As effective carbon sequesters, forests have a key role in curbing climate change. We have ways of safeguarding the diversity of forests and protecting their carbon sink and reservoir mechanisms, writes Professor Jaana Bäck.
Roughly two-thirds of the Earth's land area is covered by some kind of a forest. Discussion on climate change and ways of curbing it often revolves around forests, especially in Finland, one of the most forested countries in the world.
The reason for this is the excellent capacity of forests to capture carbon dioxide from the air and store it.
The carbon dioxide concentration of the atmosphere and Earth's temperature are interlinked: the more carbon dioxide there is in the atmosphere, the warmer the climate gets. Human action, especially that of industry and energy production, increases the carbon dioxide concentration, while the carbon cycle maintained by forests and other vegetation creates a vital counterweight to human activity.
This is why it is far from inconsequential what kind of forests and how many of them there are on the planet.
Forests serve as carbon sinks
A concept central to the climate effects of forests is a carbon sink, which refers to the amount of carbon dioxide captured by plants from the atmosphere in connection with photosynthesis. Carbon is absorbed by vegetation the most during the growing season, meaning that the carbon sink mechanism of forests is at its strongest in the summer.
In addition to trees, carbon is bound by dwarf shrubs and mosses that grow on the forest floor, whose role in the carbon balance of the entire ecosystem may even be surprisingly significant.
Among other things, the capacity of forest carbon sinks is affected by weather conditions, the structure of the forest's plant life and the vitality of its trees. Overall, the forests of the world absorb roughly 3.2 gigatons of carbon per year. Carbon emissions generated by fossil fuels and changes in land use amount to approximately 11 gigatons per year. In other words, a little under one-third of all carbon emissions generated in the course of a year are captured by forests.
Forests also store carbon
In terms of mitigating climate change, carbon reservoirs are a key factor alongside carbon sinks. Forests accumulate carbon as organic compounds in reservoirs, mainly in wood, ground litter and forest soil. At the moment, some 900 gigatons of carbon are stored in the Earth's forests, roughly the amount of carbon currently in the atmosphere.
The forests of the tropics and the soil of the boreal forests in the north constitute by far the largest carbon reservoirs.
Key to these reservoirs is their longevity. The better the conditions for the microbiota of the soil, the faster the carbon in rotting wood, ground litter and forest soil is released, as organisms involved in the degradation breathe it back into the atmosphere. Favourable conditions, that is warm and moist conditions, can multiply the quantity of the carbon dioxide produced by the degrading organisms.
The carbon reservoirs of boreal forest soil have a special role in curbing climate change. In cold climates, soil carbon is in a very stable form, slowly and steadily accumulating in the soil. In the hot and moist tropics, most of the carbon is cycled rapidly between the plants, the soil and the atmosphere. In contrast, the carbon stored in the soil of northern forests returns back to the air through natural degradation processes, on average, only in a few hundred years.
In addition to the carbon cycle, or carbon sinks and reservoirs, forests can directly affect the rate of progression of climate change in at least two other ways: they reflect solar radiation from Earth back to space and produce volatile vapours that form light clouds which cool down the atmosphere when they are condensed.
Climate change evident in forests
Consequently, forests are vital regulators of Earth's temperature, but how does global warming affect them?
Since the pre-industrial era, atmospheric carbon dioxide levels have grown by roughly 45%, while the mean global temperature has risen 0.8 degrees. At the moment, the carbon dioxide concentration is growing and the global temperature rising considerably faster than before. Due to their slow growth and longevity, trees are fairly poor at adapting to rapid change in their growth environment.
We do not yet know all of the ways in which climate change affects the growth of trees. Further research is needed on the topic.
Climate change may have some consequences that benefit forests. As a result of global warming, the period favourable to growth will be extended in both the spring and autumn: in recent decades, the onset of tree growth and budding has been observed to have moved three to five days earlier than before over 10 years, in addition to which there have been signs of a delay to the beginning of autumn.
This also means earlier activation of the carbon sink mechanism of photosynthesis, which may lead to increased net carbon capture for the entire growing period.
Warmer growing periods can also promote the spread of new plant species to regions where they have been less common because of cold winters and short periods of growth. For example in Finland, species with such potential include the oak, elm and ash, which are currently found in the wild only in the southernmost parts of the country.
