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Forests and trees deliver important ecosystem goods and services (Figure B3.1). The provide a continuing supply of  timber, pulp, bioenergy, water, food and medicines. They also offer opportunities for recreation, and play prominent roles in many cultural traditions. Forests are habitat for a large share of the Earth’s plant and animal species.  Tropical forests, in particular, are biodiversity hotspots (Gibson et al., 2011). Forests are crucial for sustainable agricultural development because of the role they play in the water and carbon cycles, soil conservation, pest management, the amelioration of local climates and the maintenance of habitats for pollinators.

There is a close interrelationship between climate change and forests. Air temperature, solar radiation, rainfall and concentrations of carbon dioxide in the atmosphere are major factors in forest productivity and forest dynamics. Forests, in turn, affect climate by removing and releasing large amounts of atmospheric carbon, absorbing or reflecting solar radiation (albedo), cooling  through evapotranspiration and producing cloud-forming aerosols (Arneth et al., 2010; Pielke et al., 2011).

As integral parts of broader landscapes, forests and trees contribute to ensuring the stability and vitality of ecosystems and meeting societal needs. Integrated approaches to landscape management (addressed in module A3) can increase synergies among multiple land-use objectives. By considering the perspectives, needs and interests of all stakeholders, including local communities and individual land users, landscape approaches can be instrumental in developing sustainable strategies for land use and related livelihoods. Broad stakeholder dialogue is especially important when making changes to the way land is used and managed in the landscape. For example, addressing the drivers of deforestation, which are associated with factors well beyond the forests themselves, requires following a cross-sectoral landscape approach and reaching a consensus among multiple stakeholders. Some impacts of climate change (e.g. an increased risk of fire or pests) may require managers to look beyond their own management units and integrate their management approaches with those of other stakeholders living in similar landscapes. Adopting a landscape approach can help in identifying and implementing forest-based climate change adaptation and mitigation measures that lead to optimal economic, social and environmental outcomes.

Given the many and diverse interests in forests, it is crucial that all stakeholders are involved in forest management. Partnerships and participatory approaches can operate at a range of levels, from the national to local level. They may involve state and local authorities, forest extension agencies, forest-dependent communities, non-governmental organizations, private-sector entities, research and academic organizations, and forest managers.

Local and indigenous communities have managed forests and associated landscapes over centuries or even millennia. They have done this in ways that have sustained their livelihoods and cultures without jeopardizing the capacity of ecosystems to provide a continuous supply of goods and services. The knowledge, innovations and practices of these communities have evolved through experiences gained from their encounters with changes in environmental, economic, political and social conditions (Parrotta, Youn and Camacho, 2016).

Typically, traditional knowledge is conveyed orally from generation to generation through stories, songs, folklore and proverbs, and through the direct training of youth by elders. Traditional knowledge, supported by and embodied in local languages, cultural values, beliefs, rituals, laws and governance systems, has created a diverse set of natural resource management practices that sustain food security, health and traditions (Berkes, 2008). Complex forest management practices based on traditional knowledge, including natural forest management, shifting cultivation and agroforestry systems, continue to meet the material and non-material needs of societies without putting the biodiversity and functional integrity of forests and associated ecosystems at risk.

The rate of climate change varies depending on the geographical scale under consideration. The extent of change generally increases with the distance from the equator. Locally, the rates and directions of climate change are affected by the topography and proximity to large water bodies. Forest species and forest-dependent people differ in their resistance (i.e. their ability to remain unchanged in the face of disturbances)  and resilience (i.e. their capacity to absorb disturbances and reorganize during change, so as to retain essentially the same functions, structure and identity)ⁱⁱ to climate change and in their adaptive capacity. To cope with climate change, species will need to adapt to the changed conditions or migrate to areas with suitable conditions for survival. The ability of species to migrate will depend on their capacity to disperse and the existence of physical connections to suitable habitats. The risks of species losses and ecosystem disruption in forests will vary geographically and over time. Neither the climate nor species respond linearly to changing conditions; they tend to react abruptly when certain thresholds or tipping points are reached.

