INTRODUCTION

In very general terms, gene conservation aims at maintaining the entire gene pool of species and populations for many generations. For many domesticated agricultural crop plants the original natural populations from which they developed have disappeared, and consequently an unknown proportion of their alleles has been lost. In contrast, for many forest trees, e.g. Norway spruce (Picea abies) frequently used as an example in this document, large variable natural populations still exist. In such cases, in situ genetic conservation simply involves conserving representative samples over successive generations. The purpose is not to try to fix the existing gene and genotype frequencies indefinitely, but rather to ensure the existence of broad genetic variation and adaptive potential of the population.

A successful in situ gene conservation programme must fulfill certain fundamental requirements:

1. Regeneration of the population must be assured, and the new generation of trees must (predominantly) originate from matings within the conserved population;

2. The number of genotypes in the conserved population must be large enough to include most of the common alleles (frequency >0.01) which orignially existed in the population of which it is a sub-sample; and

3. The network of conservation stands must be sufficient to cover the spatial genetic variation present in the species.

The demarcation of in situ conservation stands and the establishment of guidelines for their management must be based on the above requirements.

A great advantage of in situ conservation is that the tree community already exists. Thus, the establishment of a gene reserve need involve neither expensive measures nor delays. A priori, the continuity of natural populations might be taken for granted, however, in fact the ecological dynamics of forest communities may not function in the present as they did in the past. Through the impact of man, regeneration of forests may either be stopped completely, as in case of change in land use; or the autochthonous (local origin) population might be replaced with an introduced species and/or a non-local provenance. Thus, there is a need to clearly state a number of rules and conditions for successful conservation. Those guiding the present paper are based on two biological facts:

1. Trees are living organisms which eventually die, at which time they cease to directly contribute to the genetic evolution of the population. A very old stand of trees is in fact quite vulnerable to harmful abiotic and biotic factors. Therefore, it is important that regeneration is ensured, and that more than one age class occurs in the gene reserve forest at any one time. In many wind-pollinated temperate tree species an uneven-aged stand structure is difficult to maintain. A mosaic of smaller stands of even-aged trees with an age class distribution which is spatially diversified, is easier to aim for and more desirable.

2. Wind pollination is a random process that wastes vast amounts of pollen, but which is more effective in large stands and in stands possessing a high density of the target species. Even large pollen grains of wind pollinated trees, such as those of Picea abies, are carried over long distances by wind gusts. On the other hand, adequate pollination of female strobili requires a dense pollen cloud, which is formed only in stands of several hectares. In small stands the total pollen pool is limited and the proportion of self pollination is likely to be greater. Consequently, the amount of viable seed and overall vigour of seedling offspring is reduced. In areas where the species in question is common the pollination of small stands originates predominantly from outside sources. In the case of autochthonous populations, such "background pollination" from other natural stands does not cause any harm in terms of gene conservation. Quite often, however, stands of foreign and unknown origins exist in the surroundings, and gene flow from these stands is undesirable. The presence of related, naturally hybridizing species or provenances in the vicinity of proposed gene reserve forests is thus totally unacceptable.

Based on considerations related to reproductive biology and the need to ensure the presence of a range of age classes, it has been suggested that the ideal size for an in situ gene conservation unit of wind pollinated temperate conifers should be at least 100 ha. Although this is not much if compared with the size of most national parks and other protected areas, it is often not easy or even possible to identify such relatively large areas for genetic conservation purposes. Under certain conditions smaller areas will suffice, at least as a first step, as "nuclei" which can be enlarged in the future.

Genetic conservation in situ may take place both in unmanaged, strictly protected forest areas and in managed forests. The unmanaged category includes various types of protected area, such as nature reserves and national parks. In addition to their traditional roles and functions in ensuring the conservation of ecosystems and species, protected areas can help maintain intra-specific genetic variation of tree species. However, protected areas alone are not sufficient to ensure the conservation of genetic resources, due mainly to the following:

1. The distribution of protected areas and their coverage is seldom adequate, because they have been selected for other purposes and, rather than being representative of a range of different forest types, they often include only rather specific environments (e.g. of scenic beauty);

2. Access to genetic resources may be restricted, or may be limited by law; and

3. The continued presence, over time, of a given target tree species or population is by no means assured without management to favour such presence.

