Can't see the Tundra for the Trees

The tundra is shrinking. Credit: Ryan Danby

Tuesday, March 13th, 2007--

The tundra is shrinking. A study of trees in the Kluane Ranges of the St. Elias Mountains in Yukon, Canada revealed that the tree line has changed in response to global warming. Tree line is where the forest meets the tundra—an open expanse where the climate is too harsh, usually too cold, to support trees.

The prevailing thought was that trees respond slowly to global warming. But this study suggests that once a certain climate threshold is met, tree line can dramatically leap forward. “We may not see the real impacts of climate warming in some ecosystems for another twenty or thirty years until these threshold points are reached and then we can have rapid change,” says Ryan Danby, a professor of biology at the University of Alberta in Canada.

Ryan Danby and his colleagues report that spruce tree lines jumped as many as 85 meters in elevation on some slopes during the 1900s, in a study published in the Journal of Ecology. “What that means is that if we were to go out on the landscape in the early 20th century where there was tree line say at 1200 meters hundred altitude, it’s now at close to 1300 meters altitude,” says Danby.

Tree line leaps were only seen at south-facing slopes, however. North-facing slopes didn’t show such elevation changes, but did show differences in forest density. Danby says this is probably due to differences in soil temperatures: air temperature is relatively similar at south and north-facing slopes, while soil is significantly warmer on south facing slopes.

Climate change can impact tree growth in several ways. White spruce, one of the species Danby tracked in this study, don’t produce many seeds at tree line. Instead, reproduction takes place through “vegetative growth”—when tree’s branch takes root and becomes its own tree. “To actually move forward by this process takes hundreds of years,” Danby says.

However, following warm dry summers, white spruce will produce tons of seeds, in what is called a mast event. The mast event, combined with a few years of above-average temperatures, allows seedlings to germinate and grow more quickly. “Warming can result in earlier spring, due to more rapid snow melt. So these seedlings are able to take advantage of a longer growing season,” Danby explains.

If more trees don’t sound so bad, consider the caribou. “There are a whole range of species that don’t venture into forest ecosystems but remain in the tundra,” Danby says. “In northern ecosystems, large herbivores like caribou and dall sheep rely almost exclusively on large expanses of tundra. If their habitat area is shrinking, then it follows that their populations are likely to decline as well.”

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--Flora Lichtman