Warming trends

Trees fall to climate change

Posted: Sunday, March 26, 2006

A tree that many consider the most valuable in Southeast Alaska is finding it hard to adapt to a warming planet.

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The yellow cedar is able to withstand bugs and decay like no other tree in the 16 million-acre Tongass National Forest. Its outstanding durability and creamy texture make it a prize for Alaska Native carvers, weavers and the wood products industry.

"There's nothing more beautiful," says Gordon Chew, a Tenakee Springs home builder.

But for more than a century, the yellow cedar has been dying out of large swaths of the Tongass. More than half of the yellow cedar trees within 500,000 acres of the forest have died, according to the U.S. Forest Service.

The yellow cedar is actually a cypress, not a true cedar. A cold-tolerant tree that thrived in the Little Ice Age, yellow cedar is having a tougher and tougher time surviving in the Alaska Panhandle's warming climate.

The decline of the tree - a species that is relatively rare to begin with - appears to be related to a reduced spring snowpack.

Rising temperatures have resulted in reduced snowpack in the region. Without adequate snow covering them, the yellow cedar's roots freeze to death in the spring.

"It's kind of a paradox" that the trees are dying from freezing episodes that are ultimately caused by climate warming, said Paul Hennon, a federal scientist in Juneau who has studied the yellow cedar decline in the Panhandle since the 1980s.

Along the shores of Baranof and Chichagof islands - the heart of the species' range in the Panhandle - dead yellow cedars' bare gray trunks stick out like ghostly spines. Similar dead patches also have been noted in British Columbia.

In other places, including spots in Juneau, Haines and the northern extent of the tree's range in Prince William Sound, the yellow cedar still appears to be doing well.

But for years, the decline of yellow cedar in its core area of the Panhandle baffled U.S. Forest Service scientists.

Hennon, a forest pathologist based at the Pacific Northwest Research Station's Juneau lab, worked on a game of elimination to discover its cause. It wasn't bugs. It wasn't a fungus. It wasn't even a toxic mineral, his and others' early studies showed.

The scientists pieced together key information about the yellow cedar's annual cycle. They learned that wet soil and yellow cedar's exposure to the elements were critical to its survival.

Trees in low-elevation, open areas seemed to face the greatest danger. The likely cause of the decline began to emerge about 10 years ago, and the evidence is now piling on each year with new data, Hennon said. "What we think we are looking at is a springtime freezing injury," he added.

Basically, snow creates a cover over the shallow roots. Snow also appears to slow the cedar's annual springtime loss of freezing resistance, called dehardening, Hennon said.

In winter, yellow cedar are incredibly resistant to freezing, but the reverse is true in March and April, during the trees' dehardening process, Hennon explained.

"Snow appears to be a really key fact," Hennon said. It may also explain why the decline of yellow cedar is largely limited to low elevations, which are experiencing less snow, he added.

Hennon and his colleagues tested the hypothesis of spring freezing injury on seedlings last year at his office in Juneau. The seedlings not afforded protection from the cold during the late winter and early spring died, Hennon said.

As further evidence for the snow factor, Hennon and other scientists note a high correlation between maps indicating levels of snow accumulation and maps indicating the decline of yellow cedar. It's almost "a perfect match" that areas in the Panhandle that have low snow accumulation are also experiencing yellow cedar decline, Hennon said.

"(Yellow cedars) were doing well ... until the late 1800s. What changed was the pattern of snow in the spring," said Hennon.

With a good explanation in hand, studies of the decline are still continuing, Hennon said. For example, University of Alaska Fairbanks students are now gathering climate records and comparing them to yellow cedar tree-ring data from all over Southeast Alaska. Vermont scientists are also testing yellow cedar tissue, shipped to them from Alaska, to "see how cold they can get without injury," Hennon said.

A Juneau-Douglas high school student has also added to the body of research on yellow cedar this year with a science fair project shedding light on the tree's rarity in Juneau.

Sophomore Lauren Hopson's project - finished a few weeks ago and presented at the annual Juneau-Douglas High School science fair - also may explain why the trees are generally more scarce in the northeastern Tongass.

Before she began her project, Hopson had never seen any "wild cedars" in Juneau and wondered why.

Working with Hennon, Hopson interviewed local residents and identified four places where yellow cedars were naturally occurring - the Nugget Creek drainage, south Douglas Island, Cedar Lake by Auke Bay and near the Dan Moller Trail on Douglas Island.

Hopson and Hennon bored into the trees to collect tree-ring data. Hopson then used a microscope to count the rings and determine how old the trees were. While cedars to the south tend to be much older, most of the trees in Juneau turned out to be about 100 years old, she said.

Hopson's project demonstrates a relatively-recent concept that yellow cedar are still advancing north from their ice age refuges along the outer coast of Southeast Alaska, she said.

Scientists suspect that the yellow cedar most likely survived the last major ice age, which ended roughly 12,000 years ago, on Dall Island and other spots such as Prince of Wales Island. Recent archeological studies show that these islands of the outer coast probably were free of ice. The trees may have migrated to lower elevations during the Little Ice Age, which ended in the late 1800s. Now, they may no longer tolerate the lack of protective snow in those places.

"I think the primary reason (they are scare) is that they haven't had time to migrate up the coast to Juneau," Hopson said.

Genetic studies are now underway to determine the ties between yellow cedar on the southwestern coast of the Panhandle and those to the north, Hennon said.

In the future, the Forest Service could focus on protecting yellow cedar where it is now thriving, Hennon suggests.

Hennon and fellow scientists at the Forest Service and the University of Alaska Fairbanks will soon publish a paper that will encourage predictive modeling to determine where the decline will occur over the next few centuries.

They believe that yellow cedar can continue to thrive at higher elevations, with colder weather and less exposure to freezing during the early spring.

"Places around Juneau and to the north are where you could actually favor yellow cedar," Hennon said.

• Elizabeth Bluemink can be reached at elizabeth.bluemink@juneauempire.com.

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