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Climate Change & Forests

Introduction

Forests cover around one quarter to a third of the total land surface of the Earth. The reduction in area of this valuable environmental, social and economic resource has the potential to cause problems on a global scale. Forest resources are essential for both the development and preservation of the global environment. Using them rationally can create employment, help alleviate poverty and provide valuable range of products, whilst mismanagement is linked to degradation of soil and water resources, loss of wildlife and biodiversity, and aggravation of global warming.

Climate models have demonstrated an obvious link between forests and climate change. A change in climate may affect the stability of forest ecosystems. Equally, loss of forest ecosystems can influence climate through complex feedbacks.

Causes of Deforestation

The largest and most critical forested areas are in the tropics. Nation states in these areas are often under-developed and striving for improved economies. In an effort to obtain enhanced incomes, local people and developers often look to natural forest areas to provide the potential for profit through altering land use. This frequently results in the conversion of forested land to agricultural land, and deforestation for fuelwood, hardwood, urbanisation, mining and war.

Deforestation and its Effects on Climate

Deforestation, the process of changing land use from forestry to a non-forest use, is threatening not only the existence of many species and the livelihoods of many indigenous people, but can also influence climate. Forests are known to strongly moderate local climates and are important carbon sinks for the regulation of the natural greenhouse effect. In the 1980s global deforestation was estimated at 17 to 20 million hectares per year. In addition, considerably more primary or old growth forests are being replaced with single species forest plantations, unable to support diverse plant and animal communities. Western Europe has already lost over 99% of its primary forest. Existing tropical tree planting programmes are unsustainable with on average 6 hectares lost for every 1 hectare replanted during the 1980s.

Deforestation and greenhouse gases

Trees absorb carbon dioxide from the atmosphere for photosynthesis. Some of this carbon is used by the tree for growth and will remain locked up within the cell structure until the tree is burnt or felled. In this way, a tree acts as a carbon sink. At the end of the tree's life cycle that carbon is released back to the atmosphere as carbon dioxide. When forests are unmanaged there is no net change in the atmospheric concentration of carbon dioxide since the removal of carbon dioxide from the atmosphere through photosynthesis is balanced by the release of carbon dioxide from rotting trees. Deforestation, however, alters this balance. Carbon dioxide is released to the atmosphere by forest burning. The subsequent replacement vegetation, frequently in the form of agricultural crops, has comparatively little biomass in comparison to forests, and the potential to absorb carbon dioxide is reduced. Deforestation accounted for almost a quarter of all man-made carbon dioxide emissions during the 1980s.

Albedo and regional climate change

The albedo of a surface defines the amount of incident solar radiation reflected by it, expressed as a percentage. Vegetation, including forestry, absorbs much of the incoming radiation for photosynthesis and so has a low albedo (12-15%). Grassland has an albedo of about 20% whilst deserts reflect about 40% of incoming sunlight. An important link exists between changes in vegetation and rainfall. The increased energy absorbed at the surface by vegetation coupled with increased surface roughness acts to stimulate convection currents in the atmosphere, enhancing the production of rainfall.

The Effects of Climate Change on Forests

Effects of climatic shifts on forests

Biomes are regions characterised by their distinctive vegetation, the distribution of which is dependent upon the climatic regime. A change in global climate would be accompanied by shifts in climatic zones, thereby altering the suitability of a region for the growth of distinctive species. Trees, in particular, have long reproductive cycles, and many species may not be able to respond to the climatic changes quickly enough. A shift in climatic zones not only affects the vegetation but also affects the incidence of tree pests such as insects and diseases. These pests have less difficulty in migrating with their climatic zones than vegetation and may damage tree species with lower immunity.

Effects of increased carbon dioxide on forests

As well as the effects of temperature, precipitation and weather pattern changes, forest growth may also respond to increased atmospheric concentrations of carbon dioxide. Studies with immature forest plantations suggest that an increase in atmospheric carbon dioxide would be beneficial to tree growth. The elevated carbon dioxide concentrations enhance photosynthesis rates allowing more effective carbon fixation. This is called the carbon fertilisation effect. As a consequence of carbon fertilisation, water use efficiency may also increase. Increase in growth rates, however, would vary enormously within ecosystems and between species.

In general, it is expected that the negative impacts of climate change on forests will have a greater impact than any positive effect due to an increase in growth rates as a result of elevated atmospheric carbon dioxide concentrations. With unmitigated emissions of greenhouse gases, substantial dieback of tropical forests and tropical grasslands is predicted to occur by the 2080s, especially in northern South America and central southern Africa. If emissions are reduced enabling atmospheric carbon dioxide concentrations to stabilise at 550 ppm (double the pre-industrial level), this loss would be substantially reduced, even by the 2230s. Considerable growth of forests is predicted to occur in North America, northern Asia and China.

Associated effects

As well as the effects on forests themselves, climate change is expected to influence societies and economies dependent upon forestry. Forest products are the third most valuable international commodity after oil and gas. Trade is expected to increase in the 21st century along with demand, particularly in the developing countries. Climate change may well affect the development of such developing economies, particularly if current rates of deforestation remain unchecked and the unsustainable management of forests continues. As a consequence, societies dependent upon the income, food and shelter their forests provide them may well face increasing stresses due to crop failures, soil nutrient depletion and the effects of extreme weather events in the years to come.

Afforestation and Managed Forests

Tackling deforestation is explicitly recognised by the Framework Convention on Climate Change, signed by over 160 countries at the Earth Summit conference in 1992. The management of forests to combat global warming need not be costly. On the contrary, the Finnish forest industry has demonstrated that afforestation projects can make a profit through the export of forest-related products. Furthermore, the net growth of stock is estimated to sequester about 40% of the carbon dioxide emitted annually by energy production and traffic in the country. Forest land is managed sustainably with timber demand never exceeding supply.

Conclusion

Changes in global, regional and local climates may well affect the ability of forest ecosystems to survive in their present states. In turn, local and regional climates may be further altered by factors such as surface albedo changes, rainfall variations and consequently moisture availability. Positive effects on forestry may include enhanced growth due to the carbon dioxide fertilisation effect. Negative impacts may include shifts in climatic zones and previously unencountered pest outbreaks. Other associated effects may include impacts to the timber trade industry, reductions in biodiversity and social implications.