While legislation and practice vary from country to country, and while the severeness of restrictions for access and intervention varies among categories of protected area, the general approach to their management is passive (non-intervention), and the desired aim is generally a stationary condition of the ecosystem.

ESTABLISHMENT OF GENE RESERVE FORESTS

The constraints to conservation of genetic resources associated with protected areas can largely be avoided through the demarcation of managed in situ gene reserves. The most crucial requirement for genetic conservation is that the reserve should continue to exist into the indefinite future. This requires that the area in question is in permanent ownership and is not subject to changes in land use, such as building of highways or infrastructure. Maintenance through adequate silvicultural measures, protection against fire etc. must be regularly carried out. From an economic point of view the requirements are not as restrictive as in the case of conventional protected areas. The required area, 100 ha, is modest in relation to the size of most protected areas. Timber and wood harvesting is allowed on a sustained basis, and this will give income to the owner.

In spite of the above flexibility, the long time commitment to genetic conservation generally limits efforts to state-owned forest land and to relatively large forestry companies or owners. Ownership is thus not a major problem in countries where forests are mainly state owned.

Basic requirements

1. All stands and trees of the species to be conserved shall be autochthonous. Plantings of trees of foreign or unknown origin do not qualify. Small patches of undesirable or un-known origin should be removed in order to ensure that only trees of local origin remain. Areas exposed to heavy selective harvesting or other kinds of intensive operations should not be included. Gene reserve forests may be either monospecific for the target species, or may consist of a mixture of more than one species.

In some cases it may be acceptable or desirable to conserve well established "land races". This is especially the case when the autochthonous populations of an important introduced species are under severe threat, or have been totally destroyed.

2. The target area shall be _100 ha with the shortest diameter being _400 m. The entire conservation area need not consist of a fully intact, even-aged forest, but may consist of parcels of several age classes, even open areas, and it may include components of other, non-hybridizing, species.

In areas where the species in question is uncommon, smaller areas (<100 ha) may sometimes be accepted as gene reserve forests. Such areas must, however, have a potential to be regenerated and possibly enlarged using reproductive materials from the stand itself. The minimum size of a gene reserve must be decided on a case-by-case basis according to the circumstances. Stands smaller than 2 hectares are normally not suitable, and the potential area for expansion of the conservation stand by means of replanting or direct seeding, should be > 10 ha. Such small areas should be used only in special cases where the natural populations have been greatly reduced and survive only as remnants. Forests treated with selective cutting over generations may have suffered from genetic decline or erosion; therefore, selection forests are not likely to be suitable as gene reserve forests.

REGISTERING OF GENE RESERVE FORESTS

Once a suitable area has been identified as a gene reserve forest and an agreement on the reservation of the area has been done, the forest needs to be officially registered. Information on the location, characteristics, ownership etc of the gene reserve forest should be supplied to a national registering authority. Prior to registering the area the owner/manager of the gene reserve forest should have formally agreed to follow a set of guidelines for its management, and a management plan (also to be held by national registering authority)should have been formally approved. The area needs to be clearly demarcated and depicted on a map. While it is recommended that an appropriately positioned sign-board be erected in the stand, permanent marking in the field is not always necessary, and as a rule the area may remain un-fenced. However, fencing may sometimes be necessary to protect regeneration from grazing animals.

MANAGEMENT OF GENE RESERVE FORESTS

The objective of management is to ensure the continued existence of target populations. Thus, firstly, management should aim to protect the stand against calamities. Secondly, management should aim to create conditions which are favourable for natural regeneration of the stand.

Let us first consider the case of seed stands, as such stands often also serve the purpose of gene conservation. Seed stands of temperate conifers such as Norway spruce commonly comprise mature or over-mature, even-aged stands of tall trees of superior phenotype. Ex situ conservation, which in this case involves collection and long-term storage of viable seed, is an important back-up conservation measure. Bulked seedlots, collected from _100 trees, can provide insurance against catastrophic loss of the in situ conservation area, and can also be used as a source of germplasm for gradual artificial regeneration of the conservation stand. Truly representative material is best collected in mast years, when almost all trees carry cones. In the case of Norway spruce, such abundant seed crops are produced at irregular intervals. Therefore, it is recommended that seed be collected during the first mast seeding after designation of the gene reserve forest.

