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Subject Pollution Date 30-03-2006
Title Forests and climate change Section Planet

Better forest management has a key role to play in dealing with climate change

Climate change and forests are intrinsically linked. On the one hand, changes in global climate are already stressing forests through higher mean annual temperatures, altered precipitation patterns and more frequent and extreme weather events. At the same time, forests and the wood they produce trap and store carbon dioxide, playing a major role in mitigating climate change. And on the flip side of the coin, when destroyed or over-harvested and burned, forests can become sources of the greenhouse gas, carbon dioxide.

FAO has warned that action should be taken now to manage these complex relationships in a more holistic manner.

“ We need to stop deforestation and expand the land area covered by forests, certainly,” says Wulf Killmann, who chairs FAO's interdepartmental climate change working group. "But we also need to substitute fossil fuels with biofuels,-- like wood fuels from responsibly managed forests -- in order to reduce carbon emissions, and we should use more wood in long-lasting products to keep trapped carbon out of the atmosphere for longer periods of time."

How forests trap one trillion tons of carbon

When fossil fuels are burned they release carbon dioxide into the atmosphere, contributing to an atmospheric carbon dioxide increase that, in turn, contributes to global warming and climate change.

Trees and forests help alleviate these changes by removing carbon dioxide from the atmosphere and converting it during photosynthesis to carbon, which they then "store" in the form of wood and vegetation, a process referred to as "carbon sequestration."

Trees are generally about 20 percent carbon by weight and, in addition to the trees themselves, the overall biomass of forests also acts as a "carbon sink." For instance, the organic matter in forest soils – such as the humus produced by the decomposition of dead plant material -- also acts as a carbon store.

As a result, forests store enormous amounts of carbon: in total, the world's forests and forest soils currently store more than one trillion tons of carbon -- twice the amount found floating free in the atmosphere -- according to FAO studies.

Destruction of forests, on the other hand, adds almost six billion tons of carbon dioxide into the atmosphere each year, and preventing this stored carbon from escaping is important for the carbon balance and vital in conserving the environment, the UN agency says.

Forests could be better used in combating climate change

This can be achieved not just by preventing forests from being cut down, but through afforestation (new plantings) and reforestation (replanting of deforested areas) of non-forested lands.

Particularly in the tropics, where vegetation grows rapidly and therefore removes carbon from the atmosphere more quickly, planting trees can remove large amounts of carbon from the air within a relatively short time. Here, forests can store as much as 15 tonnes of carbon per hectare per year in their biomass and wood.

FAO and other experts have estimated that global carbon retention resulting from reduced deforestation, increased forest regrowth and more agro-forestry and plantations could make up for about 15 percent of carbon emissions from fossil fuels over the next 50 years.

Harvested wood is also a carbon sink -- wood used in construction or for furniture effectively stores carbon for centuries. High-energy construction materials used in place of wood, such as plastics, aluminum or cement, typically require large amounts of fossil fuels during manufacturing. Replacing them with wood therefore has additional benefits in terms of reducing carbon emissions.

Similarly, the use of wood fuel instead of oil, coal and natural gas, can actually mitigate climate change. Although burning wood and biomass does release carbon dioxide into the air, if those fuels come from a sustainably-managed forest, those carbon releases can be offset by replanting. Indeed, if managed properly, forests can supply bioenergy virtually without contributing any greenhouse gas to the atmosphere.

Increasing temperatures will change the distribution of the world's forests

Depending on the capacity of tree species to adapt to new climatic conditions, climate change and global warming could change the forest landscape worldwide, according to FAO.

The territorial range of any given plant species depends to a large extent on temperature and rainfall patterns, and so as climate change alters global temperatures and rainfall patterns a realignment in the distribution of tree species is likely to follow.

According to one FAO study, tree species tend to shift both to higher latitudes and to higher altitudes in response to global warming. Under this scenario, tree ranges in the northern hemisphere have the potential to expand 100 km northward, while their southernmost boundaries could retreat by the same magnitude for each degree of warming beyond current regional temperatures.

Similarly, scientists predict that loblolly pine (Pinus taeda), an important industrial forest species in the south-eastern region of the United States, could shift up to 350 km northward in response to a three degree increase in global temperatures, according to the FAO study.

Such shifts in forest distributions have already been observed. In Sweden during the first half of the twentieth century, the range of birch (Betula pubescens) expanded northward into the tundra in response to warming.

Shift to higher altitudes increases vulnerability

In addition to moving north, tree species could move to higher altitudes as a result of global warming.

FAO's study cited work by scientists in the Austrian Alps who found that alpine plant species have "migrated" toward higher altitudes at rates ranging from less than one metre to nearly four metres per year over the last century. The central Alps warmed by 0.7 degrees Celsius during that same period.

However, such a trend would make many species more vulnerable to genetic and environmental pressures, since mountain habitats are typical limited in size, which in turn would limit species populations and therefore the diversity of their gene pool.

For some species, little impact - for others, extinction

Not all tree species will respond by shifting their ranges, notes FAO. Some may have a greater capacity to adapt to new climatic conditions and could more or less continue to occupy their current distribution ranges.

For other species, however, climate change might outpace their ability to adapt, leading to extinction. In fact, warns FAO, in the coming century or so the Earth's climate is expected to change more rapidly than the rate at which many forest ecosystems would be able to adapt or re-establish themselves in more propitious climates, raising the spectre of large-scale species and forest die-offs.

Extreme weather events and intensified fires already affecting forests

One consequence of climate change that affects forests is an increase in extreme weather events, which can cause significant tree loss. The value of trees damaged by 2004's Hurricane Ivan is estimated in the hundreds of millions of dollars in just the 12 hardest-hit U.S. counties alone (counties are mid-sized administrative regions, smaller than states).

Aside from such direct impacts, floods and storms can also alter water flows on which trees depend, hurting forest health.

A changed climate also opens the way for non-native, harmful species to invade disturbed ecosystems. Changes in temperature and rainfall can favour outbreaks of insect infestations, both in northern boreal forests and in temperate and tropical timber plantations, with devastating consequences.

Forest fires aggravated by warmer temperatures

Such anticipated changes in the ranges of tree species, in forest composition and in the size and occurrence of insect populations will also affect the dynamics of forest fires.

Although forest fires are mostly caused by humans, their growing frequency may be due to warmer temperatures and shifting winds resulting from climate change.

Certainly, regional droughts seem to have a clear link with the frequency and intensity of fires. In Portugal in 2003, nearly 400 000 hectares of forest were burnt during a scorching summer heat wave, the largest forest area burnt during a single year in Portugal’s entire history.

Helping forests cope

" Forest managers should assess the vulnerability of their forests and examine whether tree species already react sensitively towards extreme weather patterns,” according to Dieter Schoene, an FAO forest and climate change expert.

" Forest managers need to start looking at measures that can help forests cope with, and adapt to, climate change," he says, "or forests will not survive in the long term."

In developing countries, forest vulnerability assessments can be undertaken under the National Adaptation Programme of Action initiated by the United Nations Framework Convention on Climate Change.

This programme involves helping poor countries identify priority actions that need to be taken in order to adapt to the adverse effects of climate change.

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