Communities differ in the extent to which they are vulnerable to climate change. Among the most vulnerable communities are those that are already struggling with poverty, have limited options for gaining employment or earning income, and depend directly on rainfed agriculture or forests for their livelihoods.

In the forest sector, adaptation encompasses changes in management practices and interventions designed to decrease the vulnerability of both forests and forest communities to climate change. See Climate Change Guidelines for Forest Managers for specific adaptation measures in forestry.

Forests and climate change adaptation are connected in two main ways (Locatelli et al., 2010). First, forests contribute to adaptation by providing ecosystem services that reduce the vulnerability of local communities and broader societies to climate change. For example, the supply of forest goods tends to be more climate-resilient than the supply of traditional agriculture crops. When disaster strikes or crops fail, forests act as safety nets that can provide the affected communities with food and income. Forests also provide ecosystem services that are essential for livelihoods, food security, environmental sustainability and national development. Climate change jeopardizes the delivery of these goods and ecosystem services. The second way that forests are linked to climate change adaptation relates to the effects climate change will have on forests and the measures that are needed to reduce the negative impacts and maintain forest ecosystem functions. Choices of adaptation measures for a given forest will be determined by the expected  impacts of climate change, the management objectives (which may shift in light of climate change), management history, and a range of other factors. All management actions to adapt a forest to climate change must be consistent with sustainable forest management. Policies, laws and governance frameworks must be sufficiently flexible to facilitate and support the actions taken to adapt forests to climate change (i.e. adaptive management).  See also chapter B8-5 on adaptation options for forest genetic resources.

The area covered by forests is likely to change as the climate changes. There will also likely be shifts in forest types due to changing temperatures and precipitation regimes. Forest area is expected to expand in the temperate zone and contract in the boreal and tropical regions, and in mountains. Similar changes have occurred in past geological eras due to natural changes in climate, but in the present era it will be difficult to isolate climate change from the other factors that are affecting the range of forest area (Lucier et al., 2009). 

Planted forests and natural regeneration have increased the forest area in China, the United States of America, many countries in Europe, and some countries in Latin America and the Caribbean (e.g. Chile, Costa Rica, Cuba and Uruguay) (FAO, 2010). On the other hand, some countries in Africa, Asia and the Pacific, and Latin America continue to experience deforestation and forest degradation, due mainly to the conversion of forest land to small- and large-scale crop and livestock production. Deforestation in the boreal forests of Siberia in the Russian Federation is due mainly to forest fires (FAO, 2009). 

Boreal forests are expected to move north due to climate change. Temperate forests are also expected to increase their area to the north but to a greater extent than boreal forests, which will reduce the total area of boreal forests (Burton et al., 2010). 

It is expected that the impacts of climate change, land-use conversion and unsustainable land-use practices will interact with each other. Changes in water availability will be a key factor in the survival and growth of many forest species, although the response to prolonged droughts will vary among species and also among varieties of the same species (Lucier et al., 2009). Climate change will increase the risk of frequent and more intense fires, especially in areas where it leads to lower precipitation or longer dry periods, as in boreal forests (Burton et al., 2010), and forests in Mediterranean and subtropical regions (Fischlin et al., 2009), and areas where traditional fire-based land-clearing practices are used, as in the Amazon (Aragão et al., 2008; Nepstad et al., 2008). 

Climate change may have profound impacts on the health and vitality  of the world’s forests. Forest health and vitality are determined by considering a range of factors (e.g. age, structure, composition, function, vigour, the presence of unusual levels of insects or disease, and resilience to disturbance). It is important ot recognize that the perception and interpretation of forest health and vitality is influenced by individual and cultural viewpoints, land management objectives, spatial and temporal scales, and the appearance of the forest at a particular point in time (Helms, 1998).

In some cases, vitality may increase due to a combination of carbon dioxide fertilization and a more favourable climate. In most cases, however, increasing temperatures will favour the growth of insect populations that are detrimental to forest health (Lucier et al., 2009), especially in forests dominated by a few tree species or where insect populations are sensitive to seasonal shifts in temperature or moisture levels (Box B3.2).

