Charred forests not growing back as expected in Pacific Northwest, researchers say

29 July 2019

CANADA – Camille Stevens-Rumann was a student of raging wildfires well before she began formally researching their impact on the environment.

The Colorado-based forestry professor fought wildfires that swept through the region while pursuing an undergraduate degree, observing how they influenced the landscape.

“After getting tired of just wielding a chainsaw, I moved back to getting degrees in fire ecology,” Stevens-Rumann, 33, told CBC News. “I got really interested in thinking about how we’re managing our landscapes, and how [the] decisions we’re making even on the fire line are influencing how those ecosystems are recovering after.”

Her research has taken her from the charred forests of America’s Rocky Mountain ranges all the way to the Pacific Northwest, just south of the B.C. border.

What she’s found: certain tree species are having a tough time growing back in areas that have been affected by wildfires due to warming temperatures — a discovery that could have major implications for both the forestry sector and long-term climate change targets.

Long term trends

Among Stevens-Rumann,’s work was a 2017 study of nearly 1,500 sites charred by 52 wildfires in the U.S. Rocky Mountains. Her research found that lower elevation trees had a tough time naturally regenerating in areas that burned between 2000 and 2015 compared with sites affected between 1985 and 1999, largely due to drier weather conditions.

More recently, a 2019 study written by her colleague Kerry Kemp found that both Douglas fir and Ponderosa pine seedlings in the Idaho’s Rocky Mountains — just south of B.C. — were also struggling in low-lying burned areas due to warmer temperatures, leading to lower tree densities.

Both studies attribute climate change to be the lead cause of why the trees are struggling to grow back in certain fire-scarred areas.

Stevens-Rumann says there are many similar forests facing the same challenges in B.C.’s Southern Interior, while repeat wildfires in the province are likely also to inhibit regrowth in many areas.

As a result, some ecosystems will no longer be able to support tree species. Instead they may convert to grasslands, she said.

“You have an old fire scar, and maybe there’s a really great amount of tree regeneration … but then in one of the fires you’ve had in the last couple of years, it’s re-burned all of those trees that are only 10-20 years old, and now you’re resetting whether or not it’s going to have tree regeneration again,” she said.

Losing forests

The concerns were echoed by University of British Columbia forestry professor and associate dean Sally Aitken, who co-authored a study that mapped out how landscapes in B.C. are changing in the face of changing climate.

Aitken said many areas in the province that were burned during the record-breaking 2017-2018 wildfire seasons were also recovering from previous wildfires.

When juvenile or seedlings burn before they’re mature enough to drop seeds, forest may experience what’s known as seed source shortfall.

“When those trees are too young to really produce much in terms of seeds, if it burns again, then you’re really in a seed source shortfall,” Aitken told CBC News.

Right now, upwards of 300 million trees are being planted in B.C. annually, a significant portion of it designated for areas that have previously burned. But Aitken says the fertility of landscape is inconsistent, as dry heat in some areas has stripped the soil of its ability to support trees.

“Some areas will likely convert from being forests to being more like grasslands at those very dry margins at the extent of the forest,” she said

Implications

While some areas at higher elevations are experiencing regrowth, both Aitken and Stevens-Rumann say some ecosystems will no longer be able to support tree species that have historically stood tall over the landscape.

With more grasslands dispersed through the province, the forests’ ability to sequester carbon from the atmosphere is hampered, they said.

“We can get unhealthier forests, and [they are] not going to grow as well, and going to provide potentially a positive feedback loop for climate change,” said Aitken.

29. July 2019/by GFMCadmin