A systematic approach to the conservation of genetic resources of trees and shrubs in Denmark

Fores;tcAogy Management ELSEVIER

Forest EcologyandManagement73 (1995) 117-134

A systematic approach to the conservation of genetic resources of trees and shrubs in Denmark Lars Graudal”,” , Erik D. Kjax”, Sonja Cangef ‘The Tree Improvement Station (The National Forest and Nature Agency), Krogerupvej bThe Arboretum (The Royal Veterinary and Agricultural University), KirkegHrdsvej

21,305O HumM&, 3, 2930 Hersholm,

Denmark Denmark

Accepted20 October1994

Abstract A Strategy for the Conservation of Genetic Resources of Trees and Shrubs in Denmark was prepared in 1991/93. The objective of the strategy is to secure the ability of the species covered to adapt to environmental changes, and to maintain the basis for future improvement work. The strategy includes 75 tree and shrub species of actual or potential use for planting in Denmark. Unless protective measures are taken, the size and constitution of the genetic variation of these species will, as a result of present human influence, most likely be affected in the medium-to-long term in a way that may reduce their adaptability and utility significantly. The article focuses on the systematic approach to gene resource conservation taken in Denmark. This approach provides an overview of the conservation effort required, estimated as a number of conservation stands with a minimum size and a specified geographical distribution for each species. A review of human influence on the forest genetic resources since early forest development provides the relevant background. Emphasis is given to prevailing silvicultural practices. The genetic resources in Denmark will, in general, be conserved in evolutionary conservation stands, in situ or ex situ. For most species a network of conservation stands is required to cover the spectrum of assumed genecological variability. A preliminary genecological zonation, the biology and the distribution of each species have served to estimate the required number and distribution of conservation stands. From 2 to 15 conservation stands are considered adequatefor the different species. The estimated number of conservation stands to conserve the genetic variation of the 75 species is approximately 600. Around 500 will be in situ and around 100 ex situ. The total area required is approximately 1800 ha (excluding isolation zones) or 0.4% of the total forest area and 5% of the natural forest area in Denmark. The size of the area corresponds approximately to the area of certified seed stands/seed production areas in Denmark. The conservation of genetic resources is not pursued by imposing restrictions on the use of the genetic material. The philosophy of the strategy is to effect an insurance and keep all options open. The wise use of the genetic resources will be encouraged by other means. Where target species can be identified and (gene) ecological zones defined, the approach described may be used elsewhere to provide a quick and realistic overview of conservation needs and costs. Keywords:

Nationalforest; Generesourceconservation;Planning

* Correspondingauthorat: DanidaForest SeedCentre,Krogempvej3A, 3050Humlebrek,Denmark. Fax: + 45 49 16 02 58. Tel: + 45 42 19 05 00.

0378-l 127/95/$09.500 1995Elsevier ScienceB.V. All rights reserved SSDIO378-1127(94)03497-4

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1. A strategy for the conservation of genetic resources of trees and shrubs in Denmark

1.1. Introduction

A Strategy for the Conservation of Genetic Resources of Trees and Shrubs in Denmark was prepared in 1991/93 (Skov- og Naturstyrelsen, 1994a,b). The strategy provides an overview of gene conservation needs and required gene conservation measures, and a plan of implementation for gene resource conservation in Denmark. The gene conservation strategy is closely linked with the Strategy for Natural Forests and Other Forest Types of High Conservation Value in Denmark, in short the Danish Strategy for Natural Forests, which was adopted in 1992 (Ministry of the Environment, 1994). The latter strategy focuses on ecosystem conservation, and although this approach contributes to gene resource conservation it will not address the conservation of forest genetic resources in a systematic manner . Both strategies can be considered as national responses to recent international agreements concerning the conservation and sustainable use of biological diversity-i.e. the Convention on Biological Diversity ( 1992), the resolutions of the two Ministerial Conferences on Protection of European Forests held in Strasbourg ( 1988) and in Helsinki ( 1993)) and Agenda 2 1 and the Statement of Forest Principles both adopted by the UN Conference on Environment and Development in 1992. The gene conservation strategy is also closely linked with the current and future programmes for the mass production of tree seed in Denmark, which ultimately depend on the availability of genetic resources. This article focuses on the systematic approach to gene resource conservation in Denmark: how conservation needs and required conservation measures have been identified. The approach provides an overview of the conservation effort required, estimated as a number of conservation stands with a minimum size and a specified geographical distribution of each species included in the strategy.

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1.2. The use of terminology

Biological dioersity is understood as the diversity OS all life on earth. Three levels of biological diversity are normally distinguished: ecosystem diversity, species diversity and genetic diversity (see, e.g.. FAO. 1989: McNeely et al., 1990). Genetic diversity or genetic variation includes genetic differences between species and within species. The genetic diversity of a species can be broken down into inter-population diversity and intra-population diversity, and further into the diversity within an individual expressed by differences between alleles in the two chromosomes of diploid organisms (degree of individual heterozygosity) . The genetic resources of a species are defined as the genetic variation of actual or potential value. The value of genetic variation can be expressed as an ecological, economical or ethical value. In practice, however, it is difficult to determine whether a specific genetic variant will be of future value. Hence, it is difficult to separate the ‘resource’ from the rest of the variation and, in reality it is impossible to distinguish between genetic resources and genetic variation. 1.3. The objective and justification

of the strategy

The overall objective of the strategy is to conserve the genetic resources of tree and shrub species in order to secure their ability to adapt to environmental changes and to maintain the basis for future selection and breeding activities. The genetic variation within and between species represents a natural ‘buffer’ against environmental changes, such as climate change, the occurrence of new pests and diseases and increasing pollution (FAO, 1989). In the short term, changes can, to a certain extent, be anticipated and offset, for example by choosing appropriate plant material. However, a permanent and significant reduction of the genetic variation of a species may reduce its long-term stability and adaptability (cf., e.g., Bergman et al., 1990, Larsen, 1990). The genetic variation represents the ‘building blocks’ that form the basis for future selection and breeding activities (FAO, 1989). The needs of society in terms of forest products and services change with

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time. Tree breeding is a dynamic process for which it is important continuously to maintain the opportunity to return to the basic material. In the past, genetic variation has been reduced in tandem with the loss of forest cover. Today, genetic resources of native and introduced woody species used for planting purposes in the forests or in other seminatural habitats of the open landscape in Denmark are-with a few exceptions-not considered as being immediately endangered. However, unless protective measures are taken, the size and constitution of the genetic variation of Denmark’s forest species will, as a result of present human influence, most likely be affected in the medium-tolong term in a way that may significantly reduce the buffer function and/or the utility of the genetic variation as building blocks. As discussed in more detail below, silvicultural activities can have a negative effect on the genetic variation of tree species and thus on their future stability and potential use. The negative effects can result in reduced genetic variation, undesirable and widespread mixing of formerly separate populations of the same species or hybridization between species. Completely uncontrolled hybridization or mixing of different original genetic units will reduce the future opportunities to choose provenances that are adapted to local conditions, and valuable land races may be lost. The possibility of climate changes as a result of the increase in greenhouse gases into the atmosphere is also of primary concern. A changing climate may significantly alter the natural distribution area of many forest tree species (see, e.g., Ledig, 1991, 1993; Schwartz, 1992). 1.4. The species and the number of populations covered by the strategy The strategy includes most of the tree and shrub species that are used in Denmark for planting or silvicultural purposes in forests and other semi-natural habitats in the open landscape (e.g. outer forest belts, shelterbelts, hedgerows and small game coverts). The selection of species included is based upon the current knowledge of present use and assumptions concerning possible future use. This means that additional species could be included in the strategy subject to significant new trends in the use of woody species.

