Peter Rotach, Department of Forest and Wood Science, Chair of Silviculture,
Swiss Federal Institute of Technology (ETH), 8092 Zürich, Switzerland
Europe's Noble Hardwoods can be considered an "overlooked" or neglected group of tree species. They include species of the genera Ulmus (elm), Acer (maple), Fraxinus (ash), Tilia (lime), Alnus (alder) and of the family Rosaceae. Noble hardwoods usually grow scattered in mixed-species forests where they make up a small proportion of the total forest cover (less than 5% on average). They require good sites and are characterized by high-quality timber. The species are found in specific habitats, having been pushed onto sites often considered marginal to plant growth - such as gorges, riverside forests, forest slopes etc. Most of the indigenous Noble Hardwood species in Europe are relatively rare, some are endangered. Since the United Nations Conference on Environment and Development in Rio de Janeiro in 1992, conservation of biological diversity and its sustainable use have become issues of major international concern, also in the field of forestry. Although different strategies of conservation for Noble Hardwood species are possible and are applied throughout Europe, in situ conservation is essential for dynamic gene conservation and conservation of species diversity. The Convention on Biological Diversity (CBD), endorsed in the Rio Conference and subsequently ratified by a number of Governments, assigns high priority to in situconservation of biological diversity at all levels (genes, species, ecosystems). In situ conservation is addressed specifically in Article 8 of the CBD.
Due to their generally low competitive ability, Noble Hardwoods need regular silvicultural interventions on most sites for their survival and adequate development. Accordingly, promotion of these species and their conservation in situ is most successful when the stands in which they occur are actively managed, incorporating considerations for site requirements of the different species as well as silvicultural needs, in addition to conservation aspects. In order to be successful, there are three prerequisites for in situ conservation: (1) efficient conservation measures can be implemented only if populations and - in some cases - even individuals are identified and mapped and if population structures and variation patterns of targeted tree species are known; (2) since Noble Hardwoods often occur as single individuals in mixed stands and since their competitive ability is generally low, periodic forest management interventions over time are likely to be needed; (3) the financial possibilities of the forest owners need to be taken into account when planning establishment and silvicultural operations, which are relatively cost-intensive.
Promotion and successful management and conservation of Noble Hardwood species need to take into consideration factors such as competing or alternative tree species and the special characteristics and requirements of target species in the environment in which they occur. General guidelines are difficult to develop because management interventions will differ according to prevailing site conditions as well as between individual species. The urgency of conservation and specific conservation needs also vary widely both between species, and from one part of Europe to another. However, some basic general principles, applicable to most situations, may be helpful to guide foresters in their efforts.
The Noble Hardwoods Network was established in 1996 within the framework of EUFORGEN (see text box). The work of the Noble Hardwoods Network, which includes management strategies as well as conservation in the strict sense, is briefly described below.
|The European Forest Genetic Resources Programme, EUFORGEN, is a collaborative programme among European countries aimed at ensuring effective conservation and sustainable utilization of forest genetic resources in Europe. It was established to implement Resolution 2 of the Strasbourg Ministerial Conference on the Protection of Forests in Europe. EUFORGEN is financed by participating countries and is coordinated by IPGRI (International Plant Genetic Resources Institute, Rome) in collaboration with the Forestry Department of FAO.|
Common to all Noble Hardwoods included in the EUFORGEN Noble Hardwoods network (see above) are the following characteristics which decisively influence both their promotion and conservation:
It follows that strategies for the promotion and conservation of Noble Hardwoods largely depend on local site conditions, which greatly influence natural stand development, competition between species, behaviour of the individual species in mixtures, types of mixed stands, quality and value of species and intensity of silvicultural interventions needed for conservation or promotion of a given species. On certain sites, which are unfavourable to the development of the targeted Noble Hardwood species, efforts will always be inefficient, expensive and most likely unsuccessful in the long run, and totally dependent on regular, frequent human interventions to continuously "correct" natural development of the stands. Thus:
As a first priority, promotion and conservation efforts of Noble Hardwoods should concentrate on 'ecologically optimal sites2, natural ecological niches3 or special habitats. Efforts to maintain these species even on 'physiologically optimal'4 sites are reasonable only in situations where a high silvicultural intensity can be guaranteed over time.
