silvicultural management strategies

Peter Rotach

Department of Forest and Wood Science, Chair of Silviculture, Swiss Federal Institute of Technology (ETH), Zürich, Switzerland


Introduction

Most of our autochthonous Noble Hardwood species are relatively rare, some are even endangered. Hence, conservation as well as efforts to promote these species in order to increase their proportion in the forests are desirable. The importance of minor and rare species is increasing because they represent a natural variation.
Since the Earth Summit in 1992, biodiversity, sustainability, stability and adaptability of ecosystems and their components are perceived as being highly important for further development and survival of human beings.

The management strategies in forestry need to incorporate such new objectives. Furthermore, to stay competitive, forest enterprises will need to comply with certain objectives imposed by society. Conformity with these objectives will be surveyed and certified, a process which has already started. Certification requires management strategies which protect and support natural processes (implying due attention to close-to-nature silviculture, mixed and uneven aged stands, and the protection of rare species) and which implement a variety of objectives other than wood production. Noble Hardwoods and other minor or rare species will most certainly benefit from this new development. Besides all this, Noble Hardwoods are highly interesting for wood production. Timber prices are increasing continuously and are less susceptible to fluctuations on the global timber market. A high economic value is an important stimulus for forest owners to promote certain species. This can support conservation activities rather efficiently.

Conservation and promotion of Noble Hardwood species is most successful within the frame of forestry. Hence, it is the primary objective of this paper to develop strategies for the conservation of genetic resources in managed forests. The strategies try to respect and implement important practical aspects of forestry. An important objective for Noble Hardwoods silviculture is the production of high-quality timber. Management strategies thus need to incorporate the production aspects (site requirements for optimal performance, educational needs, choice of provenances and breeding). Additional instruments for in situ conservation such as gene reserves, protected areas, seed stands, old growth stands or nature reserves which may also be important for gene conservation, are not discussed here. Conservation of most Noble Hardwoods ideally combines both ex situ and in situ measures. Especially for rare species and those with scattered distribution, artificial breeding populations, such as seed orchards, provide a very efficient instrument for conserving or even increasing genetic variability. In this paper we primarily concentrate on in situ activities; ex situ measures are mentioned only if they relate to in situ activities. 

The physiological optimum of most Noble Hardwoods is found on sites where beech or other climax species (oak, spruce) dominate naturally. Therefore, competition is the most decisive factor for the existence and survival of Noble Hardwoods on these sites; they naturally survive only under conditions which are not optimal for the climax species. Consequently, measures for in situ conservation of minor species theoretically are straightforward and easy. For survival and adequate development, Noble Hardwoods need sufficient light and space, especially in the older age classes. On most sites, silvicultural interventions thus need to regularly control competition from other species by thinning. However, three prerequisites are necessary if in situ conservation measures are to be successful in practical forestry:

1.Sufficient demographic data are needed. Efficient conservation measures can be implemented only if populations, population structures or, in some cases, even individuals are known. 

2.Since Noble Hardwoods often occur as single individuals in mixed stands and since their competitive ability is generally low, periodic interventions over time are needed.

 
3.The financial possibilities of the forest owners need to be sufficient to pay for a forestry organization and for the necessary, relatively cost-intensive thinning and planting operations. 

The last point is very valid since increasing economic problems in forestry may drastically limit conservation efforts applied in the practice. Effective conservation and promotion measures depend on the existence of a forest organization with sufficient qualified personnel and financial means. In the long run, conservation and promotion may only be possible if the highly valuable Noble Hardwoods contribute to the revenues of forest enterprises. This means that mature individuals of sufficient quality need to be harvested to pay for the interventions in the young stands. Short-term conservation objectives and long-term promotion efforts are sometimes conceived to be contradictory. Therefore, conservation and promotion can only be successfully implemented in a dynamic fashion and over a long period of time. A central problem in this context is the rarity of some of the species. With the exception of Fraxinus and Acer, populations are often too small to allow for genetic diversity in natural regeneration. In these situations, natural regeneration needs to be accompanied by planting material with high genetic diversity. Hence, in a dynamic approach, sufficiently broad populations need to be established ex situ in order to produce the required planting material.

