Carbon Sequestration and The Boreal Forest:
Boreal Carbon Sequestration and Predicted Changes
Boreal Carbon Sequestration and Predicted Changes
Boreal Carbon Sequestration
Boreal forests are an important part
of the global carbon cycle and are
currently thought to be acting as a carbon sink—that is they take up
more carbon from the atmosphere than they release. Measurements
from different boreal forest regions indicate both spatial and temporal
variability in the amount of carbon sequestered. For example, the
amount of carbon sequestered in Siberian boreal forests ranged from
99.7 to 269.9 g C m-2 during the growing season, while the amount of
carbon sequestered by a Canadian boreal aspen forest varied from 130 to
200 g C m-2 annually.
There appears to be one especially important component to boreal carbon sequestration—the soil. Boreal soils probably account for most of the forests ability to act as a carbon sink. In fact, boreal soils contain from 200-500 gigatons of carbon! So why does so much carbon end up in these soils? The answer lies in the fact that they are frozen for much of the year. This results in really slow rates of respiration by the microbes and other creatures living in the soil. This also means that the decomposition of carbon-rich soil organic matter occurs at a fairly slow rate.
There appears to be one especially important component to boreal carbon sequestration—the soil. Boreal soils probably account for most of the forests ability to act as a carbon sink. In fact, boreal soils contain from 200-500 gigatons of carbon! So why does so much carbon end up in these soils? The answer lies in the fact that they are frozen for much of the year. This results in really slow rates of respiration by the microbes and other creatures living in the soil. This also means that the decomposition of carbon-rich soil organic matter occurs at a fairly slow rate.
Predicted Changes
So what might potentially happen to
the ability of the boreal forest to
act as a carbon sink in the face of climate change? To answer
this, first will look at the component processes of carbon
sequestration individually, and then in conjunction with one
another.
In response to warming temperatures, which are already being experienced in the Mackenzie River Valley in Canada, it would be expected that photosynthesis and respiration by plants, soil respiration and the rate of decomposition of organic matter, and fire frequency and severity will all increase. It is likely that increased atmospheric CO2 levels will directly benefit photosynthesis and therefore indirectly benefit respiration both by plants and in the soil. As long as the photosynthetic rates increase at rates equal to or greater than the rates of carbon release from the ecosystem, then boreal forests will continue to serve as a carbon sink.
However, looking at the interplay between the processes of interest, this likely will not be what we see happening. Several lines of evidence suggest that the increase in photosynthesis associated with elevated CO2 levels is only temporary. Furthermore, increased respiration in the soil may lead to competition plants and microbe for nutrients. This may or may not be alleviated by increasing atmospheric nitrogen deposition.
Increasing temperatures as well as alterations in climate patterns may cause one factor to tip the balance, leading to a shift from sink to source by the boreal forest. This factor is fire. It is already becoming apparent that fire frequency and severity are increasing in response to climate change. Severe and frequent fires can alter conditions in boreal soils to the point where they are no longer able to store as much carbon as they are currently able to. With this factor taken into consideration, models currently predict that the boreal forests of the world will shift from a carbon sink to source sometime in this century.
In response to warming temperatures, which are already being experienced in the Mackenzie River Valley in Canada, it would be expected that photosynthesis and respiration by plants, soil respiration and the rate of decomposition of organic matter, and fire frequency and severity will all increase. It is likely that increased atmospheric CO2 levels will directly benefit photosynthesis and therefore indirectly benefit respiration both by plants and in the soil. As long as the photosynthetic rates increase at rates equal to or greater than the rates of carbon release from the ecosystem, then boreal forests will continue to serve as a carbon sink.
However, looking at the interplay between the processes of interest, this likely will not be what we see happening. Several lines of evidence suggest that the increase in photosynthesis associated with elevated CO2 levels is only temporary. Furthermore, increased respiration in the soil may lead to competition plants and microbe for nutrients. This may or may not be alleviated by increasing atmospheric nitrogen deposition.
Increasing temperatures as well as alterations in climate patterns may cause one factor to tip the balance, leading to a shift from sink to source by the boreal forest. This factor is fire. It is already becoming apparent that fire frequency and severity are increasing in response to climate change. Severe and frequent fires can alter conditions in boreal soils to the point where they are no longer able to store as much carbon as they are currently able to. With this factor taken into consideration, models currently predict that the boreal forests of the world will shift from a carbon sink to source sometime in this century.