Leaves or Needles, Trees or Tundra
A family friend lives in the smallest cabin on our road, usually tending to his own affairs but always reliable when it comes to tending neighbors' house plants and pets when any of us is away for a while. That is, he's very reliable when he's in town. He's a firefighter by trade, and so--depending on the weather--often spends chunks of time battling blazes in distant forests.
His occupation may explain a quirk in his house-minding performance. While he tends a place, the surrounding spruce trees tend to back off and shape up. Scruffy lower branches vanish; encroaching trunks disappear altogether.
"You really don't want spruce anywhere near a house," he'll explain. "They catch fire easily, go up like torches, send burning trash everywhere. If you want trees close by, plant birches. Growing willow doesn't burn well, nor poplar either. But keep the spruce back."
Our spruce-wary friend would probably enjoy the results of a study carried out in Canada's Northwest Territories by researchers from the University of Alberta. Simon Landhausser and Ross Wein have been studying burned-over areas of forest in the generally tundra-covered area near Inuvik, in the northern Mackenzie River country. They've found that spruce trees are losing ground after fires.
At first glance, this finding wouldn't surprise an ecologist familiar with northern forests. In the right habitat, spruce is the so-called climax species--the kind of tree that covers the terrain if it's left undisturbed from natural or human-caused disturbance for a long enough period. Usually white spruce ends up dominating warmer, drier areas, and black spruce grows in the wetter, colder regions. After a disturbance, the quick-growing plants predominate first--think of fireweed, or even dandelion along a new road. Deciduous trees like poplars are, in that sense, weedier than spruce, so soon they come to dominate. Eventually the slower-growing spruce finally shade them out and otherwise make conditions unsuitable for seedling birches and poplars to take hold. The final forest--the climax of the succession occurring as one tree community succeeds another--thus consists of spruce.
But in their study plots, the Canadian researchers found some differences in this typical successional pattern. Where the trees had recolonized burned but once forested areas, birch and cottonwood were resisting the return of black spruce and white spruce both. The conifers were less common than in unburned sites, while the resident deciduous trees--balsam, poplar and paper birch--were more common.
More significant, the scientists believe, is that the poplars and birches had extended their range. They now can be found in burned-over areas that had once been covered by tundra. In the past, it hasn't been particularly unusual to find occasional adventurous spruce trees standing outside the forest perimeter. (The high tundra in Denali Park and along the Richardson Highway near Summit Lake offer many Alaskan examples of such picturesque pioneers standing shoulders above the lower grasses and shrubs. These outliers don't mark the beginnings of a spruce expansion--or, at least, they haven't yet.) But near Inuvik, the hardy colonists are deciduous, not coniferous. The birch and poplar groves are taking more territory.
Landhausser and Wein think the deciduous trees are gaining ground because of climate change. The growing season near Inuvik has become slightly warmer, slightly drier, and slightly longer; these are changes favoring birches and poplars over spruces. The scientists predict that global warming will encourage the spread of deciduous trees. After each fire, they think these trees will become more abundant and will take over a little more tundra.
Moreover, warmer and drier conditions favor the occurrence and spread of wildfire, so the leafy trees should have even more new ground to colonize. Of course, as our fire-fighting friend has pointed out, the leafy trees don't burn as easily or exuberantly as the needle-bearing ones. Eventually, a new balance will arise between spruce and birch, tundra and trees.
