For more than a century, the yellow-cedar, a culturally and economically valuable tree in Southeast and British Columbia, has been dying. These trees, known best for their rich yellow coloration and naturally hardy properties, have slowly been decomposing, instead of thriving in area rain forests.

But until now, no one could say why.

“The cause of tree death, called yellow-cedar decline, is now known to be a form of root freezing that occurs during cold weather in late winter and early spring, but only when snow is not present on the ground,” Paul Hennon said.

Hennon works as a scientist at the Pacific Northwest Research Station in Juneau and is co-lead of a synthesis paper recently published in the February issue of the journal BioScience.

“When present, snow protects the fine, shallow roots from extreme soil temperatures,” Hennon said. “The shallow rooting of yellow-cedar, early spring growth and its unique vulnerability to freezing injury also contribute to this problem.”

Yellow-cedar decline affects about 60 to 70 percent of trees in forests covering 600,000 acres in Alaska and British Columbia. Large stands of dead trees amid an otherwise healthy forest have prompted researchers from all over the Pacific Northwest to take a closer look at the reasons why and what can be done to help the species recover.

The paper, “Shifting Climate, Altered Niche, and a Dynamic Conservation Strategy for Yellow-Cedar in the North Pacific Coastal Rainforest,” summarizes 30 years of research and offers a framework for a conservation strategy for yellow-cedar in Alaska.

Some key findings include:

• The complex cause of yellow-cedar decline is related to reduced snow, site and stand characteristics, shallow rooting and the unique vulnerability of the roots to freezing in low temperatures.

• Low snow levels and poor soil drainage lead to impact root injury and the eventual death of yellow-cedar trees. The tree thrives in wet soils, but its tendency to produce shallow roots to access nitrogen on these sites made it more vulnerable when spring snow levels were reduced by climate warming.

• Yellow-cedar health depends on changing snow patterns, thus locations for appropriate conservation and management activities need to follow the shifting snow patterns on the landscape.

• Some responses to shifting climate are expected to be complex and difficult to anticipate. Long-term multidisciplinary research was needed to determine the true role of climate in the health of yellow-cedar and untangle it from other processes and natural cycles in forests.

The yellow-cedar is a slow-growing tree; many are 700 to 1,200 years old. Additionally, the tree has long been culturally significant to Native Alaskans who use it to make paddles, masks, dishes and woven items, to name a few. The wood is also very valuable commercially (for home and boat building) because of its straight grain, durability and resistance to insects.

According to a release from the U.S. Forest Service, attention is now directed toward a solution to protect and manage yellow-cedar, as coastal Alaska is expected to experience less snow but a persistence of periodic cold weather events in the future.

Scientists are working with partners in the Alaska Region of the Forest Service to use this new information as the framework for a comprehensive conservation strategy for yellow-cedar in Alaska in the context of a changing climate.

“We also have ongoing projects with colleagues in the Tongass National Forest in Alaska on planting and thinning to favor yellow-cedar on suitable habitat,” Dave D’Amore said.

D’Amore is co-lead author and a station scientist who specializes in soils.

“Especially on well-drained productive soils where most of the commercial forestry exits,” D’Amore said. “Silvicultural techniques can be used to nudge the ecological niche of yellow-cedar, making it more competitive on these favorable sites.”

Other coauthors of the synthesis are Paul Schaberg, U.S. Forest Service, Northern Research Station; Dustin Wittwer, U.S. Forest Service, Alaska Region; and Colin Shanley, The Nature Conservancy.