Human Intervention Unnecessary in Forest Regeneration After Wildfire

Researchers and fire managers are speculating about whether forests will be able to recover from these emerging fire patterns, and whether they will need human assistance to do so because high-elevation forests in the Central Cascade mountains of the Pacific Northwest are burning more frequently and extensively than in the recent past.

Green Islands: Fire Refugia

Researchers from Portland State University have just published a study that described the function of fire refugia, or the green islands of living trees that remain after forest fires, in the process of forest regeneration in the High Cascade Mountains of Oregon and Washington.

The findings of this study can be used to determine when and when not to use human intervention in the form of tree replanting, as well as where to concentrate planting efforts and which species to give priority to.

This is crucial to know because replanting can be expensive and overcrowded forests with abnormally high tree densities and/or tree species that are grown in nurseries can worsen future fire risks or reduce the quality of the habitat.

Andrés Holz, an associate professor of geography, and Sebastian Busby, a former PSU Ph.D. candidate and current postdoctoral researcher at the US Forest Service, carried out the research.

Holz said that these fire patterns in high-elevation forests are largely brought on by climate change.

He noted that following the fires, these verdant islands, which we refer to as refugia, might be the only remaining hope for the forests to naturally recover.

Due to the region's wet and chilly climate, forests in the High Cascades may have historically burned only once every century, or even less frequently.

As the climate has continued to warm, some areas have burned twice or more in the past 20 years.

Each fire is severe enough to destroy the majority of the forest's trees in one area.

After being burned, forests in the Cascades repopulate primarily through the spread of seeds from living trees inside fire refugia.

If an area has few live trees seeding it, it might be difficult for the forest to naturally regenerate, and these areas might change into another type of ecosystem, like shrubland, grassland, or another kind of forest.

Regeneration

In this study, Busby and Holz combined fieldwork and satellite imaging to examine how various fire refugia characteristics affect their capacity to regenerate the surrounding forestland.

The researchers discovered that specific refugia characteristics promoted more tree regeneration.

For instance, tree regeneration was increased in refugia with taller, presumably older, and larger trees.

When compared to other species like subalpine and Pacific-silver firs, some species, like mountain hemlock, were able to disperse a greater quantity of seeds further into burned areas, resulting in a significantly higher rate of tree regeneration.

According to Busby, the composition and structure of fine-scale refugia are significant and can trigger very different post-fire forest regeneration responses.

Additionally, Busby and Holz developed metrics for the accessibility of seed sources using their data.

According to Busby, the data allows them to estimate the number of seeds that are likely to reach a given location and the likelihood of tree regeneration for any given area of the landscape.

Unnecessary and Wasteful

The researchers discovered that when predicting natural tree regeneration across burned landscapes, taking into account both the proximity to and density of fire refugia, as well as how those attributes interacted with local environmental and climatic conditions, was extremely important.

More specifically, after fires, cooler, wetter, or mesic conditions favored tree regeneration while hotter, drier, or arid conditions inhibited it.

He added that artificial tree planting after a fire may not only be a waste of resources but can result in overly dense forests that are more susceptible to mortality from drought, insects, and fire, especially in a warming climate.

This is true in many areas of the High Cascades.

The findings of this study implied that some earlier replanting initiatives, like the one that took place in 2010 at the site of the 2008 Gnarl Ridge Fire on Mount Hood, may not have been necessary.

Busby claimed that the natural regeneration in that area was adequate in light of our modeling and fieldwork.

It is possible that the replant has increased tree density in a way that reduces the forest's ability to fend off insects, pathogens, and fire.

Forest managers now have a new tool at their disposal to determine whether or not natural tree regeneration will take place after fires thanks to the study's findings and methods.

Overplanting will be avoided, which can be unnecessarily expensive and raise the risk of future forest fires.

Busby later noted that the forest might become extremely dense, making it more likely for a fire to break out and spread quickly and severely, Science Daily reported.