Promotion efforts are less straightforward than conservation activities. Although Noble Hardwoods may frequently be established by natural regeneration or may be planted when natural regeneration is not possible, these measures alone do not guarantee a higher proportion of such species in the future. In contrast to climax species, Noble Hardwoods dominate only under very special conditions. In undisturbed forest ecosystems they either occur on very special 'niche' sites or in mixed broadleaved forests, especially during certain stages of succession. In undisturbed forest ecosystems on beech-dominated sites, mixed stands or stages are rather an exception than a rule (Korpel 1995). While Noble Hardwoods may show higher proportions in early stages of development, their proportion drastically decreases in later stages owing to the strong competition of beech. In late stages of natural development they occur only occasionally and in very low proportions.

Consequently, management strategies need to consider the competitive environment as well as the special properties and requirements of these species. In this paper we try to summarize some silvicultural considerations and to outline some general management strategies for a successful conservation and promotion of Noble Hardwood species in multifunctional forests. Since these strategies may differ according to site conditions and vary for the individual species, and because rarity and needs for conservation vary widely between species and from one part of Europe to another, generally valid guidelines are difficult to develop. Consequently, the considerations outlined here cannot be very specific. Moreover, because many factors may influence management decisions, different optimal strategies are conceivable or necessary under differing conditions. Our experience is primarily based on sites which offer excellent growing conditions for all
Noble Hardwoods, i.e. rich, young soils, a favourable climate with sufficient precipitation and excellent conditions for natural regeneration. Strategies as well as the examples given are primarily valid for conditions found in Central Europe and for sites where beech is the main competitor. Strategies may somewhat differ and may need to be adapted for other competitive environments, site conditions, situations of rarity and threats.

Adaptation is especially important in marginal areas on the northern and southern border of the species distribution areas.

General management strategies: potential solutions

The following characteristics, essential for both conservation and promotion efforts, are common to all Noble Hardwoods as defined by the EUFORGEN Network:

• with some exceptions, these species rarely dominate under natural conditions 

• the species mostly occur as single and scattered individuals in mixed stands or on special sites, where they may be slightly more abundant owing to reduced competition from their major competitors 

• the species are mostly light demanding and consequently do not survive shade and strong competition 

• the species have a low ability to recover from unsatisfactory growing conditions, especially in the second half of their lifespan 

• compared with beech, the species have a low competitive ability 
  sociability (social behaviour in mixed stands) of these species is related to site conditions. 

From these general characteristics it follows that strategies for the conservation and promotion largely depend on site conditions. Site conditions greatly influence natural stand development, competition between species, behaviour of species in mixtures, types of mixed stands, value production and silvicultural intensity needed to conserve or promote these species. On certain sites, conservation and promotion efforts have to be considered inefficient, expensive and are most likely to be unsuccessful in the long run because regular human interventions would be necessary to continuously correct an "unfavourable" natural development. 

Recommendation 1

As a first priority, conservation and promotion efforts should concentrate on ecologically optimal sites, natural niches or special habitats. Efforts placed on physiologically optimal sites are reasonable only in situations where a high silvicultural intensity can be guaranteed.

There may, however, be situations or conditions where it might be reasonable or even necessary to undertake conservation measures regardless of site conditions, especially in marginal areas where
species are rare or in situations where species are severely threatened.

Noble Hardwoods are generally rather demanding species with regard to site conditions. Their physiological optimum is found on highly productive sites, i.e. in the center of ecograms (Ellenberg, 1986). In nature, however, they hardly ever occur in their optimum. Because of competition, they naturally occur only on suboptimal sites, either in special habitats where competition is reduced artificially (due to human activity), or during certain stages of succession when the competitors are absent or not yet fully developed, or in niches where competition is less vigorous. Conservation and promotion efforts are most efficient on sites where the species occur in their natural ecological optimum (only Fraxinus and Alnus) or where they have their natural niches. A low competitive ability combined with low tolerance to competition, and a habitat preference which they share with their major competitors, are major causes for the natural rarity of these species (Gaston and Kunin 1997).

Consequently, conserving or promoting these species on sites outside their ecologically optimal range actually means enlarging their natural distribution. Although this is possible, it requires much greater effort, investment and more continuity to be successful. Economically it can only be justified if the expected qualitative value of production is higher than the investments, which certainly is the case for some of the Noble Hardwoods like Sorbus torminalis, Juglans regia or Prunus avium.

