(Photo: Lauren E Oakes, Stanford University, 2011)
Something is killing the majestic Yellow Cedars of southeastern Alaska and parts of British Columbia. Scientists have been baffled for decades but now they have an answer. Paul Hennon, a researcher at the U.S. Forest Service Pacific Northwest Research Station, tells host Bruce Gellerman what’s going on.
GELLERMAN: To the Native people of Southeast Alaska, yellow cedar is more than just a tree. It's deeply rooted in their culture and mythology. They use yellow cedar to fashion canoes and weave the bark into baskets, carve totem poles and make masks. But it is no myth that for the last hundred years, something has been killing vast stands of yellow cedar. And only now have scientists figured out what it is.
Paul Hennon is a scientist at the Pacific Northwest Research Station in Alaska. He's devoted his entire career to solving the mystery of the dying yellow cedars.
HENNON: Yellow cedar is a beautiful tree. It has kind of droopy, hanging, dark green foliage and dark green crowns. The bark is a beautiful silver color. The yellow cedar gets its name from the yellow heart wood that it has and it has this kind of spicy aroma to it, which gives it great decay resistance.
And, you know, there's a couple remarkable things about the cedars. One is that they can be very old. They can live for over a thousand years. They're actually not the tallest trees in the Pacific Northwest or Alaska. They might be the oldest, but not the tallest. And because of, again, that aromatic chemistry in the wood and that extreme decay resistant, we're finding that the wood properties, meaning strength properties, decay resistance, are retained long after death.
GELLERMAN: But these trees, as you say, can grow to be a thousand years old. So what's killing yellow cedars?
HENNON: Cedars are okay as long as they're protected by snow. We do know that the cedars that are dying now and the living cedars, the larger cedars, grew up in a snowier time called the Little Ice Age. So part of the problem for the cedars was that they're adapted to - not necessarily colder conditions, but snowy conditions.
Whenever snow is covering the ground, the soil temperatures equilibrate to right around freezing, which is cold, but not nearly cold enough to kill the cedar roots. The cedar roots are shallow. Cedar grows on wet soils. We've learned that these roots, the fine roots, are just not very cold hardy. So this is a freezing injury to the fine roots and it's cold weather events that kill the tree. So snow provides a blanket of protection that keeps the soil from getting too cold.
GELLERMAN: So what happened to the snow cover?
HENNON: The weather station data from 1910 on is showing a warming trend in these key months of February and March. The pattern of snow is being reduced. It's essentially turning more of the snow into rain.
GELLERMAN: That's kind of ironic, odd. You've got global warming causing these trees to freeze.
HENNON: I think it's an irony or paradox. It's probably part of the reason it took us a while to figure this out. It suggests that some of the effects of climate change will be unexpected, and probably complex, difficult to figure out.
GELLERMAN: So how many trees are we talking about here?
HENNON: Well, we're talking about extensive areas. In coastal Alaska, we've mapped 500,000 acres of dead cedar, about a half million acres. And then within these patches of dead forest about 60 to 70 percent, on average, of the cedars are dead.
GELLERMAN: Why do 30 or 40 percent live?
HENNON: That's a good question. We don't know very much about the genetic makeup of the cedar forest. There may be individual trees that are more cold hardy or slower at becoming physiologically active in the winter. But we also, our really strong suspicion is that there are microsites, small areas where the cedars are growing in patches of dead forest that have deeper soils and the roots grow more deeply.
GELLERMAN: So what, if anything, can be done to protect the remaining trees?
HENNON: Well, the key thing is we've learned what kills the cedars, so there are strategies we can use. We can move yellow cedar, or conserve it, in areas where it's growing well and doing okay. Those tend to be higher elevation areas with more snow and those are the deeper soils where the trees grow deeper roots so it's kind of avoiding this problem. You know, one of our goals is to use this knowledge to maintain and really adapt and probably increase the populations in areas where the tree seems better suited.
GELLERMAN: Well, Dr. Hennon, thank you so much.
HENNON: Okay, thank you, Bruce!
GELLERMAN: Paul Hennon is a scientist with the U.S. Forest Service's Pacific Northwest Research Station. He spoke to us from the studios of KTOO in Juneau, Alaska.
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