Download Impacts of forest management practices on forest carbon

Impacts of forest management practices on forest carbon
Valeria Gelman, Ville Hulkkonen, Roni Kantola, Mitja Nousiainen,
Vesa Nousiainen, Michael Poku-Marboah
HENVI Workshop 2013: Interdisciplinary approach to forests and climate
change
Helsinki University Centre for Environment, HENVI
University of Helsinki
8.4.2013
ABSTRACT
Forest management is an important activity that affects the global carbon stock, and
therefore needs to be studied to further understand how its different practices can aid in
greenhouse gas reduction efforts. Attaining sustainable harvesting potentials without
sacrificing forests' ability to store carbon is very complicated. This report examines forest
management practices and forest carbon in relation to possible climate change mitigation.
Based on the review of scientific literature regarding forest systems' carbon sequestration,
we discuss implications of the results of different forest management practices in the world
and in Finland, and also touch upon the legislation as a regulatory measure. We conclude
that carbon sequestration can be increased by management techniques such as prolonged
rotations, increased thinning, continuous forest cover, supporting litter production and
natural ecological conditions, keeping the right water level and cleaning the high emission
ditches in peatlands. Preservation of large carbon stock of boreal forest soil is crucial,
whereas in tropical forests, preventing deforestation is more relevant. In peatlands, forestrydraining can have a negative effect on the soil carbon pool in the long run in fertile sites.
Carbon sequestration could be increased by storing carbon in wood products and using it as
an alternative to fossil fuels. Forest management usually focuses on increasing the forest
productivity and growing stock. However, forest management practices should be
optimized for other factors too, such as the ecological quality of the forests and the soil
carbon balance.
TABLE OF CONTENTS
1. Introduction .................................................................................................................................. 4
2. Global terrestrial carbon stocks ...................................................................................................4
3. Connections between forest management and carbon sequestration in Finland’s forests .......... 5
4. Forest growth and forests as a carbon sink .................................................................................. 7
5. Forest management impacts on soil carbon ................................................................................. 9
5.1. Background on the belowground carbon cycle........................................................................................ 9
5.2. Soil carbon pool................................................................................................................................... 10
5.3. Stabilization of soil carbon ................................................................................................................... 10
5.5. Site preparation ................................................................................................................................... 11
6. Forestry and forest carbon in peatlands..................................................................................... 11
7. Forest management impacts on climate change and best solutions for policy making ............. 14
7.1. What to do with the forests and deforested areas? ................................................................................. 14
7.2. Global agreements on forest management ............................................................................................ 15
8. Conclusions ................................................................................................................................. 16
References ....................................................................................................................................... 18
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1. Introduction
In light of growing awareness of environmental changes related to climate change, the issue of
carbon balance, as one of the main greenhouse gases, is of extreme importance. The increase of
deforestation due to urbanization effects and agriculture, coupled with the continuous discussions
on climate change and how it may affect the wellbeing of the Earth’s ecosystem, have generated
increased attention to forests, as major regulators of the carbon pool. Forest systems worldwide
have the ability to store carbon and therefore mitigate the effects of climate change.
This report explains the essential facts about carbon stocks, as well as basic forest system functions
in regards to the carbon cycle. It also investigates the effects of forest management practices in the
world and in Finland that effect carbon balance. Further, it pays a detailed attention to evaluating
peatlands as crucial players in carbon stock regulation. Finally, there is a discussion on legislative
issues that control forest management and forestry production, that consequently affect the state of
carbon balance. In recent decades, decision makers have become more aware of forest management
practices effecting carbon stocks, and are increasingly integrating forestry agenda into
environmental and climate change regulations.
The research covered in this report demonstrates the importance of appropriate forest management
practices in carbon stock regulation worldwide, and illustrates the challenges in establishing the
balance between sustainable forest industry production, carbon balance and the ecological quality of
forest systems.
2. Global terrestrial carbon stocks
Carbon stock is defined as the quantity of carbon in a “pool”, meaning a reservoir or system which
has the capacity to accumulate or release carbon (FAO, 2005). Carbon may be stored in reservoirs
through physiochemical and biological processes. Depending on the context, a “pool” could be a
vegetation zone, an entire country or a land area (WGBU, 1998). In the context of our topic
“Impacts of Forest Management Practices on Forest Carbon”, carbon pools may mean living
biomass (including above and below-ground biomass); dead organic matter (including dead wood
and litter); and soil carbon (soils organic matter).
The removal of atmospheric carbon and storing it in the terrestrial biosphere is one of the main
options that have been proposed to compensate for greenhouse gas emissions. The Kyoto Protocol
recognized that some terrestrial ecosystems have the potential to sequester large amounts of carbon
and thus slow down the increase of atmospheric carbon dioxide concentrations (Ardö and Olsson,
2004). This has made the potential of systems to release (serve as a source) or assimilate (serve as a
sink) carbon - crucial in the climate change debate.
Studies outlined in the GRID-Arendal/UNEP (2013) indicate that, generally, the largest amounts of
carbon are stored in the tropics and in high latitude ecosystems. In the tropics carbon is stored
mostly as biomass, and in high latitude ecosystems carbon is primarily stored in permanently frozen
layers of soil (permafrost) and in peat. However, different ecosystem types store different amounts
of carbon depending on their species compositions, soil types, climate and other features. Compared
to the atmosphere, terrestrial ecosystems store about three times as much carbon, which is stored in
living organisms, litter and soil organic matter. It is estimated that the quantity of carbon stored in
the terrestrial ecosystem is 2100 Gt C (GRID-Arendal/UNEP, 2013).