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At present the strategy covers a total of 75 species, of which 55 species are indigenous and 20 are exotics. A comprehensive list of the species included is shown in Table 1, where the species are listed within three major groups: broadleaved trees, coniferous trees and shrubs. Despite the general criteria of actual or potential use for planting purposes, the 75 species can be divided into two major groups, where different selection criteria have been used. The two groups are ‘traditional production forestry species’ and ‘trees and shrubs used for planting purposes in the open land areas’. The forestry species include all species used, native as well as exotic. Trees and shrubs for other purposes include primarily native species. The need for gene resource conservation aimed at exotic shrub species is at present considered less urgent. There are several reasons for this. There are fewer naturalized exotics among trees and shrubs used for other purposes than forestry. There is in general an increasing interest in using native species, in particular in the open landscape; an interest which is being encouraged by the National Forest and Nature Agency in Denmark. Finally, it is, of course, also a matter of setting priorities. The advantages of using exotics in the open landscape would, in general, seem less obvious than in production forestry, where a number of exotic species have proved their productive superiority during a period of some 200 years. However, in connection with any future adjustments of the strategy, it is quite likely that some exotic shrub species may be included. The estimated number of conservation stands to conserve the genetic variation of the 75 species is approximately 600, of which around 500 will be in situ and more than 100 ex situ (see Table 1) . Section 3 explains how the figures in Table 1 have been estimated. The total arearequired is approximately 1800 ha (excluding isolation zones), or 0.4% of the total forest area and 5% of the natural forest area in Denmark. The total size of the area corresponds approximately to the total area of certified seed stands/seed production areas in Denmark (cf. Table 1) . The average size of a conservation stand will be around 3 ha (excluding isolation zone), but this will vary considerably with, for example, species and species composition. A mature beech (Fagus syluatica) stand on a good site will consist of some 100 individuals per hectare, Norway spruce (Picea abies) on a medium good site around 500 per hectare, and

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Table 1 Number of stands proposed for conservation of genetic resources of trees and shrubs used in the.forests and the open landscape in Denmark, and the number of existing approved/selected seed stands and seed production areas for use in forestry and the open landscape. The species are listed in alphabetical order within three major groups: broadleaved trees,coniferous trees, and shrubs (sources: Plantedirektoratet, 1991; Brander et al., 1993; Skov- og Naturstyrelsen, 1994a) Species

Number of stands proposed for conservation of genetic resources

No. of existing selected seed production areas ( 1993)

In situ stands

Forestry

Ex situ stands

Clonal material

Qw landscape

Broadleaved trees Acer campestre Acer platanoides Acer pseudoplatanus Afnus gfutinosa Betula pendula Betula pubescens Carpinus betulus Fagus sylvatica Fraxinus excelcior Populus tremula Prunus avium Quercus petraeu Quercus robur Quercus rubra Tilia cordata Ulmus carpinifolifl Ulmus giabra Ulmus laevis

Total broadleaved

8-10 S-10

2-I II-15 8-10 S-10 s-7 5-7 S-10 s-1 II-15 5-l 8-10

4 2 9

9 I 36 13

5 3

2 4

1 3

3 s

2-4

2 3 5 -7

2

9 LO

83

18

1

II-15 24

I

2

8-10 24

115-149

I3 4 5

2 1 f

3941

II

156

81

Coniferous trees Abies alba Abies grandis Abies nordmanniana Abies procera Chamaecyparis lawsoniana Lark decidua Larix leptolepis Picea abies Piceu glauca Picea omorika Picea sitchensis Pinus contorta Pinus mug0 Pinus nigra Pinus sylvestric Pseudotsuga menziesii Thuja plicata Tsuga heterophylla

Total conifers

5-l

2 I

* * *

5-l 5-7 2-4 5-7 5-l 5-7 211 2-4 5-7 2-4 2-4 5-7 5-l 5-7 2-4 2-4

2 3

65-99

27

5 2 5 s1

2

I 4 21

2

43 I

52 2 2 7 12 2s 5 6 263

3

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Table 1 (continued) Species

Number of stands proposed genetic resources In situ stands

Shrubs Corms sanguinea Cot$us auellana Cotoneaster integerrimus Cotoneaster melanocarpus Crataegus laeoigata Cratuegus monogyna Euonymus europaeus Hedera helix Hippophae rhamnoides Rex aquifolium Juniperus communis Loniceru periclymenum Lo&era x.ytosteum Maius syklestris Prunus padus Prunus spinosa Pyrus communis Rhamnus cathartica Rhamnus frangula Ribes alpinum Ribes nigrum Rosa canina Rosa dumulis Rosa rubiginosa Rosa spinossissima Rubus fruticosus Su1i.x capreu Salk cinerea Salk pentundra Salix rosmarinifolia Sambucus nigra Sarothamnus scoparius Sorbus aria var. rupicola Sorbus aucuparia Sorbus intermedia Sorbus torminalis Syringu mdgaris Taxus baccata Viburnum opt&s

for conservation

Ex situ stands

1 l-15 S-10 5-7 5-7 S-10 8-10 S-10 S-10 S-10 S-10 11-15 S-10 S-10 S-10 11-15 S-10 S-10 S-10 1 l-15 5-7 S-10 1 I-15 1 l-15 1 l-15 1 I-15 S-10 S-10 S-10 S-10 5-7 S-10 5-7 5-7 S-10 5-7 5-7 S-10 S-10 1 l-15

of

No. of existing selected seed production areas ( 1993)

Clonaf material

Forestry

Open landscape

3 3

1 4

1 3 5

5 12

3

3

1

2 2 1 1 2 2 1 2

4 I 2

1 2

3

Total shrubs

315-393

0

38

0

59

Grand total

430-542

102-138

76

386

144

*Land-race

formation.

Some of the ex situ stands may be considered

in situ.