The impact of climate change on forest productivity (i.e. the potential of a particular forest stand to produce above-ground wood volume) varies according to geographic area, species, stand composition, tree age, soils (particularly their capacity to retain water), the effects of carbon dioxide and nitrogen fertilization, and interactions between these factors (Girardin et al., 2008; LeBauer and Treseder, 2008; McMillan et al., 2008; Ollinger et al., 2008; Phillips et al., 2008; Reich and Oleksyn, 2008; Saigusa et al., 2008; Clark et al., 2003). Some changes may be temporary, with conditions reverting back to their previous status once saturation levels are reached. This is projected to be the case for water availability, where a reduction in availability generally reduces plant growth. In areas of water surplus, however, there may be an initial increase in growth if there is less waterlogging. Similar reactions have been noted for carbon dioxide (Ollinger et al., 2008; Clark et al., 2003), nitrogen fertilization (LeBauer and Treseder, 2008), and increased temperatures (Reich and Oleksyn, 2008).

In general, in most forest areas, productivity has been found to increase with higher temperature, which is probably due to carbon dioxide fertilization. In contrast to temperate areas, however, increases in productivity in tropical forests will be temporary and will decline when carbon dioxide saturation is reached. Some studies have already reported decreasing growth rates in tropical forests (Feeley et al., 2007; Clark et al., 2003). 

Water deficits over extended periods have been shown to decrease productivity (Malhi et al., 2008) and may be the cause of the reported productivity declines in the studies cited above. Based on palaeontological evidence, Some authors have argued that reduced productivity may not result in the forest dieback, which is often mentioned in connection with expected changes in the Amazon due to climate change (Mayle and Power, 2008). Natural disturbances often decrease forest area and, through the damage they cause to standing trees, they may also decrease productivity (Chakraborty et al., 2008; Jepsen et al., 2008; Kurz et al., 2008; Nepstad et al., 2008).

It has long been recognized that forests contribute to water and soil protection. In several countries, recognition of this fact has translated into schemes to pay landowners, or offer them other incentives, for providing these ecosystem services (Postel and Thompson, 2005). However, foresters and hydrologists still debate the nature of the influence that forests have on water regulation (Kaimowitz, 2001; Innes et al., 2009). Climate change may make the role of forests in water regulation and soil protection more important, but the capacity of forests to fulfil this role may also be affected. Reductions in rainy-season flows and increases in dry-season flows are of little value when total annual rainfall is low and significant quantities of water are lost through evapotranspiration and are consumed by forests. 

In areas, with frequent fogs, the clouds (horizontal rain), from which trees absorb moisture, may contribute significantly to total rainfall (Stadtmüller, 1994 ). A palaeoecological study of changes in Amazonian vegetation (Mayle and Power, 2008) indicated that, in cloud forests, where trees are often submerged in fog, warmer temperatures may cause the clouds to rise above the trees, reducing the potential for horizontal rain. 

Water management and sustainable soil and land management are addressed in modules B6 and B7, respectively.

Climate change may increase forest growth in some areas and decrease it in others. The expected global increase in wood production could lead to lower prices, which would benefit consumers. However, lower prices and regionally differentiated impacts on productivity will have varying effects on incomes and employment derived from timber (Osman-Elasha et al., 2009). On all continents, except Australia, timber production may increase by up to 50 percent. Most of this increase, however, is expected to come from plantations with increasingly shorter rotations, and is therefore likely to be distributed unevenly among regions (Osman-Elasha et al., 2009). In South America, where the greatest increase is expected, current plantation production is concentrated in Argentina, southern Brazil, Chile and Uruguay. However, the possible dieback of natural tropical forests in South America may decrease timber production in the tropical zone. 

The harvesting of non-wood forest products (NWFPs) has three major functions: 1) supplying part of the daily necessities of forest-dependent people; 2) generating off-farm income; and 3) providing a safety net in times of adverse conditions for agricultural production. Osman-Elasha et al. (2009) have suggested that climate change could have impacts on the productivity of NWFPs, and that greater numbers of people seeking emergency supplies or alternative sources of income would increase the importance of NWFPs. The value of NWFPs is likely to increase in areas where there is high poverty and an already high dependence on NWFPs, and where there is expected to be an increase in the frequency and intensity of extreme climate events (e.g. droughts, storms and floods) and other natural disturbances (e.g. pests, diseases and fire). The impacts of climate change on NWFPs and the subsequent socio-economic consequences require further study.