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There is an uneven distribution of the Earth’s carbon stocks over its land area. Countries with colder
climate (Annex I countries, industrialized countries and countries with economies in transition) in
the Kyoto Protocol tend to have more carbon particularly in their soil. They constitute about a third
of the terrestrial land surface, yet they also contain approximately 50% of terrestrial carbon, mainly
in the soil. On the whole, the ratio of terrestrial carbon storage between vegetation and soil and its
organic layer is 1:4 (WGBU, 1998).
The forest ecosystem is no different in terms of carbon stock distribution. Out of a total of 46%
carbon stock in the forest ecosystem, 39% is stored in forest soils, including their organic layer.
Nevertheless, there are distinctions between tropical and boreal forests in this regard. In the tropics,
50% of the carbon stock is stored in the soil whereas in boreal forests the amount of carbon stored
in the soil is 84%. The boreal forests of Russia, Canada, and Alaska account for about half of global
forest carbon while the tropical forests together account for 37%.
The global proportion of temperate grasslands and savanna land also play an important role in
regards to carbon stock. Global temperate grasslands and savanna occupy approximately 23% of the
land surface and serve as a reservoir for 26% of the Earth’s terrestrial carbon stocks. By
comparison, temperate grassland has 2 - 4 times more carbon pool per unit area than temperate
savanna. Contrary to forests, the major part of the carbon pool of temperate grasslands and savannas
is stored in the soil rather than biomass (Atijay et al., 1979; Houghton, 1995).
The wetlands ecosystem has been defined differently by different stakeholders, however,
irrespective of the definition that is adopted, it covers between 3–6% of the Earth’s surface. In spite
of their relatively small area of coverage, wetlands contain a disproportionately large amount of
global terrestrial carbon stock. They are credited with holding 10–30% of global terrestrial carbon
(Lugo et al., 1990; IPCC, 1996b; Mitsch and Wu, 1995). This makes them a much more significant
player in carbon stock storage compared to forests. The ratio of carbon stock storage between
wetlands and forests, in terms of unit area, is 3:1 (Mitsch and Wu, 1995). Peatlands are credited
with holding soil carbon stocks of 541 Gt C, which is equivalent to 34.6% of total terrestrial soil
carbon. This excludes its biomass which holds 25.7 Gt C. (Zoltai and Martikainen, 1996).
The carbon stock of a “pool” is not absolute but rather depends on the carbon flux which is defined
as the mass of carbon per unit time that is absorbed by the “pool” or that is released into the
atmosphere by the “pool” (FAO, 2005).
3. Connections between forest management and carbon sequestration in
Finland’s forests
Improving rates of carbon sequestration is essential in the process of climate change mitigation.
Forest systems are powerful carbon sinks that exchange with the atmosphere 7 times the amount of
anthropogenic carbon emissions annually (Jandl et al., 2007). At the present, forests in Europe
absorb 7-12% of total carbon emissions, and this needs to be increased to comply with international
regulations like the Kyoto Protocol, and offset global warming effects (Jandl et al., 2007).
Afforestation efforts and expansion of forest areas can be constrained by area limitations, therefore,
other methods that stimulate carbon uptake, such as certain forest management techniques, have to
be taken into consideration as possible solutions.
Currently, Finland is the most forested country in Europe with 23% of its territory covered by trees
(Schuck et al., 2002). Nonetheless, this has not always been the case. About two centuries ago
forests in Finland were severely degraded, which was mostly the result of agricultural practices and
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felling of trees for profit. Pastoral and agricultural fields have traditionally been cleared by slash
and burn techniques, and no forest management practices were applied in commercial logging
(Kauppi et al., 2010). Additionally, natural disasters also contributed to the devastation, and wild
fires, often caused by men, regularly ravaged through Finland’s forests (Kauppi et al., 2010).
Since those days, the situation has been turned around, even though it took nearly two hundred
years to achieve the point of recovery. The need for sustainable forestry that could serve as a
continuous, renewable source of income prompted the birth and expansion of silviculture. The
implementation of basic forest management practices, coupled with the later abatement of slash and
burn agriculture, allowed for the slow regeneration of Finland’s forests. Forest fires and other
natural disasters have been largely prevented, and effective management has increased growth rates.
Furthermore, the striking increase of tree growth in Finland in recent decades was caused by active
programs established during the preceding decades involving well-planned harvesting operations,
effective regeneration of forest stands, fertilization, and peatland drainage (Kauppi et al., 2010).
According to Kauppi et al. (2010), the growing stock more than doubled from 1.6 to 3.4 million m3
between 1912 and 2005 in forests on an area of 387 km2 in southern Finland.
All of these changes had direct influences on the biomass carbon stock, according to the study by
Liski et al. (2006), who examined carbon accumulation in Finland’s forests from 1922 to 2004.
Increased levels of carbon sequestration into tree biomass in Finnish forests have been achieved
through several mechanisms which include: a recovery of understocked stands towards the fullstocking potential; a shift in forest management decisions about harvesting levels, harvest sites,
rotation lengths and forest regeneration decisions, actions which affect forest tree demography; and
expansion of forest areas (Kauppi et al., 2010).
The rehabilitated forest conditions and increased growing stock resulted in higher levels of carbon
sequestration from the atmosphere, as well as litterproduction, which, consequently led to the
additional increase in accumulation of carbon in litter and soil. The study by Liski et al. (2006)
suggests that the total biomass carbon stock increased by 50% and the forest area expanded by 16%
in the 82 years (1922 - 2004) when most of the forest management approaches have changed. It is
important to note that the economic interests in timber production were the motivation that
triggered an improvement of forest management. During the forest management transition time (end
of 19th, beginning of 20th century), there was no consideration of biodiversity, carbon sequestration,
water resources protection, or recreation as a management objective (Kauppi et al., 2010). Hence,
both the expansion of forested areas and the increased carbon density in Finland’s forests were an
unintended co-benefit of forest management, which was motivated by the potential of sustainable
timber harvests, improved timber yield, and higher profits (Kauppi et al., 2010). Nonetheless, Liski
et al. 2006, conclude that looking at the history of Finland’s forests, it could be implied that
appropriately managed timber production can actually be beneficial for the carbon balance of
forests.
