Trees 'Duke It Out' With Global Warming

Heavily instrumented forest plot mimics late 21st Century conditions
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Mar 8, 2010

A longer version of this story originally appeared in DukEnvironment magazine.

Clear plastic ducts twine through the tall, roofless, cubicles and black cables jut into the soil. An inelegantly perched propane tank provides fuel for humming heaters and radiators. The forest floor is crisscrossed by grids of string.

This strange encampment is a $2 million U.S. Department of Energy-funded project to assess how today’s forest trees will adjust if global climate change elevates temperatures near the ground by between 2 and 8 degrees Celsius during this century, as some models predict.

“We’re taking tree species that occur at three different geographic latitudes (Georgia, North Carolina and Massachusetts) and planting them at all three sites,” said lead scientist James Clark of the Nicholas School of the Environment. “When we warm them at Duke Forest, it will be interesting to see, for example, whether plants and species from further south will do better than local ones in their new environments.”

Sensors will track temperature, soil moisture, air humidity and light levels as each treelet competes to grow within its assigned 10-foot by 16-foot grid.

Crucially, the ducts convey warm air from the heaters and radiators to elevate temperatures around the closely monitored plants by either 3 or 6 degrees Celsius above normal. The jutting cables house 220-volt electrical lines that warm the soil.

The study aims to refine forecasting maps of how forests will adjust to changing climates. In the past, these maps have been built by locating the distributions of each of today’s species and logging the summer and winter temperatures and precipitation levels where those now thrive. The maps then predict where the optimal conditions are likely to shift with climate change.

“Maps showing which species will likely make it in a given place are pretty much based on that kind of logic,” said Clark, the H.L. Blomquist Professor of biology and statistical science in the Nicholas School.

“The problem is that where a given species will be found on a future map depends not only on climate but also on the other species it’s competing with,” he added.

“For instance, if you plant a spruce tree in your yard in this part of North Carolina, chances are it will do just fine,” Clark said. “But it won’t reproduce here. That’s because it just can’t compete with Southern species at these higher temperatures outside its natural range.”

As rival trees grow rapidly to shade it out, the struggling baby spruce will be left buried in their shadows and starving for the sunlight it needs to photosynthesize, he explained. Spruce can reproduce naturally only in the wilds of Dixie’s highest and coldest elevations—if even there.

In other words, plants not only have to adapt to the climate. They also have to slug it out with their neighbors as temperatures, humidity and precipitation levels change. Nature provides no free lunch. And with global warming, caused in part by growing amounts of  carbon dioxide (CO2) produced by humans, outcomes should be even more complex.

That’s why Clark is overseeing coordinated warming experiments at a Duke Forest plot near Hillsborough, N.C., plus another at Harvard Forest in Massachusetts and a third near the University of Georgia at Athens.

In spring 2009, scientists, students and technicians from all three locations and universities began busily pruning unwanted vegetation at Duke Forest and planting seeds of about a dozen different kinds of trees in grid spaces marked with plastic swizzle sticks. In some cases, they allowed pre-existing treelets to continue growing, marking them with plastic strips of various colors.

Some seeds were hand-collected in the forest while others came from supply houses. Seeds from Georgia and Massachusetts were planted in Duke Forest, and vice versa.

Increases in surface temperatures brought on by global warming are expected to alter the germination, growth and mortality of trees like these.

Depending on availability, species in the experiments might include red, black, white and chestnut oaks; sugar and red maples; loblolly and white pines; and tulip poplar, sweet birch and southern magnolia.

Species like sugar maple, sweet birch and chestnut oak already are near their southern limits in North Carolina, while black oaks, white oaks and tulip poplar are near their northern limits in Massachusetts. The study seeks to determine if species near the ‘warm’ end of their range will decline in abundance during the coming 100 years or whether trees near the ’cool’ end of their ranges might extend their range.

The protocols of the three experimental sites are designed to address nature’s inherent messiness and unpredictability.

“Every year there’s a different climate, with different temperatures and different patterns of rainfall,” Clark said. “And it’s difficult to know what’s affecting different species in a particular year unless we manipulate conditions.”

Consequently, some plants will be warmed more than others, and still others will not be heated at all. Some trees also will receive more water than they would get naturally, while others won’t.

“So if, for example, we have plots where we don’t warm we can compare those directly to plots we do warm to gauge response to the treatments,” Clark explained.

To compare how light levels affect tree growth, some chambers will be situated out in the open and others in dense shade. A third group will be in sun-dappled semi-shade beneath the tallest mature trees.

Meticulous computerized records collected for analysis will include tracking nitrogen use (vital for the photosynthesis process) and measuring movements of carbon and sugars as the young trees prepare for winter and spring.

Monte Basgall is senior writer in Duke’s Office of News and Communications.