Inadequate snow cover linked to yellow cedar decline in Southeast

If you have traveled around Southeast in the past couple of years, you have probably noticed the white, almost ghostly looking trees heavily peppered among what was once healthy forest. Those are dead yellow cedar, and the remarkable thing about the dead trees-also known as snags-is they can remain standing for up to 100 years.

The decline in yellow cedar came on the radar of the US Forest Service (USFS) in the 1980's, but USFS research plant pathologist Paul Hennon says the problem was documented long before then. In 1909, a hunter first noticed the snags of the species and wrote about it.

"As we started looking back historically at this, we could see it on old aerial photographs," he says. "It turns out there is a lot of cedar mortality around Petersburg."

Hennon has been studying yellow-cedar decline for almost 35 years. His background is researching forest diseases, and while other scientists and researchers have come and gone, Hennon has stayed the course and dedicated much of his life to finding out what causes the decline.

Coastal forests are comprised of a limited number of species, so each species of tree plays a vital role in the health of the forest. Yellow-cedar foliage, for instance, is high in calcium and when it drops it seems to have some positive chemical effect on soils, other vegetation and even stream water.

Early research focused on what was causing yellow-cedar mortality. Researchers believe the problem began in the late-1800's or early-1900's. However, Hennon says for the first 15 years of attempting to find the cause researchers had no clue why the cedars were dying.

"We started to rule things out. We looked at insects and fungal diseases, the normal things that injure and kill trees," Hennon says. "In the last 10 years really started to understand that this was a freezing problem of the fine root system."

What helped Hennon and fellow researchers discover this was what he calls a "series of clues on the landscape," basically digging trees up to look at the roots. Once they began focusing on the root systems, they also enlisted the help of some scientists in Vermont that specialize in freezing injury of trees. After conducting field trials and experiments it was discovered that yellow-cedar roots are really sensitive to drops in temperature, especially the fine feeder roots.

Hennon says cold days with blue skies in February and March mean high pressure systems resulting in cold winds traveling from British Columbia toward the ocean. This puts yellow-cedar stands in wetter climates at risk for injury and mortality because they develop shallow root systems. But the biggest risk to yellow-cedar stands is not having adequate snow cover.

"You think of snow as being cold, but it's actually keeping the soil temperature from reaching that colder temperature that injures the roots," he says. "It's the lack of snow that allows that cold temperature to penetrate into the soil and kill the roots."

It's a problem easily seen on the landscape, allowing the USFS to actively map and follow its progression. Hennon says while the future is uncertain, researchers have ways of modeling the future spread of the freezing, and evidence suggests the problem is slowly emerging to the north. Currently, dead yellow cedar can be found throughout Southeast, but mortality seems to slow along the outer coast of Glacier Bay. The problem then stops at nearby Cape Fairweather. From Cape Fairweather north to Hawkins Island, up by Cordova, yellow-cedar forest populations are healthy and thriving. The same can be said for areas like Valdez and Whittier. The common trend in places where yellow cedar is not declining is simple, adequate annual snow to protect the roots.

Hennon says the average decline in a yellow-cedar stand is around 70 percent die off, leaving about 30 percent of healthy cedars. In the future, researchers desire to better understand how some trees are adapting and surviving. It could simply be that surviving trees are located in ideal spots within the stand, or some may have genetically adapted to withstand a lack of snow cover, but it's all speculation at this point.

In the meantime USFS management plans include planting new stands in areas showing acceptable growing conditions. Ideal areas for planting and expansion have well-drained soils that allow deeper rooting, and possibly higher elevation stands placed further north.

But what about the numerous snags already on the landscape, and why do they stand for so long?

"If you think about the tree species and the woods that have a nice or spicy aroma, things like cedars, cypresses or redwoods," Hennon says. "What your smelling is these natural compounds in the wood of the tree that makes it really decay resistant."

These long-living species all have different aromas and colors, suggesting different mixture of chemicals. Hennon says this is also the reason for limited decay from fungi and other harmful organisms. In yellow cedar, five or six chemicals make the species resistant to decay and also act as insect deterrents. Once the tree is dead, the compounds are still present, and the snag remains standing.

"There is a growing interest that these stands of dead yellow cedar might represent a really valuable timber source," Hennon says. "We've done a couple trials in Wrangell of salvage recovery of the dead trees and it looks pretty promising."

Hennon says a research team from University of Alaska Southeast is currently looking at the economics of salvage potential. They are trying to figure out the best ways to potentially harvest the resource and which areas are best suited and most economically responsible to access. Areas with established road systems would bring an obvious advantage in accessing forests with machinery, he says.

USFS recently began merging data with researchers in British Columbia, including mapping extensive patches of yellow-cedar decline and trying to understand how it's affecting forests.

"It's almost a paradox, where what we're really talking about is the climate is warming, but we're looking at a freezing injury," Hennon says. "Every spring, even in a warming climate, we get this cold air mass off of the interior of British Columbia that injures the trees."