The abatement of selective logging of the oldest trees was one of the pivotal changes in Finnish
forest management approaches in efforts to ensure effective sustainable productivity of forests. This
practice was replaced with even aged silviculture with rotation length on average of 80 years, which
decreased the share of forests older than 100 years by 39 % from 1950 to 2010 (Vilén et al., 2012).
Therefore, Vilén et al. (2012) argue that the raise in carbon stock in Finland is not due to the
changing age of trees, but can rather be attributed to other factors. These may include changes in
forest management practices, recovery from the legacy of irresponsible forestry of the 18th and 19th
centuries, and increased N deposition and atmospheric CO2 concentration, forest tree species
composition, forest density and possibly other factors (Vilén et al., 2012). The authors also note
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that, most likely, at the beginning of the century European forests were less dense and perhaps also
more diverse in age compared to the forest regeneration that replaced them after the regeneration
cut (Vilén et al., 2012). Between 1950 and 1980 the share of old forests declined continuously, but
because low density forest stands were replaced by dense and fast growing young and medium aged
forests, the average carbon stock increased despite of the declining average age (Vilén et al., 2011).
It is necessary to note, however, that older forests that are allowed to grow to their maximum
biomass have a carbon stock up to three times higher compared to the forests that are managed for
maximum sustained yields and financial returns (Karjalainen, 1996). Calculations on carbon
balance may vary drastically, depending on whether carbon stock in wood products is added to the
equation or not (Karjalainen, 1996).
As a result of improved forestry methods and peatland drainage, Finland’s forests currently contain
more wood than they did at the beginning of the century. Peatland drainage, fertilization, successful
implementation of well planned harvesting operations, and effective regeneration of forest stands
have finally reached their peak in the 1970s, due to which there has been a spike in forest
productivity and coverage in the latest decades (Liski et al., 2006). In the last twenty years,
however, several important legislative steps have been taken to ensure sustainable future for forests
in Finland, despite continuous commercial forestry activities. As a result such heavy-handed
practices like peatland drainage, deep ploughing of forest soil and using herbicides to kill
undergrowth have been banned. Additionally, habitats of importance for the preservation of
biodiversity are now excluded from forestry and felling operations, and both living and dead trees
have been left in felled areas to promote natural regeneration (Liski et al., 2006). Nonetheless,
Finnish forest management is far from perfect from the ecological point of view and improved
collaboration between foresters, scientists and legislators is needed to ensure that the balance
between forests’ quality, forest industry, and forest carbon balance is achieved.
There are some forest management practices, identified in the study by Jandl et al. (2007) that are
clearly positive or negative for carbon sequestration. Therefore, carbon stock can be regulated with
appropriate forest management techniques. Practices like prolonged rotations and increased rate of
thinning could enhance carbon sink ability of forest systems (Jandl et al. 2007). In general, more
nature-oriented silviculture and continuous cover forestry with fewer canopy openings, and a high
rate of above ground and belowground litterproduction have a positive effect on carbon balance
(Jandl et al. 2007).
4. Forest growth and forests as a carbon sink
The basic requirements for photosynthesis are water (H2O), carbon dioxide (CO2) and solar
radiation. During photosynthesis, plants absorb CO2 from the atmosphere through stomata and use
the energy from visible light to oxidize water. When stomata are open, a plant loses water because it
is evaporating at the same time. By oxidizing water, a plant is able to form an electron flow and that
flow of electrons is used as a reducing power to convert NADP+ to NADPH. At the same time, H+
protons are moving from lumen to stroma through the ATP synthase generating ATP. ATP and
NADPH can be used to make carbohydrates in the Calvin cycle (Taiz and Zaiger, 2010). In addition
to water, CO2 and solar radiation, other factors also affect the photosynthetic activity, e.g. the
amount of foliage, the light use efficiency of the foliage and temperature, and water and nutrient
availability of the soil (Boisvenue and Running, 2006). A growing plant is always a carbon sink
because in order for it to be alive it needs to maintain a positive net carbon balance. The trunks of
live trees act as carbon sinks. In other words, the productivity of the plant has to be higher than its
respiration. Soil, however, is also very important, and when intact in forest, it holds significant
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amounts of carbon (Jandl et al. 2007). Finally, forests occupy ca. 52 % of Earth’s land surface
(Boisvenue and Running, 2006).
Kolari et al., (2004) investigated managed different aged Scots pine (Pinus sylvestris) stands in
southern Finland to investigate the carbon fluxes in different aged stands between 2000 and 2002.
The sites were 4, 12, 40 and 75 year old forest stands. With eddy covariance and chamber
measurements they measured the productivity of the whole forest ecosystem. During the growing
season, the photosynthetic capacity of the 4 year old site often exceeded the respiration coming
from the soil, however overall, the site was a carbon source of 400 g of C m-2 a-1. The maximum
photosynthetic capacity was 8 µmol m-2 s-1 in the 4 year old stand site, while the average value for
older sites was 12 µmol m-2 s-1. The 40 and 75 year old sites were carbon sinks with net ecosystem
exchange (NEE) of -192 and -323 g C m-1 a-1 (negative value indicating carbon intake). The 12 year
old sapling site was becoming a carbon sink with the NEE of -24 g C m-1 a-1. During winter, all of
the forests were sources of carbon (Kolari et al. 2004). In this study, the large part of carbon
fluctuations of the sites were estimated with a model and measurements were taken only during the
3.5 summer months. It is important to note that the NEE of the 75 year old stand was smaller (the
sink was larger) than that of the 40 year old stand, despite the fact that it was labelled as old and
near the end of its rotation cycle.