A low competitive ability combined with low tolerance of competition and a habitat preference shared with major competitors, are among the major causes for the rarity in nature of Noble Hardwood species. Promoting and conserving these species on sites outside their ecologically optimal range thus actually means enlarging their natural distribution. Although this is possible, it requires a much greater effort, a higher investment and longer-term continuity to be successful than actively managing stands on ecologically optimal sites. Economically, such action can only be justified if the expected value of production is higher than the investments. This has proven to be the case for some of the Noble Hardwoods, including Sorbus torminalis, Juglans regia and Prunus avium. Such action may also be justified in situations in which a species is rare or highly threatened, as is the case at the margins of the natural distribution of a number of tree species.
Another possible solution would be to limit in situconservation measures to populations which have a given minimum size, for example, to demes in which more than 20 individuals of the targeted species are found. This strategy may however not meet expectations in a number of situations and for several species (for a detailed discussion, please see the original paper quoted in footnote on page 1 of the article). We recommend instead:
For rare and very rare species, all individuals found in ecologically optimal sites or niches should be conserved. For species that are still found in relatively large populations, such as Fraxinus, Acer and Prunus species, concentration of efforts on demes with more than 20 individuals seems reasonable.
Growth and quality are not necessarily optimal on sites in which the species have their ecological optimum. With few exceptions optimal growth and quality are found on highly productive sites, on which also beech (Fagus) is at its optimum. From a production point of view, Noble Hardwoods should be managed on the most productive sites, either on riparian sites (Fraxinus, Acer, Ulmus, Prunus), or on sites optimal for beech. In most cases, however, this means promoting Noble Hardwood species on sites on which they are not dominant or where they do not occur naturally. Although Noble Hardwoods can be established easily on most such sites, either by using natural regeneration or by planting, the high competitive ability of beech has important consequences for success, which will primarily depend on two factors: the intensity of silvicultural management; and the type of mixture. Mixed stands have many advantages, including the fact that trees in such stands are better able to withstand the effects of harmful abiotic and biotic factors and therefore have lower associated risks of failure. In many cases mixed stands also provide for higher adaptability, more diversity, and better quality and growth of component species. Mixed stands may also provide opportunities for flexibility in product marketing. Noble Hardwoods are prime candidates for mixtures with beech, and may increase the value of production of beech stands considerably. It should however be realized that highly diverse mixed stands do not occur naturally on sites which are optimal for beech. Active silvicultural management is therefore necessary to guarantee a substantial proportion of Noble Hardwoods at maturity of such stands, and to attain reasonable management objectives (minimum diameter, good wood quality, acceptable rotation age). Hence, only where intensive silviculture and continuity of management interventions can be guaranteed, should other hardwoods be promoted on sites presently dominated by beech.
If a high silvicultural intensity is guaranteed, valuable hardwoods may 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 characteristics of each species. In the case of the latter, a number of factors are important including dynamics of growth over time of the individual species, their final height, differences in height growth between them and changes in such relationships over time, whether they are shade tolerant or light demanders, capacity of a species to expand its crown in a competitive environment and ability to restore the crown after release through thinning. The most decisive factor among these is height growth relationships among species and relative shade tolerance. In the case of Noble Hardwoods, which generally have a low competitive ability, we recommend:
On sites where competition is high, Noble Hardwoods should be mixed in patches (groups of trees) rather than as single trees. They should be either established in groups, or a patchy structure of the stand should be favoured from the first silvicultural interventions.
