Birch Trees To Edge Out Aspens In Warming World

Carbon dioxide is emitted when fossil fuels are burned, and it's a heat-trapping gas blamed for global warming. But rising carbon dioxide levels also have a fertilizing effect on trees and other plants, making them grow faster than they normally would.

At a 38-acre experimental forest in northeastern Wisconsin, U-M microbial ecologist Donald Zak and his colleagues have been pumping extra carbon dioxide into the tree canopies since 1997 to simulate atmospheric conditions expected in the latter half of this century. The forest contains several thousand trembling aspen, paper birch and sugar maple trees.

Mixed aspen-and-birch stands bathed in extra carbon dioxide grow about 45 percent faster than their untreated neighbors. To sustain that speedy growth, the experimental trees had to find a way to extract more of the essential nutrient nitrogen from the soil.

It appears that the extra carbon dioxide (CO₂) helps the trees do just that, by allowing them to grow more roots and "forage" more successfully for nitrogen, said Zak, a professor at the U-M School of Natural Resources and Environment and the Department of Ecology and Evolutionary Biology.

The birch trees seem better at nitrogen foraging than aspens, said Zak, one of the lead scientists at the federally funded experiment in Rhinelander, Wis. In mixed stands of aspen and birch subjected to elevated carbon dioxide levels, birch trees increased recent nitrogen acquisition by 68 percent, compared to a 19 percent increase among the aspens.

"The implication from that experiment is that it could alter the abundance of birch and aspen—in places like Michigan—by favoring birch," Zak said.

In another study being published online in the Proceedings of the National Academy of Sciences, Zak and colleagues at three other experimental forests say current models about how ecosystems will respond to mounting carbon dioxide fail to include their critical recent findings about nitrogen foraging.

Climate researchers and ecosystem modelers assume that in the coming decades, CO₂'s fertilizing effect will boost the growth rate of northern temperate forests for some unknown period of time. But eventually, current models say, the growth spurt will grind to a halt because the plants won't be able to get the soil nitrogen needed to sustain it.

But in the PNAS paper, Zak and his colleagues say enhanced nitrogen-foraging abilities should allow trees to keep the fast-growth train chugging right along.

And that's significant because forests are important carbon "sinks." They pull carbon dioxide gas out of the air during photosynthesis and lock the carbon away, for a time, in tree tissues.

In doing so, forests could help dampen the heat-trapping effects of rising carbon dioxide -- if they sustain this greater growth and nutrient uptake as they mature. If the period of CO₂-enhanced tree growth persists longer than scientists previously believed, then the climate-buffering effects of northern temperate forests could be greater than they'd assumed.

"Some of our initial assumptions about ecosystem response are not correct," Zak said. "Elevated CO₂ increases the ability of these forest trees to get limiting nutrients out of the soil. And that's a mechanism that's not in our current conception of how CO₂ will affect the way forests work."

The journal of Global Change Biology article was published online August 6.  Zak and former U-M researcher William Holmes are authors of both the PNAS and Global Change Biology papers. Other authors of the PNAS article are from Boston University, Oak Ridge National Laboratory, Duke University, the University of Tennessee, the U.S. Forest Service, the College of Charleston, and several European institutions. Other authors of the Global Change Biology paper are from Michigan Technological University, North Carolina State University, and the U.S. Forest Service.