Similar results were obtained from a study from central Oregon USA, where Law et al. (2003)
studied a chronosequence of ponderosa pine (Pinius ponderosa var. Law). They divided stands into
four groups - initiation (ages from 9-23 years), young (ages from 56-89 years), mature (ages from
95-106 years) and old (ages from 190-316 years) stands. The averaged net ecosystem productivity
(NEP) in those stands were -124 g C m-1 a-1 in the initiation (mean age 20 years) stands, 118 g C m1 -1
a in the young (mean age 70 years) stands, 170 g C m-1 a-1 in the mature (mean age 100 years)
stands and 34 g C m-1 a-1 in the old (mean age 250 years) stands. These results indicate that the old
ponderosa pine stands are fairly poor carbon sinks (in NEP negative values indicate loss of carbon).
Law et al. (2003) conclude that the maximum NEP in ponderosa pine is between 150-200 years and
declines after that. Ponderosa pine initiation stands are still a carbon source despite some of them
being 20 years old or older compared to young Scots pine stands measured in southern Finland. The
semi arid climate of the area might be responsible for this, as seedlings grow very slowly in such
conditions. The total ecosystem carbon storage and above ground biomass grows rapidly until the
stand age of 150-200 years and does not decline after that. This means that old ponderosa pine
forests do not lose carbon when they grow older, but that the sequestering of the carbon becomes
slower. The rotation length in forest economy for ponderosa pine has traditionally been 80-120
years (Law et al. 2003) allowing for the forest could accumulate more carbon to the ecosystem.
Taking into consideration the results of the study, the rotation length should be extended, for
example to 150-200 years, if the goal is to sequester carbon to the maximum potential of the
ponderosa pine forests.
Different areas around the northern hemisphere vary greatly in their ability to sustain forest growth
and there are significant differences in the NEE and net primary productivity (NPP) mainly due to
climatic and nutrient variability (Schulze et al., 1999). For instance, Scots pine grows much faster
in European Russia than in Siberia (Schulze et al. 1999). According to Schulze et al. (1999) the
NPP in Siberian boreal forest is only 123 g C m-2 a-1 compared to the European deciduous forests
with the NPP of 460 g C m-2 a-1.
The impact of climate change on forest productivity is an interesting issue, which is receiving more
and more scientific attention. For decades scientists have been documenting the rate of forest
growth and recently Boisvenue and Running (2006) reviewed studies from the middle of 20th
century to the present. They also analyzed satellite information to evaluate forest productivity. The
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results revealed that forest productivity has increased in areas with abundance of water and also on
sites (less than 7 %) where water is scarce, thus limiting the growth of vegetation. There are many
reasons for why the productivity has increased. Boisvenue and Running (2006) mention that there is
a trend of increasing amount of growing degree days (the average temperature over 5 Celsius which
there are 13% more in British Columbia) in northern hemisphere which can be seen e.g. from more
productive forests in southern Alaska and Canada. Also the amount of CO2 in the atmosphere has
increased which most probably positively affects the growth of forests. In FACE (Free air
concentration enrichment) experiments, where forest sites were grown in elevated CO2 of 550 ppm,
the median NPP was 23% higher compared to sites with CO2 of 370 ppm (Norby et al., 2005).
When people think about how forests act as a carbon sink they usually think about young forests
that grow rapidly and the general thought has been that old forests are carbon neutral and
insignificant in their ability to function as carbon sinks, as their NPP declines drastically (Luyssaert
et al., 2008). According to Luyssaert et al. (2008), this claim is based on a single ten-year study
(Kira, T. and Sihdei, T, 1967) and does not apply to all ecoregions. Luyssaert et al. (2008)
evaluated the productivity of old growth forests and the result demonstrated that forests over 200
year old continue to sequester carbon and their biomass increases in boreal and temperate forests for
centuries. The authors estimated that the maximum amount of biomass per hectare would be around
500 and 700 tons. 15% of the northern temperate and boreal forests are unmanaged, but they might
be responsible for 10% of the NEP in the world, according to their analysis by Luyssaert et al.
(2008). Disturbing that kind of a pristine forest area might be detrimental for several reasons,
including converting the area from a carbon sink to a massive carbon source.
Trees are the responsible autotrophic organisms that produce litter onto soil and this litter is the
basis of the soil carbon pool. There is not a single opinion on how different tree species affect the
soil carbon pool. Coniferous trees have more shallow roots vs. deciduous trees. Due to this fact,
oaks, for example, contribute more to the soil carbon stock than spruces because their roots go
deeper to the ground below organic layer and therefore do not decompose that easily (Jandl et al.
2007). Mixed tree stands are assumed to be more productive because they can fill in different niches
and are expected to be more tolerant against pests and diseases (Jandl et al. 2007).
5. Forest management impacts on soil carbon
5.1. Background on the belowground carbon cycle
Litter produced by plants, such as dead roots, leaves, branches and stumps, is deposited on the
ground and becomes a part of soil, supporting a complex food web of organisms. Different fungi,
bacteria and invertebrates have their own specialized functions in the consumption and
decomposition chains, where each organism preferentially breaks down only certain kind of organic
compounds. Most of the energy released during the decomposition process is used in respiration:
CO2, H2O and nutrients are released as byproducts of this process. Eventually, the compounds that
are most resistant to decomposition become soil humus and enter the soils stable carbon (C) pool. In
addition to the C flux from plant litter to the soil, there is also a direct C flux from trees to the
fungal part of mycorrhizal roots. This process is called rhizodeposition.