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some shrubs may consist of 25 individuals or fewer per hectare. Of the approximately 500 in situ stands (roughly 1500 ha), more than two-thirds are expected to be covered by areas which are also included in the Danish Strategy for Natural Forests referred to above. Of the remaining approximately 150 stands, 100 are expected to be in State forests and 50 in private forests. The selection of in situ stands in private forests will require compensation payments. Of the more than 100 ex situ stands, half are expected to be selected among existing stands and the other half to be established as new stands in State forests. Some of the existing stands will be on private land. 1.5. The implementation measures

of the proposed

conservation

The strategy for the conservation of genetic resourcesof trees and shrubsis planned to be implemented within the next 10 years. Implementation will include (i) collection and mapping of existing information on distribution and genetic variation of each speciescovered by the strategy, (ii) development of guidelinesfor preparation of managementprescriptions and management protocols, (iii) field inspection, selectionanddemarcationof in situ standsand existing ex situ stands,(iv) field inspection, selectionand collection of reproductive material for the establishment of new ex situ standsand (v) the establishmentof new ex situ stands. The total cost of implementing the plan is estimated to be about 14 million Danish crowns (approximately 2.2 million US!$) over 10 years, excluding the managementof in situ standson sitescovered by the Danish Strategy for Natural Forest, and including 2.5 million Danish crowns to support and catalyze research.

2. The status of forest genetic resources in Denmark and the need for conservation

The needfor conservation of forest geneticresources and the measureschosen for their conservation must reflect their current status, which is the result of early forest development and past and presenthuman influences on the forests of Denmark and their genetic resources.

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2. I. Early forest development

in Denmark

Denmark is a naturally wooded country and most of the country would-without the presenceof manhavebeencovered by forestor bein successiontowards forest cover. The country has mainly been formed by the last Glacial (Weichsetian) and the development of the forests hastaken place during the last 10 000 years. For a period of 4000 years starting about 6000 years BC, the forests were dominated by lime (Elia CUTdata). The beech(Fagus syluatica) immigrated 1OOO2000 years BC and replaced lime as the dominating forest tree species(see Fig. 1j Approximately 5000 years ago, substantialhuman impact on the landscapewas initiated by agriculture and grazing, and the forest areareacheda minimum ot 3% of the total land area towards the end of the 18th century. Sincethen the forest areahasincreasedto 12%. largely through the establishmentof coniferous plantations. Today, the forest areais cultivated intensively and to a large extent characterized by the useof exotic speciesand genetically improved material. 2.2. Deforestation

and exploitation

The main human activities that affected forest biological diversity in Denmark until aroundthe year 1800 were as follows (see. e.g., Iversen, 1973; Fritzboger. 1992):

( 1) Slashand burn agriculture which favoured certain tree speciesover others and which in someareas was not followed by natural regenerationbut resulted in a long-term replacementof forest by other types of vegetation cover, e.g. heathland. (2) Cutting for firewood and construction wood, which generally meantsystematiccutting of treeswith straight stemform. (3) Cutting of fencing material and grazing, which resultedin deforestationor very open forests. This exploitation and mismanagementof the forest until the beginningof last century seems,in a historical perspective,to be the most radical anthropogenici&uenceon the forests.The deforestationchangedprevious continuous forests into small and fragmented populations of forest trees, often open forests with changed microclimate and unfavourable conditions for regeneration. Many areas lost their tree cover completely, while othersmusthave passedthrough bottleneckswith

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ground

vngr

-

0 - -1000 - -2000

Iron Age BronzeAge Neolithic

- -3000 - -4000 - -5000 Mesolithic - -6000 - -7000

b

Xi%

1 -8000

50%

Fig. 1. Pollen diagram from small wet hollows in Eldrup Forest, Eastern Jutland, Denmark (drawing by H.K. Pedersen, Geological Survey of Denmark, after Andersen et al., 1983; reproduced with the permission of the Geological Survey of Denmark / DGU). The diagram shows local forest development and reflects the main trends of general forest development in Denmark (birch, Betula; pine, Pinus; aspen, Populus; hazel, CO&U; lime, Tilia; oak, Quercus; beech, Fagus). -

only a few trees per hectare, maybe only regenerated vegetatively. Both ecosystemsand specieswere lost, as was the genetic diversity of the remaining species. The loss of Pinus syhestris from the Danish flora towards the end of the 17th century representssuchan example. The lossof genetic variability and the introduction of ‘founder effects’ must have taken place in this period. The forests which avoided deforestationduring that time were mainly located in areaswhere the pressure on the forests for different reasonswas limited, for example becauseof low population density. Most of the forest landsdid passthrough periodswith only little forest cover. However, these periods seldom lasted more than a few tree generations(Fritzbgger, 1992). The reduction of genetic diversity that took place in thesestandsfrom exploitation most likely varies within Denmark. Systematic changesin the genepool from exploitation is likely to have taken place for Quercusrobur and Q. petrea. Poor stemform, for example, is found in the degradedQuercuspetrea forests in the western, nonfertile, partsof Denmark (Jensen,1993). Theseforests have been heavily degradedthrough logging and cop-

picing, and the poor stemform of offspring from these seedsourcesis believed to be the result of the selective harvesting of straight trees. Field experiments with Quercus robur suggestthat standsfrom the southeastern islandsof Denmark have better stem form than those with more northern origins. This is believed to be the result of low anthropogenicpressureon the forestsin the southeast(Jensen,1993). Similar evidence is not reported for other Danish forest tree species,but the issuehasnot beenthoroughly investigated. Towards the end of the 18th century, silvicultural practicesand regulatedforest managementwere introducedin Denmark. At this time the forest areain Denmark reachedan all-time minimum of 3% of the land area.Hence, this period representsa genetic bottleneck for many native forest tree speciesin Denmark. 2.3. Silvicultural practices The lasttwo hundredyearsof controlled silvicultural practices have in general continued the reduction of diversity at the ecosystemlevel, especially through the draining of wet biotopesandreducingthe speciesdiversity by converting ‘native’ stands to monocultures

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(especially to pure Fagus syluatica or coniferous stands). On the other hand, the introduction of exotic species has of course increased the diversity. Since the beginning of the 19th century, natural forests have been reduced by two-thirds, while the total area of forests has been multiplied by four (Ministry of the Environment, 1994). The change from open and degraded forests to closed stands further reduced the habitats of a number of herbaceous species, which became common in the periods of forest exploitation and which may also have been common earlier in areas grazed by large animals (Buchwald, 1994), but today many of these species are rare or endangered (Skov- og Naturstyrelsen, 1991; Lojtnant, 1993). The impact of silviculture at the gene level is more complicated, as it has both increased and decreased the genetic variability. Increase originates from outcrossing between native stands and plantations of the same species established with genetic material of foreign origin (from non-local seed sources). Increasing focus on the importance of using the best seed sources has, on the other hand, resulted in a practice and legislation which for a number of important forest tree species favour commercial seed harvest from relatively few selected stands. This may have reduced the genetic variation for some species. Tree improvement activities may have either reduced or increased the genetic variability of some stands, as discussed below. From an economic point of view, tending practices, which aim at selective removal of economically inferior trees, might have upgraded the genetic quality of some species slightly through a reduction of the frequency of economically undesirable alleles (Wilusz and Giertych, 1974; Ledig and Smith, 198 1) The following considerations only address the impact of silvicultural practices on genetic diversity. 2.3.1. The introduction of foreign species and seed sources (provenances) Many exotic species have been tested in Danish forestry and a number of coniferous species in particular have found wide application (Picea abies, P. sitchensis, P. glauca, Abies alba, Larix decidua, L. kaempferi, A. procera, A. nordmanniana, Pinus sylvestris, P. contorta, P. nigra, Acerpseudoplatanus and to some extent Populus spp.) . Today, stands of these species are based on seeds from direct imports, or-very frequentlyseeds that originate from older stands based on previous

