1 Faculty of Forestry University of Zagreb, 10000 Zagreb, Croatia
² Forest Research Institute, 10450 Jastrebarsko, Croatia

At the second meeting of the EUFORGEN Noble Hardwoods Network held in Laurizan, Spain from 23 to 25 March 1997, it was concluded that is necessary to work on European gene conservation strategy for the genus Alnus. This task was given the participants from Croatia (Ante Krstinić and Joso Gračan). This strategy document was planned to be finished for the third meeting in Sagadi, Estonia, 13 to 16 June 1998. However, at the meeting it was noted that the strategy for this species could not yet be presented, and it would be discussed at the next one.

Black alder (Alnus glutinosa /L./ Gaertn.) is very important forest tree species in Croatia, especially in the floodplain ecosystems along the Sava and Drava rivers. Because that black alder is very variable and fast growing species found all over Europe, in our opinion it can be used as a example for Conservation strategy models of gene resources for other species of genus Alnus.

The genus Alnus includes about 36 species mostly located in the Northern Hemisphere. Generally, alders grow in the regions of floodplain forests, even though in mountains of central Europe they occur along water flows up to 2800 m above sea level. Species of this genus grow either as bushes or high trees, showing great variability in their morphological and physiological properties.

Black alder is becoming a more and more interesting forest tree species due to its multiple uses both in the forestry and wood industry. It is known as a fast growing and meliorative species, which has the ability of fixing atmospheric nitrogen. Black alder stands with longer rotations can produce logs of very good quality for wood industry, but in short rotation (cycles) in so-called special-purpose plantations it is very desirable for biomass production (Krstinić 1994). The basic function of these black alder plantations is a reduction of the pressure on natural forests of valuable hardwoods species (oak, beech, ash, maple), conservation of genetic resources and effect on global climate changes. Owing to these characteristics black alder proved to be very suitable for tree cultivation either in the form of pioneer plantations or in mixed plantations with other coniferous and deciduous tree species which grow better than in monocultural plantations (Hansen and Dawson 1982; Heilman and Stettler 1983; Krstinić and Komlenović 1986).

As the natural distribution of black alder is not compact but markedly disjunct and widely spread both in the horizontal and vertical directions, the natural populations of this species exist in different edaphic conditions, and it should be supposed that climatic, edaphic and altitudinal races are differentiated. This hypothesis is proved by the results of provenance research which have showed very distinct genetic differences in survival and biomass production (Krstinić et al. 1992.). The genetic differences between provenances are found not only at the European level but also at a broader one. As a result of what was mentioned above there is a need to protect existing biodiversity of natural populations and to spread the range of black alder by establishing plantation on suitable sites.

Species of the genus Alnus are widely spread in the Northern Hemisphere from Northern Africa in the South up to 70^o N in Europe and Asia. In south America, species of the genus Alnus occur south from the equator in Peru as well as in the northern part of Argentina up to the latitude of 25^o S. They usually grow in cold and moderate climate conditions, but they can be also found in the warm regions of the Mediterranean. Alders grow on the banks of streams and lakes in a humid habitats, although their occurrence in dry sites have been observed too. Most species prefer rich, moist but well drained soils with a high water table. Stagnant waters are tolerated well by some species only.

Black alder is spread all over Europe from Ireland in the west to western Siberia in the east, as far as northern Africa in the south and up to 65^o in the north. It has been introduced in the Azores and in the United States of America. Its range both in Europe and Africa is not compact but markedly disjunct. In the mountainous regions of the central Europe it reaches altitudes between 1500 and 1800 m. Its preferred climate is moderate to cold one, it grows best in deep soils where the water table is high. Optimum amount of precipitation is from 800 to 860 mm per year. It does not tolerate stagnant water or high soil acidity (Fig. 1).

Alnus glutinosa /L./ Gaertn. belongs to the genus Alnus, of the Betulaceae family. Some 36 species of the genus Alnus are divided into various number of subgenera, subspecies or varieties.

In its natural range in Europe, black alder usually reaches the height of 25 m and in some rare cases up to 40 m. Diameters in old trees normally are between 35 to 40 cm, but a maximum diameter at breast height of 175 cm was found. Black alder is a highly variable forest tree species, therefore, it has been the subject of research carried out by many scientists.

Black alder is becoming an increasingly interesting forest species due to its multiple uses both in the forestry and wood industry. It is a known as a meliorative species which, owing to a symbiotic actinomycete, Frankia alni, located in nodules, has the ability of fixing atmospheric nitrogen. The nodules act not only as a nitrogen fixatives but also as regulators of photosynthetic products (Wheeler and Lawrie 1976). This ability makes it very suitable for tree cultivation either in pioneer plantations or in mixed plantations with other coniferous and decidous tree species which grow better than in monocultural plantations (Hansen and Dawson 1982; Heilman and Stettler 1983; Krstinić and Komlenović 1986, and others).

Black alder may also be used as a pioneer species in establishing common oak plantations, common ash tree plantations and black poplar plantations. However, our experiences have shown that the establishment of mixed plantations with mentioned species is not easy since black alder fast outgrows common oak. Therefore, pioneer plantations for this purpose should be used mainly for biomass production and soil enriching with nitrogen, that will then improve the growth of oaks and ash.

Black alder wood is used in mechanical and chemical processes. Thus, it is used in mining and hydraulic engineering, in production of veneer, utility plywood and joinery boards, and for the pulp and paper and furniture industry. It is also used in many crafts, musical instruments, models, toys, barrel stopcorks, wooden shoes, etc. Being very sensitive to the presence of SO₂, black alder may find an application as a bio-indicator in case of air pollution by this chemical compound. By cultivating black alder in mixed or pioneer plantations, the use of nitrogen fertilizers is avoided and the biological control of weed vegetation is ensured by shading thus making the application of herbicides unnecessary. In this way, it protects the soil and underground water (Trinajstic et al. 1991).

Growth of black alder is very intensive between the fifth and the tenth year, as well as between fifteenth and twentieth year (Mlinsek 1960, Ehrenberg 1979, Nemesszeghy 1986.). After reaching the age of 20, a stagnation of growth in height occurs while diameter growth remain satisfactory in old age. In the Table 1, data describing black alder production in Sweden, Germany and Croatia for various age and different habitat are given according to several authors. In this table the data show that the number of standing trees per ha depends in the first place upon the age (Schwapach 1902, Rauš 1971, Vidakovic and Krstinic 1984). According to the some authors, at the age of 40 the number of trees per ha varies from 568 to 1352, but at the age of 60 from 363 to 595 trees/ha.

The annual average volume increment of black alder stands is lowest in the north but becomes higher towards to the south. The maximum volume increment per annum in Sweden is 3,9 m³/ha, in Germany 6,4 m³/ha, in Slovenia and Croatia from 7,7 to 9,7m³/ha.

The first records of the black alder plantations originate from eastern Germany (Ehrenberg 1979). Later, plantations of this species have also been established in other countries where black alder is autochtonous as well as in countries where it is cultivated as an exotic species (USA, Canada). In order to improve its cultivation, special of black alder breeding have been prepared in several countries in Europe in States and in Canada (Hall 1985). When establishing black alder plantations, it must be taken into consideration that this species is very sensitive to unsuitable sites (the environmental stress) and to some arboricides (Tordon 10 k), also it is sensitive to NaCl and to herbicides.

Table 1: Black alder production (Alnus glutinosa /L./ Gaertn)