Coarse roots, that anchor the tree to the ground and transmit nutrients and water to the foliage and
photosynthate to fine roots, usually begin to decompose only after the tree is dead. Fine roots,
however, are in a constant process of growing and dying, so they can acquire water and nutrients
effectively. Studying the exact amount of C flux from fine roots is difficult due to their
inaccessibility and short life span (Lukac and Godbold, 2011).
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Soil releases carbon in a process called soil respiration, which is the combined respiration of all
organisms in the soil. It can be divided into autotrofic and heterotrofic respiration. Autotrofic
respiration is done by plants, while heterotrofic respiration is done by microbes and animals during
the decomposition process. Soil respiration is a complex process and is difficult to study, as it is
affected by many factors such as temperature, humidity, the amount of carbon, the amount of roots,
the structure of microbial populations, the physical and chemical properties of the soil, and the plant
growth rate (Lukac and Godbold, 2011).
5.2. Soil carbon pool
The total soil C pool is determined by the balance between soil respiration, where C is released
from the soil, and the incoming C from litterfall and rhizodeposition. Both of these factors can be
influenced by forest management practices such as thinning, harvesting, soil preparation and
fertilization. Therefore forest management practices can have an effect on soil carbon sequestration
(Jandl et al., 2007).
The actual turnover rate of soil organic matter differs greatly between regions. In boreal forests it is
limited by the short growing season, in peatland forests it is restricted by excess moisture, and in the
Mediterranean systems summer droughts inhibit the soil C cycle (Jandl et al., 2007).
As the global temperature rises, both the primary productivity and the decomposition of organic
matter are expected to accelerate, so the total turnover rate of C would rise. One meta-analysis by
Rustad et al. (2001) concluded that warming stimulates soil respiration slightly more than plant
production. This will lead to the loss of soil C in the long run, and would turn forest soils from
carbon sinks to carbon sources. It has been hypothesized that soil respiration would be stimulated
by warming more in colder regions, but the meta-analysis found no proof for this. A 10 year study
in an even aged mixed hardwood forest in central Massachusetts suggests that the loss of soil C is
only temporary: the labile soil C pool is exhausted but the stable soil C will not be affected by
temperature. The quality of soil organic matter affects the rate of soil respiration: labile soil C
fractions are quickly mineralized when the temperature is warm enough, but stable soil C fractions
are not affected by temperature changes (Melillo et al., 2002).
5.3. Stabilization of soil carbon
Soil organic matter can stabilize when it is not available for decomposition. This can happen due to
the inherent recalcitrant quality of the compounds as they are bound at oxide and clay mineral
surfaces, or simply the inaccessibility of the soil organic matter for the decomposers. In some soils
C can stabilize more easily than in others. Additionally, reactive surfaces in clay minerals and
oxides allow C to form complexes with a low turnover rate (Jandl et al., 2007).
An interesting study of a 100-year-old soil archive from the Russian steppe shows minimal changes
over a century in the stable C stock, despite cultivation and climate change (Torn et al., 2002). The
single most defining factor of the soil C pool was the amount of amorphous minerals. It appears that
once the soil reaches equilibrium between the C input and the output, the stable C pool does not
change even with great changes in land use and climate. If that is the case, soil properties would
play a stronger role in the stabilization of soil C than the productivity of a site. 13C tracer
experiments proved that loamy soils with low productivity forest accumulated more new treederived C than sandy soils with a higher NPP (Hagedorn et al., 2003).
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5.4. Nitrogen fertilization and Liming
Nitrogen (N) is most commonly used in forest fertilization in boreal areas, as it is usually the
nutrient that limits tree growth. Therefore, fertilizing with N will potentially increase the input of
organic matter into soil through increased litterfall and rhizodeposition. However, fertilization also
stimulates decomposition by increasing nitrogen content of the litter material (Fog, 1988). On the
other hand, many studies have indicated that the input of N decreases the decomposition rate of
more recalcitrant organic compounds (Fog, 1988; Berg and Matzner, 1997; Magill and Aber, 1998;
Hagedorn et al., 2003). In other words, N fertilization could increase the decomposition of new
organic matter and slow down the decomposition of older, more stable compounds. This would
increase the amount of stable humus in soils, thus increasing the stable soil C pool.
N fertilization has been shown to have widely varied effects on the soil C pool. A meta-analysis of
48 experiments concluded that a significant increase of soil C was found in the upper mineral soil
and the total soil pool after both mineral N fertilization and capturing of N by N-fixing plants
(Johnson and Curtis, 2001). The C/N rate has been observed to decrease, probably owing to the fact
that soils are more efficient at retaining N than sequestering C (Johnson and Curtis, 2001).
Liming is the application of calcium- and magnesium rich materials into the soil, and it has been
used in Central and Northern Europe to neutralize soil acidity and mobilize slowly decomposing
organic material. This is however in conflict with the target of C sequestration. One literature
review showed that liming caused a net loss of C in temperate and boreal forests (Lundström et al.,
2003).
5.5. Site preparation
Site preparation includes an array of methods to prepare soil for the establishment of a new tree
generation. Most of the methods cause a great disturbance to soil by exposing the mineral layer and
mixing different soil layers. This stimulates the decomposition of soil organic matter and nutrients
are released (Johansson, 1994). The increased soil respiration may decrease the soil C pool. Some
studies indicate that the loss of C increases with the intensity of soil disturbance (Johansson, 1994;
Örlander et al., 1996). However, as site preparation also increases biomass production by
supporting the growth of seedlings, the overall effect might balance or even outweigh the loss of
soil C when looking at the total ecosystem.
