Technical guidelines for genetic conservation and use Silver birch Betula pendula Betula pendula Betula pendula Pekka Vakkari Finnish Forest Research Institute, Finland

These Technical Guidelines are intended to assist those who cherish the valuable silver birch genepool and its inheritance, through conserving valuable seed sources or use in practical forestry. The focus is on conserving the genetic diversity of the species at the European scale. The recommendations provided in this module should be regarded as a commonly agreed basis to be complemented and further developed in local, national or regional conditions. The Guidelines are based on the available knowledge of the species and on widely accepted methods for the conservation of forest genetic resources.

Biology and ecology and efficiently wind-dispersed. The species is monoecious, male and female catkins are found Silver birch (syn. Eu- in the same tree. Male catkins ropean white birch develop at the end of the pre- (Betula pendula vious summer and are visible Roth)) is a fast during winter whereas female growing, medi- catkins overwinter in the shelter um-sized decid- of buds and become visible at uous tree with budburst in spring. Silver birch white bark, fair- is self-incompatible, a chemical ly narrow crown mechanism prevents the growth and drooping of pollen tubes on the stigmas of outer branch- the same mother tree. lets. Trees European birches are a spe- rarely ex- cies complex, where correct ceed 30 m identification of silver birch may in height, be a challenge. Silver birch is even on the characterized by hairless leaves best sites in and young shoots and by very northern Eu- thick bark at the base of old rope, and the av- trunks. Although quite similar, erage height of pure downy birch (Betula pubescens) stands is usually less has hairy leaves and shoots. Leaf than 25 m. The average shape of silver birch is most of- maximum biological age of ten triangular and the base of the silver birch is approximately 100 leaf is at a right angle to the stalk, years, although sometimes trees whereas the base of downy birch can survive up to the age of 150 leaf is rounded. Chemical and years. molecular markers are presently Silver birch is regenerated by available for more definitive iden- seed, which are numerous, small tification. SilverBetula birchBetula pendulaSilver birch pendulaBetula pendulaSilver birchBetula pendulaSilver BetulabirchBetula pendulaSilver birch Betulapendula pendulaSilver birchBetula pendulaSilver birchBetulaBetula pendulaSilver birchBetula pendulapendulaSilver birchBetula pendulaSilver birch BetulaBetula pendulaSilver birchBetula pendula

These two species can also Distribution Importance and use be distinguished cytologically, silver birch has 28 chromosomes (a diploid species), whereas The distribution of silver birch Silver birch is among the most downy birch has 56 chromo- covers almost all Europe from commercially important tree spe- somes (a tetraploid species). De- the Mediterranean in the south cies in northern Europe. Its wood spite this difference, the nearly to latitude 70° in is used in plywood-making and two species may oc- the north. The species carpentry as well as for pulp- casionally hybridize. is most abundant in wood and fuel. The wood of Silver birch has northern Europe, silver birch is pale-colored and soil requirements where the distri- there are no differences of any similar to those of bution is more or importance between the spring- Norway spruce (Picea less continuous and summer wood or heart- and abies); it thrives on fairly in mixed forests sapwood. The wood specific fertile and well-drained soils and also fairly large density is very high. The wood and does not survive on wet clay pure stands can be found. In fibers are very short and birch or peat soils. the western and southern parts pulp is mixed with conifer pulp to of the range the distribution is improve paper quality. more patchy and in the south Curly birch (var. carelica), a silver birch is found mostly at the special form of silver birch, is higher altitudes. The species is highly valued for the decorative missing from Iceland, and colour pattern of the wood. There most of the Iberian penin- are several heritable pattern types sula and Greece. and some of them can also be identified by visual inspection of the trunks. Curly birch is used in carpentry as veneer and for vari- ous small handicrafts. Special forms of silver birch (f. bircalensis, f. crispa, ‘Dalecarlica’) are used in ornamental plantings because of their decorative leaf forms. In the southern fringe of the distribution silver birch is used for reforestation purposes and in some areas as fodder for cattle. Silver birch (and birches in general) is an essential compo- nent of temperate and boreal forests and has positive effect on the diversity of ecosystems. A large number of herbivorous insects and symbiotic fungi are dependent on the presence of birches. SilverBetula birchBetula pendulaSilver birch pendulaBetula pendulaSilver birchBetula pendulaSilver BetulabirchBetula pendulaSilver birch Betulapendula pendulaSilver birchBetula pendulaSilver birchBetulaBetula pendulaSilver birchBetula pendulapendulaSilver birchBetula pendulaSilver birch BetulaBetula pendulaSilver birchBetula pendula

