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Fraxinus Excelsior Technical guidelines for genetic conservation and use Common ash Fraxinus excelsior Alfas Pliûra1 and Myriam Heuertz2 1 Lithuanian Forest Research Institute, Girionys, Kaunas, Lithuania EUFORGEN 2 Centre de Recherche Public – Gabriel Lippmann, Luxembourg These Technical Guidelines are intended to assist those who cherish the valuable ash 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 recom- mendations 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 Common ash (Fraxinus excelsior L.) is the largest tree in the genus Fraxinus and at maturity (90-120 years) it can reach 20-35 m (maximum 40 m) in height. The mean stem diameter varies from 30-70 cm (maximum 150 cm) in adult specimens. The crown is irregu- lar with massive branches, elongated in forest stands. Common ash is wind pollinated. Flowering starts at 15-20 years on single trees and at around 30 years within stands at irregular intervals. The breeding sys- tem is polygamous, rang- ing from male to female individu- als with hermaphroditic interme- diates. Morphologically hermaph- roditic individuals are often pre- dominantly male or female. Inter- annual variation in sex expression is observed. The fully developed seeds start to disperse by wind in the autumn. Seed dormancy usu- orommon ashFraxinusFraxinus excelsiorCommon ashFraxinus excelsior excelsCommon ashFraxinus excelsior ally lasts for two winters but can Distribution Importance and use last for up to six. Stored seed requires combined warm-cold stratification to germinate. The natural range of common Of the four different ash species Common ash requires a rich ash covers most of Europe from growing naturally in Europe, soil and tolerates a pH as low as the shores of the Atlantic Ocean common ash is the most impor- 4.5, but prefers soil above 5.5. It in the West to the Volga River in tant commercially. Despite the is highly tolerant of seasonal the East, with the exclusion of high demand for this quality tim- water-logging and favours flood- the most northern and southern ber, only a few European coun- plain forests. It is also a typical parts. The northern limit of the tries have gene conservation or species of slopes and ravines, natural range is about 64° North tree breeding programmes in growing in association with other in Norway, the southern margin place for this species. characteristic species such as reaching 37° North in Iran. In Common ash timber is hard, maple, lime and elm. Although mountainous areas, common elastic and withstands pressure, dormant trees are very cold ash is found in the Pyrenees at shock and splintering. It is used hardy, the young shoots are sen- 1750-1800 m above sea level mainly for tool handles, and for sitive to frost. and in the Swiss Alps at 1630 m. sports equipment such as hock- Common ash exhibits inter- In Asia (Iran), it can be found at ey sticks, oars and hurdles. It has mediate properties between a much higher elevations of up to a straight grain and there is little pioneer species and a permanent 2200 m. distinction between sapwood forest component. Although dis- and hardwood, making it very persal and natural regenera- valuable for furniture, veneer and tion are efficient, the com- flooring. The formation of “black petition ability of the heart”, a dark stain of the hard- species is only strong wood, can occur in mature trees. when the ecological This varies within and between requirements are individual trees and different met. Vegetative sites and reduces the economic regeneration is value. Ash bark and leaves are strong after cop- astringent and the leaves are picing. used in modern herbal medicine for their laxative properties. siorrCommon ash FraxinusFraxinus excelsiorCommon ashFraxinus excelsiorCommon exce ashFraxinus excels Genetic knowledge Iberian Peninsula, the Alps Threats to and/or in Italy and in the Balkan genetic diversity Peninsula. This result is in agree- The genetic variation of a species, ment with fossil pollen data and On the European scale, common both within and between popula- the postglacial re-colonisation of ash is not an endangered tions, is influenced by many fac- Europe from these refuges. species. However, the natural tors. The mating system, pollen Nuclear DNA markers reveal lev- range and area of ash forests has and seed dispersal, role and suc- els of intra-population differenti- decreased during the last 4000 cessional stage in forest ecosys- ation similar to other Noble years as the area of agricultural tems, site conditions and histori- Hardwoods such as chestnut, lands has increased. Silvicultural cal influences such as colonisa- elm and walnut, but higher than methods have promoted com- tion patterns after the glacial peri- is found in oak and beech. High mon ash in the last 30-40 years ods of the Quaternary are particu- differentiation was particularly due to the high economic value, larly important. Two types of observed between populations supporting natural regeneration, genetic variation can be distin- in southeastern Europe and in planting and thinning. guished: (i) adaptive variation (i.e. northern central Europe, indicat- Common ash genetic enabling adaptability) and (ii) a ing the high conservation value resources are threatened by priori neutral variation (i.e. not of ash genetic resources from deforestation, loss of suitable affected by natural selection). these areas. habitats, unsustainable exploita- Adaptive variation, expressed tion and improper management in the phenotype in response to (i.e. uncontrolled transfer of natural selection, has been reproductive material), natural investigated through provenance climatic changes, global warm- and progeny trials. These studies ing, air pollution, competition revealed that variation between with other species, pests and families within provenances/ game damage. These pressures populations was generally as could lead to population extinc- high as variation between prove- tion. Despite the high regenera- nances/populations. tion potential, the reproduction Phenological traits, including of some valuable autochthonous the date of bud burst and set, populations is not ensured, and displayed geographic patterns the health status of mature on a large spatial scale, whereas stands in some countries has growth and form, which are influ- deteriorated significantly in enced by soil and competition recent years. Therefore, common conditions, varied at the local ash is considered to be threat- level. The existence of different ened at the population level by ecotypes, (i.e. floodplain, hillside, most countries. slope and limestone ecotypes), has never been proven by proge- ny studies. Chloroplast DNA data used to investigate neutral variation suggests ice age refuges in the elsiorsiorCommon ashFraxinus excelsiorFraxinusCommon ashFraxinus excelsiorCommon exc ashFraxinus ex Guidelines for genetic tions). Whenever possible, in situ regeneration of the target conservation and use conservation activities should be species, creating multi-age struc- undertaken jointly for other ture and habitat diversity, and Genetic conservation aims at Noble Hardwoods. increasing generation turnover. ensuring continuous survival and Where common ash occurs in To conserve an even-aged adaptability of the target species. large populations in a country, in mature stand in situ, parts of the These objectives are met when situ conservation is sufficient, population should be opened the Multiple Population Breeding with the selection of up to three (thinned or cut in narrow strips of System (MPBS) is applied. gene conservation populations/ 15-30 m width) to create condi- Ideally in MPBS, a breeding pop- gene reserves of 5-15 ha in size, tions for natural regeneration. ulation is subdivided into sub- with at least 100 flowering trees Preferably, this should be under- populations which are then in each provenance. A high den- taken in the year following the grown over a wide range of site sity of in situ gene conservation mast, when maximum seed is conditions. populations should be estab- produced by the stand. An area In each country where com- lished in Southeast Europe, adjacent to the gene reserve mon ash is found, an inventory especially in Romania and could be set aside for natural should be undertaken to define Bulgaria, which have been regeneration, and could later be the geographical distribution of colonised by populations from incorporated as part of the the species, conservation status, different ice age refuges. In these reserve. threats and potential use pat- regions, neutral genetic markers To promote regeneration in terns. Ecogeographic zones show high differentiation among clear-cut strips, randomly select- (provenance regions) should be populations, suggesting that they ed, abundantly flowering seed delimited according to climatic may have different potentials to trees should be left. If the popula- variation, topography, soil and cope with future climatic condi- tion consists of some stands or vegetation. Trees are generally tions. Specific conservation groups of trees of different ages best adapted to the ecological efforts are also recommended in but there is no regeneration,
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