6. Forestry and forest carbon in peatlands
The total C pool of global peatlands is estimated to be over 600 GtC, which is over one third of the
total soil C pools. The sink intensity of peatlands has varied over time, but over the Holocene, they
have been a sink of 5 GtC per century on average. The C pool is 547 (range 473–621) GtC in
northern peatlands, 50 (44–55) GtC in tropical, and 15 (13–18) GtC in southern peatlands (mostly
in Patagonia, southern South America) (Yu et al., 2010).
Peatland forestry often includes drainage by ditching. Approximately 4 % of the total area of
northern peatlands has been drained for forestry (Minkkinen et al. 2008). Undrained peatland
forestry is also practiced, mostly in North America, but it is not included in the estimates. In
Finland, ca. 55 % of peatlands are drained for forestry, comprising 25 % of the total land area used
for forestry (Minkkinen et al., 2008).
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When peatlands are drained for forestry purposes, carbon dioxide (CO2) emissions may increase
and methane (CH4) emissions decrease. In the short term, the overall climate impact in forestrydrained boreal peatlands is cooling, because of the increased tree growth. However, if the peat layer
starts to degrade, there will be loss of carbon in the long term (Minkkinen et al., 2008).
Drainage increases the depth of the oxic peat layer, and the presence of oxygen accelerates
decomposition of peat and can lead to carbon loss in soil. In contrast, it also increases the growth of
trees, which sequester carbon in large quantities. Therefore, the net primary production (NPP) and
litter production increases. In Finnish peatlands, on average, the speed of decomposition is lower
than the peat production. The C balance varies from the loss of 120 g C m-2 yr-1 to the sink of 320 g
C m-2 yr-1 (Minkkinen, 1999).
Fertile soil, on average, is a CO2 source in forestry-drained boreal peatlands, so the peat layer
degrades. Production of CO2 in fertile soils increases if the water table deepens or temperature rises.
They are originally fens, which are minerotrophic peatlands, which means that their water supply
comes mostly from streams. They usually have a high water table level, so the change in water table
level can be dramatic. Fertility increases production of CO2 probably because nutrient availability in
peat is better than at the poor sites. Peat is easier to decompose, and because the bulk density of peat
is higher than at the poor sites, there is more peat to decompose in the oxic layer, and the ecological
changes are more drastic than at the poor sites, where typical peatland vegetation, like the C
accumulating moss layer, remains (Ojanen et al., 2012).
Nutrient poor peatlands accumulate peat even in the case of drainage. This is because trees need to
allocate more C to root systems, and moss and other ground layer production is higher because of
the open tree stand that allows light penetration. Also, high decomposition rate requires many
nutrients which are absent. Drainage on nutrient poor sites is often weaker because they are mostly
bogs which are ombrotrophic peatlands, which means that they are rain-fed, and lower tree growth
and respiration are not sufficient to compete with precipitation (Ojanen et al., 2012).
The net emissions of CH4 from peatlands decrease after drainage because of oxidation, but the
emissions will stay the same in ditches. Methanogenesis by methanogenic archae happens in the
anaerobic water-saturated peat layers. Methanogenic archae acquire their carbon compounds mostly
from deep-rooted plants. Plants, in turn, also transport CH4 into the atmosphere via their porous
tissues (aerenchyma). In contrast, methanotrophic bacteria oxidize a part of CH4 and turn it to CO2
in the oxidative peat layer, and this oxidation increases with the drawdown of the water table level.
Also, deep rooted mire plants disappear after drainage, which decreases methanogenesis, because
they bring new carbon to the soil which is a major C source for methanogenesis. In forest
management, the knowledge of impacts of different water table levels and vegetation changes in
different kinds of peatlands is important, because, for example, a water table level drop of more
than 30 cm cannot be compensated by the increased NPP in boreal peatlands. The tree stand volume
is a good indicator of vegetation changes and predictor of CH4 fluxes, for which the data is easily
accessible. Emissions from ditches can, however, neutralize the CH4 reduction caused by drainage,
at least if they are dammed and in nutrient rich peatlands. Additionally, vegetation in ditches also
decreases water movement. Only few thinnings for forestry are done before the felling, which takes
place usually 50–100 years after drainage, because peatlands have naturally uneven tree stand
structure which already carries the purpose of thinning, and because ditch cleaning is additional
maintenance burden (Minkkinen, 1999).
If the ditches are not dammed, they might transport CH4 and dissolved organic carbon (DOC)
compounds outside of the peatland. Thus, a considerable, but currently unknown amount of C
12
emissions from peatlands can occur outside peatlands, especially immediately after their drainage.
Long-term leaching is still quite small (ca. 10 % or 1 g C m-2 yr-1). Water table level fluctuations
caused by rainfall events can increase leaching of C compared to undrained peatlands, but it is not
likely to have importance to the C balance. (Minkkinen et al., 2008)
Water table levels are relevant for increased heterotrophic respiration and CO2 efflux when they are
near the surface, but when the water table is below 30 cm, temperature is a more relevant factor
(Minkkinen et al., 2008).
There are also factors that decrease the decomposition of peat after drainage. Soil temperature
decreases after drainage, which is caused by the increased shading by trees, and by decreased
thermal conductivity of dry peat. It decreases the otherwise increased peat decomposition. Also, soil
pH decreases after drainage because of the protons that are released from oxidation. Ditches also
prevent influx of cations which neutralize the acidity. The increased tree stand takes up cations as
nutrients, and acidity depresses biodegradative enzymes. Also, the vegetation changes and becomes
woodier, which reduces litter quality by increasing lignin content. Increased drought periods could
also inhibit decomposition (Minkkinen, 1999).
