Conservatoire Génétique des Arbres Forestiers ONF, Centre INRA, 45160 Ardon, France

The genetic resources present in the thousands of useful tree species on the earth constitute an intergenerational resource of vast social, economic and environmental importance. Most of the forest species are characterized by extensive natural range and high levels of diversity. This high genetic variation at the level of individual trees, within and among the populations, enables adaptation to the environmental conditions they face during the very long life cycle of forest trees and forest stands. Nevertheless, many tree species are considered to be threatened because of global climate change, pollution, reduction of the forest and uncontrolled seed transfer, and require the intensification of joint efforts for genetic conservation. These efforts must be based on a better knowledge of the diversity and the genetic structure of the species. This strategy emphasizes the preservation of genetic variation and hence evolutionary adaptability of populations to a changing environment over generations, as the main goal of gene conservation. The preservation of genetic variation is also crucial for its effective use for human needs in the future. From a more practical point of view, the conservation of forest genetic resources must be taken into consideration not only for timber and other forest products, but also for a whole range of values provided by forest ecosystems, including the ecological and social benefits not limited by national boundaries.

The genus Sorbus complex (Sorbus torminalis, Sorbus aria, Sorbus aucuparia and Sorbus domestica) belongs to the Rosaceae family. This indigenous genus exists in Europe from temperate to boreal countries and from mountain to plains. These widespread species typically grow in mixed hardwood forests. They are characteristic of ancient and undisturbed lowland. Despite their weak competitive ability they are mostly to be found in extreme sites where their competitors do not survive. Most of the Sorbus species are interfertile, even sometimes with related species of Malus and Pyrus. In most countries represented in the Noble Hardwoods Network, S. torminalis and S. aria are not considered to be really threatened as species, but sometimes as populations. On the contrary, S. domestica is considered under threat by most of the countries. The countries in which Sorbus species are considered to be threatened are shown in Table 1C.

The main causes contributing to the endangerment of Sorbus are:

• extensive cutting for commercial purposes

• narrow genetic base for small effective population sizes

• lack of natural regeneration

• competition with other species

• inadequate silvicultural management

• uncontrolled seed transfer.

Some countries have already developed gene conservation programmes on Sorbus, but most have only just started.

Sorbus species flower regularly everywhere in their distribution area and seed production is not a limiting factor. Although seed production is often abundant, germination and regeneration are frequently low, especially for S. domestica. For the time being, it seems that the species are only insect-pollinated. New studies on S. aucuparia seem to show that self-pollination is not possible (O. Raspe, pers. comm.). Nevertheless, partial self-pollination for isolated individuals cannot be excluded.

For Sorbus, vegetative regeneration is important. Moreover, these species are very sensitive to competition; pure stands are non-existent. Isolated trees or small patches are most common.

Because of the high economic value of timber and especially for S. torminalis, the increasing demand on the market, uncontrolled seed production and uncontrolled seed transfer takes place. Indeed an early exploitation of stands before their regeneration may promote plantation with uncontrolled material. Seed transfers occur among provenances and countries. For these species the demand for afforestation material is higher than the supply. The seed transfers may pose a threat to local populations because of introgression with other populations which can be genetically very different. The other species less intensively used for wood production are in the same situation, for example S. aucuparia used for landscaping. Important seed transfer occurs across Europe for ornamental plantations.

For a realistic programme of gene conservation, one of the key issues is to gain better knowledge of the genetic variability of Sorbus. Very little is known about these species. Some countries are starting to undertake research on this topic (Table 1D). The first results of genetic analyses indicate that S. torminalis has relatively high population differentiation compared with other disseminated species and especially Prunus avium. But the geographical structure seems to be very weak (Prat and Daniel 1993). New studies must be carried out with more populations all over the range and with more markers including adaptive traits which are an important component of the diversity. Moreover, these characters are often strongly linked with genetic structure. The characterization of all aspects of diversity is necessary for an efficient strategy or a policy of gene conservation.

1. To preserve sufficient genetic variability to maintain the adaptive potential of Sorbus, it is necessary to:

• identify and protect very small autochthonous populations under threat because of their small effective size

• protect and conserve the genetic variability of autochthonous populations.

2. In situ conservation

In situ methods, the main component of gene conservation, maintain the genepool of natural populations and species over many generations. In situ conservation, where evolutionary forces in populations are very close to natural conditions, is appropriate for saving populations and maintaining their genetic adaptability within individual species. The following guidelines can be given:

• network of gene conservation stands must be sufficient to cover spatial variation of the species

• effective population size must be maintained for a good system of regeneration

• the regeneration stock must originate from matings within each population

• if artificial regeneration is necessary, seeds must come from the same stand or neighbouring stands in the same area

• adequate forest management must be promoted.

3. Ex situ conservation

Ex situ methods aim at maintaining the material from endangered stands (destruction by natural or anthropic effects, or lack of regeneration) with no influence of evolutionary forces on the collections. This is the case with clone collections or seed banks. The main aim for ex situ conservation will thus be to maintain or enhance the existing genetic structures (and thus the existing adaptability). Ex situ conservation can also be used to develop breeding programmes for high-quality wood.

Ex situ conservation is complementary to in situ methods and is especially important when the conditions for in situ conservation cannot be applied. Indeed it can be recommended for:

• Endangered populations: within the distribution range of Sorbus the further existence of small populations under threat, and especially when the efficient size of the populations is too low. The populations can be easily maintained by planting or grafting.

• Establishment of clone collections: the material must come from the whole range and must be planted in different areas. The different species of Sorbus must be conserved separately in order to avoid hybridization. The collection can serve for static conservation and also for seed production; for its establishment it would be recommended to:

  • sample with respect to genetic variation within and between the stands

  • sample at least 10 stands within each region of provenance

  • collect seeds in a year of abundant seed crop and from 20-30 trees well distributed in the population and 30-50 m apart

• Establishment of seed banks: Gene conservation can be achieved through the storage of bulked seed lots with precise identification of sampled individuals in a region of provenance. An adapted procedure of seed drying methods can be used. Temperature above zero (2-4°C) can be recommended for short-term storage. Another purpose for seed storage can be to ensure good supply of afforestation material. For the time being, only the principal forestry species benefit from national and international rules on seed transfer. No long distance transfers should be encouraged.

The conservation strategy for the genus Sorbus can not be considered as a whole and has to differentiate among the needs of individual species. It seems that S. domestica is more endangered than the other species, few populations remain and the recommendations for this species are more focused on ex situ conservation than in situ. On the contrary, S. torminalis and S. aria populations are not threatened and in situ conservation is more suitable for them.

Of course all these required activities are implemented on a voluntary basis. It is clear that Noble Hardwoods conservation is just beginning, and no programme is totally sufficient at the moment.

Prat, D. and C. Daniel. 1993. Variatilité génétique de l'alisier torminal et du genre Sorbus. Rev. For. Fr. XLV(3):216-228.