Climate change threatens the carbon storages in forest soil

As soil temperature rises, forest litter decomposes more quickly. This means that more carbon dioxide is released into the atmosphere.

An ordinary person who goes out to a Finnish upland forest walks on fallen leaves, needles, shrubs and grasses and stumbles on branches. A soil researcher walks on an important, centuries-old carbon storage.

One square metre of Finnish upland forest – the trees and ground vegetation together – sequesters slightly under 400 grams of carbon every year. This comes up to 4,000 kilos per each hectare. Some of this carbon will be stored in the growth; and some falls down in litter, such as dead leaves, needles and so on.

Trees as well as the ground vegetation produce forest litter. The biomass of the ground vegetation makes up only a couple of percent of the forest biomass, but it produces as much as one fifth of the forest litter.

This is because a large part of the ground vegetation is annual: it dies in the autumn and grows up again the next spring. As for trees, only a fraction of their biomass falls down to the ground every year, explains Mr. Jari Liski, Senior Researcher in the Finnish Environment Institute.

Slow but steady growth of carbon storage
In forest soil, bacteria and fungi break down forest litter with their enzymes into a form more useful to them. At the same time, some of the carbon sequestered in the litter is released back into the atmosphere as carbon dioxide.

The activity of the bacteria and fungi depends on the temperature. The warmer the day, the faster the decomposing. The climate change increases the decomposition process by two routes: the days will be warmer and the number of warm days will increase.

How much litter is formed and how much of it decomposes, depends on the growing tree stock and ground vegetation, for example. The more litter is left undecomposed, the faster the carbon storage in the soil grows. According to Liski, the carbon storage grows fastest in forests of 40–50 years of age.

Speed is a relative concept, however. In an old forest, where growth and decay proceed at the same rate, all of the 400 grams sequestered annually can fall down. Only a few grams, ten at most, are left undecomposed per square metre per year.

To a layman this may seem like quite a slow rate: ten grams of carbon per year per square metre. “Well, yes, so it seems, but forest soil is an extensive and important carbon storage. Part of the forest litter decomposes so slowly that it is actually difficult to measure. The average age of the carbon in forest soil is several hundred years,” Liski says.

Soil can turn from sink to carbon source
If the Finnish climate warms up by five degrees Celsius as predicted, the speed of decomposition will be 1.5 times the current rate, as was found in a study that was made jointly by the Finnish Environment Institute and the Finnish Forest Research Institute and funded by the Academy of Finland.

This would mean that nearly 600 grams of carbon per square metre would be released into the atmosphere annually. The soil of Finnish upland forests would turn from a carbon sink into a carbon source.

Luckily, the growth of forests and forest litter production are also predicted to increase. If the forest litter production increases at the same rate as the decomposition process accelerates, the current situation will be maintained and the forest soil will remain a carbon sink.

However, other factors make the situation more complex. As the climate gets warmer, the litter that has been preserved in the forest soil for hundreds of years may start to decompose. If this happens, the calculations have to be changed radically.

The problem is that no one knows how sensitive old forest litter is to temperature changes. It is also difficult to measure and model its decomposition rate.

Forest litter production must increase
Mitigating the climate change requires fossil fuels to be replaced by renewable energy sources, such as wood. But wood used as fuel can also be regarded as forest litter removed.

Liski says that in the future, when assessing the amount of logging residue that should be left in the forests, factors to be taken into account include its impact on the carbon stored in the soil and the increasing rate of decomposition. This will be a challenge, as measuring the amount of carbon stored in the soil and its changes is both laborious and expensive.

Researchers in several countries are developing models to predict the changes in the amount of carbon in soil. However, even the best models contain uncertainties.

Liski points out that the United Nation’s climate goals include the requirement that the carbon storages in soil must not diminish if a country wants to benefit from the forests’ ability to sequester carbon when determining its carbon dioxide emissions.

Liski’s advice to decision-makers is not to play a zero-sum game. The amount of forest litter and logging residue left in forests must be more than the decomposition capacity of bacteria and fungi. Why is this? Because the models used to calculate the decomposition rate could well be wrong by as much as is allowed by their margin of error.

By Krista Kimmo

Taittoelementti, vihreš vaaka pisteviiva

WWW-sivustoThe research programme's homepage
Link to another page in FOREST.FI service Growing forest stores carbon in trees and also to the soil

Slow but steady
growth of carbon storage

Soil can turn from
sink to carbon source

Forest litter
production must increase

Links to further information

ari Liski. Photo: Finnish Environment Institute.
 About twice as much carbon is sequestered in the forest soil in Finland as in trees, says Jari Liski, Senior Researcher in the Finnish Environment Institute.

Jari Liski. Photo: Markku Vuorikari
The Finnish forest soil is sensitive to changes in temperature, as was found in a joint study by the Finnish Environment Institute and the Finnish Forest Research Institute. In a warmer soil, the decomposition of forest litter speeds up and the amount of carbon dioxide released into the atmosphere increases, Jari Liski explains. An increase of five degrees Celsius in the average temperature would increase the decomposition rate to 1.5 times the current one.
Photo: Markku Vuorikari
Publisher: Finnish Forest Association, 08/15/2008

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