Genetic knowledge ing season. At the end of sum- mammalian herbivores cor- mer, cessation of height growth, relates clearly with the which is the first step in the number of resin Genetic structure of silver birch hardening process, is primarily pockets in the in the main distribution area is controlled by photoperiod and shoots and is shaped by the abundance (com- modified by temperature and therefore monness) of the species and other factors. Birch populations effective seed and pollen dis- from different latitudes differ re- persal. Pollen can travel several garding their critical night length, hundreds of kilometers and birch i.e. the shortest night length seeds are very light and easily to trigger growth cessation. eas- dispersed. Although the majority In the spring, bud burst ily as- of the seeds fall down close to and onset of new growth sessed, the mother tree, a smaller frac- is primarily controlled by but so far tion will be dispersed over con- temperature. has not been siderably longer distances. A considerable amount of used as a trait for As a result, the genetic pat- genetic variation has been de- breeding. tern of silver birch is similar to tected in many economically The genetic basis common conifers (Scots important growth and quality of the very valuable spe- pine (Pinus silvestris) characters. The provenances cial form of silver birch, and Norway spruce, f ro m Finland and Swe- curly birch (var. carelica), is (Picea abies)) and den (omitting the still not completely understood. pedunculate oak most northern mar- The proportion of curly pheno- (Quercus robur). ginal populations) type in the progenies is 60-70% Levels of adaptive- usually have better if both parents are curly birch- ly neutral genetic stem form than prov- es; in open-pollinated seedlots variability are high enances from Central about 50% of progenies show within populations Europe, but there is curly phenotype. There is con- and local popula- also genetic variation siderable amount of variation tions in northern Eu- within populations. among mother trees regarding rope are very similar Conventional breeding the proportion of curlyness in to each other, i.e. has considerably im- their progenies. the differentiation proved both the vol- among populations ume growth and quality is very low. However, of planting material in for adaptive traits a differ- Finland. ent pattern is observed. There is also consid- Strong genetic differ- erable genetic variation entiation of local Scandi- in herbivore resistance, navian populations occurs both against mammals along south - north axis in and insects. The genetic the case of phenological variation in resistance is mostly traits. This is evidently an adap- within populations, there is no tation to the local climate and to known differentiation among the timing and length of grow- populations. Resistance against SilverBetula birchBetula pendulaSilver birch pendulaBetula pendulaSilver birchBetula pendulaSilver BetulabirchBetula pendulaSilver birch Betulapendula pendulaSilver birchBetula pendulaSilver birchBetulaBetula pendulaSilver birchBetula pendulapendulaSilver birchBetula pendulaSilver birch BetulaBetula pendulaSilver birchBetula pendula