At the same time, new plant species increase the diversity of habitats, but we may also lose some naturally occurring species in the process.
Drought and pests threaten forests
In order to effectively photosynthesise and grow, plants need not only carbon dioxide, but also water and other nutrients, which may become scarce due to climate change. As temperatures rise, droughts become more common. Forests stressed by drought are sensitive to a variety of destruction. As a result, the carbon sink mechanism is diminished, and in extreme conditions forests can turn into carbon sources, releasing stored carbon back into the atmosphere.
Already now, there are indications that mild winters can consume the energy resources of trees so that part of the carbon they have captured in the summer is released back into the air, which also affects growth the following summer.
In Europe, unprecedented droughts are already occurring, and they are expected to become increasingly common. For northern plants accustomed to cool and humid climate conditions, droughts and heatwaves pose a tough challenge. Forest fires are also forecast to become increasingly common and destructive; fires release huge amounts of carbon dioxide into the atmosphere, not only from trees but also from the soil where it has been sequestered for periods as long as thousands of years.
One of the clearest risks associated with climate change is the increasing abundance of naturally occurring pests and their spread to new areas. This has recently been experienced in Central Europe, where bark beetles have destroyed spruce woods in extensive areas after several successive dry summers.
Wind damage is also expected to increase. Both the intensity and frequency of storms can change in the future, and in connection with mild winters associated with unfrozen and wet soil, local storm damage can be substantial.
How are carbon sinks and reservoirs managed?
The preservation and cultivation of natural carbon sinks and reservoirs provided by forests are among the key means of eliminating carbon from the atmosphere. There is no other 'technology' that is as widespread and reliable, and which even offers other benefits at the same time, including preventing erosion, protecting water resources and providing recreation and experiences for all in equal measure.
How can the ingenious mechanisms of forest nature be put to as effective use as possible? On the one hand, the answer is complex, on the other, simple: by ensuring the functioning of diverse forest ecosystems. Only diverse ecosystems are resilient enough to face the risks posed by climate change. We also need to take care of the long-term persistence of forest carbon sinks and reservoirs.
Success requires multidisciplinary and diverse understanding of both climate change as well as the functioning of forests, forest management and potential additional necessary maintenance measures. We have to be prepared to take a new perspective on forests and demand comprehensive planning instead of partial optimisation.
The scholarly community is fairly unanimous on the central role of forests in curbing climate change. Instead, there is disagreement about the means needed to reach the goal. Some of these differences of opinion stem from different timelines associated with the measures.
Action to slow down climate change must be taken right now, but the time span in the planning of forest management often extends over the entire turnover time of commercial forests, which is usually several decades.
However, climate-conscious forest owners or land-use planners have at their disposal a number of methods for protecting carbon sinks and reservoirs: land areas can be reforested, and tree species that effectively bind carbon can be chosen for cultivation.
Commercial forests can be maintained in wooded condition, while thinning can be optimised and logging delayed. The biodiversity of forest nature can be safeguarded by stopping the destruction of habitats and species.
These are ways of both mitigating risks and increasing carbon sinks. There is no single answer to which of these means should be utilised at any given time, nor should there be. The same solution does not apply everywhere, and this diversity is easily overlooked in the discussion.
Centuries-long utilisation of felled trees
One of the methods in the toolbox is prolonging the period that cut wood serves as a carbon storage. While using wood as a bio-based renewable fuel and a substitute for fossil fuels is an attractive idea, the felling of trees destroys carbon sinks and reservoirs, especially if they are logged only to be burned.
At the moment, only one-sixth of logged wood ends up in a slightly longer cycle as raw material for the sawmill and construction industries. The lifespan of carbon in wood harvested from forests is very short, in some cases only five years. This means that the carbon captured by a tree during its growth in a forest has returned to the atmosphere within five years of logging.
Whatever we do in commercial forests, it is important to make sure that the wood harvested forms as long-lived a carbon reservoir as possible. The final product can be a timber building, whose lifespan is calculated in hundreds of years, another durable good, or a book manufactured from recycled material. The key is to considerably extend the current lifespan of products.
