Swiss Federal Institute for Forest, Snow and Landscape Research, Birmensdorf, Switzerland

Introduction

Norway spruce is the most abundant tree species in Switzerland. The total forest area of the country covers 1.2 million hectares and Norway spruce constitutes 49% of the total growing stock (Brändli 1996). The species fulfills important protective, social and economic functions. Its stands help to prevent avalanches, erosion and landslides, reduce the risk of flooding and block rock falls. Many stands are recreational areas and contribute to the scenic beauty of the cultural landscape. The annual harvest of Norway spruce is approximately 2.3 million m³.

Norway spruce as a species is not considered to be threatened in Switzerland. Norway spruce stands, however, were largely affected by extensive exploitation in the past (Leibundgut 1986) and human impact continues mainly by environmental pollution. This and the predicted climate change are likely to have negative effects on genetic variation of the stands. To maintain the ability of stands to adapt to a changing environment, preservation of a broad genetic variation is an important factor for maintaining their stability (Müller-Starck et al. 1995). Here we report on the activities performed in Switzerland to conserve genetic variation of Norway spruce.

Norway spruce is common and widespread in most parts of Switzerland (Table 1). The main range of the species is in the Alps, the Pre-Alps and the western Jura. It is less frequent in the southern Alps and the remaining Jura and is rare in the southern and western parts of the Tessin. Norway spruce is the predominant forest tree in the upper montane and the lower subalpine vegetation zones (Brändli 1996). In the lower subalpine vegetation zone it represents 75% of the total growing stock. In this zone Norway spruce often grows in pure stands whereas in the upper montane vegetation zone it is associated with silver fir and beech. Norway spruce stands are found at elevations of up to 2000 m whereas small groups of trees are encountered up to 2200 m (Leibundgut 1984).

Distributions of fossil pollen indicate that Norway spruce in Switzerland is derived from the glacial refugium in southeast Europe (Huntley and Birks 1983). Norway spruce colonized the country in the east and southeast some 8000 years ago and expanded to the valleys of the Alps within 3500 years (C. Burga, unpublished). The expansion in the region north of the Alps was much slower and took more than 6000 years because of competition with beech, oak and silver fir. Recent results from pollen analyses indicate that Norway spruce in the Jura immigrated from west either from relic populations in France or following an east-west route to Savoy and then westwards to the Jura (C. Burga, in preparation).

In the past, forest stands in Switzerland were largely affected by human activities. Destruction of stands started in Roman times and continued in the Middle Ages by cuttings for timber and creation of pasture land. In the late Middle Ages extensive cutting was done to obtain timber for construction, firewood, salt works and ore mining. In the Alps more than two-thirds of the subalpine forest stands were eliminated (Leibundgut 1986).

Source: Bachofen et al. 1988.

Destruction of stands caused an increase in erosion, avalanches and flooding. Following extensive flooding in 1865, the first federal forest legislation was established in 1876. The law forbade clear-cutting in the Alps. This and the import of coal reduced the pressure on the forest stands. The devastated Norway spruce stands recovered naturally or were regenerated by planting or sowing. The material used for plantations was often not of local origin and imported from foreign countries (for example Müller 1990). In addition, plantations were established outside the natural range of Norway spruce.

The present forest law forbids clear-cutting in the whole country and protects forests as ecosystems close to nature. Particularly in the montane and subalpine vegetation zones, silviculture aims at maintaining natural stands. This is achieved by promoting natural regeneration and using natural growth patterns to obtain uneven-aged and well-structured stands.

Extensive exploitation of Norway spruce stands in the past may have eliminated many local races. In addition, fragmentation and decreased size of populations may have affected genetic processes and thus genetic variation. At present, genetic variation may be influenced mainly by environmental pollution. There is growing evidence that environmental pollution causes changes in patterns of genetic variation (Scholz and Bergmann 1994).

The ability of populations to survive and reproduce in a changing environment largely relies on their genetic variation (Müller-Starck 1995a). The supply of genetic variation determines the potential of populations to generate new variation and thus to adapt to a changing environment. Conservation of as much genetic variation as possible is therefore an important factor for maintaining stable forest stands.

