Wildfires have long been known to destroy large swathes of forests, wreaking havoc on tree canopies and surrounding communities, but a new study from Oregon State University finds that the effects of heat on new tree growth have not been adequately taken into account.
The research, published in the Canadian Journal of Forest Research, examines the stress that temperature spikes can have on young trees (also called seedlings).
Researchers developed a metric called Stress Degree Hours (SDH), which measures the stress that above-average temperatures can exert on young plants over a certain period of time.
Data were collected using Douglas fir seedlings in western Oregon during the region’s heat dome in 2021.
“Our data clearly demonstrate the role that extreme weather events are likely to play in our ecosystems in the face of climate change and global change,” said Amanda Brackett, a research associate at Oregon State University and leader of the study.
Brackett said that as part of forest management, it is important to maintain higher canopy cover to better protect seedlings.
The study shows that the maximum temperature two centimeters above the ground drops by 1.3 degrees Celsius for every ten percent increase in tree canopy cover.
Yukon’s native tree species, such as white spruce and black spruce, are likely less heat-tolerant than Douglas fir because they are less adapted to higher temperatures, Brackett said.
She also said the SDH metric could help researchers better understand what temperatures are most dangerous for northern species.
Northern meaning
Some believe that Oregon-related research could be of great benefit to forestry in the Yukon.
Jill Johnstone, founder of the Northern Plant Ecology Lab in Whitehorse, said it’s becoming increasingly common in northern Canada for trees to fail to regrow after fire. Johnstone noted an associated decline in the density of conifer seedlings.
“We create firebreaks around communities to reduce fire risk, and there are different ways to design those firebreaks,” Johnstone said. “The research in this study is really helpful in thinking about the potential impacts on our own forest harvest planning.”
Black spruce is one of the most common trees in the Yukon. Johnstone says spruce and pine are the best fuel for wildfires. (Submitted by Jill Johnstone)
According to the Parks Canada website, forest fire prevention involves removing conifers and other wood that could catch fire from the forest floor. Johnstone said spruce and pine, both common in Yukon forests, are the best fuel for forest fires.
One possible solution to improve tree canopy cover while fighting wildfires is to create shade firebreaks. The Yukon government defines these firebreaks as areas where conifers are removed and/or planted more widely spaced, while retaining more fire-resistant broadleaf trees such as aspen and birch.
Johnstone said it is common practice in forestry to inhibit the growth of these deciduous tree species because they provide less light to the conifer seedlings.
She said aspen and birch can contribute to faster canopy growth because they grow faster than conifers such as pines and spruces.
Slopes in all directions
Brackett, who conducted the research as part of her master’s degree, said the study could have been even more meaningful if researchers had been able to observe the seedlings over a longer period of time.
Johnstone said there are some “next steps” that could be taken to make the study applicable to a wider range of regions, including examining the effects of temperature on other directional slopes.
The study itself focused on south-facing slopes in western Oregon, but the Yukon-based researcher said studying other slope directions could help adapt forest management approaches to a variety of landscapes.
Jill Johnstone, founder of the Northern Plant Ecology Lab, says the study could help with future forestry projects, such as an ongoing collaboration with the Teslin Tlingit Council. (Nicole Lavergne-Smith/CBC)
The study used temperature sensors to capture climate conditions near the ground, where air temperatures are more influenced by soil surface temperatures and better reflect the climate experienced by young seedlings. According to the study, seedlings are most at risk of heat injury at this stage.
In the future, Brackett wants to combine temperature data with moisture data from the soil and humidity. She said moisture can be an important factor in a plant’s response to heat.
Johnstone said the ideas presented in the paper could have implications for projects she is currently working on, particularly a collaboration between the Northern Plant Ecology Lab and the Teslin Tlingit Council.
“The type of forest that is encouraged could depend on how we try to build greater climate resilience,” Johnstone said. “The ideas presented in this paper could be really useful for us at Teslin to explore whether we should completely remove the canopy when harvesting these areas, or whether partial canopy removal with trees left standing for shade would help support that strength.”