The impacts of climate change on forest-related cultural and recreational services are difficult to measure, and have not been extensively studied. Osman-Elasha et al. (2009) have reported on studies of well-defined recreational services in forested landscapes. One example comes from mountainous areas, where skiing activities at lower altitudes is likely to be affected by higher temperatures. The recreational value placed on forests is usually local, and most countries lack reliable projections of the impacts of climate change at the local scale. Osman-Elasha et al. (2009) have also indicated a need for further study of the impacts of climate change on forest biodiversity in Africa and its effects on tourism in national parks.

The expected increases in extreme weather events, such as heat waves, droughts and floods, and the increased risk of fire, pests and diseases will cause additional stress for large forest-dependent populations. The forest-dependent poor, who often rely directly on forests for their livelihoods and for meeting domestic needs related to energy, food and health, will be most vulnerable to these stresses. NWFPs can provide a safety net for rural and urban communities during food shortages. Agricultural crop failures may become more common due to climate change. This will increase the role forests play in providing a safety net and put more pressure on forest resources, especially during crises caused by extreme weather events. Unless properly addressed, the increasing difficulty faced by the forest-dependent poor in meeting basic needs for food, clean water and other necessities is likely to deepen poverty, lead to a deterioration in public health and increase social conflict. Given the risk that crop failures will increase due to climate change, diversifying livelihoods through forest-based products and services could increase the resilience of rural people, especially in areas where the potential of forests to provide livelihoods, for example, through NWFP production and ecotourism, is not yet fully realized. 

In many parts of the world, climate change scenarios project that forest fires will be more frequent and that fire seasons will be longer. The intensity of the fires is also expected to be greater, which could have  significantly harmful effects on human health. Changes in forest cover and biodiversity could reduce access to forest foods, medicines, other NWFPs and timber. This could also affect human health, directly, for example, by lowering the availability of medicinal plants, and indirectly, for example, through the loss of marketable goods. The impact on human health could be felt over the long term, if, for example, indigenous knowledge about medicinal plants is lost.

Changes to the global carbon cycle and their effect on concentrations of carbon dioxide in the atmosphere are crucial in shaping the global climate. Forests play important roles as both sinks and sources of carbon dioxide. Forest vegetation and soils (see module B7) contain about half the planet’s terrestrial carbon, and terrestrial ecosystems have the potential to sequester more carbon dioxide than they currently do. Forests absorb carbon dioxide through photosynthesis, store it as carbon, and release it through respiration, decomposition and combustion. The capacity of a forest to act as a carbon sink increases with the forest’s rate of growth and its ability to retain the carbon on a permanent basis. Vigorous young forests may sequester a great deal of carbon as they grow. In contrast, the vegetation and soils of old-growth forests typically store large quantities of carbon but add to these stocks at a slower rate.

Forests are also sources of greenhouse gas emissions, mainly carbon dioxide. These emissions, which are associated with deforestation and forest degradation, account for an estimated 17 percent of global greenhouse gas emissions. Climate change and increased climate variability have direct and indirect effects on forests and forest-dependent people. For example, there is a disturbing synergy between forest degradation caused by poor logging practices, forest fragmentation and increasingly severe droughts, which has made many Amazonian and Southeast Asian forests more prone to fire. In both boreal and tropical regions, climate change is increasing the susceptibility of forests to stresses that have long been present but which previously posed much lesser threats. When forests and associated social systems are unable to cope with the direct and indirect stresses associated with climate change, they may be considered to be vulnerable to it.

Reducing emissions from deforestation and forest degradation and the role of conservation, sustainable management of forests and enhancement of forest carbon stocks (known as REDD+) will be vital for global efforts to combat climate change. In the December 2015 Paris Agreement on climate change (UNFCCC, 2015), countries agreed to conserve and enhance carbon sinks and reservoirs, including forests. Accordingly, many nationally determined contributions, in which countries set out their responses to climate change, will require action related to agriculture, forests and other land uses.