In Switzerland, a programme was initiated in 1987 aiming at the conservation of genetic variation of Norway spruce, silver fir and oak and it is foreseen to include rare and endangered species (OKOK Genreservate 1988). The partners of this programme are the Swiss Forest Agency, the Swiss Federal Institute for Forest, Snow and Landscape Research and the Swiss Forest Service. Because Norway spruce is common and widespread in the country and natural stands are still present, the programme aims at maintaining genetic variation in situ in gene reserves. Ex situ conservation measures are not envisaged at present.

Efficient conservation of forest genetic resources largely depends on information on patterns of genetic variation in natural populations. Strong differentiation of a population from other populations may indicate its adaptive specialization or its distinct origin. On the other hand, a population revealing low differentiation from the remaining populations may well represent the species (Gregorius 1985). Such information is valuable for selecting populations for establishing gene reserves.

As a basis for establishing gene reserves, natural Norway spruce populations were investigated by isozyme gene markers. A total of 20 populations located in the montane and subalpine vegetation zones were investigated (Fig. 1) (Müller-Starck 1995b). For each of these populations, multilocus genotypes were identified at 18 loci from 100 trees. The results obtained revealed large genetic variation within populations in contrast to relatively small variation between populations. The variation observed was not smaller than in populations located at lower elevations in Germany and Italy (for references see Müller-Starck 1995b).

The populations investigated showed substantial differences in heterozygosity, number of alleles per locus and hypothetical gametic multilocus diversity. This latter measure is suggested to quantify the ability of populations to create genetic variation and thus to adapt to a changing environment (Gregorius et al. 1986). The highest level was found in a population of the Jura (no. 1). Additional high levels were observed in three populations distributed over the entire country (nos. 3, 14, 16) (Fig. 2).

Genetic variation observed between populations revealed evidence that populations in the west of the country (nos. 1, 2, 3) and also from regions in the southeast (nos. 15, 16) are differentiated from the remaining populations (after Müller-Starck 1995b). Differentiation of the western populations was most evident at the locus SKDH-A encoding shikimate dehydrogenase. In two of these populations the allele SKDH-A4 was at least seven times more frequent than in the remaining populations. These findings are consistent with results from pollen analyses indicating that Norway spruce stands in the west of Switzerland may have a distinct origin or followed a separate colonization route (C. Burga, unpublished).

Additional investigations are being performed by studying variation in mitochondrial DNA. Analyses of two intraspecific crosses indicated that mitochondrial DNA is maternally inherited in Norway spruce (Mátyás et al., unpublished). Mutations of mitochondrial DNA arising in different individuals are thus not recombined during sexual reproduction. This together with the fact that mitochondrial DNA shows a low rate of evolution in plants (Wolfe et al. 1987) suggests that variation in mitochondrial DNA persists over many generations and may show high levels of differentiation between populations. DNA sequence analyses of a non-coding mitochondrial DNA fragment indicated that there are at least three different mitochondrial DNA types present in Switzerland (Sperisen et al., unpublished). The spatial distribution of mitochondrial DNA variation will be investigated and used to identify post-glacial migration routes.

Ten of the 20 populations analyzed were selected for establishing gene reserves (Fig. 1) (Bonfils et al. 1996). Populations showing high numbers of alleles and high levels of heterozygosity and hypothetical gametic multilocus diversity were selected. They included populations showing strong differentiation and populations showing little differentiation. Additional criteria for the selection of populations were their autochthonous character, size, distribution within ecogeographical regions and post-glacial migration routes.

For each of these potential gene reserves contracts between the owners and the Swiss Forest Agency are under negotiation (Table 2). These contracts include the local name of the stand, the geographic location, a map of the area covered, the ownership and forest management regulations. The contracts will last 50 years. At the end of this duration they will be extended for periods of 20 years. The gene reserves will be incorporated in the general management plan of the forest owner. In case of financial losses due to the special management of the gene reserves the federal government and the cantons will compensate the owners.

