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Prof. Dr. Sebastian Hein

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Hochschule für Forstwirtschaft Rottenburg
Waldbau (Waldbautechnik, Forstpflanzenanzucht, Ertragskunde)

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Article

Author(s): Jari Hynynen and Pentti Niemistö
Editorial office: FVA, Germany
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Silviculture of Silver Birch in Finland

Birkenbestand am Seeufer
Fig. 1: Birch stand on the lakeshore. (All photos: Erkki Oksanen/Metla)

Birches are the most important broadleaved tree species in Finland. There are two commercially important birch species; downy birch (Betula pubescens Ehrh.) and silver birch (Betula pendula Roth). The proportion of birch of Finnish forest resources (volume of the growing stock) is 16 %, equals to 363 mill m3. Although downy birch is more frequent (12 %), silver birch (4 %) is economically more valuable.

Two-third of Finnish birch resources are growing in mixed forests dominated by conifers. In addition, silver birch is also planted and managed as pure stands. The proportion of planted silver birch stands of the artificially regenerated forests has been ca 5 % during the last ten years. Birch is an important tree species both for chemical and mechanical forest industry. Downy birch is mainly used for pulp, but silver birch is valuable raw mechanical woodworking industry. Slender stem form, moderate branchiness, and light colored wood are valuable characteristics of silver birch.

Ecology

Birches are typical light-demanding, shade-intolerant pioneer species, which easily occupy open areas after forest fires and clear-cuttings. Birches have rapid early growth. In managed birch stands growth is vigorous until the stand age of 40‑50 years. The vitality of birch trees decreases before the age of 100 years, and susceptibility to decays and other defects increase. Birch can maintain vitality and vigorous growth only when growing as dominant tree in a stand with relatively wide spacing and low degree of within-stand competition.

The most suitable growing sites for silver birch are fertile forest site types, and afforested abandoned fields. The most important site characteristics for vigorous growth of silver birch are adequate moisture and air content. Best forest sites for silver birch are sand and silt moraine soils and fine sandy soils. Clay and silt soils are often too compact for silver birch, which also suffers from flooding. Site requirements of downy birch are not strict as those of silver birch. Downy birch can survive also on compact soils and on wet peatlands.

Management of silver birch

Silver birch can be grown either in pure stands or in mixed stands together with the conifers. In both cases the goal of the management is to produce high quality saw timber or plywood. Therefore, the silviculture of birch aims at the production of large diameter, straight and defect-free stems.

Management of pure stands

Das waldbauliche Ziel: Birken-Wertholz
Fig. 2: High quality silver birch stand.

Planting with genetically improved planting stock is the main method of artificial regeneration of silver birch. As the result of intensive Finnish birch breeding program, yield and stem quality of silver birch has been significantly improved. Large-scale production of genetically improved birch seed in polythene greenhouse seed orchards, has become a well established and profitable practice.

Being a shade-intolerant tree species, crown development and tree growth of silver birch is retarded when growing in high densities. Therefore, relatively wide initial spacing and intensive thinnings are required in order to ensure high yield of good quality timber in final fellings, and adequate removals of merchantable wood in thinnings.

In planting, container grown seedlings are used applying the planting density of 1600 seedlings/ha, which has found to be b the maximum number of silver birch trees per hectare that can reach the merchantable stem size. Therefore, no precommercial thinnings are usually needed in birch plantations, but weed control is recommended especially on the most fertile sites.

In management of silver birch stand, the rule of thumb is that the proportion of live crown of birch tree should be at least 50 % of the tree height for vigorous growth. Thinnings speed up stem diameter growth and saw timber yield, shortens the rotation, and thus increase the cutting revenues. In silver birch stands, fast diameter increment does not impair wood quality, which may be the case in conifer stands. Besides, shortened rotation decreases the risk of decay, which is typical defect of old birch stands.

The typical management schedule of a planted silver birch stand includes two commercial thinnings during the rotation. First commercial thinning is usually carried out at the dominant height of 13‑15 m to the density of 700 trees per hectare. By that time, the stems of dominant silver birch are clear of live branches along the length of butt log. Thinning removal is mainly pulp or energy wood. The correct timing and careful logging is important in thinning of birch stands, because the growth of young birch trees is highly sensitive to stand density, and birch stems can be seriously damaged in careless logging.

The second commercial thinning usually takes place ca 15 years after the first thinning. The stem number after thinning is ca 350‑400 trees per hectare. In the second thinning, the removal includes some saw timber in addition to pulp wood.

Typical rotation in silver birch plantations varies between 40 to 60 years depending on site productivity, and the quality of growing stock. In stands, where high quality saw timber can be produced, rotations tend to be longer than in birch stand of poorer timber quality.