Concentrating the efforts on ecologically optimal sites has two advantages. First, silvicultural interventions need to be less intensive and less regular over time (continuity) to guarantee success. Second, we increase the chance of concentrating our efforts on the genetically most important natural populations, the so-called 'core populations', which are most important for the survival of the species (if they still exist). This essential consideration illustrates the differences that exist between climax species and Noble Hardwoods. Climax species such as spruce, fir or beech do not only form extended populations, they also differ from Noble Hardwoods with respect to management strategies. While gene conservation of climax species can be accomplished in unmanaged stands (or stands which require minimum management interventions to ensure health and regeneration), and since natural development does not endanger the survival of such a population or its genepool (only of certain individuals), the in situ conservation of Noble Hardwoods primarily means managing the survival of the species in a given location. Hence, gene conservation programmes for these species not only require a different concept, they also require more effort and investment than climax species. In situ conservation of Noble Hardwoods should thus concentrate on sites where they are in their ecological optimum or they should be grown in special habitats.

Another possible concentration of efforts could be achieved by limiting in situ measures to populations of more than 20 individuals as proposed in the report of the first meeting of the Noble Hardwoods Network (Turok et al. 1996). This strategy may, however, be questioned for several reasons. Population genetic considerations clearly indicate that the minimum viable population size for genetic conservation purposes needs to be much larger than 20 individuals (Lawrence and Marshall 1997). Moreover, for many of the species in question it would be impossible to find in situ populations in the sense of interbreeding individuals that fulfill this condition. For example, for S. domestica, Pyrus pyraster, Malus sylvestris, J. regia, Ulmus laevis or Ulmus minor, such effective populations are very rare. In addition, to define a population in a genetic sense, we would need to know the distances of effective geneflow. Owing to fragmentation, geneflow may be rather restricted.
Hence, in such species, genetic differentiation may be higher than in other species and a large part of genetic diversity may thus reside among the single fragments. Furthermore, it is generally agreed that in situ gene conservation should respect the 'metapopulation structure' of these species. In the metapopulation model (Levins 1970), in which a number of local demes interact through geneflow and migration, local extinction may occur but may not be decisive for the survival of the species since the genepool is maintained, as a whole, through recolonization of new habitats from certain 'core' populations. If we accept this model, then the demography, the patterns of distribution and the interaction of the single demes are essential for conservation rather than the size of the single demes.
From this model it also follows that only a dynamic conservation over a large area (essentially over the complete metapopulation) will be successful. In this sense, a small but genetically effective deme may eventually be more valuable for gene conservation than a group of 20 or more trees because the effective population size of such a group is not necessarily larger (especially in species which frequently regenerate vegetatively and which often are dominated by competitors and thus hardly fructify). Even if metapopulation structures are disturbed for most species as a consequence of human activity, it would be counterproductive, especially for the rare species, to limit in situ conservation to groups of more than 20 individuals. It is doubtful whether generalized population genetic principles can be applied to rare species with a scattered distribution. Drift, inbreeding and geneflow may play a different role in naturally rare and dispersed species than in species which occur in extended populations. For example, species which occur naturally in low densities (such as tropical species with less than one individual per ha), show a surprisingly high genetic diversity (Bawa 1992; Loveless et al. 1992; Hamrick et al. 1994). The same can be observed in very rare autochthonous species such as S. domestica or S. torminalis (Wagner et al., Menn et al., in preparation). As long as we do not have a better understanding and knowledge about the demography, the genetic structure and geneflow patterns, conservation measures for the rare species should include as many individuals as possible, regardless of their association with certain genetic demes. Even if genetic drift and inbreeding may occur in small demes, a lot of among-deme variation or certain rare or interesting genotypes may be conserved this way. In contrast, for hardwood species which still occur in larger populations such as Fraxinus excelsior, Acer pseudoplatanus and Prunus avium, a concentration of conservation efforts on stands with populations size between 20 and 50 individuals seems reasonable.

Recommendation 2

For rare and very rare species, all individuals on their ecologically optimal sites or niches should be conserved. For species that still occur in larger populations such as Fraxinus, Acer and Prunus, a concentration of efforts on demes with more than 20 individuals seems reasonable.
Growth and quality are not optimal on sites where the species have their ecological optimum. With few exceptions, optimal growth and quality production are encountered on highly productive sites, i.e. where beech is in its optimum. From a production point of view, Noble Hardwoods should be managed on the most productive sites, either riparian (Fraxinus, Acer, Ulmus, Prunus) or optimal beech sites. In most cases, however, this means promoting the species on sites where they do not occur naturally. Although populations of Noble Hardwoods can be established easily, either from natural regeneration or by planting on most of the beech-dominated sites, the high competitive ability of beech has important consequences for a successful promotion. Success primarily depends on two factors: the possible silvicultural management intensity and the type of mixture.