After thinning and clear-felling the soil, temperature rises, causing the increase of root respiration.
This could reduce heterotrophic respiration, so the decomposition might not increase. If the summer
temperature rises due to climate change, the peat in permafrost peatland areas of Canada and Russia
could decompose at a much higher rate, and the melting could release fossilized methane that is
stored in the permafrost. Hence, effects of initiating forestry in those peatland areas should be
studied. Still, the C accumulation has been the strongest during the highest winter–summer
seasonality. This peak in C accumulation could result from the increased biomass production in the
warm summers and reduced soil respiration in the cold winters of this region (Yu et al., 2010).
Intense soil disturbance increases C losses in peatlands on average. The soil is soft and easily
disturbed, so harvesting needs to be done in the winter when the soil is frozen. If the soil organic
layers are mixed, soil temperature and aeration increase, which causes the nutrients to be easily
decomposable. Also, previously deposited easily decomposable organic material becomes available.
The peat subsides after the water table level drawdown and the plant structures collapse. One
centimeter of subsidence leads to emissions of ca. 13 t C ha-1 yr-1. In forestry-drained peatlands, it is
mostly physical compaction and not compaction by increased decomposition, because the plant
cover and the new litter production protect peat from C losses. However, if the plant cover is
disturbed or litter production ceases, it can lead to subsidence of peat. Strong winter–summer
seasonality and low temperatures prevent subsidence (Minkkinen et al., 2008).
Whole tree harvesting reduces C stocks more than sawlog or stem harvesting, where residues are
left. Residues might be rapidly covered by mosses in peatlands, therefore becoming better
preserved. Stumps and main roots, especially, can persist after fellings for decades. It is suggested
to leave residues at the site rather than to use them for biofuel, although leaching of DOC and
nutrients increase if the residues are left to decompose. However, recent study shows that retention
of logging residues in clearfelled sites increase soil heterotrophic respiration and peat
decomposition, and which is the situation in where logging residues should be used as biofuel
(Mäkiranta, 2012).
Fires on peatlands can be a problem, because peatland protective vegetation is scarce and the peat
layer can also burn, releasing more CO2 than forest fires (Minkkinen et al., 2008: Gorham, 1991).
13
Even if forest management practices could increase the C pool of peatlands, natural mires might
have ecosystem values that can be more significant than the mitigation of climate change, so it is
not recommended to drain more natural mires in Finland (Minkkinen, 1999).
7. Forest management impacts on climate change and best solutions for policy
making
As this report has so far presented, there are several ways to research and estimate the climate
impacts that forests and their management may have. Since in the 21st century the global climate
has been changing and the temperatures rising at an alarming rate, it is important to do not only
long term predictions, but also short term policy analyses for global forest management.
This chapter will first introduce the main impacts of different types of forest management. Then it
will summarize the knowledge available trying to present a hypothesis on a relevant policy
analyses, and finally, it will briefly cover the ongoing global forest policy issues and agreements.
7.1. What to do with the forests and deforested areas?
Forests are known to be important carbon sinks of the planet. Still, there is a lot of pressure to fell
vast areas of natural forest in different parts of the world. What would be the best solution for
managing global forests - boreal, temperate and tropical areas? Would the best short term solution
be to use renewable forests as energy resource substituting the traditional fossil fuels, or should the
forest be left as such?
The annual global wood harvest is about 3 billion m3, and it has been stable for the last 15 years. In
addition to that, there is a parallel large scale illegal timber market that is not recorded officially,
and thus there could only be estimations on how much is felled. Approximately 60% of all wood
removed, is for industrial purposes and the rest is used for non-commercial wood fuels (FAO,
2006).
The traditional suggestion to slow global warming is to either prevent deforestation or promote
afforestation. Deforestation releases CO2 to the atmosphere but also changes the biophysical effects
of forest systems, which include land surface albedo, evapotranspiration and cloud formation (Bala
et al., 2007). This is especially evident in the winter time when forests absorb significantly more
solar radiation than deforested areas. The global-scale deforestation is thought to have a net cooling
effect on the climate, however, with the exception of tropical forests, where afforestation is
suggested to have the opposite result (Betts, 2000).
This only shows how complex the question of forest management can be. Furthermore, it is not
only a question whether or not to leave the forests as such, but also how to use them once they are
felled. One thing is the optimal rotation length to provide best possible carbon sequestration. It has
been studied that shortening the rotation of the Scots pine from the recommended 90 years did
decrease the carbon sequestration whereas for the spruce the shortest rotation was the best (Liski et
al., 2004).
Since the use of fossil fuels is responsible for the major part of carbon emissions, it is crucial to
look at the issue of replacing those fuels with renewable sources such as wood. This, however, is
very complicated because felling forests reduces their capacity to sequester CO2. Therefore, it is
crucial to maintain a careful balance of mitigation practices for carbon emissions, such as
14
afforestation and reduction of deforestation, and the use of forests as a renewable source of energy
(Nabuurs et al., 2007). It should also be noted that not only forests impact climate but vice versa.
Climate warming is estimated to increase wood growth especially in the boreal region. The average
biomass of wood stock of 72 Mg ha -1 in 2005 may increase to up to 104 Mg ha-1 by 2100 in the
northern regions (Pussinen et al., 2009).