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imports. In the latter case, the offspring is second, third or in some cases even fourth generation in Denmark. Natural selection and domestication activities may have resulted in the formation of land races which arc especially well adapted to Danish conditions and therefore very interesting from a conservation point of view. The indication of land-race formation is found in Picea abies (Bornebusch, 193S), Abies procera (Larsen. 1986)) Picea glauca (Pedersen, 199 I ) and Piceu .sitchensis (Nielsen, 1993). Hybridization between the introduced and native species has not been reported for forest tree species. Hybridization between introduced species, however. has turned out to be a serious problem. Picea sitchensis and P. glauca hybridize frequently and hybrids are normally considered to be undesirable (Nielsen et al.. 1992). P. glauca has mainly been planted in the west,ern part of Denmark, and today it is difficult to tind t-‘. sitchensis stands suitable for seed collection in this region, as most stands give offspring with many hybrids. Hybridization between Abies alba and A nordmanniana is also of negative economic importance. Old adapted stands of A. nordmanniana are seldom sufficiently isolated from A. alba stands to avoid hybridization and the breeding work is complicated by a supposed introgression of the A. alba in the breeding populations. Foreign seed sources of species that are native to Denmark have been imported and widely used for two reasons. Field experiments with Quercus spp., and to some extent Fagus sylvatica. have shown that specific foreign seed sources are economically superior compared to the local seed sources, as they produce stands with better stem form. Secondly, most forest tree species flower irregularly, and problems with insufficient seed production and/or seed storability have resulted in large temporary shortages of forest seeds from the approved Danish stands. These gaps have been filled with imported seeds from sources presumed to be suitable for Danish conditions. It is difficult to find areas in Denmark completely without stands of non-local origin of Fagus sylvatica, Fraxinus excelsior. Betula pubescens and especially Quercus robur and Q. petraea. There are no known barriers against hybridization between the imported provenances and the original seed sources. Outcrossing between the native stands and the imported origins are thus taking place

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and will be prevalent unless specific conservation measures are imposed. Hybridization between Danish and imported seed sources represents a particular problem within the context of conservation. The hybridization may actually result in increased vitality of the hybrid offspring, as inbreeding depression will disappear as a result of the outcrossing process (heterosis) . Heterosis can, however, be replaced by outcrossing depression if the foreign seed source is adapted to conditions of growth which are quite different from the native stand (cf., e.g., Ledig, 1986, 1993; Ellstrand, 1992). Seed sources of forest tree species have generally been imported to Denmark from origins which are expected to be well adapted to Danish conditions. Often this expectation is based on results from field experiments, although this it not always the case. But even field experiments are not necessarily a guarantee for a correct choice of seed source. Trees are slow-growing organisms, and a full rotation-age experiment would last 50-100 years. Large-scale introduction has therefore often been based on relatively short trials from which the life-span development under Danish condition could not be determined. An illustrative example of the latter situation is the introduction of Rumanian Picea abies in the 1970s and 1980s. Trees of Rumanian origin were considered a superior alternative to the Danish land race as relatively short field experiments had indicated fast growth, good quality, and spring frost resistance in the Rumanian trees. Later this juvenile behaviour turned out to be misleading, as the Rumanian Norway spruce revealed poor health at older ages (Wellendorf, 1988). Today Picea abies of Rumanian origin is avoided completely, but some areas in Denmark have been afforested with these trees and now they are beginning to flower. Natural selection will eventually reduce the frequency of genes responsible for poor health, but a risk of unknowingly using the depressed hybrids in the meantime is an unpleasant prospect. A special problem concerns species with small population sizes in Denmark, e.g. Tilia platophylla or T. cordata. Random genetic drift is relatively important compared to natural selection if the populations are small. Populations of only a few trees may therefore by chance be fixed for non-optimal alleles simply as a result of the small population size. Such alleles may be introduced through outcrossing from a non-local stand

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to a small native one. These small populations are therefore also especially vulnerable to hybridization. 2.3.2. The use of genetically improved planting material A number of the coniferous-and a single broadleaved-species have been subjected to more or less intensive tree improvement activities, and seed from seed orchards are meeting an increasing part of the Danish demand for these species (especially Picea abies, P. sitchensis, Pseudotsuga menziesii, Pinus sylvestris and Fraxinus excelsior, and in the future Abies nordmanniana and A. procera). Most of the clones in the breeding populations are selected for good performance in a number of quantitative traits (see, e.g., Wellendorf, 1988, for Picea abies) which are all expected to be under polygenic control, in general with low or intermediate heritability. The seed orchards are typically based on 10-50 superior trees selected in several different, well-adapted stands. Tree improvement can reduce the genetic variability of stands originating from seed orchards if the number of clones is low. However, this is not automatically the case.The genetic variation (e.g. asmeasured by heterozygosity in isozyme loci) may well be larger in such seedorchard offspring than in each of the originating native stands (see, e.g., Mouna and Harju, 1989,for Pinus sylvestris) . Of course,continued generationsof recurrent selection for improved performance must eventually reduce the additive genetic variation in the traits favoured by selection (Fisher, 1958). However, resultsfrom repeatedselectioncycles with short-lived organismssuchasLkosophila or corn show that heritability generally decreasesslowly, and substantialadditive variation canbemaintainedfor several generationsaslong asthe effective population size at generationturn-over is not too small (seeNamkoong et al., 1988, or Falconer, 1989, for references). The gain from the subsequentselectioncycles must therefore be achieved through many small, simultaneous changesin gene frequencies of alleles with additive effect rather than by fixations of a few major genes. Furthermore, work by Lande and others (e.g. Lande, 1975, 1988; Lynch, 1988) showsthat for a varity of speciesand charactershigh mutability of quantitative traits will rapidly replaceadditive genetic variation in an evolutionary perspective.The practical implication of these results is that the present tree improvement

