The Forestry Source - March 2012 - (Page 20)
Solving a Forest Mystery: Snow Cover Linked with Alaska Yellow-Cedar Deaths
aul Hennon and Dave D’Amore, researchers with the US Forest Service’s Forestry Sciences Laboratory in Juneau, Alaska, are two of a handful of science detectives who have solved the mystery of a rash of suspicious deaths in the northern coastal forests. “Shifting Climate, Altered Niche, and a Dynamic Conservation Strategy for Yellow-Cedar in the North Pacific Coastal Rainforest,” recently published in Bioscience, is a compelling story. Hennon, D’Amore, and three coauthors found that a dearth of snow on the ground around Chamaecyparis nootkatensis has led to high levels of mortality in the trees across a quarter-million hectares of southeast Alaska and northwest British Columbia. In some areas, 70 percent of the mature yellowcedar trees are dead. “The cause of tree death, called yellowcedar 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,” said Hennon, a plant pathologist. “When present, snow protects the fine, shallow roots from extreme soil temperatures. The shallow rooting of yellowcedar, its early spring growth, and its unique vulnerability to freezing injury also contribute to this problem.” The researchers believe that the onset of yellow-cedar decline began at the end of the Little Ice Age, a period between roughly 1350 and 1900 during which temperatures in the Northern Hemisphere were between 1° C and 2° C cooler than at present, according to the National Oceanic and Atmospheric Administration.
As the climate in the southeast Alaska region has slowly warmed since the end of the Little Ice Age, snowpacks in the yellow-cedar’s range have declined. When relatively cold air from the interior of the continent moves westward over the area, as often happens in the spring, yellow-cedar roots are more prone to freezing if little or no snow is on the ground. Other species in the region appear to be faring well, in spite of changing snowfall patterns, and yellowcedar at higher elevations, where spring snows are deeper, do not show signs of the decline. Hennon and his coauthors suggest that foresters have some options for managing yellow-cedar in the future. “We’re thinking that for conservation purposes or especially for active management, we can shift our planting to higher elevations, where there’s more snow and look at the way we favor yellow-cedar in thinning at lower elevations,” said Hennon. “Part of the solution will involve running snow models not only for current conditions, but also where they might be in 50 or 100 years, because these are going to have to be long-term solutions for this very slow-growing tree.” Researchers have learned that yellowcedar mortality is higher on wet, poorly drained soils, where roots typically grow nearer to the surface. “In the low-snow zone, if we shift yellow-cedar over to the better drained soils, where we know that the roots grow more deeply, we know that it isn’t affected as much by this root-freezing problem and that it can do quite well there. However, yellow-cedar is not quite as competitive
Yellow-cedar mortality is widespread in the West Chichagof–Yakobi Wilderness Area in southeast Alaska.
on those sites as western hemlock and Sitka spruce, so although it can occupy some of the better drained, more-productive sites, it’s not the dominant tree there. But that’s where active management can come in, through planting and thinning to give yellow-cedar a competitive edge,” Hennon said. One lesson learned so far is that there may be cases in which only one species out of several is significantly impacted by climatic changes. Said Hennon, “We need to recognize that some species may be more vulnerable than others, and so in modeling the effects of climate on forest types or cover types, there may be times
when we want to take a look at the effects species by species.” Hennon also suggests that forest managers will need to look beyond broad climate changes at other factors that affect tree and forest health. “The thing we learned with yellowcedar is that climate by itself didn’t explain why all the trees were dead,” he said. “We had to dive down and look at what you might call the niche factors for the species. In this case, the soils turned out to be very important.” The paper is available on the Source Extras page, www.safnet.org/members/ archive/source_extras.cfm.
News for forest resource professionals published by the Society of American Foresters • March 2012 •Vol.17, No. 3
5400 Grosvenor Lane • Bethesda, MD 20814-2198 • www.eforester.org
Paul Hennon/US Forest Service
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