In most cases, single tree mixtures of Noble Hardwoods and other species are not stable and are dependent on periodic, intensive silvicultural interventions. Growth rhythms of Noble Hardwood species differ considerably from those of beech (Fagus), oak (Quercus) and spruce (Picea) species, and most Noble Hardwoods have rather weak competitive ability Even if some of the Noble Hardwoods may outgrow e.g. beech in the first 30 to 40 years following establishment, they are all clearly dominated by beech in the second half of rotation period. In groups of trees, growth differences are less pronounced, since competition between different species is restricted to the edges of the groups.
Some Noble Hardwood species cannot be integrated into high-forest stands, not even as groups. These species include species of the genera Malus, Pyrus and Sorbus. These species need special stand structures or habitats to survive. Suitable habitats for them are, for example, forest margins, small woodlots outside the forest area, stands in which associated species are managed as "coppice with standards" with low standing volumes, alluvial forests, or pine stands. Hence:
Species that are difficult to integrate with other species should be given priority in special stand structures or habitats, which are especially favourable. Such 'special habitat'-management should be used primarily in the case of endangered minor or rare species.
Experiences in Switzerland indicate that field staff is poorly aware of the existence and/or the potential of some of the Noble Hardwood species. On the other hand, examining inventory results and comparing documented records with the actual existence of various tree species in the field, it is evident that more publicised rare tree species are perceived quite differently from others. While the foresters in the country knew of nearly all of the existing, rare and much-talked about Taxus baccata populations and even had information on individual trees, only about 20% of them had information on stands and individuals of Sorbus torminalis, another rare but less publicly talked-about species. In addition to being easy to recognize, Taxus was perceived as important, being rare, "special" or otherwise especially valuable. Sorbus torminalis, on the other hand, was hardly noticed and was evidently not perceived as a valuable species, although market prices for wood of this species have been very high in the past, much higher than those of any other species. This experience seems to demonstrate that awareness is related to information, knowledge, and motivation for promotion or conservation. Promotion of Noble Hardwood species will neither be efficient nor successful unless field personnel are well-informed, trained and committed.
For successful promotion and conservation of minor hardwood species (with special reference to the Noble Hardwoods group), the perception of staff of the role and importance of these species needs to be improved. Filed staff need to be better informed, trained and motivated.
In most cases, the proportion of minor Noble Hardwood species could be increased considerably just by changing routine silvicultural interventions and perceptions of their value. Individuals of all Noble Hardwood species, even the rare ones, are often found in certain proportions in the natural regeneration of mixed stands. The proportion of most Noble Hardwoods could be considerably increased by training and supervising field staff and by giving clear directives for early interventions in young stands, which favour targeted minor species. Many of the Noble Hardwood species have been generally considered as economically unimportant in the past. Even worse, the so called "minor species" have frequently been perceived as competitors for the main economic tree species and have been actively eliminated through silvicultural interventions carried out early on in the rotation. The number of individuals and the proportion of these species found in young stands today do not reflect their potential numbers at the end of the rotation, and the proportion of most of them can be increased by appropriate tending operations.
For some Noble Hardwood species, natural regeneration may not be sufficient to meet the needs of genetic conservation. Some of these species, notably Pyrus pyraster, Malus sylvestris and Prunus avium may, due to their occurrence in small demes, be primarily pollinated by pollen from domesticated individuals outside of the forest. Due to a resulting high degree of introgression from cultivated forms, it is doubtful if Malus and Pyrus species still exist as wild forms. Some introgression may also have occurred in Sorbus and Ulmus species. For genetic conservation purposes, these species need to be enriched by infusion of genetically diverse material, containing a high proportion of undomesticated genotypes. A possible solution is the establishment of seed orchards, followed by use of reproductive materials originating from these seed orchards in plantation establishment. In other words, two different strategies can be applied, depending on the characteristics of the target species and the actual situation in the location in which they grow, as outlined below.
In species which still occur in relatively large populations and which do not suffer from introgression from domesticated populations, the following recommendation is given:
If a given species still occurs in relatively large, wild populations, efforts and financial means should primarily be invested in favouring natural regeneration and in early silvicultural interventions. Planting of these species should be restricted to the most productive sites, on which production can be expected to be highest. Only select or improved material of the best possible phenotypic quality of any given provenance should be used in plantation establishment.