Mixed stands of these species have many advantages, i.e. higher stability with regard to the effects of abiotic and biotic factors and associated lower risks, higher adaptability, higher biological diversity, better quality, better growth in some cases, flexible timber marketing (Lanier 1992; Schütz 1994). Noble Hardwoods are prime candidates for admixtures with beech since they may increase the qualitative value of production considerably. It has to be realized, however, that highly diverse mixed stands do not occur naturally on these sites. An active silvicultural management is thus necessary in order to guarantee a substantial proportion of Noble Hardwoods at maturity and to attain reasonable silvicultural objectives (final diameter, quality, rotation age). Hence, only where a certain silvicultural intensity and continuity can be guaranteed should hardwoods be promoted on beech-dominated sites.

Recommendation 3

If a high silvicultural intensity is guaranteed, valuable Hardwoods may also be promoted on highly productive sites. Growth and value production will be optimal on such sites and return of investments will be high for many of the species. Competition in mixed stands depends on the type of mixture and the social behaviour of the species. Several factors are important (Schütz 1994): growth dynamics of the species over time on a given site. Relationships in height growth between species change over time and in relation to site conditions. Competitive ability and survival largely depend on these growth relationships  light demand or shade tolerance determine the reaction of a species in a competitive environment  capability to expand the crown in a competitive environment is essential for competitive ability  ability to restore the crown after release from competition reflects tolerance to competition and determines silvicultural intensity which is necessary to conserve the species in a mixture  final height determines survival, growth capacity and vitality in older age classes.  Most decisive factors for competition in mixed stands are height/growth relationships among species and shade tolerance of the respective species.

Recommendation 4

On beech-dominated sites, Noble Hardwoods should be mixed in patches (groups of trees) rather than as single trees. The hardwoods are either planted in patches or they are favoured by means of the first interventions.

In most cases, single-tree admixtures of Noble Hardwoods are not stable and depend on periodic, intensive silvicultural interventions. Growth rhythms of these species differ considerably from that of beech or oak and competitive ability is generally rather weak for most of the species in question. Even if some of the Noble Hardwoods may outgrow beech in the first 30-40 years, they are all clearly dominated by beech in the second half of the rotation period. In groups of trees, growth differences are less important since competition is restricted to the contact zone between the species. If the patches are chosen such that their size at least equals the necessary growing space of an adult tree, at least one individual of the less competitive species should survive without human intervention even under severe competition. Occasionally, single mixtures are possible, for example for Fraxinus, Acer and Ulmus on humid, riparian sites where they dominate naturally and integrate well in mixtures. For all other sites and species, group mixtures are recommended since they are more stable, silviculturally less intensive and thus more efficient than tree-by-tree mixtures. Patch size should be chosen according to 'sociability' and rotation age of the species and with respect to the possible silvicultural intensity. Patches should, however, not be smaller than 10 m in diameter. Even when Noble Hardwoods occur as patches, attention has to be paid to the choice of the accompanying species. Depending on the site, suitable species need to be selected. Two considerations are important. First, if patches are small, growth rhythms and final height should not differ too much. Second, rotation age should be similar for reasons of future regeneration. If rotation age differs considerably, patches of the species with short rotation should be very small. Otherwise, when they need to be harvested, unproductive gaps will result.

Some Noble Hardwood species cannot be integrated into high forest stands, not even as groups. For example, Malus, Pyrus and certain Sorbus species have considerably slower height growth than any other species and generally reach final heights of less than 20-25 m. An integration in closed, high-stand structures is therefore hardly successful. These species need special stand structures or habitats where they find optimal growth and light conditions.

Recommendation 5

Species that are difficult to integrate with other species should be favoured in special stand structures or habitats which are especially favourable. Such special habitats should primarily be managed for the endangered minor or rare species.

The following are interesting special habitats for species with a very low competitive ability, i.e. species which are light-demanding, slower growing and of small stature, such as Malus, Pyrus or Sorbus aria, S. aucuparia and S. torminalis.

