RESEARCH HIGHLIGHT

Measuring CO₂ Emission In an Alaska Black Spruce Forest

August 3, 2009

Even small changes in soil and stem respiration rates may have profound impacts on the atmospheric CO₂ budget.

IARC scientist Yongwon Kim is working at a research site set up in an Interior Alaska black spruce forest to measure respiration rates and thus reach a better understanding of their role in terrestrial ecosystems. Through continuous monitoring, researchers are able to record average daily respiration, or CO₂ flux, both from the soil and from the black spruce trees themselves.

To measure the amount of CO₂ that moves from the soil to the atmosphere, a series of seven light chambers 50 cm in diameter and made of transparent plastic are arranged on the forest floor. The chambers are set in a variety of vegetation types—sphagnum moss, feather moss, lichen, and tussock—on the black spruce forest floor. The chambers are equipped with an automatic control system and air compressor, making it possible to take timed measurements. The chamber lids stay closed for five minutes and open to give Kim a measurement of how much CO₂ moves from the soil to the atmosphere in a fixed time.

Temperature is the most important factor affecting soil respiration. As soil temperature rises, decomposition of roots and other organic material is enhanced, and the release of CO₂ increases rapidly. The organic layer is 30–50cm thick, and it acts as insulation, meaning that ambient air temperature has little or no effect on permafrost.

Black spruce account for about 30% of the overall forest land in Interior Alaska. The trees are extremely slow growing and susceptible to wildfire. Trees in Kim’s experimental forest range from 4.3 to 14.5cm in diameter at breast height. The largest of these trees may be as many as 150 years old.

The reproduction cycle of black spruce is dependent on fire. The trees produce lots of cones, and it is only with the extreme heat of a wildfire that the seeds are released and scattered by wind. Massive amounts of CO₂ are released to the atmosphere during fire.

However, a black spruce forest is in general a CO₂ sink. The uptake of ambient CO₂ through photosynthesis is far greater than the release through stem respiration. That is unique to black spruce, Kim explained. Unlike white spruce and several species of deciduous trees, black spruce have extremely slow metabolism. They absorb large amounts of CO₂ and release it slowly. Birch and aspen, for example, grow very fast and may have a total life cycle of only about 60 years.

Kim has found that stem respiration in black spruce is dependent on age. Younger trees, 25–50 years old, have a very active metabolism, and that rate slows down considerably as the trees age. The metabolism rate of young black spruce may be up to three times higher than that of older trees.

Yongwon Kim measures black spruce stem respiration.

Stem respiration measurements show that metabolism is more than three times higher in younger black spruce than in older trees. This chart shows measurements from seven stems varying in diameter from 4.3cm to 14.5cm from day of year 140 to 242 in 2008.

IARC scientist Yongwon Kim demonstrates how a light chamber measures soil respiration on a lichen-covered forest floor. Other chambers measure soil respiration from sphagnum moss, feather moss, and tussock.