Freezing to death in a warming climate: yellow-cedars in trouble
For decades, a mysterious force called yellow-cedar decline has decimated hundreds of thousands of acres of our iconic native yellow-cedars.
When it hits a vulnerable stand, most yellow-cedars die. Recently a team of researchers with the U.S. Forest Service published a paper based on three decades of study that revealed the culprit: by burning oil, coal and natural gas, humans are stealing the trees’ winter blankets, leaving them to literally freeze to death.
With grey papery bark, an evocative spicy-smoky smell, a buttery smooth carving wood and a life span that can exceed a thousand years, these trees have woven themselves deeply into both ancient and modern human culture.
I personally have a deep fondness for yellow-cedar trees born of many long quiet encounters with them in the wilds.
One of my favourite groves of yellow-cedar: their curving trunks fight to keep vertical in a steep, dark col high in Strathcona Park.
Yellow-cedar beast. Part of a cathedral grove of massive, column-straight trunks on the flanks of the Golden Hinde – Vancouver Island’s tallest peak.
To me the yellow-cedar is a tree of our high mountain wilderness. But as you head north along our Great Bear Rainforest coast and up into the Alaska panhandle, these yellow-cedars come crowding down towards the tide water. And it is here that the biggest die-offs are taking place.
Along this 1,000 kilometer stretch of coast there are thousands of locations – totaling over 600,000 acres -- in which an average of 70 per cent of our mature yellow-cedar trees have been killed. In some areas nearly every single adult yellow-cedar is dead.
Map of yellow-cedar decline. From the report: “Shifting Climate, Altered Niche, and a Dynamic Conservation Strategy for Yellow-Cedar in the North Pacific Coastal Rainforest” in biosciencemag.org
Decline, death and banishment
It happens like this. First the fine roots of a yellow-cedar die. Next its coarse roots fail, followed by “necrotic cambial bole lesions”. Fungi, oomycetes, insects, nematodes, viruses and mycoplasmas attack and eat away on the faltering tree. Its crown dies back. Finally the last remaining foliage gives up.
Crown die-back caused by yellow-cedar decline. Photo: BC Ministry of Forests
Left behind, the bleached skeletons of trunks and branches can stand for over a century before toppling.
If it was just a onetime die off we would at least have hope that the species would rebound to its former health and range. But this is different.
We have so altered the weather that an increasing amount of yellow-cedars’ prime habitat has become a death trap for them. The strongholds where yellow-cedars have held their greatest competitive advantage for millennia have morphed into graveyards.
How warming leads to a tree freezing to death
We have known for a long time that fossil fuel pollution is rapidly warming our area, especially during our winter nights. Indeed, a just published review of 65 years of Environment Canada weather data by University of Victoria's Pacific Climate Impacts Consortium (PICS) revealed that our area is warming twice as fast as the rest of the planet.
This rapid warming is reducing snow cover in much of yellow-cedars’ territory, just as it is here in Vancouver. PICS estimates that spring snowfall across BC will decline another 30 per cent by next decade and around 60 per percent by 2050. The greatest declines will be along the coast where yellow-cedars live.
It turns out that snow is a very good blanket. Tests show that soil around yellow-cedars -- if covered with a blanket of snow -- won’t freeze even when the bitter cold outflows from the interior sweep across the landscape.
But the same ground without a blanket of snow can plunge below -5 C. The exposed fine roots of yellow-cedar freeze to death at those temperatures and yellow-cedar decline unfolds.
How warming leads to banishment for the species
Yellow-cedars have specialized in colonizing shallow, wet, poorly drained soils. Survival in this habitat requires a large network of fine roots much closer to the surface than in other soils. This extensive fine root network in shallow wet soils is what makes them so vulnerable to freezing events.
For thousands of years this habitat was survivable because a snow blanket could be relied upon to shelter their roots. Not anymore.
Now in areas where global warming is removing the snow blanket, this prime habitat turns lethal when cold snaps occur. Attempts by yellow-cedars to re-populate their ecological niche are doomed to fail repeatedly as shrinking snow cover leaves them exposed more and more often.
A tale of one mountain
The researchers describe a detailed study of one mountain, Mount Edgecumbe, to understand more precisely how much snowfall is needed.