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practices probably only reduce the genetic variation to a minor extent, because (i) improved seed sources originate from the first or second selection cycle, (ii) the breeding for most species is based on a relatively large number of clones (usually more than 500 in the breeding programme and 25 or more in each seed orchard), and (iii) of the possible replacement of additive genetic variation through polygenic mutation. The average performance in the improved traits is of course changed. This may by itself call for conservation activities as the economic and environmental conditions change with time, and the possibility of returning to conserved, unimproved, gene resources in order to look for new traits must be maintained. The risk of an unintentional increase in an undesirable character, which correlates positively to the improved trait (correlated response) is probably an even more serious problem, and the value of maintaining unimproved stands should therefore not be neglected. 2.3.3. Tending procedures The standard procedure in Danish forestry for the last two centuries has been to remove the inferior individuals by selective thinning leaving the superior phenotypes for final rotation age and thus regeneration. Evidence exists that such silvicultural practices can result in positive genetic changes from the point of view of human economy ( Wilusz and Giertych, 1974; Ledig and Smith, 198 I ) . However, this does not necessarily mean that genetic diversity has been either reduced or increased. Mass selection can theoretically be of high intensity, especially for species that are regenerated through natural seed fall. A Danish stand of beech ( Fagus syluatica) with 200 trees per hectare at the time of regeneration may thus have started out with 200 000 individuals 1 year after germination. The reduction from 200 000 to 200 implies a selection of one out of each 1000. Such a selection intensity is. however, purely theoretical, as a large amount of the early reduction is directed mainly towards increased spacing, and later the selection is only performed by comparing a few trees in each local neighbourhood. The selection is purely phenotypic, and the phenotypical variation of traits (e.g. growth) is mostly the result of environmental variation. Experience from conifers in Sweden suggests a single tree heritability for growth of 5-10% (on average 0.075, Danell, 1990). The Danish experience is within the same range. The broadleaved species may

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have even lower heritability. The selection is to a large extent performed on young trees, and low juvenilemature correlations will therefore reduce the response on trees even more at the time of regeneration (see, e.g., Lambeth, 1980). All these considerations suggest that one or two generations of selective thinning have had only a small influence on the genetic resources 01 the grown trees. The influence of tending on the genetic diversity of, for example, the native, but managed, Danish stands of Fagus sylvatica may therefore be small and possibly not significant.

3. Conservation method&gy and the method of estimating required conservation areas 3.1. Evolutionaq

in situ and ex situ stand.5

The conservation strategy chosen in Denmark is a combination of in situ and ex situ conservation. The two methods each have their advantages (see, eg., FAO, 1989, pp. 21-22). The choice of conservation method will depend on different factors, for example the size of the population considered, its genetic history, isolation, possibility of regeneration, and security in terms of long-term survival. Generally speaking, in situ conservation is the ideal method of conserving wild plant genetic resources. When evaluating security. however, a combination of in situ and ex situ conservation will often be considered. In the case of Denmark, where major parts of the flora are exotic, there will often be ‘transitional’ cases between in situ and ex situ. Where exotics have developed land races. one would speak of in situ stands, in other cases of ex situ stands. For most exotic species in Denmark the development of land races is not sufficiently well proven, although indications exist, as described above. In Denmark con-servation of exotics is in general considered to be ex situ. Development and adaptation are a dynamic process and the genetic resources will therefore generally be conserved in evolutionary conservation stands (et’. Guldager, 1975). Evolutionary conservation stands arc ‘living’ stands, in situ or ex situ, or ecosystems where the genetic composition of target species is allowed to adapt to the prevailing environmental conditions and their changes with time.

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Static conservation, for example in the form of seedlots in long-term storage and vegetatively propagated clones in clone collections, will be used as a supplement where relevant, for example if pollen contamination from surrounding stands cannot be satisfactorily limited or in connection with tree breeding. Commercially utilized clones will usually be conserved this way. The concepts of evolutionary and static conservation introduced by Guldager ( 1975) correspond to themore commonly quoted differentiation of conservation and preservation made by Frankel and Soul6 ( 198 1) ‘Conservation in use’, i.e. the conservation of genetic variation through existing silvicultural practices, is not an explicit part of the conservation strategy in Denmark. The conservation of genetic resources is thus not pursued by imposing rules for, and restrictions on, the use of the genetic material. The philosophy of the strategy is rather to effect an insurance and keep all options open. The wise use of the genetic resourceswill be encouragedby other means.Nevertheless, ‘conservation in use’ will in practice be an important meansof conserving genetic resourcesfor somespecies.In Denmark, this appliesparticularly to beech (Fugus syluatica), which through generationshas been subject to natural regenerationand only light massselection (cf. above) . 3.2. A network of conservation stands:deciding the required number of standsand their distribution To conservethe genetic variation of a target species, where genetically different populations have evolved through adaptationto different ecological andenvironmentalconditions, it is necessaryto cover the spectrum of ecological variability within the areaof distribution of the species (Frankel, 1970). For most speciesa network of conservation standsis thus required. Such ‘conservation in multiple populations’ (Namkoong, e.g. 1986; cf. alsoErikssonet al., 1993) underlinesthe evolutionary nature of the conservation stands. The different standswill be subject to different selection pressuresand, thus, with time continue to develop in different directions. To conserve the genetic variation of a speciesadequately, we should ideally know this variation. For someforestry speciesa considerableamount of knowledge of genetic variation in adaptive traits has been gained in Denmark through about 100 years of prove-

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nance research.More recently, isozyme studieshave added to this knowledge for a few species.For most treesand bushes,our factual knowledgeof their genetic variation is still minimal. The central dilemmaof gene resourceconservation is a recognized need for conservation without knowing exactly what to conserve. A total survey of the speciesinvolved is not practical or economically possible.The study of genetic variation in adaptive traits requiresin generalthat the speciesbe tested for decades.A survey basedon ecological data in combination with biochemical markers and data from already establishedfield trials is probably the only possibleway to approachthe problem within a realistic timespan.This approachis currently being adaptedfor Quercus robur, Quercuspetrea, Fagus sylvatica and Tilia cordatu, but suchsurveys are still not possiblefor all speciesincluded in the strategy. For the present,the required number and the optimal geographicdistribution of the conservationstandsmustbedecidedby other means. Following Frankel ( 1970), we assumethat similarity of ecologicalconditionsimpliessimilarity of genetic constitution. A comparisonof a speciesdistribution with well-defined ecological zoneswill thus provide a framework for selection of conservation stands.This method is an application of the widely usedseedzone concept (Barner and Willan, 1983) to gene resource conservation. For each of the 75 speciesidentified for gene conservation in Denmark, a preliminary estimate of the required numberof conservationstandshasbeenmade basedon the criteria listed below. 3.2,l. Distribution in Denmark Continuous distribution throughout the country necessitatesa number of standsrepresentingthe ecogeographic variation of the country, unless factual knowledge of genetic variation prompts something else. More limited geographical distribution will of course reduce the required number of standsaccordingly. A strongly fragmented distribution area will in general require relatively more standsto be selected than an even area of distribution. Although Denmark is a small country (approximately 43 000 km’), there is considerablevariation in a number of factors of importancefor the distribution of treesand shrubs.The country is here tentatively androughly divided into 47 genecologicalzones (see Fig. 2). Genetic conserva-

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Fig. 2. Tentative genecological zonation of Denmark and the natural distribution of sessile oak (Quercus petraeu). The country is divided into four main zones, Two of the four zones are further divided into two and three subzones, respectively. The zonation is based on Jacobsen (Jacobsen, 1976; Nordisk Minister&l, 1984) and 0dum ( 1987). The distribution of sessile oak is according to Ovum ( 1968). Dots represent findings according to the literature, flora lists or herbarium material.

tion of a species evenly distributed over the whole country would thus require a minimum of 4-7 stands. This zonation is based on a compromise between the variation in ecological factors, which considered alone would result in more than 4-7 zones, and expectations of gene flow. A division of Denmark into seven primary physiogeographic zones and 30 subzones has been made by Jacobsen (Jacobsen, 1976; Nordisk Ministerr&d, 1984) based on climate, geomorphology and soil. 0dum ( 1987) has divided the country into 13 regions

based on the woody flora. However, expectations of gene flow tend to reduce the number of zones, as setective differences resulting from environmental heterogeneity can only result in local adaptation if the barriers to gene flow are sufficient. The validity of the tentative zonation will be tested by classification of environments based on existing provenance and progeny trials with special emphasis on adaptive traits (cf., e.g., Wdlendorf, 1992)) and by allozyme studies of the variation for selected species within and between the zones.