As mentioned above, old stands are often vulnerable to harmful abiotic and/or biotic agents. Pollutants, pests and diseases adversely affect older trees to a greater extent than younger, more vigorous trees. Conservation stands will need to be periodically regenerated. In many species such as Norway spruce, this is a rather complex process. Large openings in the stand may induce wind throw damage and promote vigorous growth of ground vegetation which competes with natural regeneration. Careful opening up of the stand through cutting of strips, not straight ones; or small patch clearings, seems to be the best way to encourage regeneration. Regeneration can be promoted by means of site preparation, weed control; sowing or planting can also be resorted to using seed from the stand itself. After 5-10 years, a second phase of regeneration adjacent to the first regeneration strip/patch can be initiated, and so on. Felling and removal of large trees is difficult and easily causes damage to remaining trees. However, simply leaving Norway spruce forest in an unmanaged state will eventually lead to a blind end. In nature, the natural succession of spruce forests involves complete destruction of the stand by wildfire, followed by regeneration of broad-leaved species and, some 50 years later, re-colonization by spruce in a manner which is often not compatible with genetic conservation because of the extent of forest fragmentation. Thus, only if the gene reserve forest is >100 ha it may be acceptable to leave a portion of the stand in an unmanaged condition.

Thinning is not only permissible in gene reserve forests, but a necessary measure to ensure the continued vigour and regeneration of stands. It is sometimes argued that thinning is a form of unnatural selection which can modify the genetic composition of the stand. This, however, will depend on the procedures used to select trees for thinning and would hold true mainly for the current population; the next generation will, in any case, have another likely different genetic composition, regardless of thinning.

It is impossible to give detailed thinning prescriptions for gene reserve forests to cover all circumstances. Timely thinning is important in order to avoid over-crowding and its negative consequences on health, vigour and seed production. Most frequently, gene reserve forests will be predominantly old or mature forests. Thinning is then mainly from below, removing suppressed, damaged and un-thrifty trees. If the stand is originally very dense, the operation must be carried out gradually and requires careful planning. Falling large trees may injure remaining ones, and heavy machinery may damage bark and roots. Excessive thinning, leading to wide spacing of trees, may expose the stand to excessive solar radiation or wind throw. Steep slopes create special problems. Small openings or narrow, not straight, strips may be suitable, and will allow gradual regeneration of the stand. In every case, the local professional forester needs to be involved in selecting the most appropriate mode of thinning.

Younger stands are easier to manage. Moderate thinning enhances the growth of dominant trees and maintains the stand in a healthier condition. The thinning should be mainly from below, with probable "losers" being removed in advance. On the other hand, if any special or rare phenotypes are present, then it may be decided to retain these. In some cases uneven-aged stands are designated gene reserve forests. In such cases, thinning is mainly from above.

Timber harvesting is the operation which might most obviously distinguish gene reserve forests from protected areas; however the purpose of harvesting, in this case, is not to maximize net income. Harvesting is an essential part of active and dynamic forest management, and aims to maintain stand vigour and regeneration potential. Selling of merchantable timber can partly offset economic losses associated with conservation. Needless to say, extreme care needs to be exercised in harvesting operations: damage to remaining trees must be kept to an absolute minimum. "Clear cutting" is acceptable, but the openings must be rather small (<5ha). In particular on exposed sites and loose soils, the stability of the adjacent trees must be taken into account. As a rule, discrete openings or corridors are preferable to selective cutting, i.e. the removal of individual trees. Selective cutting is both less cost efficient and more likely harmful from a genetic standpoint.

In younger, even-aged stands the first thinning may be delayed until trees have reached dimensions of merchantable round wood. Normally the first thinning is however from below, and consequently economic returns are not great. At this stage of development of the stand, harvesting of the tallest, best-formed trees of higher value is incompatible with the gene conservation function.