Forest margin. A special treatment of the margin is necessary in order to promote these species. A zone of 20-30 m in depth should be reserved for these minor species. In most cases they need to be planted. Patch size planting is again the favoured solution since competition from shrubs will be severe in the phase of stand establishment. The border zone needs to be managed with low standing volumes which should not be higher than 150 m3/ha. Locally, all trees should be removed to create small patches with ideal light conditions. 
Hedges or small 'forest patches' scattered within the agricultural land. It is well known that such forest patches are not only important elements of the landscape, they are also valuable as retreats for plants and animals, as wind barriers and much more. Regarding the conservation of rare and disseminated species, hedges and scattered forests are especially important because they may serve as "stepping stones" for geneflow, linking fragmented core populations. 
Coppice with standard. Many of the rare and endangered hardwood species used to be relatively abundant in special stand structures such as coppice with standard. This management system was practised all over Europe until around the turn of the century. It was especially suitable for the less competitive species for several reasons. First of all, standing volumes were very low (in order to allow a sufficient development of the coppice that produced the highly important fuel wood; Schütz and Rotach 1993), which provided excellent growing conditions for all light-demanding species. Secondly, competition of beech was absent because beech normally was not tolerated (since it produced a less valuable timber than oak, pine or spruce and had negative effects on development of the coppice; Mathey 1998). Finally, every 20-30 years, the coppice was removed but the valuable fruit trees and hardwoods were kept (for different reasons) and allowed to grow freely without any lateral competition. Since coppice with standards type stands show a considerably lower qualitative value of production than high forests, this ancient system cannot be reintroduced in larger areas. As long as it is feasible from an economic standpoint, all remains of old coppice stands should be conserved and heavily thinned to standing volumes below 150 m3. Interventions should favour all the existing hardwood species. Eventual natural regeneration of these species needs to be protected from deer and to be supported periodically from competitive shrubs and stool-shoots. 
Pine stands. Sorbus aria, S. torminalis, S. domestica, Pyrus and Malus are often relatively frequent in the understorey of older pine stands. Mature pines provide favourable growing conditions even if stands are closed because light penetrates the crowns easily. The mentioned species can grow and survive in the understorey for a rather long time. If sites are not too poor, such stands offer good conditions for these species. The hardwoods often show insufficient quality, however, because they have been suppressed for too long. Even if these species may be quite frequent, such populations commonly are of little value for gene conservation. In most cases, they do not contribute genes to future generations since they hardly fructify in the understorey. In general, however, pure pine stands offer excellent possibilities to favour the mentioned species. Natural regeneration is rather abundant. Moreover, S. torminalis (and to a lesser degree also S. domestica) generally develop a better form if they are tended under a light canopy for the first 10-20 years (Drapier 1993). Crown cover should not be too dense, and young trees finally need to be completely released when a straight axis of 6-8 m has been formed. Hence, to obtain good phenotypes and individuals that fructify, rather heavy interventions are necessary, removing older pines in the upper storey. 

Experiences in Switzerland indicate that the personnel in the field are hardly aware of some of the species. Inventories where the 'populations' that were known to the forest service were compared with the actually existing ones in the field, have clearly shown that rare species are perceived quite differently. While the local forest service knew nearly all of the existing Taxus baccata populations and even individuals, only about 20% of the actually existing Sorbus torminalis individuals were known to them. Obviously, Taxus is easily recognizable and/or it is perceived either as rare, as special or as especially valuable. Sorbus torminalis, on the other hand, is hardly "noticed". Evidently it is not perceived as a valuable species, although market prices have been extremely high in the past, much higher than for any other species. It is our experience that perception or awareness seems to correlate with knowledge, information and motivation. Consequently, promotion of rare species will neither be efficient nor successful unless the personnel working in the field are trained, motivated and committed. 

Recommendation 6

For a successful conservation and promotion of minor hardwood species, the perception of most of these species needs to be improved. People working in the field need to be better trained and motivated.
In most cases, the proportion of minor hardwood species could be increased considerably just by changing habits, perception and objectives. It is our experience that all Noble Hardwoods, even the rare ones, are very often found in certain proportions in natural regeneration (only S. domestica being an exception). The proportion of most Noble Hardwoods could thus be considerably increased by training and supervising field staff and by giving clear directives for early interventions in young stands.