To achieve the relevant Sustainable Development Goals and implement the actions needed to combat climate change, there is an increasing urgency to gain a better understanding of the drivers behind the conversion of forests to agriculture and vice versa.

Mitigation strategies in the forest sector can be grouped into four main categories: 1) reducing emissions from deforestation; 2) reducing emissions from forest degradation; 3) enhancing forest carbon sinks; and 4) product substitution. Product substitution involves the use of woodfuel instead of fossil fuels for energy and the use of wood fibre in place of materials, such as cement, steel and aluminium, whose production emits larger quantities of greenhouse gases.  

Climate change mitigation measures, including those undertaken in forests, are needed urgently to help reduce anthropogenic interference in the climate system. However, these measures will only begin to have an effect on global mean surface temperature decades from now. Adaptation measures in forests (see chapter B3-1.2) will be required for many years to come to secure the continued delivery of forest goods and ecosystem services.

The forest sector can make a major contribution to the mitigation of global climate change, but realizing this potential requires coordinated actions across crop, livestock and forestry production systems. See module B1 and B2 on, respectively, crop and livestock mitigation actions. The special case of forestry in agriculture has been recognized in recent international efforts to support and coordinate national REDD+ efforts. This recognition reflects the significant greenhouse gas mitigation potential of forestry and agroforestry relative to other agriculture-related mitigation options. Forest degradation and deforestation are driven by two forces: the excessive harvesting of forest products (e.g. timber and woodfuel) and the comparative economic advantage of clearing forests for crop and livestock production (Cattaneo, 2008). The extensive literature on the indirect, or underlying, drivers of deforestation (e.g. Geist and Lambin, 2002; Hosonuma et al., 2012; Pacheco et al., 2011) shows that developments in agriculture outside forest areas, whether they are related to pastures, crops or the production of biofuels from agricultural crops, can have a large impact on the drivers of deforestation (Cattaneo, 2005; Barona et al., 2010; Lapola et al., 2010; Cohn et al., 2014). To manage deforestation and forest degradation, the indirect effects of developments outside the formal forest sector must be taken into account and require landscape-level coordination of management strategies (see B3-3.1). 

Many sustainable forest management practices have clear benefits for reducing forest degradation in forest landscapes (Boscolo et al., 2009). These within-landscape management approaches have important implications for the mitigation of climate change (see B3 Annex 4). However, their large-scale adoption may face multiple financial, institutional and policy-related barriers.

There is an important interaction between the potential for forests to store and sequester carbon and changes in temperature and precipitation. On the one hand, the more carbon that is stored in forests, the less there is in the atmosphere. Increasing stocks of forest carbon, therefore, will help reduce the rate of global warming. This relationship has become important in global discussions on climate change. Many tropical countries are preparing to reduce forest-related emissions and increase forest carbon stocks to capture part of the international funding that has been pledged for reducing greenhouse gas emissions. In Costa Rica, recognition of the carbon storage services of forests led, in the mid-1990s, to the implementation of innovative financing mechanisms for forest management, planted forests and forest conservation (Sánchez Chávez, 2009), and an increased effort to measure the extent of existing natural and planted forests and their carbon content. On the other hand, increasing temperatures, longer dry seasons and increasing carbon dioxide concentrations in the atmosphere are expected to reduce the capacity of forests to store and sequester carbon, possibly converting forests from carbon sinks to carbon sources (Nepstad et al., 2008; Ollinger et al., 2008; Saigusa et al., 2008; Clark et al., 2003). Because the rate of carbon sequestration partly depends on forest productivity, all factors that affect productivity will also affect carbon sequestration. In the short term, increasing temperatures may reduce carbon storage capacity. However, in temperate regions, the effect may vary by season. Early spring warming, for example, has been found to increase carbon sequestration in terrestrial ecosystems, and early autumn warming increases respiration more than the rate of sequestration (Keenan et al., 2014).