Vedrötor i stadsträd : biologi, detektionsmetoder och förebyggande åtgärder

Sammanfattning: Urban trees have very little in common with trees on natural sites, such as forests or pastures. Trees in urban environments are often planted in packed soil and in small volumes. This can lead to decreased availability of water and oxygen, as well as deteriorated ability for the soil to store nutritional elements. Also above ground the tree usually have limited space in cities. Regulations on free height over roads and cycle tracks demands high stems at an early stage of their development. Wrong tree in the wrong place make massive pruning necessary in trees which grows tall in nature. In many communities in southern and middle Sweden salt is used to keep the roads free from ice. The salt can harm the trees, but also decrease their uptake of nutritional elements. All these factors stresses and weakens the trees, and increases the susceptibility for pathogen attacks. In forestry the main problem with wood decay is the decrease of quality and reduction in value. In urban environments the big problem is the risk of branches and stems breaking and harming life or property. Most wood decays are caused by fungi. The exception is wetwood, where bacterias are the main patogen. Wood rots in living trees are usually categorized in two types, (1) brown rot and (2) white rot. Only 6% of all known wood decaying fungi causes brown rot and they mainly attack conifers. Brown rot fungi mainly consumes the polysaccharides, i.e. cellulose and hemicellulose, while the brown lignin remains. Wood attacked by brown rot cracks in cubic pieces, and the fungi decreases the strength of the wood. Examples of fungi causing brown rot are Laetiporus sulphureus and Sparassus crispa. Fungi causing white rots on the other hand, breaks down both polysaccharides, i.e. cellulose and hemicellulose, and lignin. The breakdown is heavily dependant on the fungi species, which breaks down different molecules in different rates. White rot make the wood stringy och fibrous. Examples of common white decay fungi are Merpilus giganteus and Armillaria sp. (honey fungus) which both cause big problems in urban environments. Fungi can infect trees in many ways, usually through wounds in the stem, in roots or branches, made by wind, mecanichal injuries, traffic related wounds or wrong pruning. The majority of the wood decaying fungi produce huge amounts of spores that easily infect wounded trees. To detect rots in trees it is important to know both trees and the wood decaying fungi. Many damages are possible to detect visually, while others demand more thorough methods. Since different fungi makes different damage in living trees it is important to identify the fungi species in order to take future steps. Trees are able to restrict the damage by preventing the growth of the pathogen. Trees can make three "walls" in the wood, (1) in the xylem, (2) in the annual rings and (3) in the rays. These three walls are structurally weak and can only arrest, not stop the pathogen. The strongest wall is the barrier zone, which is formed by the new tissues of the tree and makes it impossible for the fungi to grow into the wood formed after the tree was infected. The only way to prevent fungal attacks in living trees is by minimizing the amount of possible sites of attack. This may be done by correct pruning and by providing good conditions for the tree, both above and below ground.