Due to the self-pruning of branches, silver birch stems are usually free of living branches up to 5‑7 meters height, i. e. along the length of butt log by the time of first commercial thinning. Therefore, high pruning of birch stems is not always necessary in order to produce high quality saw timber. However, if goal is ensure top quality timber production, pruning is recommended, and it should be started as a part of young stand management practices. Pruning is recommended to be carried out in two phases. First pruning takes place at the height of 6‑7 meters up to the pruning height of 2.5‑3 meters. The number of pruned trees is 600‑700 stems per hectare. The second pruning takes place, when stand height exceeds 10 meters. Then 400‑500 birch trees per hectare are recommended to be pruned, with pruning height of 5‑6 meters. The final number of pruned trees at the time of final felling is ca 350 trees per hectare. In pruned stands with top quality trees, longer rotations are applied then in the stands of average timber quality. In pruning of birch, clippers are recommended instead of pruning saws, in order to avoid bark and stem damages. The risk of damages is low, if the diameter of pruned branches is below 2 cm, if the work is carried out carefully. Birch trees should be pruned during the growing season in July, or well before the growing season during the late winter or early spring.

Fertilization is not a common practice in the management of birch stands. Those few results from the empirical fertilization trials suggest that fertilization of silver birch is not profitable due to weak growth response.

Management of mixed stands

Kiefer-Birken-Mischbestand
Fig. 3: Pine-birch mixtures can only be found on good site for pine.

In mixed stands dominated by Scots pine or Norway spruce, dominant birch trees are capable to maintain their vitality during the commercial rotation period. Moreover, when growing in mixed stands, the stem quality of silver birch is often even better than when growing in pure birch plantations.

On fertile mineral soil sites, silver birch and Norway spruce is the most common species mixture in commercial stands. Both species have quite similar site requirements, and both species have good productivity. The different growth patterns and shade-tolerance of birch and spruce decreases the level of competition between the species. The growth of birch starts to decline at the time when the growth rate of spruce has not yet reached its culmination. As a shade-tolerant tree species, spruce does not suffer too much from the shading of birch mixture, and can survive even when growing in under-storey.

The typical sites for Scots pine are too poor for silver birch. However, mixtures of silver birch and Scots pine can be found on the best site types for pine. Because both species are shade-intolerant, the competition between pine and birch is heavier than between birch and spruce. However, with the help of intensive silviculture, silver birch and Scots pine can be grown successfully in a mixed stand.

A small birch mixture in a spruce dominated mixed stand can even slightly increase the total yield compared to that of a pure spruce stand. Small birch mixture in Scots pine dominated stand has a negligible effect on yield. However, a high proportion of birch in a mixed stand has found to result in decreased wood production compared to pure conifer stands. Silver birch requires wide spacing and heavy thinnings in order to maintain its vitality. Heavy thinnings in turn result in lower wood production during the rotation.

Birken mit sichtbarer Maserung
Fig. 4: Curly birch stand.

Successful management of an even-aged mixed birch-conifer stand requires skillful and an intensive silviculture, because of the different growth patterns of the tree species. Being fast-growing pioneer species, birch trees start easily to suppress conifers in a sapling stand. Therefore, conifer seedlings should be advanced in their early growth compared to birches. In practice, the difference in tree ages between conifers and birches should be 5 to 10 years in naturally regenerated stands. In spruce plantations, where site preparation have been carried out, naturally regenerated birches emerging on the site after planting do cannot suppress planted spruce trees, and can be later grown in same tree-storey together with spruce.

Stands, where fast-growing birches have already suppressed spruce seedlings, easily develop into two-storey stands with birch over-storey and spruce under-storey. With intensive management and careful logging operations, it is possible to grow birch over-storey to saw timber dimensions, and harvest it without causing too severe damages to spruce under-storey. Then, after felling the birch storey, a stand will be managed as single-storey spruce stand.

Management of curly birch

Curly birch (Betula pendula var. carelica (Mercklin) Hämet-Ahti) is a variety of the silver birch. Based on the exterior of the trunk, one distinguishes between curly grain with protuberances, necks, rings and stripes. Also mixed types can occur. Typical features of curly-grained wood include faulty cell orientation, broader-than-usual wood rays and ingrown bark, resulting in a brown, flamy pattern.

Curly-grained wood is especially valuable, and instead of cubic meters it is sold by the kilos. With a fresh wood density of more than 900 kg/m3 and a current (2008) price of curly-grained stem wood (3‑5 €/kg), the price of one cubic meter can easily exceed 3000 euros. However, prices can vary considerably, depending on the quality of the curly grain, log length and diameter, and consignement size. Even good-quality curly grained branch wood is in demand. Small or bushy forms of curly birch not suitable for wood production can be used as decorative garden and park trees.

The establishment procedure for a curly birch stand is similar to that for normal silver birch. Repeated thinnings of the curly birch are needed to ensure enough light (more than for normal birch). The final felling will take place at the age of 40‑50 years. Artificial pruning of the stems must be started, preferably, two-three years after establishment and then continued in stages. July has proved to be the best time to carry out pruning.

Ein Maserbirken-Funierblatt
Fig. 5: Curly birch wood.

Silver birch – the national tree species of Finland

Besides wood production, birch is an important species from the point of view of non-wood products and services. Birches are important to the biodiversity of northern coniferous forests. A large number of species live together with birches, including mycorrhiza forming fungi, herbivores, wood decaying fungi and saproxylic insects. Finally, silver birch is the national tree species of Finland with a great importance to the Finnish culture and landscape. Light green birch stand at the lakeshore in the early summer is one of the most famous Finnish landscapes.

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