Following on from my earlier post on deforestation and climate change, I will now discuss the role of trees and forests in mitigating anthropogenic climate change in more detail.
The potential key role of forests in mitigation of anthropogenic climate change is widely acknowledged both by scientists and policy makers. International policies to ensure the protection and sustanable management of forests are emerging rapidly, with increasing focus on reforestation and afforestation activities. However, carbon sequestration by forests is a complex mechanism, effected by numerous factors, which makes the accurate calculation of potential and actual carbon sequestration challenging. This issue is especially relevant for policies and mechanisms that allow countries to untilize forest carbon sequestration to meet their emission reduction targets as failure to properly calculate carbon sequestration can lead to inappropriate global emission reductions.
Forests play a crucial role in the global carbon cycle. Through photosynthesis they assimilate carbon dioxide (CO2) and fix carbon (C) in their biomass releasing oxygen (O2) as a waste product. Forest ecosystems store an estimated 638 gigatonnes (Gt) of carbon, 283 Gt of which in forest biomass alone, which is approximately 50% more than all the carbon in the atmosphere. This makes forests the single largest terrestrial carbon sinks (1). As a result, minor changes in forest ecosystems can have great impacts on the carbon cycle and therefore on the concentration of CO2 in the atmosphere (2). According to some estimates, the terrestrial biosphere currently sequesters around 3 billion tons of anthropogenic carbon annually, 20-30% of global anthropogenic CO2 emissions (3). Forests, however, can switch between being a net sink of carbon to being a net source, depending on succession stage, disturbance or management regime.
While the role of undisturbed forests as climate protectors is not questioned, it is hard to determine both the forest carbon sink and reservoir that can be managed and the exact amount of carbon forests at any point may remove from the atmosphere (4). As Canadell & Raupach (2008) describes, the upper limit of carbon sequestration can be estimated by the carbon emitted from historical land conversion. If 75% of this came from forest conversion, they suggest, and therefore can be returned by reforestation over the course of the 21st century, a mitigation potential of about 1.5 Pg carbon annually can be calculated. This would reduce atmospheric CO2 concentration by 40 to 70 parts per million (ppm) by 2100. While these numbers are very imprecise, the method is still useful for providing estimates of global forest sequestration potential.
At a more localized scale, however, there can be great variability in sequestration potential of different forest types as well as between natural and production or plantation forests. This makes the calculation of sequestration potential challenging, as models and methods should be adjusted to specific forests and environments (5). For example, Vyskot et al. (2011) notes that, depending on various factors such as site conditions or species composition, the average biomass of production forests only reaches about 20-60% of biomass in natural forests.
The potential key role of forests in mitigation of anthropogenic climate change is widely acknowledged both by scientists and policy makers. International policies to ensure the protection and sustanable management of forests are emerging rapidly, with increasing focus on reforestation and afforestation activities. However, carbon sequestration by forests is a complex mechanism, effected by numerous factors, which makes the accurate calculation of potential and actual carbon sequestration challenging. This issue is especially relevant for policies and mechanisms that allow countries to untilize forest carbon sequestration to meet their emission reduction targets as failure to properly calculate carbon sequestration can lead to inappropriate global emission reductions.
Forests play a crucial role in the global carbon cycle. Through photosynthesis they assimilate carbon dioxide (CO2) and fix carbon (C) in their biomass releasing oxygen (O2) as a waste product. Forest ecosystems store an estimated 638 gigatonnes (Gt) of carbon, 283 Gt of which in forest biomass alone, which is approximately 50% more than all the carbon in the atmosphere. This makes forests the single largest terrestrial carbon sinks (1). As a result, minor changes in forest ecosystems can have great impacts on the carbon cycle and therefore on the concentration of CO2 in the atmosphere (2). According to some estimates, the terrestrial biosphere currently sequesters around 3 billion tons of anthropogenic carbon annually, 20-30% of global anthropogenic CO2 emissions (3). Forests, however, can switch between being a net sink of carbon to being a net source, depending on succession stage, disturbance or management regime.
While the role of undisturbed forests as climate protectors is not questioned, it is hard to determine both the forest carbon sink and reservoir that can be managed and the exact amount of carbon forests at any point may remove from the atmosphere (4). As Canadell & Raupach (2008) describes, the upper limit of carbon sequestration can be estimated by the carbon emitted from historical land conversion. If 75% of this came from forest conversion, they suggest, and therefore can be returned by reforestation over the course of the 21st century, a mitigation potential of about 1.5 Pg carbon annually can be calculated. This would reduce atmospheric CO2 concentration by 40 to 70 parts per million (ppm) by 2100. While these numbers are very imprecise, the method is still useful for providing estimates of global forest sequestration potential.
At a more localized scale, however, there can be great variability in sequestration potential of different forest types as well as between natural and production or plantation forests. This makes the calculation of sequestration potential challenging, as models and methods should be adjusted to specific forests and environments (5). For example, Vyskot et al. (2011) notes that, depending on various factors such as site conditions or species composition, the average biomass of production forests only reaches about 20-60% of biomass in natural forests.
Finally, carbon sequestration by forests can be affected by a number of physical, chemical and biological factors, such as climate warming, the concentration of CO2 or ozone in the atmosphere, respiration rates, nitrogen deposition, biological invasions, or management regime. This means that in order to provide accurate estimates of sequestration potential of forests over time, not only their present state but potential future environmental changes should also be considered.
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