Carbon cycle

All living organisms are made up of molecules that contain carbon: carbo-hydrates, proteins and lipids.

The carbon cycle includes all the reactions that allow living organisms to use carbon to manufacture their tissues and release energy.

Plants are the starting point of the carbon cycle. Through the process of photosynthesis, plants absorb carbon from the air (CO₂) and incorporate it into their biomass (leaves, wood, roots, flowers, fruits). This organic matter provides food for heterotrophic organisms (consumers). By releasing energy when they respire, heterotrophs and autotrophs return carbon to the atmosphere (CO₂).

In forest ecosystems

A growing forest is a carbon sink; in other words, it fixes more carbon through photosynthesis than the amount it releases via respiration. When the forest reaches maturity, an equilibrium is created between the quantity of carbon fixed and the amount released.

While a forest contains carbon in its trees, in a northern climate, carbon is mostly stored in forest soils as:

humus (stable organic matter, rarely attacked by decomposers);
roots in the soil;
non-decomposed plant litter on the ground;
heterotrophic organisms on the ground.

In forest ecosystems, natural disturbances as well as those induced by human activities lead to changes in the rates of carbon fixation and release (photosynthesis and respiration). For example, climate warming could accelerate decomposition of plant litter by enhancing the respiration of decomposers. In such a case, the forest soil could become a source of carbon; in other words, more carbon would be released than fixed.

Carbon cycle at the Montmorency Forest

Description

The Montmorency Forest, Université Laval's experimental forest, is located in the mountains north of the Quebec City region. The forest is comprised primarily of balsam fir and black spruce. Precipitation is abundant both in winter and summer, and the growing season lasts three months.

The ECOLEAP Project has located one of its sites within the Montmorency Forest. This study site, measuring approximately 1 ha (10 000 m²), has made it possible to measure the quantities of carbon circulating in this ecosystem.

The mature ecosystem

The Montmorency Forest study site is a balsam fir stand that is approximately 60 years old and can be considered to be a mature ecosystem. Why?

Because an equilibrium has almost been achieved between:

the quantity of carbon fixed by plants during photosynthesis;
the quantity of carbon released by the respiration of all the organisms (plants and animals) living in the forest.

Carbon in the ecosystem

The forest in the study site has been fixing carbon for 60 years, storing it in its wood, roots and plant litter. As long as it was growing, the forest was a carbon sink.

At the present time, the quantity of carbon present in this ecosystem, i.e. its carbon pool, is stable. Photosynthesis by plants still fixes carbon, but an equal amount is released by the respiration of all the organisms living in the forest. The ecosystem is no longer growing; it is maintaining itself.

The carbon pool at the Montmorency Forest site contains 17.3 kg of carbon/m² (or 173 t of carbon/ha), distributed as follows:

7.3 kg in the vegetation
- 4.5 kg in the wood
- 1.5 kg in the branches and leaves
- 1.3 kg in the roots
10 kg on and within the soil
- 3 kg in humus
- 7 kg in mineral soil

Circulation of carbon in the ecosystem

At the Montmorency Forest site, tree photosynthesis allows 1 kg of carbon/m² to enter the ecosystem (in the form of atmospheric CO₂) each year.

Tree respiration alone releases 0.5 kg of carbon/m²/yr, half of which comes from root respiration. Roots respire a great deal because they expend a large amount of energy to absorb nutrients from the soil.

A very small proportion of the carbon (0.06 kg/m²/yr or 6% of the carbon fixed by photosynthesis) accumulates in trunks and roots; the quantity of carbon in the leaves and branches of mature trees remains stable.

The remainder (0.44 kg/m²/yr) makes its way into the plant litter (dead leaves, branches and trees) and the soil (roots).

In the soil, respiration by the decomposers that consume plant litter releases a quantity of carbon equivalent to that supplied by the plants: 0.44 kg/m²/yr.

Therefore:

Soil carbon pools are in equilibrium because the amount of carbon released into the atmosphere (respiration) is equivalent to the quantity fixed in the plant litter.
In this type of forest, vegetation represents the only carbon sink. However, it is not very efficient, accumulating barely 0.06 kg carbon/m²/yr.
Overall, equilibrium has almost been achieved, but could be disrupted by a disturbance, such as fire or insect infestation, that could cause the site to become a major carbon source.

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