Deforestation

Globally, deforestation contributes to around 18% of anthropogenic carbon emissions, with an estimated 6 million hectares of primary forest being lost or modified each year. The conservation of forests world-wide is thus an immediate and highly effective form of climate change mitigation. In this context, conservation can be thought of as any strategy that conserves existing carbon reservoirs and thereby prevents emissions of greenhouse gases into the atmosphere. Strategies should include, but are not limited to, the reduction of deforestation, the management and conservation of existing forests, and the encouragement of more sustainable forest land uses.

Deforestation is broadly defined as the clearance of forests by people and the conversion of land to another use. These activities are estimated to result in the loss of 13 million hectares of forest per year, and over the past 140 years, we have seen a total reduction in global area of forests of almost 20%. When forests are cleared to convert land for agricultural or pastoral use, it is common for a large proportion of the above ground biomass to be burned, which rapidly releases carbon into the atmosphere. Since the Industrial Revolution, the conversion of forests to grasslands has led to a net release of approximately 121 gigatonnes of carbon. Of this, 40% was released during the first half of the 20th century in mid and high latitudes, and roughly 60% has been released in the tropics from the 1950s onwards. Although deforestation is offset to some extent by forest replanting, landscape restoration, and the natural expansion of forests, the net global change of forest cover between 2000 and 2005 was still -7.3 million hectares.

Causes of Deforestation

Land-use change is a dynamic process and it varies according to region. It is carried out at different scales by different people for very different reasons. Some of the major causes of deforestation and the impacts of the change in land-use are set out below.

The main cause of deforestation is the clearing of forests and its conversion to pasture and cropland for agricultural use. The conversion of forests into fields is essentially an investment on the part of the farmer or landowner, based on the return they will get on the crops that are grown. Since the widespread clearance of tropical forests first captured the attention of international commentators in the 1960s and 70s, there have been various suggestions of the possible drivers of agricultural land conversion, including population increases, political instability, and issues of land tenure and property rights. Although it is agreed that certain conditions do result in increases in agricultural clearance, for example, the building of roads which creates access to new markets, it is now accepted that each situation is temporally, politically and spatially specific, with a multitude of interacting factors.

Shifting cultivation is a cropping practice which involves the periodic clearing of natural vegetation for short periods of cultivation, followed by long fallow periods. Estimates of the numbers of small farmers practicing this form of subsistence agriculture range from 300–500 million. If, as historically has been the case, population densities are low and rotation rate long, shifting cultivation can be practiced sustainably. If fallow periods are long enough for forests to regenerate, shifting cultivation releases less carbon than more permanent forms of forest clearing. However, in recent years, increasing land pressures have led to the growing intensification of shifting cultivation, with some estimates associating it with as much as 60% of tropical deforestation.

Forest plantations are defined broadly as forests of introduced species, and in some cases native species, established through planting or seeding, with few species, even spacing and/or even-aged stands. The FAO distinguishes between two types of plantation: Productive forest plantations, established predominantly for the provision of wood, fibre and non-wood products; and Protective plantations that are predominantly for the provision of services such as protection of soil and water, and the rehabilitation of degraded lands. Plantations currently constitute 4% of the world’s total forest area. Of this, 78% are productive forest plantations, and the remaining 22% are protective.

Plantations of agricultural and industrial crops are providing new sources of raw materials throughout the world. Plantations of rubberwood (Hevea brasiliensis), coconut palm (Cocos nucifera), and oil-palm (Elaeis guineensis) cover extensive swathes of Asia, totalling an estimated area of over 24 million hectares. Although grown in these areas for centuries, technological advances have seen the uses of these crops evolve. For example, while in the past rubberwood was grown throughout Southeast Asia for the production of latex, developments in seasoning and preservative treatments have made it a viable utility timber. Rubberwood harvests in Asia are now estimated to exceed 8.9 million hectares, making it the most widely planted tree species on the continent. In 2005, protective forest plantations constituted 0.82% of total forest area, totalling a reported area of over 30 million hectares.

The impact of plantation establishment on local environments and forest carbon stocks and flows is largely dependent on the use of the land prior to establishment, levels of precipitation and tree type. On average, the conversion of forest to plantation reduces soil carbon stocks by 13%, although the effect of conversion to coniferous plantation is far more pronounced than conversion to broadleaved.

The establishment of plantations also has much wider impacts on the functioning of ecosystems and the services that they provide. In South Africa, for example, the expansion of commercial plantations of Eucalyptus and Pinus has led to significant declines in several endemic and threatened species of native grassland birds, as well as the suppression of indigenous ground flora. In addition, the increased water uptake of the introduced species has caused problematic reductions in the water yield of surrounding catchment areas.

Plantations are often more vulnerable to the direct and indirect effects of climate change. For example, the conversion of natural forests to palm oil plantations in a number of Southeast Asian countries, such as Indonesia, has been associated with increases in uncontrolled forest fires. Within the Tropics, it is also predicted that changes in water availability, and the effects of this on the fecundity, growth rate and survival of pests and the increased risk of fire hazards, could significantly reduce silvicultural yields. In some areas, an increase of mean annual temperature above 4.5ºC could result in a crop decrease in rubber of up to 15%.

Contrary to popular opinion, only a very small proportion of deforestation is the direct result of logging. In nearly all countries of Europe, the ‘clearcutting’ of forestry for timber has to be followed by obligatory reforestation, or is regulated as part of sustainable forest management. Similarly, when logging is carried out in timber plantations, replanting is a vital to ensure a continued product supply, with the result that there is an implicit obligation to reforest. Even within the Tropics, most logging is done selectively involving the extraction of carefully selected, valuable trees. When carried out a at a suitable intensity and frequency, sustainable selective logging can leave up to 70% of trees in an area undamaged and, if left undisturbed for long enough, it is possible for the biomass in selectively logged forests to recover to pre-harvest levels. However, as land pressures increase and local timber markets diversify, logged areas are repeatedly revisited and up to 50% of the canopy cover can be destroyed. In some parts of Southeast Asia, the problem is exacerbated by a failure by the state to enforce timber concession regulations. Not only does the indirect disturbance of soils and biomass from logging release carbon into the atmosphere, it also greatly increases the risk of forest fires. In addition, through the creation of new forms of access, logging can increase colonisation and increase the likelihood of further deforestation.

The international timber trade is estimated to be worth some US$140 billion, and although there have been no attempt to establish the magnitude of direct impacts of climate change on the timber trade, it is likely both timber availability and quality will be affected. In turn, changes in the amount of timber available could have a knock on affect on local employment. Illegal logging is estimated to result in losses from public lands of forest resources worth between US$10 billion and US$15 billion per year, and serves to depress prices, undermine legitimate enterprises and fund war and civil conflict.