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curly birch and research

Information about curly birch and research on it.

Birch burl (Betula pendula var. Carelica) are almost always individuals of the slope birch (Betula pendula). There are rarely bur birches (Betula pubescens) that develop characteristics of burlss. Botanically, the burl birches are not a species or subspecies, but a genetic variety. A variety differs little from the species proper and its range is more limited than that of a subspecies.
Wild grained wood is most evident in white birch. Significantly less burl-grown wood has been found in burl birch, gray alder, black alder, mountain ash, goat willow, and maple. Burl-grown wood is a variant inherited through seed, resulting from mutation.
Burls are thickenings, often spherical formations in tree trunks, branches, or roots. Sometimes a burl resembles overgrowth in wood; these two phenomena are similar. Burled wood is characterized by abnormally wide medullary rays and unusually wild, often circularly oriented growing tissue. This phenomenon is also accompanied by the incorporation of bark fragments into the wood of the trunk, where they appear as brown spots and streaks. The formation of burl wood has the effect of slowing down the growth of the tree.
The causes of this burl formation are difficult to determine. Possible causes include injury from insects and mechanical damage (often observed on street trees at the level of bumpers), as well as local mutation in the form of alteration of genetic factors. One cell in the cambium layer of the tree divides faster than the rest of the cambium cells. The abnormal growth of the tissue can continue throughout the life span of the tree, thus burls can become quite large, sometimes over 1 meter in diameter.
However, there are other reasons for burls. Linden and willow trees often have burl-like formations at the base, initially caused by constantly growing and dividing dormant buds. A common fungus that causes decay in birch trees is the oblique schillerporling (Inonotus obliquus), which forms dark, tuber-like formations on the trunks of birch trees. However, these formations are fruiting bodies of this parasitic fungus that preferentially attacks birch trees. The woody tissue of the true burls, on the other hand, is hard and very resistant. It is very suitable as raw material for heavily used objects, e.g. handles and small vessels. Burls of willow are especially valued as raw material in carving burl cups (kuksa). Very large burls are even used for making furniture and veneers.
Some hardwoods, often birch, occasionally exhibit a wavy grain pattern that is different from burl. The tissue structure of wavy formations in the wood of trees is normal except for the wavy radial or tangential growth of wood fibers. This wavy structure in birch wood is called mottled or figured and is in great demand.

The characteristics of burl birch

Burl birch is a very multifaceted variety of birch, which is sometimes difficult to distinguish from ordinary birch. The external characteristics of burl can vary greatly from tree to tree. A tree with external signs of burl need not have burl growth. At the other extreme, a normal-looking white birch may turn out to be burl birch upon closer inspection after felling: Despite the long and smooth trunk, the cross-sectional area then exhibits a fine burl texture. The truth becomes apparent only after the tree has been felled.

The most important characteristics of curly birch are:

Growth habit: the trunk is shorter than that of the normal burly birch, it tapers more and is usually slightly curved. Budding is unusually abundant, causing the trunk to branch and often resulting in a bushy growth habit. The crown of such trees becomes round and often flat. The branches are generally thick and crooked. Also, the tree tends to lean in the direction where there is more room for growth and more light. The growth of the curly birch has much in common with that of apple trees.
The surface of the trunk is generally wavy; nodular, bulbous or ring-shaped thickenings, bulbs and necks called narrow spots occur. The bark at the base of the tree is generally deeply furrowed and thick, often shaped like briquettes, black in color, rough and coarse. Often the signs of furring occur only along a few meters at the base of the tree, while in other trees these signs extend to the branched parts that form the crown of the tree (cf. burls).

The wood of curly birches

The growth rings are wavy and irregular, with brown, bark-like wood cells often forming a V-pattern in cross-section. If this pattern continues evenly over the entire surface, the result is a star-shaped pattern. In longitudinal section, the trunk exhibits lenticular patterns. The wood of the curly birch is dense and very heavy, approx. 950 kg/m³ when freshly cut, and approx. 700 kg/m³ at 10% relative humidity. Burl birch has some unusual leaf shapes, but these cannot be used as a reliable identifying characteristic. The wild budding associated with burl growth results in abundant branching and bushy growth in birch.
Burl growth begins at different times. While some seedlings show signs of burl growth in the nursery, it is often not apparent until later years. On the seedlings and young plants, the abnormal base curvature is visible at the base of the branches, based on which it can be suspected that it is a burl birch. However, in most cases, the formation of burls begins at the age of about ten years.

