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Forest genetic resources refers to the heritable materials maintained within and among tree and other woody plant species that are of actual or potential economic, environmental, scientific or societal value (FAO, 2014b). As mentioned in the Millenium Ecosystem Assessment (MEA), forests are home to the vast majority of the Earth’s terrestrial biodiversity (MEA, 2005), and trees are the keystone species of forest ecosystems. Forest trees differ from other plant species in their capacity to maintain high levels of genetic diversity within populations rather than among populations (Hamrick, 2004). This results from their outcrossed mating system, extensive gene flow and large population sizes (Petit and Hampe, 2006). Forest trees and other woody plant species provide wood, fibre, fuel and many non-wood forest products. They also contribute to a broad range of ecosystem services and fulfil environmental functions. According to Botanic Gardens Conservation International (BGCI), there are approximately 60 000 tree species (BGCI, 2017), but only very few have been studied in any depth for their present and future potential. Globally, around 2 400 species of trees, shrubs, palms and bamboo are actively managed for products and/or services, and approximately 700 tree species are subject to tree improvement programmes (FAO, 2014b).

Tree populations can respond to climate change in three different ways: exhibiting phenotypic plasticity^xix; adapting to new climatic conditions; or migrating to new areas with more suitable climates (e.g. Aitken et al., 2008). Most tree species have a high degree of phenotypic plasticity (e.g. Rehfeldt et al., 2002). This means that tree populations can grow well under a range of climatic conditions around their climatic optimum. When tree populations adapt to local conditions, they generally maintain high genetic variation in their adaptive traits (Savolainen, Pyhäjä and Knurr, 2007). For this reason, some scientists (e.g. Hamrick, 2004) consider that many tree populations have sufficient phenotypic plasticity and genetic diversity to enable them to adapt reasonably well to climate change. However, others foresee significant problems (e.g. Mátyás, 2007; Rehfeldt et al., 2002). 

The migration rate of forest trees is often over-estimated. This is due to the fact that many species have extensive gene flow, especially through pollen. In wind-pollinated forest trees, gene flow can be 100 kilometres or more. Climate and species distribution models indicate that migration rates should be more than 1 kilometre per year to allow plants to follow the predicted shifts in their current climatic niches (Malcolm et al., 2002). However, it has been estimated that the post-glacial migration rates of temperate forest trees were less than 100 metres per year (McLachlan, Clark and Manos, 2005). Therefore, it is unlikely that forest trees would be able to cope with the current rate of climate change through natural migration. Most scientists agree that measures, such as modified silvicultural practices and assisted migration, are necessary to facilitate the survival, adaptation and migration of forest trees under changing climatic conditions, and that selection and breeding strategies need to be redesigned (Alfaro et al., 2014). 

In addition to affecting physiological and genetic processes in forest tree species, climate change is expected to affect forest genetic resources through its impact on ecological processes. Changes in temperature and precipitation may inhibit the capacity of trees to regenerate and alter the composition of tree species in forests. Climate change may also break the synchronism between the flowering periods of trees and the active periods of pollinator species. A decline in the availability of pollinators limits gene flow and reduces the effective size of tree populations, impeding their capacity to adapt to climate change. Invasions of problematic species may also become more common, with native trees being outcompeted by species that can migrate and reproduce rapidly.

As the climate changes, the distribution range for some tree species are expected to expand, while for others it may shrink. In temperate regions, the ranges of tree species are likely to shift towards the poles and towards higher elevations. The retreat at the receding edge of species’ distributions is likely to be more rapid than the advance into new areas. In tropical regions, changes in rainfall regimes may be the most important climatic factor influencing tree distribution. Research has indicated that a dry climate is a particular barrier to migration for tropical tree species (e.g. Muchugi et al., 2006, 2008). As in temperate regions, natural migration rates in the tropics will not be sufficient to keep pace with the predicted rate of climate change.

Some invasive species may be exceptional in this regard, as they may have a greater capacity to respond rapidly to changing conditions, either because their seeds are dispersed over very long distances or because they reach maturity very quickly. 

In the future, extreme weather events that kill large numbers of trees may become more common. In some places, pest and disease attacks may become more severe as climatic conditions become more favourable for pests or because climate-induced stress makes trees more susceptible to attack. Climate change is also likely to have a significant effect on the distribution of insect pests of trees.  

