Finnish Forest Research Institute, 01301 Vantaa, Finland

Introduction

In most European countries Norway maple and sycamore are not considered to be threatened as species, although two countries noted that the species are threatened mainly by forest management practices and competition by other species. A survey of which species are considered to be threatened in various countries is presented in Table 1A. Many more countries consider the maples to be threatened at population level, the tendency being more evident for Norway maple than for sycamore. The most common threats seem to be forest management practices, competition by other species and, especially for Norway maple, mixing origins due to the transfer of reproductive material. Species hybridization is exceptionally considered to be a threat to both species. The intensity of genetic conservation reflects the general feeling that maples are not immediately threatened.

Some European countries already have a gene conservation programme for maples but in most there are no national strategies in place so far. Table 1B gives an overview of the conservation activities in various countries based on a questionnaire that was sent to members of the Noble Hardwoods Network (mid-1996) as well as on the country reports provided for the first Network meeting (Turok et al. 1996).

When replying to the Norway maple and sycamore questionnaire, many countries stressed the diversity of maple species in Europe. It was pointed out that consideration should be given to maple species which are endemic in some parts of Europe (e.g. A. heldreichii in Balkan and A. lobelii in southern Apennines). Although these species may be already protected as such, their genetic structure deserves to be given some thought as well and activities towards their conservation need to be started. Some countries also expressed their desire to develop a conservation strategy for Acer campestre.

Sycamore is, to some extent, an important forestry species in many European countries. It is used for saw wood and a little as pulp wood. There are some breeding activities both in countries where the species is indigenous and where it is non-indigenous. Since sycamore has higher economic value and generally more controlled seed procurement than Norway maple, it may be easier to start genetic conservation programmes for sycamore. Forest exploitation is probably not the most serious threat either to sycamore or to Norway maple, although some countries have reported that harvesting over increment could be risky in the future.

Norway maple is used less intensively for wood production, but the wood is considered to be valuable and there seems to be growing interest in silviculture and breeding of the species. The wood is often used for special purposes like musical instruments, decorations and furniture but in some countries even for firewood. Norway maple is commonly used for landscaping or for protective belts and it seems to be fairly resistant to air nitrogen pollution. Generally it is seen to possess considerable ecological value.

Both species regenerate naturally without problems and sycamore may even be too vigorous. Planting is used in addition to natural regeneration. Maples are usually grown and also grow naturally mixed with other species. Pure stands of sycamore are more common than pure stands of Norway maple.

The wide use of Norway maple as ornamental trees for gardening and for landscaping indicates that seed transfers are under less strict control than principal forestry species. This stresses the urgent need for genetic conservation. The growing general interest for the species is also an advantage for genetic conservation, since it gives an opportunity to increase the species’ proportion and to promote public awareness. However, there is a risk that uncontrolled trade will lead to insufficient records of afforestation material and its origin or even to the use of unsuitable material. This may cause threat to the local populations and it will definitely complicate future gene conservation activities.

The prime objective of a gene conservation strategy is to preserve enough genetic variation in order to ensure future evolution in a changing environment. This long-term adaptability should be ensured both in species and in populations within species, the latter deserving particular attention with regard to maples. In addition, there are objectives that vary according to the economic and ecological value of the species in each country and these have an effect on the national conservation programmes.

The knowledge of the genetic structure of maples is vague since there has been very little population genetic research on these species as on any other insect-pollinated trees from the temperate zone in Europe. Some assumptions and generalizations can be made based on the ecological characteristics of the species although, for example, the proportion of insect or wind pollination in maples is not known. Scattered distribution, partial or complete insect pollination and specific ecological require-ments indicate that the populations are probably more differentiated from each other than wind-pollinated species with continuous distribution (e.g. Hamrick et al. 1992).

Neutral characters can be used to measure the overall genetic variability and its distribution in the species. This reflects the adaptive potential of the species or populations and makes it possible to estimate the impact of evolutionary forces that act on the species. Results from isoenzyme studies on Norway maple populations in Finland indicate relatively high levels of population differentiation (F_st=0.126) confirming some of the theoretical expectations (Rusanen et al. 1996). Perry and Knowles (1989) have studied allozyme variation in sugar maple (Acer saccharum) populations in Canada. Although the F_st value in their study was only 0.03, they observed significant allelic heterogeneity among populations. Even though there is a lack of reliable estimates for population genetic parameters, educated guesses can be made to start with. Since maples naturally form only small populations and the pollination ecology strongly suggests that the effective population sizes are even smaller, it may be expected that drift is an important evolutionary force which should be taken into consideration when designing conservation strategies.

The variation in adaptive characters is important, because any gene conservation programme wants to capture a wide range of well-adapted existing populations. A key issue is to obtain information on phenology and growth capacity. The general tendency is that southern material has greater capacity for high growth rates, which has been explained as a photoperiodic effect on growth rate, as well as differences in the critical night length for growth cessation (e.g. Håbjørg 1972; Koski and Sievänen 1985). Unfortunately, very little is known about the genetic variation in phenology and growth capacity between stands or between families within a stand of insect-pollinated forest tree species with scattered distribution. Kriebel and Wang (1962) found a clinal pattern in the bud burst of Acer saccharum. The autumn temperature effects on the induction of dormancy in Acer platanoides were studied by Westergaard and Eriksen (1997), who also found latitudinal variation in the timing of bud burst.