The distribution curly birch

Curly birches grow native in Northern Europe, and even here only in isolated areas. In southern Finland it grows mainly in former bog areas, in other Nordic countries in parts of southern Sweden and in the most southeastern parts of Norway. In addition, curly birch is found in Karelia (Russia), in the Baltic countries, in Belarus, and in some other western parts of Russia. There are also scattered populations in Central Europe, but it is most widespread in Finland and the Karelian part of Russia.
An old Finnish proverb claims: "The curly birch grows only so far from a church that you can still hear the sound of the church bells.
Because curly birch is a light-demanding, slow-growing tree, it competes with other, faster-growing tree species on cleared areas. The short rotation periods that were once common created favorable competitive conditions for the bur birch. The end of the clearing practice, the development of modern silviculture, and the great demand for the wood of the curly birch due to its very limited availability have significantly reduced the natural occurrence of the curly birch. As a crop, the curly birch thrives best in southern and central Finland, but if slower growth and greater risks of failure are accepted, the curly birch can also be grown on good sites in northern Finland.

Habitat requirements

The best sites for growing such birches are fertile forests with herb-rich sites and the best arable soils (fine sand and gauze). The less fertile the site, the more curly bircesh tend to branch out more. Sites unsuitable for growing curly birch are peaty soils, shallow clay soils, or sites where the water table is high. The curly birch is in its element on sites that are considered valuable for landscape and ecological reasons:
- Fertile woodlands
- Good fine sand and fallow land
- Alder stands that are to be converted into productive commercial forests
- Fertile soils with low frost risk
- Landscape valuable sites
Location and stand density have been observed to influence the occurrence and degree of burrowing. Geographic location does not play a role in the occurrence of curly birch growth. The northernmost birch burl plantations in Finland have developed well in Koli, Pyhäkoski, and Rovaniemi. Isolated groups of bur birch were found as far north as Ivalo.

Research on the curly birch

Johan Grundberg from Western Finland studied at the Turku Academy, he submitted his dissertation on the properties and uses of birch in 1759. In this work, written under the supervision of Prof. Pehr Kalm, Grundberg covered topics such as the use of birch wood burls at the time as a material for various utensils. (Kosonen 2004)
In the early 1900s, the first Finnish researcher to draw attention to burl birch was T.J. Hintikka, also inspired by the ideas of A. K. Cajander.
He gave a lecture on the subject at a meeting of the Finnish Society of Forest Sciences in 1916. The first scientific research report onr measles birch, entitled "The Visa Disease of Birch Trees in Finland," was written by him and appeared in a German journal in 1922. The actual research on birch burl and its cultivation began in the 1930s at the Finnish Forest Research Institute, where Olli Heikinheimo started his extensive experiments investigating the genetic causes of this burl disease, as the phenomenon was then called. It was proven that the measles growth was transmitted to new generations of trees by seeds. In controlled experimental plantings, this hereditary trait was transmitted to over 50% of the offspring. Other Finnish researchers on the subject include Etholén, Raulo, Saarnijoki, Sarvas, and Saarnio,

The oldest curly birch plantation in Finland is located near the village of Vesijako, close to the Romo research facility of the Natural Resources Institute Finland. The director of the Evo Forestry School, Aaltonen, planted the stand of curly birch according to the ideas of T. J. Hintikka. Seeds were collected in 1920 from naturally occurring birch burl trees in the Vanhakartano forests in Lammi municipality, and then sown in the planting beds of the forestry school's nursery to produce seedlings. With a spacing of 1.20 m x 1.20 m, the 2-year-old seedlings were then planted out on a freshly plowed and prepared site in close proximity to the experimental station. However, the development of this planting was slow: mostly the naturally growing birch and spruce overgrew the young curly birch. Evaluating only the results of the stand at Vesijako, it is not possible to make any far-reaching conclusions about the development and yield of curly burl birch. The best stands of curly birch in Metla are located at Aulanko, Hauho (Vitsiälä), Kerimäki and Punkaharju. These plantations were mostly established in 1932-1938. The economics and profitability of growing curly birch is largely based on indicators obtained from the study of these stands.