severe wildfires in Cascade Range forests are reforming


More information:
Sebastian U. Busby et al, Determining Interactions Between Fire Refugia and Climate-Environmental Conditions Mesic Subalpine Forest Recovery After Large and Severe Forest Fires, Borders in forests and global change (2022). DOI: 10.3389/ffgc.2022.890893

Provided by Portland State University


Quote: New study shows how ‘green islands’ help forests recover after fires (2022, June 9,), retrieved June 9, 2022 from https://phys.org/news/2022-06-green-islands-forests -regenerate.html

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