In the case of those species and situations in which natural regeneration is not sufficient to meet the needs of genetic conservation, planting is not only an option but a necessity. The following recommendation will apply:
For species which occur in small demes and which are subject to introgression from domesticated populations, natural regeneration needs to be complemented by artificial regeneration. Planting is also necessary on sites on which relying on only natural regeneration is not economically feasible. Reproductive materials used in these cases should be highly diverse and should contain a large proportion of material from wild genotypes of the same provenance as that targeted for promotion and conservation.
Without active silvicultural interventions, most individuals of Noble Hardwood species are apt to disappear over time following natural regeneration, even on sites which are not optimal for their main competitor, beech. With the exception of species of the genera Fraxinus, Acer and Ulmus, most of these species will lose out in competition with other species within the first 20-30 years after establishment. Early silvicultural interventions are thus essential for their survival and for their continued existence in more mature stands. A minimum of one silvicultural intervention in the sapling or pole stage will be necessary to regulate the mixture and to favour targeted minor Noble Hardwood species.
The first 20-30 years are decisive for the survival of most Noble Hardwoods. At least one silvicultural intervention in the sapling or pole stage is necessary to promote and conserve targeted minor species. Such species should be favoured in natural regeneration, regardless of their quality, their proportion in the mixture and length of rotation period.
Due to financial constraints, early interventions in practice need to be kept to a minimum. At present, the first silvicultural interventions are frequently postponed until the revenues from thinnings cover the costs. Since this break-even point is not reached until age 40 to 50 in Noble Hardwoods, most minor species will disappear from the stand if this principle is applied. Two solutions are possible to confront this situation: the first one consists in limiting early interventions to stands in which a high proportion of targeted minor Noble Hardwood species are present; the second one includes the selection of plus trees at an early stage, at final spacing. If individuals of target species are selected and marked, their chances of being favoured in subsequent interventions, and thus their survival throughout the rotation, will be considerably increased with little investment. The two strategies can be combined, thus further reducing tending costs.
In young stands with a high proportion of minor species targeted for promotion and conservation, the first silvicultural interventions should strongly favour such minor species, and this should be complemented by carrying out selection of plus trees already in the sapling stage. Although selecting plus trees at young age carries a certain risk of making mistakes, this is the most efficient strategy to guarantee the promotion and thus survival of individuals of such species up to the first thinning and beyond.
|Note from the Editor: Although limited to a group of species in Europe, the principles and methodology presented in the present paper could be applicable to tropical hardwood species, in which many of the problems and constraints expressed in the paper also apply. See also paper by Purnell, R.C. and Kellison, R.C. 1987, Tree Improvement Programme for Southern Hardwoods, Forest Genetic Resources, No 15 p 72-76 FAO, Rome.|
Readers interested to learn more about the work of the EUFORGEN Noble Hardwoods Network should refer to:
Turok, J., Erickson, G., Kleinschmit, J., S. Canger (Eds). Noble Hardwoods Network. Report of the First Meeting held in Escherode, Germany, 24-27 March 1996. International Plant Genetic Resources Institute, Rome 1996.
Turok, J., Collin, E., Demesure, B., Erickson, G., Kleinschmidt, J., Rusanen, M., Stephan, R. (Eds). Noble Hardwoods Network. Report of the Second Meeting held in Lourizán, Spain, 22-25 March 1997. International Plant Genetic Resources Institute, Rome 1998.
Turok, J., Jensen, J., Palmberg-Lerche, C., Rusanen, M., Russel, K., de Vries, S., Lipman, E. (Eds). Noble Hardwoods Network. Report of the Third Meeting held in Sagadi, Estonia, 13-16 June 1998. International Plant Genetic Resources Institute, Rome 1999.
The above and related publications can be obtained from:
The EUFORGEN Coordinator, IPGRI
Via delle Sette Chiese 142
00145 Rome, Italy