If mature stands contain a certain proportion of the desired hardwoods, natural regeneration should be possible for many of the Noble Hardwoods. On suitable, fertile sites, for example, Fraxinus and Acer can be easily established by natural regeneration. Since these species are light-demanding, and since they need to have a head start over beech, fast regeneration techniques are the method of choice in order to favour their proportion in future stands. On suitable sites, shelterwood regeneration, removing 60-70% (or less if weed competition is important) of the standing volume in a first cut and a second and final cut following after 4-6 more years, should produce a natural regeneration with a rather high proportion of the desired species. Alternatively, a direct and complete removal of the stand is possible in cases where sufficient hardwood seedlings are already established, which is very often the case on beech-dominated sites.

On suitable sites, single individuals of other Noble Hardwoods are rather frequent in most of the natural regeneration (for example Prunus avium, Acer platanoides, Acer campestre, Ulmus glabra and Tilia species). Even species with heavy seeds such as J. regia may be found in astonishing quantities where there are old trees nearby (due to bird seeding). A successful integration of such species in future stands would need little effort in many cases.

Promotion of some species primarily depends on vegetative propagation. Species such as S. torminalis, S. aria and S. domestica and Prunus avium frequently produce root suckers. Early interventions and regular tending could thus produce stands with rather high proportions of these minor species without the need for plantations. In contrast to P. avium which also regenerates from seed, Sorbus species only occasionally occur as seedlings established from seed. If they do, seedlings are not vigorous, are slow growing and thus do not withstand competition as well as root suckers (Drapier 1993; Germain 1993). Vegetative propagation is an easy way to regenerate these species naturally and to avoid plantations that are rather expensive. Consequently, promotion of Sorbus species is especially promising in stands where a certain number of mature individuals occur. Root suckers can develop as far as 25 m from the trunk. Since root suckers are produced without the mother tree being cut (Germain 1993), immature individuals should not be cut but kept as standards until they reach maturity. Sufficient light, however, is necessary for the propagation and development of the root suckers. Protection against browsing is necessary in most cases since the minor species are especially vulnerable.

Many of the hardwood species have been considered as economically unimportant in the past. Even worse, the so-called minor species have primarily been perceived as competitors for the main species and have been actively eliminated early on. The proportions found in the younger stands today do not reflect the potential proportions of these species. The naturally occurring proportions of most of these species can be rather high and can be increased by tending operations.

For certain species, natural regeneration may, however, not be sufficient for gene conservation. Some of the species, especially Pyrus pyraster, Malus sylvestris and Prunus avium may, owing to their occurrence in small demes, primarily mate with domesticated individuals outside the forest. Because of a high degree of introgression from cultivated forms, it is for example doubtful if Malus and Pyrus still exist as wild forms (Kleinschmit 1998). Some introgression may also occur in Sorbus and Ulmus species. For gene conservation, these species need an enrichment with genetically diverse material, containing a high proportion of undomesticated genotypes. A reasonable solution for the mentioned species thus is the establishment of seed orchards, followed by the reintroduction of the desired material in plantations. Hence, regarding regeneration, two different strategies, depending on the species and the actual situations, are necessary. For species which still occur in larger populations and which do not suffer from introgression by domesticated material, the following recommendation is given.

Recommendation 7

If species still occur in large, wild populations, efforts and financial means should primarily be invested into supporting natural regeneration and early interventions. Planting for these species should be restricted to the most productive sites where value production can be expected to be highest. For these plantations, only improved material with the best phenotypic quality should be used.
Breeding and conservation are often considered contradictory. We do not see any contradiction if minimum requirements for the breeding populations are respected. On the contrary, we consider ex situ breeding populations as an ideal supplement to in situ measures. The value of production can be extremely high for most Noble Hardwoods if quality is sufficient. Quality primarily depends on stem form and branching habit. For many Noble Hardwoods, these traits are often unsatisfactory in average populations. Prunus avium, for example, very often shows an insufficient qualitative value of production due to poor form caused by frequent forking and sinuous stem form. Improving these traits by breeding would thus considerably enhance the value of the products of certain species. Since conservation and promotion of Noble Hardwoods in practical forestry is probably most successful if owners and foresters are convinced that these species produce highly valuable timber, improved reproductive material should be used for all plantations. Genetic improvement and seed production are efficiently achieved in seed orchards containing selected and tested clones. If appropriate breeding programmes are used, improved material may be genetically as diverse as material collected in the wild. Considering the difficulties of collecting seed from a large number of trees in mature stands, orchard seed may even be more diverse since genes of more trees may be represented in the offspring.
Moreover, compared with in situ populations where individuals may be scattered over a large area, chances for random mating in the orchard populations are increased over natural conditions. In addition, within the artificially formed orchard population, individuals are allowed to exchange genes that would never have had a chance to mate in the wild. Breeding orchards may thus also serve as ex situ instruments for gene conservation. This is especially so if they contain a sufficient number of clones, if several (regional) breeding populations are established and/or if multiple population breeding concepts (Eriksson et al. 1993) are used.