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3.2.2. Mode of reproduction

Insect-pollinated species or species with apomixis are likely to have developed more differentiated populations than wind-pollinated species and will thus in general require a relatively greater number of conservation stands. Seed dispersal, which depends on the interaction between seed (form, weight and survival) and seed dispersal vector (water/wind/soil/animals/ humans), is of similar importance. 3.2.3. Conservation status

For common species which are naturally regenerated, the silvicultural practice will contribute to the conservation of the genetic resources and the need for pure gene conservation stands will be less. The Danish Strategy for Natural Forests (Ministry of the Environment, 1994) will protect the genetic resources of alarge number of species, where it will be sufficient to select a number of natural forest areas as gene resource conservation stands, and monitor and manage them as such. 3.2.4. Origin

The distribution history of a species is important for its genetic variation. Native species, for example, are likely to have developed a higher degree of variation between populations than exotic species introduced from a single source or only a few sources. Conservation of exotics will thus in general require fewer conservation stands. 3.2.5. Insurance and economy

The replication of conservation stands is necessary to minimize the risk of loss due to unforeseen external events, such as windthrow, for example. At the same time, the number of stands should be limited to allow them to be monitored and managed appropriately and securely. Based on the above considerations, it has been estimated that from 2 to 15 conservation stands are required for each of the different species. The number of stands for each species is shown in Table 1. 3.2.6. An example

To illustrate the species-specific considerations behind the estimates of required conservation stands presented in Table 1, an example of one species, sessile oak ( Quercus petraea) , follows. Similar considerations have been made for all species.

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Two oak species occur naturally in Denmark, sessile oak (Quercuspetraea) and pedunculate oak (Quercus robur). Both species are wind-pollinated and have a natural distribution covering most of Europe, pedunculate oak with a larger and more continental distribution than sessile oak. Within the natural distribution area the two species often grow together. Oak came to Denmark following the last Glacial approximately 8000 years ago, together with Alnus, Ulmus and Tilia (cf. Fig. 1). Oak probably arrived some 500 years earlier in the western part of Denmark than in the eastern part (cf., e.g., pollen diagrams in Iversen, 1973). Sessile oak established itself primarily on sandy hills in Jutland, whereas pedunculate oak spread to more humid areas all over the country. Today, pedunculate oak is found all over the country, whereas sessile oak has a much more scattered distribution. The distribution of sessile oak according to 0dum ( 1968) is shown in Fig. 2 with the genecological zones overlayed. Sessile oak occurs naturally in all 4-7 zones; in two of the zones, however, on only one site in each. The species is most common in the western zone, where it primarily occurs in so-called oak ‘purs’. The conservation status in the western zone is good because the ‘purs’ are protected as natural oak forest according to the Danish Forest Act. In this area, either one or a few of the ‘purs’ should be designated as in situ gene conservation area(s) and protected against pollination from external sources. Otherwise conservation efforts will concentrate on the remaining zones where one to two stands in each should be conserved in situ. To summarize, 5-7 stands are considered sufficient for the in situ conservation of the genetic resources of sessile oak, a wind-pollinated, naturally occurring species with a limited and widely scattered distribution, and with a good conservation status in one to two of the genecological zones. Sessile oak is widely used for planting, and seed demand has so far primarily been covered through the import of provenances from Norway. At present, only two stands in Denmark are approved for forest seed collection (cf. Table 1) . The in situ conservation measures will be complemented by the establishment of two clonal seed orchards of Danish sessile oak, one clonal seed orchard of Norwegian sessile oak from Agder, and the conservation of one stand of German origin (Spessart) approved for seed collection in Denmark.

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3.3. The design and management of the individual conservation stands The design and management of each stand will be registered in management protocols. Management prescriptions will vary with species and the site-specific characteristics of each stand. The general guidelines for design and management are listed below. ( 1) A conservation stand can consist of one or several species, i.e. either a pure stand or a mixed stand. In a mixed stand, one may target one or several species for gene conservation. The stand can be established artificially (planted or sown), or through regeneration (natural or assisted by silvicultural interventions). (2) Conservation stands should have a size that reduces the risk of losing genetic variation. The size will depend on species and site-specific conservation aspects. Mixed stands where the objective is to conserve the genetic variation of one or more species will in general have to be larger than pure stands. (3) As a ‘rule of thumb’, an in situ stand of a windpollinated species should initially consist of at least 150 and preferably more than 500 interbreeding individuals of each of the speciesto be conserved. For species naturally occurring in small populations (e.g. adapted to small ecological niches), fewer individuals may be accepted. In general, the aim should be a final stand size of 500-1500 individuals per species,This may be achieved by enlarging the population through regeneration on adjacent areasin order to minimize the loss of genetic variation in future generations.The size of the area should be sufficient to maintain the minimum number of individuals required at generationturnover. (4) Seedcollection from one speciesin a stand for the purpose of establishing an ex situ stand should involve at least 150 trees if their relationship is unknown. Seed should be collected from at least 25 randomly chosenand supposedlyunrelated individuals (i.e. half-sib families). If the individuals in the mother stand are supposedly unrelated (e.g. if the stand is establishedartificially from seed), the ‘rule of thumb’ is that it shouldconsistof at least50 trees.When establishing ex situ conservation stands,the aim should be a final standsize of 500-1500 individuals or more. (5) The conservation standsshould be regenerated with genetic material originating from sexual reproduction in the standsand with aslittle genetic influence from outside in the form of contamination with pollen