It is important not to overlook another critical issue for climate impacts due to forests use, which is
the endproducts made of wood. These products include biofuels, paper and wood materials. Due to
changes in governmental policies and thepromotion of renewable energy, the use of wood biomass
for bioenergy has increased in recent years. It is estimated that if traditional fossil fuels are
substituted with more renewable bioenergy sources, it could have an overall net cooling impact on
the climate (Nabuurs et al., 2007).
The use of wood for construction is popular in many parts of the world and it is very traditional in
the Nordic countries. A study from Finland and Sweden has shown that constructing residencies
with wood elements can have a beneficial role in regards to carbon stock and reduce significantly
net carbon emissions. This mitigation approach is even more effective when wood is used as biofuel
after its service life as construction material (Gustavsson and Sathre, 2006).
7.2. Global agreements on forest management
Forest impacts on global climate are significant and therefore should be evaluated closely and taken
into consideration in climate mediation policies. Additionally, due to the urgency of climate
warming, policies must consider forest management on local and global scales, and should be
catered to both, short-term and longer-term solutions. International legislation on the three forest
related issues discussed earlier (forests as carbon sink, biofuels and wood construction) need to
have a global legal framework provided by international agreements and collaboration.
Increasing forest areas and their maintenance have relatively high immediate monetary costs,
however, the benefits are also significant. Maintenance of forests provides for increased carbon
sinks in the short term, and new forest areas insure positive impact in the long run. Forests can
serve as stockpile for carbon when in their young age. Later, when their relative growth decreases,
their capacity to store carbon may not increase any further. Moreover, increasing the use of
bioenergy resources instead of fossil fuels provide for environmental benefits, while at the same
time cutting down the costs on fossil fuel production. At the present, there are already some signals
that indicate that fossil fuel production may be losing competitiveness compared to bioenergy
production (Nabuurs et al., 2007).
There is a strong historical legacy of international agreements on afforestation and land-use.
However, these agreements have been signed for non-climate reasons. Due to the stronger
institutional tradition in industrialized countries, they had have better possibilities to ensure
sustainable forest management practices, and thus follow the international agreements.
Prevention of deforestation and forest management has been playing an important role in global
climate negotiations. Many different forestry projects have been set, especially in the developing
countries, which have mainly tropical forests, and may thus have net cooling impacts on the
climate. These forestry projects provided by international institutions and organizations, such as the
UN and the World Bank, have included strengthening forestry and legislative authorities in the
developing countries, reducing illegal logging, and providing technical support and lawenforcement. National governments have only restricted possibilities to affect privet forest areas,
15
and typical attempts to affect private owners have been through taxation subsidies (Nabuurs et al.,
2007).
Regarding biofuels, they may have other important political aspects than just climate impacts. For
many countries they play an important role as a provider of self-sufficiency in energy resources, and
thus, are supported nationally. This shows how national interests and climate change mitigation can
sometimes go hand-in-hand. Taxation and subsidies are also seen as the most effective policy tools
regarding promotion of biofuel production.
Building codes and other national and international policies, such as for example the Lacey Act in
the United States, can control the use of illegal timber and promote applications of sustainably
harvested in construction and other wood products. The consumer market should also be provided
by better standardization of sustainable harvesting (Murphy, 2004).
Finally, forest management policies should be especially aimed at three different dimensions:
insuring that forests maximize their ability to provide net carbon reductions per unit area, be
potentially used at a large geographic scale, and have a relatively low potential for leakage (Niesten
et al., 2002).
8. Conclusions
As we have seen, sequestration of carbon by forests and later the accumulation of that carbon into
stable soil carbon pool is not a simple issue. Carbon in the forest soil is sensitive for large scale
disturbances, for example fires (especially peatlands) and logging (chapters 4, 5 and 6) because
after the trees are gone, the respiration from the soil increases and the plant production of the
clearfelled areas decreases. Continuous cover forestry would not leave such large openings to the
forest and therefore the temperature on the forest floor would not increase, leading to lesser soil
respiration by the microbes. We certainly need more information about how continuous cover
forestry affects the sequestering of the carbon compared to traditional economic forestry that is now
the usual practice.
To address climate change effectively there needs to be an increase in the stable pool of soil C,
which, unfortunately, accumulates slowly. The increase in less stable soil C is also relevant,
especially when the pool is continually sustained by the input of new organic matter (Jandl et al.,
2007).
When evaluating forest management practices for carbon sequestration, it is difficult to classify a
procedure as clearly positive or negative. Many management measures may increase the
aboveground production but also cause a loss of soil C. The ideal aim is to increase the productivity
of forests while avoiding strong soil disturbances at the same time.
In order for the forests to be as efficient carbon sequesters as possible we need different types of
management tailored individually for different forests types. For example the optimal rotation
length for carbon sequestration differs from commercial rotation lengths for forests of certain
species (chapter 4 and 7).
As we have read from chapter 2, forest management has increased the amount of forests and the
amount of carbon in Finnish forests. So we clearly have knowledge how to grow forests efficiently
for the economic purposes but we do not have that much knowledge on how to combine the forest
16
management taking consideration also the ecological purposes. That is also a question for the policy
makers to decide (chapter 7). Mixed species stands could be one solution but it complicates the
management of the stand.
Warming climate tends to create positive circumstances for forest growth (chapter 4) and the
productivity of the forests have increased. Although a warmer climate is beneficial for forest
growth, it also increases the amount of diseases and pests. Decomposition of the litter might also
increase because of the rising temperature, unless the microclimate in the forest floor remains the
same.
Questions to be discussed with visiting teacher
Will soils turn into carbon sources and accelerate climate change as global warming increases soil
respiration?
How can we ensure the sustainable production of biofuels that wont affect the carbon balance
negatively?
17
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