For forest genetic resources, it is important to characterize genetic diversity both at a broad scale that includes the entire distribution range of a species, and on a finer scale within the species itself. This is needed to increase the understanding of the adaptation of forest trees to different climatic conditions. Characterizing genetic diversity is done for two main purposes: to support conservation planning and forest management; and support tree breeding and improvement. In the first case, the activities may include identifying tree populations with high genetic diversity for in situ or ex situ conservation, and describing the relationships between genetic variation and environmental variables to define seed zones within which transfer of genetic material is recommended. In the second case, activities include the identification of individual trees with desirable characteristics for breeding and the selection of tree stands for production of forest reproductive material (i.e. seeds, cuttings and other propagating parts of the tree).  

In addition to characterizing morphological differences within tree species, provenance and progeny trials have long been used in forestry for analysing genetic variation in quantitative traits related to growth (e.g. diameter increment), physiology (e.g. nutrient or water use efficiency) and phenology (e.g. bud burst). Laboratory-based techniques based on molecular markers have also been applied for studying genetic variation among tree populations. However, as many of these techniques use neutral markers, they mostly reveal historical and demographic processes. New genomics tools offer possibilities to link genetic diversity at the molecular level, or even individual genes, to adaptive traits (Neale and Kremer, 2011). Progress is being made in whole genome sequencing and marker-assisted selection in several tree species. The next challenge is to link the increasing amounts of gene-level data to phenotypic data from tree populations. 

Genetic monitoring of tree populations is needed to verify how well genetic diversity is maintained over time, and how this diversity is shaped by forest management practices and climate change. However, considering the number of tree species, it is impossible to implement genetic monitoring in all or even most tree species. A genetic monitoring system should be based on a sample of tree populations, such as permanent forest monitoring plots (Konnert et al., 2011) or conservation units (Aravanopoulos et al., 2015).

Forest genetic resources are used by people to grow trees for many purposes from obtaining wood and non-wood products to providing a range of other ecosystem services. The utilization of forest genetic resources started millennia ago. However, the use and international transfer of forest genetic resources have been more extensive during the past 200 years (Koskela et al., 2014). The oldest form of research and development is testing of tree species and their provenances for different purposes and under different environmental conditions. The main purpose of provenance research is the identification of well-growing and sufficiently adapted tree populations to be used as seed sources for tree planting (König, 2005). The results of provenance research have also been used for initiating tree breeding programmes (FAO, 2014b) and, since the 1990s, for studying the impacts of climate change on tree growth.

Tree breeding typically aims at a gradual improvement of breeding populations rather than development of new varieties. It is a slow process, as one cycle of testing and selection may take decades. Traditional tree breeding is based on the phenotypic selection of individual trees in wild populations (referred to as ‘plus trees’), testing their progeny and then reselecting the best individuals for the establishment of seed orchards and further breeding. The testing is focused on growth, wood properties, resistance or tolerance to pests and diseases, and other traits of commercial interest. Traits related to climate change adaptation, such as plasticity and drought tolerance, are also increasingly being considered by tree breeding programmes (FAO, 2014b). New breeding approaches, such as molecular marker-assisted selection, have raised hopes for reducing the long breeding cycles. However, the fact that a trait is typically influenced by a large group of genes in forest trees and the variable expression of quantitative trait loci across environments have slowed down progress in applying this new approach to forest trees (Neale and Kremer, 2011). 

In response to climate change, many countries have pledged to restore millions of hectares of forests. These forest restoration efforts will further increase the demand for forest reproductive material. Unfortunately, there are not enough seed orchards for many tree species to meet the current demand for reproductive material. Seed collection from wild tree populations (seed stands) remains a common and much needed practice. For trees species in which the germination capacity of seeds starts declining right after their maturation (recalcitrant seed behaviour), naturally regenerated seedlings (wildings) can be collected for tree planting elsewhere. Forest reproductive material is also produced through vegetative propagation, with multiplication from rooted cuttings being the most frequently used method (Wilhelm, 2005). Micropropagation methods, such as microcuttings and somatic embryogenesis, are increasingly being applied in forestry (FAO, 2004).

Key approaches for using forest genetic resources to promote the adaptation of forests to climate change are presented in box B8.3.

As discussed in module B3 on climate-smart forestry and in chapter B5 - 3.1 on the contribution of agroforestry to climate change mitigation, conserving and enhancing existing carbon stocks in forests, establishing new forests, integrating forestry into crop and livestock production systems, and planting trees on agricultural lands offer considerable opportunities for mitigating climate change (Alfaro et al., 2014). 