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from external sources as possible. In practice this requiresisolation. In Denmark an isolation beh of 500 m is recommended.The international standardis 330 m (cf., e.g., FAO, 1992). Natural regenerationshould be preferred, if the ecological conditions allow. Small standsshould be avoided to achieve adequateregeneration units. (6) In some cases,the conservation effort can be combined with different forms of forest utilization if the usedoes not markedly change the genetic constitution of the stands.In somecases,conservation may be combined with ordinary forest management.For somepopulations the conservation effort will consist of a certain managementsystem, which may include, for example, cutting of competitive speciesor, for certain bushes,controlled animal grazing. The different numbersof individuals given asguidelines to achieve populations of a sufficient size to reduce the risk of losing genetic variation are derived from basicpopulation genetic theory. A total of 50 unrelated,panmictic interbreedingindividuals (i.e. an effective population number) are required for this strategy for short-term conservation, as a population size of 50 will result in only a slow increasein the inbreeding coefficient and in minimal genetic drift. Five-hundred unrelated, panmictic interbreeding individuals are required for long-term conservation, as this number will reduce the expected long-term genetic erosion (see, e.g., Franklin, 1980; Frankel and Soule, 1981) When larger numbers(multiples of 50 or 500) are given, it is in order to compensate for the difference between actual and effective population size. Differences between the individual contribution of gametesto the offspring at generation turnover can generate substantial deviation between actual andeffective population size (Crow and Kimura, 1970). Such deviation hasbeenreported for many tree species (see, e.g., El-Kassaby et al., 1989; Innes, 1994)) and the actual number of trees in the conservation stands in this strategy are therefore recommendedto be up to three times the required minimum effective population size. The number required for short-term conservation (50) is in principle accepted as a sufficient starting point for a stand to develop into a long-term conservation population if the population is increasedin size, becausethe ‘bottleneck’ of 50 individuals in a few generationsis not consideredto be severe in terms of

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loss of genetic variation. An effective population size of 500 is generally considered sufficient to adjust to long-term changes in natural selection factors, but it will not be sufficient to conserve rare alleles, where effective population sizes of 2000-3000 are recommended (Krusche and Geburek, 1991). However, rare alleles are hardly of significant importance for adaptive traits under polygenic control, and rare alleles are therefore not targeted in the Danish strategy. Based on theory and empirical examples, Lande ( 1988) suggests that demography is usually of more immediate importance than population genetics in determining the minimum viable sizes of wild populations. This point of view is taken into consideration by ‘replicating’ conservation stands and by ensuring that conservation stands constitute adequate regeneration units, i.e. have a microclimate favouring regeneration. Beech stands with edges exposed to wind, for example, would have to be larger than more protected stands.

4. Discussion The required number of conservation stands and their distribution is of course estimated with considerable uncertainty, and with larger uncertainty for some species than for others. However, at present there is no realistic alternative to the empirical indirect method of surmising genetic variation as a basis for identifying conservation needs. It is a basic feature of gene resource conservation that it focuses on target species. When applying the method on a conglomerate of species in one country, as is done here, it becomes national gene resource conservation planning. Because of the central dilemma of gene resource conservation, such planning can never be perfect. A national gene resource conservation plan or strategy should therefore be flexible. Flexibility should not only encompass the ability to adjust as new knowledge becomes available, but also address the need for seeking new knowledge. The genetic variation of a species can be assessed with different techniques. It is possible to study morphological and metric characters, and biochemical and molecular markers. The study of metric characters or adaptive traits in field trials was previously the dominating technique, and today it is still the most robust

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and only truly valid way of assessing genetic variation in these characters. There are theoretical and practical problems connected with the extrapolation and implementation of results from such experimental field trials to a heterogenous forest environment. A theoretical problem is, for example, the interpretation of provenance X environment interactions. More practical problems may arise because of the usually long timespan between the establishment of the trials and the occurrence of the experimental results. True identification, and even availability of the tested genetic material, may be difficult or impossible if the stands of origin have been depleted or even disappeared. Caution should therefore be taken not to mistake ‘preliminary findings’ for ‘final truths’. Nevertheless, information from this type of experiment is very valuable when assessing adaptive genetic variation as a basis for conservation activities. The recent rapid development of biochemical and molecular markers allows fast surveys of genetic variation within and between populations. These techniques will be powerful in connection with traditional field trials and ecological surveys, but cannot replace them, as the biochemical markers deal with neutral, rather than adaptive, genetic variation (Millar and Westfall, 1992). When a species has been identified as a target for gene resource conservation, the objective of the conservation measures will, generally speaking, be to conserve as much of the genetic variation as possible. The concept of resource in a traditional narrow sense of being something useful (to human society) is thus not entirely applicable to the gene level, because in principle all genes are considered potentially useful. It does, however, make a considerable difference whether rare alleles are targeted or not. Apart from the question of rare alleles, the ‘resource’ in gene resource conservation is really identified at the species and population level. The gene conservation strategy does not per se include threatened or endangered species. The focus on the usefulness and the interand intra-specific genetic variation of target species distinguishes the conservation of genetic resources in situ from both ecosystem conservation and more traditional nature conservation with a focus on endangered species. The different approaches to conservation cannot replace one another, but will often be complementary.

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In Denmark the conservation of natural ecosystems and endangeredspeciesare-or will be-covered by other measuresthan the presentstrategy. The Strategy for Natural Forests and other Forest Types of High Conservation Value (Ministry of the Environment, 1994), aimed at ecosystemconservation, has already beenmentioned. A large part of the geneconservation standswill be part of the ecosystemscovered by the strategy for the natural forests. In general this will improve their conservation status.However, where the conservation of specific gene resourcesrequires management interventions, these have to be compatible with the natural forest conservation objectives. This may not always be the case, but in general will be so in Denmark. A very critical issuein gene conservation planning is obviously how to identify the group of target species to be included. In the Danish strategy presentedhere, the basicprinciple has been actual or potential use for planting purposes.Therefore emphasisis on native species and naturalized exotics. The emphasison native speciesand naturalizedexotics doesnot imply any ban on using new exotics-but speciesnot present in Denmark can obviously not be part of a national gene conservation programme.This doesnot meanthat speciesor populationsgrowing outsideDenmark are of no potential interest to Denmark, but their conservation will have to be assuredin their respective countries. However, national gene conservation programmesshould not be seenin isolation, but shouldrather be consideredasparts of an international network. Such networks have already beenestablished for somespecies(Arbez, 1993). The objective of the Danishstrategy is closely linked to securing a long-term supply of adequatereproductive material. As a consequenceof this, the identification, monitoring and managementof the network of geneconservation standswill be carried out by the Tree Improvement Station of the National Forest andNature Agency. The Tree Improvement Station is the Danish national tree seedcentre, and as suchis responsiblefor the long-term tree improvement programme in Denmark. The systematic approach to gene conservation in Denmark may b an inspiration to others. In countries with more complex ecosystems,many more species, and much lessknowledge of the biology and potential useof individual speciesthan is available in Denmark,

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there will often be a well-justified tendency to focus primarily on ecosystemconservation. This may carry the danger of overlooking the need for gene resource conservation, and of ignoring the responsibility to set up national seedcentresto safeguardthe future baseof reproductive material. However, this doesnot make the needfor geneconservation Iesscritical. In many developing countries, the most importanr step in a strategy for conservation of genetic resources will be the reconciling of conservation activities with immediate and basic human needs (cf., e.g., FAO. 1989). At present,the fulfilment of theseneedsoften resultsin the destruction of the resource,the on-going deforestationin the tropics being particularly notable. The situation in any country will be more or less unique ascomparedwith other countries. Thus, in gen-era1generesourceconservation strategieswill have to be country-specific, depending on such things as the size, geography, topography, ecology, infrastructure, level of development, extent of tree planting, administrative and organizational system, etc., but to the extent that target speciescan be identified and (gene) ecological zones defined, the approach used in Denmark can provide a quick and realistic overview of conservation needsand a basis for an estimation ot costsin other countries as well.

