Heat, wildfires could alter Alaska's forest composition

Aug. 26 (UPI) -- If global warming continues unabated and wildfires become bigger, stronger and burn more frequently among northern climates, scientists worry the composition of Alaska's northern forests could be permanently altered.

According to a new study, published this week in the journal Nature Plants, warmer temperatures and the threat of fire is likely to enable the spread of deciduous tree species at the expense of evergreens.

In simulations, the well-tested ecosystem model called ecosys predicted northern Alaska's conifer population, including black spruce, will decline by 25 percent by the end of the century. The abundance of moss and lichen will decline by 66 percent, according to the ecosys model. Broadleaf deciduous trees like aspen will eventually come to dominate.

The shift in forest composition is likely to impact the region's carbon cycles and ecosystems.

"Expansion of the deciduous broadleaf forests in a warmer climate may result in several ecological and climatic feedbacks that affect the carbon cycle of northern ecosystems," study author Zelalem Mekonnen, a postdoctoral fellow at the Department of Energy's Lawrence Berkeley National Laboratory, said in a news release.

Scientists predict that the forests of northern Alaska will remain a net carbon sink, but they're less certain whether they will become more or less of a carbon sink as their composition changes. An increase in microbial decomposition and transpiration, the loss of moisture through leaves, will increase warming, but bare trees in winter will allow the snow to reflect more of the sun's rays, reducing warming.

While wildfire patterns are difficult to predict, scientists used a variety of simulations to produce a range of possibilities. Even modest increases in the frequency, size and intensity of wildfires would alter the region's forest makeup.

"Fires deepen the active layer, which is the zone of soil that remains unfrozen. That leads to an increase in soil nutrients available for plants. Increases in soil nutrients favor deciduous plants, which is one reason why we predict they will do so well under a warming climate," said William J. Riley, a senior scientist at Berkeley Lab. "Higher deciduous tree cover has happened under previous climates; paleoecological studies of the last 10,000 years suggest that Alaskan forests have undergone similar shifts in dominant tree species."

As deciduous trees become more common, their falling leaves will accelerate carbon turnover, which benefits broadleaf species. Deciduous seedlings are also better able to establish themselves and out-compete evergreens for sunlight in the wake of forest fires.

In addition to altering carbon cycles, shifts in the makeup of northern Alaskan forests could impact the other plants and animals that make up the region's rich ecosystems.

"Broadleaf deciduous trees have a large canopy which covers underlying vegetation, potentially decreasing herbaceous plant cover," Mekonnen said. "Those plants, especially moss, are very important forage for wildlife."