Technical guidelines for genetic conservation and use Eurasian aspen Populus tremula Populus tremula Populus tremula Georg von Wühlisch Federal Research Institute for Rural Areas, Forestry, and Fisheries Institute for Forest Genetics, Germany These Technical Guidelines are intended to assist those who cherish the valuable Eurasian aspen genepool and its inheritance, through conserving valuable seed sources for 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 for 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 Populus tremula L. (European aspen, common aspen or Eura- sian aspen; section Populus (syn. Leuce) (subsection Trepi- dae (Dode)); family Salicace- ae) is a wide-spread colo- nising pioneer species. It is a medium-sized to tall deciduous tree growing to 40 m height with a trunk attaining over one meter in diameter. The bark is pale silvery-grey to green and smooth on young trees with dark grey diamond- shaped lenticels, becoming dark grey and fissured on older trees. The adult leaves, produced on branches of mature trees, are nearly round, slightly wider than long, 2–8 cm diameter, with a coarsely toothed margin and a laterally flattened petiole 4–8 cm long. The flat petiole allows them to tremble in even slight breezes, and is the source of its scientific name. The leaves on seedlings and fast-growing stems of root sprouts are very different, heart- EurasianPopulus aspenPopulus tremulaEurasian aspen tremulaPopulus tremulaEurasian aspenPopulus tremulaPopulusEurasian aspenPopulus tremulaEurasian tremulaaspenPopulus tremulaEurasian aspen PopulusPopulus tremulaEurasian aspenPopulus tremula Eurasiantremula aspenPopulus tremulaEurasian aspenPopulus tremula shaped to nearly trian- increment is slower and cul- Distribution gular, and often much minates at about 30 years of larger, up to age. Aspen can reach an age 20 cm long; of 200 years. Root suckers are Eurasian aspen has a large dis- their petiole produced abundantly on the tribution range. It is native to is also less shallow lateral roots after an cool temperate and boreal re- flattened. individual has been damaged gions of Europe and Asia, ex- The flowers or destroyed, e.g. by cutting, tending from the British Isles are wind- fire or diseases leaving an open and Iceland eastwards to Kam- p o l l i n a t e d space exposed to sunlight. The chatka, and from north of the catkins pro- trees growing from the suckers Arctic Circle in Scandinavia duced in early form clones and mature stands and northern Russia, south to spring before reproduce vigorously by this central Spain, Turkey, the Tian the new leaves vegetative means. Aspen clones Shan, North Korea, and northern appear. The spe- vary widely in many characteris- Japan. The aspen in the latter cies is dioecious, tics, members of a clone being regions of East Asia is consid- with male and fe- indistinguishable but can be ered by a large number of male catkins on distinguished from authors to be sufficiently different trees. The those of a neigh- different to justify clas- male catkins are bouring clone. sification as different patterned green Clones typically species; P. davidiana and brown, 5–10 have many ram- (Dode) Schneider. In cm long when ets over an area the south of shedding pollen; of a few tenths its range, as- the female catkins are green, 5-6 of a hectare, in pen occurs at cm at pollination, extending 10- rare cases up to high altitudes 12 cm long at maturity in early several hectares. in moun- summer to bear 50–80 capsules tains (Alps: each containing numerous tiny 1 3 0 0 - 2 0 0 0 seeds embedded in downy fluff. m, Pyrenees: The fluff assists wind dispersal to about 1600 of the seeds when the capsules m; Caucasus: to split open at maturity. 1900 m above sea level). Eurasian aspen is a distur- Eurasian aspen occurs bance-adapted species and is where annual precipitation often a principal coloniser of exceeds evapotranspiration. It areas after fire or clear cutting, favours moist soils so long either by seeding or root suck- as they are well aerated and ers. It is capable of rapid growth not stagnant and is able to under conditions favoured by grow in a wide variety of soils this light and water demand- (mainly Alfisols, Spodosols, ing species. Fast growth con- and Inceptisols) ranging from tinues until the age of about 20 shallow and rocky to deep years when crown competition loamy sands and heavy increases. After that, growth clays. EurasianPopulus aspenPopulus tremulaEurasian aspen tremulaPopulus tremulaEurasian aspenPopulus tremulaPopulusEurasian aspenPopulus tremulaEurasian tremulaaspenPopulus tremulaEurasian aspen PopulusPopulus tremulaEurasian aspenPopulus tremula Eurasiantremula aspenPopulus tremulaEurasian aspenPopulus tremula Importance and use Genetic knowledge For aspen breeding pro- grammes, parent trees have been selected phenotypically mainly The light, soft wood has very lit- The genetic diversity of for favourable stem and branch- tle