Threats to Guidelines for genetic be larger at low elevations in genetic diversity conservation and use Central European countries. An additional measure to pro- The main threats to the genetic Because silver birch is a wide- tect the natural genetic compo- diversity of silver birch are found spread species, the environmen- sition is to select in situ areas at the margin of its distribution, tal conditions and hence priority for genetic conservation either where occurrence is discontinu- and methodology of ge- for birch alone or as mixed for- ous and consisting of isolated netic conserva- tion are est with other species. Such ar- populations. In particular, south- different in various eas can be nature conservation ern (Spain, Italy, France) and, to parts of the distribu- areas or gene reserve forests a smaller extent, northern edges tion area. In north- under commercial forestry. In of the distribution area are sub- ern Eu- rope silver both cases natural regeneration ject to this fragmentation proc- birch is common should be favoured or, if artificial ess. Very little is known about a n d has almost regeneration must be applied, the amount and organization of continuous the material originating from the genetic variation in the southern distribution same forest should be used. marginal areas. over large areas In the areas of scattered Within the main distribution without any im- distribution the use of gene re- area there are no immediate mediate threats to serve forests may not be pos- threats to the genetic diver- the amount of ge- sible. The local stands may be sity of silver birch. Natural netic diversity, hence too small and threatened by regeneration has prevailed and the nature of the gene various environmental hazards large-scale planting started conservation measures to such a degree that an ex situ only after the importance in these areas is mostly pre- conservation strategy is more of genetic knowledge and cautionary. These measures applicable. Ex situ collections the control of the origins include restrictions on the use may be based on either grafts of the planting material was re- of single seed sources or veg- or seedlings. alized. Special forms, however, etatively reproduced clones and A special case for gene con- need more attention and native the selection of properly adapted servation is curly birch, which curly birch genotypes may be at forest reproductive material. is usually found as single trees risk of extinction. In the northern parts of the or as groups of a few trees in Unbalanced use of forest re- distribution area (Finland and natural forests. In such cases a productive material can lead to Sweden), controlling the dis- population-based approach is loss of genetic variation. In the tance of provenance shifts not functional and an ex situ case of silver birch this possibil- is crucial for the suc- collection of indi- ity has to be considered seri- cessful use of vidual clones ously, because of the extremely planting mate- (genotypes) high seed-producing capacity of rial. In order to is a more ap- birches. The production of clonal avoid the risk propriate ap- seed orchards in good condi- of late spring proach. The tions is so high that one orchard and early autumn frosts, collections can be may produce enough seeds for the recommended maximum created either by grafting extremely large areas and limita- transfer distance in Finland is or seedlings. tions to the usage of seed must 150km either north or south. be considered. The transfer distance could BetulaSilver birchBetula pendulaSilver birchpendulaBetula pendulaSilver birchBetula pendulaSilver Betula birchBetula pendulaSilver birch Betulapendula pendulaSilver birchBetula pendulaSilver birchB

Distribution range of silver birch BetulaSilver birchBetula pendulaSilver birchpendulaBetula pendulaSilver birchBetula pendulaSilver Betula birchBetula pendulaSilver birch Betulapendula pendulaSilver birchBetula pendulaSilver birchB

This series of Technical Guide- Selected bibliography lines and distribution maps were produced by members of the EUFORGEN Networks. The Hynynen, J., Niemistö, P., Viherä-Aarnio, A., Brunner, A., Hein, S. & Velling, P. 2009. Silviculture of birch (Betula pendula Roth and Betula pubescens objective is to identify minimum Ehrh.) in Europe. Forestry (in prep) requirements for long-term Rusanen M, P. Vakkari & A. Blom, 2003. Genetic structure of Acer platanoides genetic conservation in Europe, and Betula pendula in northern Europe Canadian Journal of Forest in order to reduce the overall Research – Revue Canadienne de Recherche Forestiere 33 (6): 1110-1115 conservation cost and to improve Viherä-Aarnio, A. & Heikkilä, R. 2006. Effect of latitude of seed origin on moose quality standards in each country. (Alces alces) browsing on silver birch (Betula pendula). Forest Ecology and Management 229: 325-332. Viherä-Aarnio, A., R. Häkkinen, & O. Junttila, 2006. Critical night length for bud set and its variation in two photoperiodic ecotypes of Betula pendula. Tree Physiology 26: 1013-1018. Viherä-Aarnio, A. & Velling, P. 2008. Seed transfers of silver birch (Betula Citation: Vakkari P. 2009. EUFOR- pendula) from the Baltic to Finland - effect on growth and stem quality. GEN Technical Guidelines for Silva Fennica 42(5): 735-751. genetic conservation and use of silver birch (Betula pendula). Bio- versity International, Rome, Italy. 6 pages.

Drawings: Betula pendula, Gio- vanna Bernetti. © Bioversity, 2009.

ISBN 978-92-9043-803-8

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