Climate Change Decimates Yellow Cedar

Warming winter weather in southeast Alaska has created a combination of conditions that's eliminating yellow cedar from low-elevation rainforests.

Dead yellow cedar trees cover 200,000 hectares (770 square miles) of coastal Alaska and extend into northern BC.

Scientists have now directly linked weather patterns with tree response over the last hundred years to document how climate change has pushed yellow cedars into a lethal predicament.

Unlike other trees in the region, yellow cedar's frost hardiness is triggered by temperature, making it particularly vulnerable to climate change. Its response to spring temperatures enables yellow cedar to get an early start on growing, giving it an edge over other northern rainforest species. There's a drawback, though. Intermittent late-winter melting can prompt yellow cedar to end its frost-hardy state prematurely.

When a maritime air mass moves over southeast Alaska, winter temperatures can climb above freezing enough to inspire yellow cedar to deharden. Periods of late-winter thawing have become more frequent and pronounced in recent decades. But bouts of freezing weather, brought by cold arctic air, have continued returning at a steady pace.

It's becoming more common, then, for freezing temperatures to be preceded by a winter warm spell. Such a cycle destroys any unprotected fine roots. With fewer roots feeding it, the tree crown suffocates, causing foliage to turn brown and die. Eventually, the entire tree succumbs. Cedars growing in bogs where water forces them to keep roots near the ground surface are particularly vulnerable to freezing.

In past centuries, yellow cedar roots were protected from cycles of thawing and freezing by an insulating blanket of snow on the ground. But as another consequence of a warmer climate, the ample precipitation in this region during winter is increasingly landing as rain rather than snow. The annual snowfall along the southeast Alaskan coastline has declined by an estimated two metres (6.5 feet) since 1900. Meanwhile, late-winter rainfall rose by 6.2 millimetres (0.08 inches) a year. It's where snow accumulation has diminished most that 79 percent of dead and dying yellow cedars have been found.

The suffering cedar trees average 240 years old and almost all began growing during the Little Ice Age, a cold period lasting several centuries. The onset of dying yellow cedars corresponds with the end of the Little Ice Age in Alaska around 1890.

Conditions lethal for yellow cedar have arisen especially often during the last two decades. The worst period when alternating thawing and freezing temperatures coincided with little snow on the ground was early 1987. Similarly harsh circumstances also occurred in 1997, 2001 and 2003. The 1987 episode is marked by snags dating from that time and extremely little growth in those cedars that did survive.

So far, most Alaskan forests above 300 metres (985 feet) elevation have escaped the trauma induced by climatic warming. Farther up the mountain sides, the weather remains cool and the snowpack gets deep. As the climate continues warming though, researchers are already observing the dying cedars spreading up Alaska's hillsides.