A review of the promises and constraints of breeding silver birch (Betula pendula Roth) in Finland

VEIKKO KOSKI 1 2 MATTI ROUSI 0 1 0 Finnish Forest Research Institute, Punkaharju Research Station , Finlandiantie 18, FIN 58450 Punkaharju, Finland 1 Institute of Chartered Foresters , 2005. All rights reserved 2 Finnish Forest Research Institute, Vantaa Research Centre , Box 18, FIN-01301 Vantaa, Finland In comparison to the breeding of boreal conifers, silver birch (Betula pendula Roth) has several advantages from the biological point of view. Flowering at an early age allows faster succession of generations, which together with rapid juvenile development can shorten the breeding cycle. Occurrence of natural polyploidy, inter-specific hybridization and the ability to propagate asexually offer optional breeding methods besides conventional recurrent selection. On the other hand, environmental factors have a strong impact on the performance of silver birch. Even though silver birch is ecologically a strong pioneer species, plantations in boreal forests often suffer from being eaten by mammalian herbivores, such as moose (Alces alces) and voles (Microtus spp.). A large number of insect species attack silver birch, but except for cambium miner (Phytobia betulae) they are rarely economically important. Silver birch is also susceptible to some fungal pathogens. Resistance breeding is difficult because of the multitude of pests and diseases. The main stream of breeding was started with phenotypic selection in superior autochthonous stands followed by progeny testing and establishment of plastic-covered seed orchards. Considerable genetic gain has been achieved, even though more moderate than the most optimistic expectations of the early years. The utilization of advanced methods, such as gene transfers and micropropagation, is restricted because of public opinion and the stringent requirements set by the European Union directives and the Organization for Economic Cooperation and Development scheme on the marketing of forest reproductive material. The conspicuous oscillation of the interest and demand for birch wood by forest industries causes problems for longterm planning of breeding. However, the objective of improving silver birch using traditional methods is supported by public opinion. To successfully breed silver birch, close collaboration is recommended between the disciplines of genetics, silviculture and ecology and forest industries. Summary Introduction Activities and achievements of the breeding of silver birch (Betula pendula Roth.) were introduced during the 1970s and 1980s, especially in Finland (Lepist, 1973; Krki, 1976). More recently the enthusiasm and optimism have declined, but breeding still goes on and makes progress. Advanced methods, such as molecular genetics and micropropagation, as well as willingness to increase the purposes and even for mass propagation of reproductive material. However, grafting and rooting of cuttings are not generally successful in birch. On the other hand, the methods of micropropagation of adult trees developed during the 1980s provided the option to multiply specific valuable genotypes, such as curly birch, on a commercial scale (Ryynnen and Ryynnen, 1986). A problem is that all genotypes do not multiply with equal ease, and consequently there is a risk that an undesirable selection takes place on the grounds of response to micro-propagation (but see Laitinen et al., 2004). More recently silver birch has been subject to studies on molecular biology. Today many results of genetic mapping are available, and protocols of gene transfer are being developed. With regard to methods of breeding trees, silver birch offers more alternatives and also essentially faster progress than the commercially valuable temperate and boreal conifers. The broad hereditary variation of silver birch is a thankful starting point of selection and breeding. The species has a wide range of distribution in Europe and in the northern part of Asia. At the species level, birch shows considerable ecological adaptability and climatic differentiation. Most of our knowledge comes from experiments on photoperiodic responses carried out on seedlings in phytotrons or greenhouses. At least young birch seedlings react to increasing night length according to the latitude of origin. There are few conventional provenance trials of silver birch set up to study how trees from various origins perform in the field. Geographic transfers are successful only within certain limits (Velling, 1979; ViherAarnio and Velling, 2001), and caution is recommended because of unpredictable biotic and abiotic factors, including edaphic interactions of exotic white birches in natural soils (Laitinen et al., 2004). The knowledge on flowering biology and mating pattern suggests that silver birch has a high rate of out-crossing and effective gene flow among stands and localities. The consequences are (1) large within-stand variation, and (2) no differentiation among stands but a clinal population structure (Rusanen et al., 2003). Furthermore, crossing with downy birch is rare, and on the rare occasions when it does occur, the hybrid is sterile due to the difference in polyploidy level of the parents (diploid and tetraploid). In a recent detailed morphological survey by Prittinen (2004), 30 720 saplings of 10 open-pollinated free-growing silver birches were checked and 51 putative B. pendula B. pubescens hybrids (0.2 per cent of the total) were found. In addition to its numerous favourable biological properties, silver birch has the advantage of being popular with the public and is of high social value. While cultivated stands of pine and spruce are often considered dull and unnatural, birch stands appeal to people. Even foresters accept silver birch for the sake of its merchantable timber and also as an alternative to spruce on fertile but contaminated sites (Korhonen and Piri, 1994). Compared with conifers, birch cover has been shown to have a favourable impact on soil, e.g. on pH, nutrient contents, microbes, earthworms and decomposition of lignin (Priha, 1999). Rapid growth does not have a strong effect on wood density or stem form of birch (for review, see Dunham et al., 1999). Problems and constraints Silver birch grows rapidly in favourable conditions. During the juvenile period, height growth can be over 1 m per year in small-scale experimental plantations after the root system is fully established (Figure 1). In commercial plantations the growth rate is rarely so rapid. On the other hand, birch is sensitive to many environmental factors. Heavy soil, high ground water level, or suppressive competition of grass and shrub retard growth and spoil the stem form (Figure 2). Plantations on former agricultural land on peat soil have mostly been a failure, although rather tolerant silver birch genotypes can be found (Laitinen et al., 2002). Even on suitable mineral soils the microvariation of the site may cause such large differences in the performance of test families that they m (...truncated)