Genetically diverse tree populations are crucial for long-term mitigation of climate change and for maintaining the capacity of forests to adapt to changing climatic conditions (Alfaro et al., 2014). However, the important role of forest genetic resources is often poorly understood by policy makers and forest managers. For example, as mentioned above, as part of global efforts aimed at restoring forests and reducing deforestation and forest degradation, countries around the world have expressed their commitment to massive tree planting efforts targeting millions of hectares. However, many countries still have problems related to the quantity and quality of forest reproductive material (FAO, 2014b). This is often due to lack of well-functioning national tree seed systems capable of supplying adequate amounts of reproductive material for producing seedlings for tree planting efforts. The establishment and maintenance of national tree seed systems have proven challenging for many countries. These systems are still struggling to reach various users of tree germplasm, in particular smallholder producers (Graudal and Lillesø, 2007).

Between 2010-2015, the global area of planted forests increased by 3.2 million hectares per year and reached 7 percent of the world’s forest area (FAO, 2016b). Considering the current and planned tree planting efforts to mitigate climate change, as well as a growing demand for forest products and environmental services, the area of planted forests can be expected to continue increasing in coming years or even decades (Box B8.4). Presently, the global supply of reproductive material for boreal and temperate trees, as well as for fast-growing tropical and subtropical trees, seems to meet or exceed demand, while for tropical hardwoods, demand is often higher than supply (Koskela et al., 2014). For agroforestry tree species, a lack of high-quality tree germplasm prevents smallholders from increasing the productivity of their agroforestry systems (e.g. Nyoka et al., 2011). As tropical hardwoods and agroforestry tree species are increasingly favoured in climate change mitigation and forest restoration, research and development efforts should focus on increasing the supply of tested and improved germplasm of these species, and developing novel genomic approaches for identifying and selecting trees with required traits for propagating planting stock and tree breeding. For practical forest management, genetic considerations, such as maintaining genetic diversity and selecting an appropriate provenance for a given site, should be given more attention in both tree planting efforts (e.g. Bozzano et al., 2014) and the management of  existing forests (e.g. Ratnam et al., 2014), as this can significantly increase the long-term success and sustainability of these actions.

The preferred approach for conserving forest genetic resources is in situ conservation because it is a dynamic process that allows for forest genetic diversity to change over time and by location. Ex situ conservation, on the other hand, is mostly static, maintaining the genetic diversity of the sampled species in a fixed state. The main goal of in situ conservation is maintaining the evolutionary processes (i.e. natural selection, genetic drift, gene flow and mutation) within tree populations, rather than simply preserving their present day genetic diversity (e.g. Lande and Barrowclough, 1987; Eriksson, Namkoong and Roberds, 1993; FAO, FLD and IPGRI, 2004a). Given that present day conditions will change along with the climate, this dynamic approach is crucial for the long-term conservation of forest genetic resources. It is also often easier and cheaper to conserve tree populations in their natural habitat than under ex situ conditions. 

In situ conservation of forest genetic resources is typically carried out in protected areas or managed natural forests by designating specific conservation stands or units for this purpose (FAO, DFSC and IPGRI, 2001). These units may harbour conservation populations for one or more tree species. Silvicultural treatments are applied, if necessary, to maintain or enhance genetic processes within tree populations. Ideally, the network of these conservation units should cover the whole distribution range of a tree species.

Forest genetic resources are also conserved ex situ in seed banks, seed orchards, field collections, provenance trials, planted conservation stands and botanical gardens to complement in situ conservation, especially when the population size is critically low in the wild. In forest trees species that have orthodox seeds  (i.e. seeds that maintain their viability when dried and stored at low temperature), ex situ conservation is relatively easy. However, many tree species produce recalcitrant or intermediate seed, which lack dormancy and are sensitive to both desiccation and low temperatures. This presents a major difficulty for conservation, especially in humid tropics, where more than 70 percent of tree species have recalcitrant or intermediate seed behaviour (Sacande et al., 2004). Ex situ conservation of these species is carried out in field collections, ex situ conservation stands and breeding populations. More technically sophisticated approaches, such as cryopreservation, seedling conservation, in vitro conservation, pollen storage and DNA storage, are also used (FAO, FLD and IPGRI, 2004b). 

Climate change is expected to alter or increase biotic threats (e.g. pests, diseases and species competition) and abiotic threats (e.g. fire and land-use changes) to tree populations. The climatic niches of tree species are also predicted to shift as a result of climate change. It is important to assess the vulnerability to climate change of individual genetic conservation units, as well as the networks of these units and their distribution range, and identify high-risk units for further monitoring and complementary or enhanced conservation measures. In Europe, for example, it has been predicted that the genetic conservation units located in lowlands and in the southern edges of distribution ranges of tree species will be the most vulnerable to climate change (Schueler et al., 2014). It is necessary to incorporate climate change considerations into the development and implementation of both in situ and ex situ conservation strategies (e.g. Kelleher et al., 2015).