Both Norway maple and sycamore flower regularly in their central distribution area, and even in the margin the seed production is not a limiting factor. Often the seed production is abundant and germination is good. There is wide variation in both the morphology and function of flowers in the genus Acer. The proportion of insect- and wind-pollination is difficult to estimate and probably varies according to the external conditions. The role of self-pollination is not known, but isolated trees have been known to produce seed, which suggests at least partial self-fertility (de Jong 1976).

It is necessary that knowledge on the genetic variation, in both neutral and adaptive characters of insect-pollinated species like maples, be widened by further research at the European level.

In order to preserve sufficient genetic variability to maintain the adaptive potential of Norway maple and sycamore in Europe it is necessary:

In situ gene conservation stands should be selected throughout the distribution area. This could be linked to seed stands as many countries have already chosen to do. The structure of the in situ network of gene conservation stands would be based on the following guidelines.

• The essential in situ network of populations is situated in the natural distribution area.

• In order to cover the variability of adaptive characters, the network should be structured according to climatic variation.

• The network should consist of at least 20 stands to ensure sufficient coverage. The absolute minimum number of regularly flowering and seed-producing trees in a gene conservation stand should be 20 trees.

• Marginal areas of the natural distribution area must be covered. Another option would be to limit the network in the centre of the distribution area, assuming that most of the variability can be found within the centre. However, since the data to support this assumption seem to be inadequate, a safer strategy would be to include the margins in the network.

• In many cases it is not reasonable to aim at pure stands; the efforts should be combined for including several species.

The in situ network could be complemented with ex situ collections, which would be designed to serve provenance research at the same time. The material would include the whole geographic variation of the species and it would be planted throughout the distribution area. Since not all of the material would have good prospects in all places, the collections would finally serve conservation of only part of the originally planted material. A disadvantage would be that these populations could not be used for seed production because the variation in the next generation might be too wide and unpredictable.

Local ex situ collections can be established to serve both conservation and seed production. They should be designed to enhance variability within a region of provenance and to avoid inbreeding. Thus they would improve the genetic quality of the seed material. In some special cases the variability of a natural stand may need to be enhanced with controlled seed transfers.

The trade with maple seed as well as other Noble Hardwoods reproductive material is generally under weak control compared with more important forest species. In addition, maples are regularly used for non-forestry purposes like horticulture and landscaping, where the tradition for controlling and documenting seed sources is recent or non-existent. The present EU directive does not cover maple species and in many countries the national legislation is limited to the principal species.

It is essential that maples moving in trade be properly documented and that the users are guided to select suitable material according to their conditions. Any long-distance transfers should not be encouraged unless they are based on knowledge from long-term provenance trials. At the moment this knowledge is generally not available. Seed transfers are usually not necessary since in most cases the seed crops are regular and abundant.

Although Norway maple and sycamore are generally not endangered as such on the European level, some countries have indicated that heavy forest utilization and management practices threaten these species and that they should be protected by appropriate measures.

Maples have several biological and cultural features which make their populations rather vulnerable to threats. However, it seems clear that neither Norway maple nor sycamore has high priority in the conservation programmes of most European countries. As resources are limited, it is probably wise to ensure a minimum conservation level through actions which do not demand high economic inputs or which can be integrated to the conservation of other species. The main approach will be to establish a network of a minimum of 20 in situ gene conservation stands. In addition, local ex situ collections may be combined with seed production and a few collections can be established to serve provenance research. The use of maples as forestry species should be promoted and special actions to be taken to ensure wise use of certified reproductive material.

de Jong, P.C. 1976. Flowering and sex expression in Acer L. A biosystematic study. Mededelingen landbouwhogeschool Wageningen 76-2, 201 p.

Hamrick, J.L., M.J.W. Godt and S. Sherman-Broyles. 1992. New Forests 6:95-124. Kluwer Academic Publishers, The Netherlands.

Håbjørg, A. 1972. Effects of photoperiod and temperature on growth and development of tree latitudinal and tree altitudinal populations of Betula pubescens Ehrh. Institute of Dendrology and Nursery Management, Agric. College of Norway, Report No. 44.

Koski, V. and R. Sievänen. 1985. Timing of growth cesseation in relation to the variations in the growing season. Pp. 167-193 in Crop Physiology of Forst Trees (P.M.A. Tigerstedt, P. Puttonen and V. Koski, eds.). University of Helsinki.

Kriebel, H.B. and C.-W. Wang. 1962. The interaction between provenance and degree of chilling in bud-break of sugar maple. Silvae Genet. 11:125-130.

Perry, D.J. and P. Knowles. 1989. Allozyme variation in sugar maple at the northern limit of its range in Ontario, Canada. Can. J. For. Res. 19:509-514.

Rusanen, M., A. Mattila and P. Vakkari. 1996. Jalojen lehtipuiden geneettinen monimuotoisuus-säilyatä ja käytä. Metsäntutkimuslaitoksen tiedonantoja 605, pp. 45-52.

Turok, J., G. Eriksson, J. Kleinschmit and S. Canger (compilers). 1996. Noble Hardwoods Network. Report of the first meeting, 24-27 March 1996, Escherode, Germany. IPGRI, Rome, Italy. 172 p.

Westergaard, L. and N.E. Eriksen. 1997. Autumn temperature affects the induction of dormancy in first-year seedlings of Acer platanoides L. Scand. J. For. Res. 12:11-16.