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Karelian Birch Burl- Insights and research

Birch Burls  (Betula pendula var. Carelica) are almost always individuals of the slope birch (Betula pendula). There are rarely hair or downy birches (Betula pubescens) that develop characteristics of bur birches. Botanically, the maser birches are not a species or subspecies, but a genetic variety. Such 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 seed-borne variant that has resulted from mutation.
Burls are usually spherical formations on tree trunks, branches, or roots. Sometimes a burl resembles overgrowths on the trunk of a tree, these two phenomena resemble each other but are two completely different phenomena. Burled wood is characterized by abnormally wide medullary rays and unusually wild, often circularly oriented growing tissue. In birch, 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 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 burl formation. 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 a raw material for heavily used objects, e.g. knife handles and small drinking vessels, e.g. the traditional kuksa in Finland. 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, especially common 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 undulating radial or tangential growth of wood fibers. This wavy structure in birch wood is called mottled or mackerel and is in great demand.

The characteristics
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 reality only becomes apparent after the tree has been felled.

The main characteristics curly birch:
Growth habit: the trunk is shorter than that of the normal maser 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 usually wavy; nodular, bulbous or ring-shaped thickenings, tubers and necks called narrow places. The bark at the base of the tree is often deeply furrowed and very thick, often shaped like briquettes, black in color, rough and coarse. Often the signs of this furrowiness 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.

The wood
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 across the entire surface, the result is a star-shaped appearance. 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 the burl does not begin until the age of about ten years.

The distribution
The bur birch grows originally 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, maser 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 the curly birch is a light-demanding, slow-growing tree, it competes on clearing areas with other, faster-growing tree species. 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, curly birch can also be grown in good locations in northern Finland.

Habitat requirements
The best sites for growing maser birch are fertile forests with herb-rich sites and the best arable soils (fine sand and gauze). The less fertile the site, the more the maser birch tends to branch out more. Sites unsuitable for growing maser birch are peaty soils, shallow clay soils, or sites where the water table is high. The maser 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 plays little role in the occurrence of birch maser growth. The northernmost maser birch 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 Curly Birch Burls
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. Peer Kalm, Grundberg covered topics such as the use of burl birch wood 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 on bur 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 growth and its cultivation started in the 1930s at the Finnish Forest Research Institute, where Olli Heikkinheino started his extensive experiments investigating the genetic reasons of this 'burl growth disease', as the phenomenon was called at that time. It was proven that the measles disease was transmitted to new generations of trees through 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 maser 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 maser birch 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 maser 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 maser birch. The best stands of maser 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 maser birch are largely based on indicators obtained from the study of these stands.