Acknowledgement The authors wish to thank an anonymousreferee for useful suggestions.

References Andersen, S.Th.. Aaby, B. and Odgaard, B.V., 1983. Environmenl and man. Current studies in vegetational history at the Geological Survey of Denmark. J. Danish Archaeol.. 2: 184-196. Arbez, M., 1993. International Report, Resolution 2. Conservation of Forest Genetic Resources. Report on the Follow-up of the Strasbourg Resolutions of the Ministerial Conference in December 1990, pp. 6044. Ministerial Conference on the Protection of Forests in Europe, Helsinki, June 1993. Mini­ of Agriculture and Forestry, Helsinki. Bamer, H. and Wibn, R.L., 1983. The concept of seed zones. In: Technical Note No. 16, Danida Forest Seed Centfee,Humlebiek. Bergmann, F., Gregorius, H.R. and Larsen, J.B., 1990. Levels of genetic variation in European silver fir. Are they related to the species’ decline? Genetica, 82: I-10.

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Bornebusch, C.H., 1935. Proveniensforstrig med Rodgran (German summary: Ein Provenienzversuch mit Fichte). Det Forstlige Forsogsvssen i Danmark (The Danish Forest Experiment Station), 13: 325-378 (Beretning nr. 112).K&enhavn. Brander, P.E., Lund, M.C. and S@mnsen,CAa., 1993. Fortegnelse over k&de, udpegede og fremavlede frokilder aftrreer og buske til landskabsform~. SP Rapport Nr. 12, Landbrugsministeriet, Statens Planteavlsforseg, Lyngby. Buchwald, E., 1994. Fortidens arv og fremtidens mulighed. Jord og Viden, 139( 9) : 6-9. Danmarks Jordbrugsvidenskabelige Kandidatforbund, Kobenhavn. Crow, J.F. and Kimura, M., 1970. An Introduction to Population Genetics Theory. Harper and Row, London. Danell, a., 1990. Possible gains in initial stages of national tree breeding programmes using different techniques. Forest Tree Improvement, 23: 11-30. Arboretet, Horsholm. DSR Forlag, Kobenhavn. El-Kassaby. A., Fashier, A.M.K. and Crown, M., 1989. Variation in fruitfulness in a dough&r seed orchard and its effect on crop management decisions. Silvae Genet., 38: 113-121. Ellstrand, N.C., 1992. Gene flow by pollen: Implications for plant conservation genetics. Giios, 63: 77-86. Eriksson, G., Namkoong, G. and Roberds, J.H., 1993. Dynamic gene conservation for uncertain futures. For. Ecol. Manage., 62: 1537. Falconer, D.S., 1989: Introduction to Quantitative Genetics. 3rd edn. Longman, Essex, UK. FAO, 1989. Plant Genetic Resources. Their Conservation In Situ for Human Use. FAO, Rome. FAO, 1992. Establishment and management of ex situ conservation stands.For. Genet. Resour. Inf., 20: 7-10. FAO, Rome. Fisher, R.A.. 1958. The Genetical Theory of Natural Selection. 2nd edn. Dover Publications, New York, 286 pp. Frankel, O.H., 1970. Genetic conservation in perspective. In: O.H. Frankel and E. Bennett (Editors), Genetic Resourcesin PlantsTheir Exploration and Conservation. IBP Handbook No. 11, Blackwell, Oxford, Edinburgh, pp. 469-489. Frankel, O.H. and Soule, M.E., 1981. Conservation and Evolution. Cambridge University Press,Cambridge, UK. Franklin, I.R., 1980. Evolutionary change in small populations. In: M.E. Soule and B.A. Wilcox (Editors), Conservation Biology: An Evolutionary-Ecological Perspective. Sinauer Associates, Sunderland, MA, pp. 135-149. Fritzboger, B., 1992. Danske skove 1500-18013 (English summary: Danish Woodland 1500-1800). Landbohistorisk Selskab, Denmark. Guldager, P., 1975. Ex situ conservation stands in the tropics. In: The Methodology of Conservation of Forest Genetic Resources. Report on a pilot study. FAO, Rome, pp. 85-92. Innes, J.L., 1994. The occurrence of flowering and fruiting on individual trees over 3 years and their effects on subsequent crown condition. Trees, 8: 139-150. Iversen, J., 1973. The Development of Denmark’s Nature since the Last Glacial. Geology of Denmark III. Geological Survey of Denmark. V. Series No. 7-C. CA. Reitzel Forlag, Copenhagen (original Danish edition 1967).

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Agency, Copenhagen (bi-lingual Danish-English edition). Mouna, 0. and Harju, A., 1989. Effective population sizes, genetic viability, and mating systemin natural stands and seed orchards of Pinus syhestris. Silvae Genet., 38: (5-6), 221-228. Namkoong, G., 1986. Genetics and the forests of the future. Unasylva, 38( 152): 2-18. FAO, Rome. Namkoong, G., Kang, H.C. and Brouard, J.S., 1988. Tree Breeding: Principles and Strategies. Springer Verlag, Berlin. Nielsen, U.B., 1993. Genetisk variation i sitkagran (Picea sitchensis (Bong.) Carr.) i hojdevrekst, stammeform og frostbaerdighedvurderet ud fra danske proveniens-, afkoms- og klonforseg. Ph.D.-athandling, Arboretet (Den Kgl. Vet. oglrmdbohejskole) og Forskningscentret for Skov og Land&b, Lyngby. Nielsen, U.B., Kjaer, E.D. and Roulund, H., 1992. Sitkahybrider. Skoven, 24(2): 72-75. Dansk Skovforening, Kobenhavn. Nordisk Minister&d, 1984. Naturgeografisk regionindeling av Norden. Nordiska Minister&et. 0dum. S., 1968. Udbredelsen aftrmer og buske i Danmark (English summary: The distribution of trees and shrubs in Denmark). Bot. Tidsskr., 64( 1). Dansk Botanisk Forening, Kebenhavn. 0dum, S., 1987. Tmxrtsvalg til nye skovbryn. Principper for artsvalg belyst ud fra en opdeling af landet i 13 egnskarakteristiske omrader. Ugeskrift for Jordbrug, 132(48): 1514-1518. Danmarks Jordbrugsvidenskabelige Kandidatforbund, Kobenhavn. Pedersen, A.P., 1991. Har hvidgran en fremtid i dansk skovbrng? Dansk Skovbrugs Tidsskr., 76( 1): 25-40. Dansk Skovforening, Kebenhavn. Plantedirektoratet og Statens Forstlige Forsegsvssen, 199 1. K&ede froavlsbevoksninger i Danmarks skove. Plantediiektoratet. WgW.

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