The gene reserves will be under strict forest management regulations (Bonfils 1995). Introduction of foreign genetic material is forbidden and natural regeneration has to be used as far as possible to ensure transmission of all genetic information to the subsequent tree generation. The gene reserves are divided into four zones (zones 0 - 3); for each zone, management regulations are defined. In zone 0 selective thinnings are forbidden. This zone covers a relatively small area (approx. 2 ha) but ensures a selection process close to nature. Zone 1 represents the main part of the gene reserve. This zone surrounds zone 0 and is 20-100 ha in size. Within this zone traditional close-to-nature silviculture is performed to enhance natural regeneration. If natural regeneration is not possible, artificial regeneration has to rely on reproductive material from the local provenance. A zone 2 is realized if trees of foreign origin grow within the gene reserve. These trees have to be eliminated at the end of the production period. Finally, zone 3 is a buffer zone that prevents or reduces geneflow from trees of foreign provenances to the gene reserve. For this zone, no particular silvicultural regulations are defined.

In addition to gene reserves, Norway spruce is also preserved in five forest reserves covering a total area of 400 ha. These forests are allowed to develop with almost no human interference, leaving the trees to reach their natural age and leaving dead wood in the forest. The forests serve as nature reserves and research populations. The structure and development of the forests are being analyzed and results from these studies will be used to establish guidelines for close-to-nature silviculture. Three of these forest reserves will be integrated in the forest gene reserves.

Planting trees from distant provenances may influence the patterns of genetic variation in natural stands by pollen contamination. To obtain seeds of local origin for plantations, a total of 384 Norway spruce stands were selected as seed stands and registered according the OECD regulations (Fürst, pers. comm.). These stands are distributed in all ecogeographical regions of the country.

The isozyme analyses performed on 20 Norway spruce stands in Switzerland revealed significant differences in intrapopulational and interpopulational variation. These results together with information on the forest history, site conditions and autochthonous origin of the stands were used as criteria for selecting stands and their establishment as gene reserves.

The results obtained indicated that patterns of genetic variation we see in present Norway spruce populations were influenced by post-glacial migration. Similar results have been described for other plant and animal species (Hewitt 1996). These analyses indicated that many plant and animal species underwent divergent evolution in glacial refugia. Following amelioration of the climate these refugial populations expanded and, depending on the mode of migration and the route of migration taken, gave rise to the present patterns of genetic variation. To conserve as much genetic variation as possible, gene reserves should include stands derived from different glacial refugia and stands along distinct post-glacial migration routes.

Isozyme markers are likely to describe only a part of the characteristics of a provenance. For selecting stands for gene reserves it is therefore necessary to use additional information such as the forest history, site conditions and autochthony of stands. Several different types of DNA markers are currently being developed for Norway spruce. They include repetitive sequences of nuclear (Morgante et al. 1996) and chloroplast DNA (Vendramin, pers. comm.) and non-coding sequences of mitochondrial DNA (Sperisen et al. in preparation). Because these markers contain DNA of the nuclear and organelle genomes, they reveal different types of inheritance and different rates of evolution and may thus give new insights into the history and genetic structure of Norway spruce stands.

Conservation of genetic variation may not only take place in gene reserves but also by silvicultural practices applied to the total forest area. From a genetic point of view these practices would aim at promoting transmission of as much genetic variation as possible to the next generation. Nursery practices would be ideally performed in a way that no variation is lost from seed collecting to outplanting. Variation in isozymes and DNA may provide useful markers for studying genetic effects of these practices. For example, genetic effects of different modes of seed collecting, culling of seedlings in nurseries and outplanting of young trees could be analyzed. In addition, natural and artificial regeneration could be compared in juvenile stands. Results from such analyses could help to include genetic aspects in silvicultural practices.

Our experience showed that an active information policy and cooperation with the forest service is of great importance for successful implementation of any kind of gene conservation measures. In this sense establishment of gene reserves contributes not only to the conservation of genetic resources but also to the forest service's awareness of the forest diversity.

We thank Silvia Fineschi and Gerhard Müller-Starck for helpful discussions. We also thank Eliane Escher for carrying out isozyme analyses, Urs Büchler, Gábor Mátyás and Esther Jung for performing DNA analyses. The Swiss programme "Conservation of forest genetic resources" is financed by the Federal Office of Environment, Forests and Landscape (BUWAL). Development of DNA markers is financed by the Federal Office of Education and Sciences (BBW).

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