REGENERATION OF GENE RESERVE FORESTS

Regeneration is a critical aspect of in situ genetic conservation. Natural regeneration comes readily to our mind in this present connection, but we cannot always rely on the success of this method. The entirely natural process, including forest fires and successional alternation with broadleaved species, is seldom applicable. For example, an uncontrolled forest fire would burn not only the gene reserve but may burn nearly everything in the vicinity.

An ideal gene reserve forest is a mosaic of stands of various age classes. This means that the whole area of 100 ha or more, would never be purposefully simultaneously regenerated. A plan needs to be made for gradual and continuing regeneration of the gene reserve forest, which includes both spatial and temporal elements. If the entire stand consists of mature or overmature trees, then establishment of new generations must be started without delay. However, if the area includes sapling stand and/or pole stage stands, then further regeneration may be delayed.

The actual operation of regeneration must be tailored to local circumstances. Detailed, universally applicable instructions cannot be given. One principle is to keep the size of the areas to be regenerated at any one time rather small, up to a maximum of 5 ha. On the other hand, several separate regeneration areas may exist simultaneously in the same gene reserve.

Regeneration from natural seeding is the most desirable and economical method. As a rule, site preparation needs to be undertaken prior to seeding, and weed control is frequently necessary during the seedling stage. In situations where natural seeding is poor or does not adequately cover the openings, direct sowing into properly prepared sites can be effective. However, in many cases the most rapid and successful regeneration method is by planting. This holds true especially on fertile soils rich in organic matter. Planting, like direct seeding, is a fully acceptable method of regeneration of in situ conservation stands, provided that the reproductive materials used are of local origin, i.e. collected in the gene reserve forest itself.

Seed is to be collected from _100 trees, preferably from the central, inner parts of the stand. Seed used should be a mixture of seedlots collected from all, or most, trees in the stand, in other words be bulked seed representing as many trees in the stand as possible. It is recommended that this kind of bulk seed be collected during the first abundant seed year and kept in safe storage for future use. To be absolutely certain that enough seed will be available to gradually regenerate the gene reserve forest or, in a "worst case scenario", to regenerate the entire stand if destroyed by natural or man-made calamities, it is recommended that a minimum of 10 kg of seed be stored in the case of Norway spruce.

UTILIZATION OF GENE RESERVE FORESTS

The main purpose of in situ gene reserves is to maintain natural genetic variation over long periods of time. It is difficult to envisage all the potential uses of genetic resources conserved in such reserves, but future generations will certainly not thank us if we do not take care and conserve our forest genetic resources. Gene reserve forests have the advantage that they can be utilized in several ways, instead of standing as protected relicts. The economically most important advantage of gene resource forests in countries such as Finland, is their continued availability and use for timber and wood production. In addition, they can serve the following purposes:

1. Seed source. Commercial quantitites of seed can be collected from standing trees and also from felled trees during thinnings and regeneration cuttings. The seed can be a highly valued or useful provenance seed source to cater for the needs of also other areas.

2. Reference population. Local natural populations provide suitable reference material for use in provenance trials and progeny tests ("controls" in field trials). In situ gene reserve forests are intended to remain as samples of dynamically evolving natural populations. Seed collected from the internal portions of a large gene reserve forest is likely to adequately represent the local, natural gene pool, even in cases in which planted stands of foreign origin, or established using genetically improved stock of a relatively narrow genetic base, become dominant within the district as a whole.

3. Research population. Genetic structure of natural populations, evolutionary processes, and co-adaptation of trees and their parasites or symbionts, are important research issues which can be addressed in gene reserve forests.

4. Complementary breeding population. It is likely that, in future, new kinds of selection procedures will be developed, such as DNA-methods which might provide a new tool to select desirable genotypes directly. Also, presently unknown or un-selected traits may turn out to be valuable in the future and, consequently, complementary selection may become necessary. The gene reserve forest can provide genetic material for such new or complementary selection.

CONCLUSIONS

In conclusion it can be noted that management of forests for in situ genetic conservation purposes, is quite close to normal silvicultural practice. Conservation of a range of locally common genes and allele frequencies is the crucial issue. This is certainly within the capacity of trained professional foresters. Awareness and a basic understanding of the objectives and methods of genetic conservation and its incorporation into forest management, can further motivate the forestry profession and the public in general to make greater efforts, and can thus provide a decisive contribution in this crucially important field.