They found that the growing season begins and ends abruptly with the temperature transitions that mark the onset of spring (usually in May) and fall (usually in October). As the surface layers of soil begin to warm and thaw, the above-ground vegetation begins to take in carbon dioxide during photosynthesis–which is the process by which plants use sunlight as energy to "fuse" carbon dioxide and water molecules into larger, more complex molecules called carbohydrates (plants’ basic building blocks). Wofsy’s team found that from late May through July the old black spruce forest "inhaled" 1-to-1.5 grams of carbon per square meter per day (Goulden et al. 1998). Yet in August and September, the hottest, driest period of the Canadian summer, the rate of carbon dioxide intake fell to almost zero. Then, in October, the forest began to "exhale" carbon back into the atmosphere at a rate of 0.6-to-0.8 grams per square meter per day, which tapered off to 0.2-to-0.3 grams of carbon per square meter per day from December to April (Goulden et al. 1998). Averaged over the course of a year, from October 1994 to October 1995, for that region (within a 500-meter radius of the tower), Wofsy’s team found that the old black spruce stand lost 70 grams of carbon per square meter (Goulden et al. 1998)! The following year it lost 20 grams per square meter.

The BOREAS team used instruments positioned at various heights on towers like this one (located in Manitoba, Canada) to measure the movement of carbon dioxide between the forest and the atmosphere. The towers also measured the transfer of heat and moisture, and housed digital cameras to study cloud coverage and type. They were erected among pine, aspen, and fir trees, and above marshy fens. (Photograph courtesy BOREAS project)

If the boreal forest is a carbon sink, how can it be losing carbon? While it is not possible to extrapolate the measured dynamics of this one old black spruce stand to the entire boreal forest, the BOREAS team’s findings raise some troubling questions.

According to Wofsy, most of the carbon in the boreal ecosystem lies deep in the soil, some 40-to-80 centimeters below the surface–below the depth to which even large forest fires penetrate. The soil at this depth remains frozen most of the year so it cannot decompose. The decomposers–bacteria–are simply dormant. Then, when the soil does thaw toward mid-summer, it is so water-saturated from melting snow that there is not enough oxygen present for the bacteria to thrive. So, when the soil thaws and then begins to dry in late summer, the bacteria wake up and begin furiously decomposing the carbon, thus releasing carbon dioxide.

"We found that what’s going on in the soils is more important than what is going on in the trees," Wofsy explains. "We found that the boreal peat is drying and thawing out. This year, the peat never froze all the way down. If that trend continues, the ‘permafrost’ [or permanently fozen layer of soil from 75-to-100 centimeters beneath the surface] will eventually melt and…the soils will release carbon dioxide faster than the trees can absorb it."

Ironically, Wofsy found that the old black spruce site was a carbon sink in 1998–the warmest year ever on record, a year you would expect the site to be a strong source of carbon. Wofsy explains what happened: "Usually spring comes at once and dries the soil quickly. But in 1998, spring came early and lasted longer than usual."

Wofsy and his colleagues observed that the soils thawed, but remained cool throughout the spring, thus allowing the trees to "inhale" carbon dioxide for a longer than usual period of time. The net result was that from October 1997-98, the old black spruce site took in 40 grams of carbon per square meter (Wofsy 1999).

"This finding speaks to the importance of understanding the hydrology (water processes) of the boreal system," Wofsy notes.
 

The above chart shows Wofsy's measurements of carbon exchange between the atmosphere and the boreal forest. Positive numbers represent an accumulation of carbon in the forest, while negative numbers show carbon released into the atmosphere. The measurements are totalled over the course of each year, and set to zero each January first.

Although this site loses a modest amount of carbon in average years (1996 & 1997), 1998's long growing season and cool, wet summer resulted in a net gain of carbon. In contrast, 1995 had a hot and dry summer, and more than three times the usual amount of carbon was released. Over the winter the forest loses carbon at a steady rate year after year, which is shown in the graph by the similar slope of each line from October—April. (Graph by Robert Simmon, based on data from Wofsy)

Hall agrees, stating that if the boreal soils were to dry, this might increase soil carbon decomposition, but the carbon loss might be offset by the intake of carbon by trees as they grow. "This is why we need [better computer] models that incorporate all of these complicated tradeoffs–to accurately predict the effects of climate change on the boreal ecosystem. Precipitation is actually increasing at these latitudes as the climate warms; thus the future may bode a warmer, wetter, and more tropical environment for the boreal ecosystem."

At the old black spruce site in Manitoba, are we already seeing the first sign of the boreal ecosystem’s transition from a net sink to a net source of carbon? In a paper submitted in January 1997 to Science, Wofsy and his colleagues reported that the boreal carbon budget is roughly in balance, but that the boreal ecosystem demonstrates an extreme sensitivity to climate. The net carbon balance at a site is the difference over a year in the amount of carbon dioxide taken up by plant gowth and the carbon dioxide lost through bacterial decomposition of dead limbs, leaves and soil organic matter. For instance, the old black spruce site consistently takes up 800 grams of carbon per square meter per year. But in 1995 it exhaled 890 grams of carbon–a net source of 90 grams per square meter–whereas in 1997 it exhaled 790 grams of carbon–a net source of 10 grams per square meter (Goulden et al. 1998). In short, subtle changes in climate can be the difference between whether the boreal forest is a carbon source or sink; and this difference can show up on a global scale when you consider annual atmospheric carbon dioxide levels.

"If you look at the rate of increase in global atmospheric carbon dioxide, it’s not constant at all," Wofsy observes. "It sometimes varies [from one year to the next] by a factor of 3. Presumably, that gives us a message about the underlying mechanisms for putting away those missing 2 billion tons [of carbon]."

According to Hall, this underscores the importance of the length of the boreal growing season, which also varies from one year to the next. "Climate variation from year-to-year affects the carbon uptake differently than carbon loss," he explains. "Years with warmer springs and falls are better for plant uptake of carbon, but result in increased rates of carbon loss." It is during these latter years that the boreal forest becomes a net source rather than a sink.

Some climatologists argue that a warming trend would simply trigger longer growing seasons and more vigorous plant growth, thereby offsetting the warming trend by enabling plants to absorb more carbon dioxide. Not necessarily so, says Wofsy. "We found no evidence of high rates of carbon accumulation associated with elevated carbon dioxide or climatic warming."

But, Hall counters, Wofsy's black spruce site is old, nutrient poor, and is located near the northern edge of the boreal forest. There may be other regions where the trees are increasingly accumulating carbon due to the lengthening growing season.

Implications for Global Climate
Some Important Clues

BOREAS scientists used probes like this one to measure the amount of carbon released from the forest floor back into the atmosphere. They also monitored soil temperature and moisture to gain an understanding of the factors that influence the rate of carbon exchange. Once the processes are understood, computer models can be developed that can predict future change. Note the carpet of moss that insulates the soil from the atmosphere and prevents the soil from drying out. (Photograph courtesy BOREAS project)