shrinkage and high grades are Eurasian aspen is gen- ing characteristics since about used for lumber and matches. erally high. Much of the 1950. Such trees are usually Aspen wood is used for pulp and diversity is found within conserved in the collections of paper, where it is favoured for its stands and less between poplar breeding institutes easy delignification, easy bleach- them. However, when com- in Scandinavian, Baltic ing and characteristics especially paring populations of different and other states, among favourable for writing paper. It is geographic regions originating them Germany. Fast also used for plywood and dif- from different refugia the di- growth, good stem ferent types of particle and flake versity may be very high. Dur- forms and disease re- boards. It plays an important role ing the most recent glaciation, sistance was found in in production of wood for renew- refugia may have existed not sources from the Ka- able energy. only in Mediterranean regions liningradskaya Oblast Aspen is of high ecological but also north of the Alps in region and is avail- value. Many insect species bene- mild micro habitats. The north- able under the name fit from aspen and it provides ward migration to England and ‘Tapiau’. habitat for a wide variety of mam- into Scandinavia, as well as into Eurasian aspen hy- mals and birds requiring young higher altitudes in mountainous bridises naturally with forests. Numerous leaf, bark and regions, took place early after Populus alba forming P. wood inhabiting insects and fun- the end of the last ice age. x canescens. Artificial hy- gi exist on aspen, among them Since systematic prov- brids have been produced Melasoma sp. an ectoparasitic enance trials are lacking, it with a number of other pop- mite feeding the leaves, Saperda can only be assumed that aspen lar species (e.g. P. tremuloides, sp., boring the trunk, fungi like has undergone selection for ad- P. grandidentata, P. davidiana, Phellinus tremulae, causing heart aptation to local environments. a.o.). Some hybrid progenies, rot, Pollaccia radiosa, causing Clinal variation in adaptive traits especially those of P. tremula shoot blight, bacterial canker, along latitudinal, altitudinal as by P. tremuloides as well as P. Xanthomonas populi, caus- well as maritime to continen- davidiana and vice versa dem- ing damage to the trunk and tal (longitudinal) gradients onstrate heterosis effects and branches, as well as Melamp- can be expected to be are significantly faster growing sora sp. causing leaf rusts. pronounced across and less susceptible to diseases Some of these are det- the whole range. than the parental species. Also rimental to operational Wind pollina- some triploid aspen clones of plantations. tion and long pure and hybrid aspen are faster range seed dispersal prob- growing. Like other poplar spe- ably serve to enhance genetic cies, aspen has been subject of variation with the result that the a variety of research, especially term “local” in this case applies in tree improvement including to quite large areas. The sucker- genetic modification since 1990. ing ability enables aspen to form natural clones, some of which may be very old. EurasianPopulus aspenPopulus tremulaEurasian aspen tremulaPopulus tremulaEurasian aspenPopulus tremulaPopulusEurasian aspenPopulus tremulaEurasian tremulaaspenPopulus tremulaEurasian aspen PopulusPopulus tremulaEurasian aspenPopulus tremula Eurasiantremula aspenPopulus tremulaEurasian aspenPopulus tremula Threats to Guidelines for genetic ensure a high amount of diver- genetic diversity conservation and use sity and a low number of clones. Particular attention must be paid Aspen is a minor forest tree spe- As a colonising species, aspen to all practices that have an im- cies. Therefore it is restricted has under unmanaged condi- pact on flowering habit and the to marginal or abandoned sites. tions only infrequent and unpre- regeneration process, which de- As a pioneer species it is light dictable availability of spaces to termines the effective population demanding and requires bare spread into, where the prevail- size. Conditions for self-seed- soils devoid of competing veg- ing forest vegetation has been ing and seedling establishment etation for natural regeneration. destroyed by disturbances from should be improved by com- Therefore, in silvicultural systems storm, fire and flood. Under pletely exposing the mineral soil favouring closed cover forests, managed conditions the ex situ within or close to aspen stands. natural regeneration of aspen is conservation of aspen popula- For restored populations, in- nearly impossible. However, in tions would depend on deliber- trogression can be limited by regions of extensive land use ately creating open areas for new creating buffer zones around the it plays an important role as a colonisations or actively planting population by keep- coloniser species after fire, storm or seeding this area.