For certain species or situations, natural regeneration is not sufficient for gene conservation and additional planting is not only optional but necessary.

Recommendation 8

For species which occur only in small demes and which are endangered by introgression from domesticated material, natural regeneration needs to be accompanied by planting. Planting is also necessary on sites where they are economically not very efficient. Material should be highly diverse and contain a large proportion of wild genotypes from the respective provenance.
Conservation of species such as Pyrus, Malus, Ulmus, Sorbus or Prunus avium may thus require more effort and financial means. The only reasonable solution for the conservation of Malus and Pyrus seems the establishment of seed orchards which contain wild, natural genotypes and which produce genetically diverse material that can be reintroduced on their original sites. In situations where sufficiently large demes are no longer available, additional plantations may also be necessary for P. avium, Ulmus or Sorbus species.

In the context of in situ gene conservation, two primary objectives are important for tending
operations: 

a maximum number of individuals should survive until fructification, and 

individuals should be able to transfer as many genes as possible into the next generation. 

Silvicultural interventions thus need to control competition in order to guarantee the survival of as many individuals as possible and to provide for a sufficient crown development and vitality for a regular fructification at the end of rotation. Without interventions, most of the Noble Hardwoods will disappear from natural regeneration, even on sites where beech is less vigorous. With the exception of Fraxinus, Acer and Ulmus, most hardwoods lose competition within the first 20-30 years. Early interventions are thus essential for their survival and their integration into future stands. At least one intervention in the thicket or pole stage is necessary to regulate the mixture and to favour all minor species.

Recommendation 9

The first 20-30 years are decisive for the survival of most Noble Hardwoods. At least one intervention in the thicket or pole stage is necessary in order to conserve and favour existing minor species. All minor species should be favoured in natural regeneration, regardless of their quality, occurrence in mixed stands and length of rotation period.

In the past, young stands were regulated in order to obtain silviculturally 'ideal' mixtures. The regulation of the mixture was an important silvicultural objective. The ideal mixture was based on site requirements of the species, their 'sociability', value production and rotation length. Interventions often reduced diversity, since they favoured primarily species which could easily be mixed with the major species (commonly beech). Slower-growing species which are difficult to integrate into the stand structure or species with a shorter or longer rotation period were not chosen as plus trees and thus were not favoured. Silvicultural objectives aimed primarily at highest possible value production.

Today, biological diversity as an objective is equally important as production. Consequently, all minor species should be favoured, even if mixtures are not ideal from a silvicultural perspective. This also holds from a purely economic point of view. The economic potential of most minor species is very high. Prices for Sorbus, for example, reached new records last year. We believe that future perspectives are even better, considering that certification should increase the demand for locally, ecologically and sustainably produced wood products. Noble Hardwoods should especially profit from certification because of their highly useful, precious timber produced in a sustainable way. Minor species such as Juglans, Sorbus or Prunus commonly are more valuable than the dominant species (Fagus, Pinus or Quercus) even if their volume production is lower. Economically, final dimensions of minor species may be smaller and logs may be shorter than what is expected for the common species. Consequently, interventions in favour of these species are economically worthwhile even if better and bigger individuals of the common species need to be sacrified.

Because of financial constraints, early interventions need to be kept to a minimum. Today, the first interventions are frequently postponed until the revenues from the removed products cover thinning costs. Since this break-even point is not reached before age 40-50, most minor species will disappear from the stand in this case. In this situation, two strategies are feasible. A first strategy is to limit early interventions to stands with high proportions of minor species. A second strategy is the early selection of plus trees (Z-trees) at a final spacing. If minor trees are selected as a first priority, their chance of survival can be increased considerably with little investment. Both strategies may, of course, be combined to further reduce tending costs.

Recommendation 10

In young stands with a high proportion of minor species, the first interventions should primarily favour minor species, using early plus tree (Z-tree) selections already in the thicket stage, followed by at least one heavy intervention. Although early plus tree selections bear a certain risk, they are the most efficient strategy to guarantee the survival of minor species until the first thinning interventions.

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