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Findings

Aim

The main objective of my study was to try to figure out which factors that increased the risk of oak mortality. By comparing dead and living oak’s growth pattern and close surroundings, it was possible to tell if the dead trees were more frequently represented by a certain factor, presumably increasing the risk of the oak dying. The study had two aims:

 
           Identifying individual tree and environmental variables that increased the risk of oak mortality
 
            Clarifying the temporal process of the oak decline and detecting a possible growth depression prior to death

Methods

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Three sites within the county of Östergötland, south-eastern Sweden, were used for the field data collection; Tinnerö, Norrköping and Sturefors. All sites were parts of grazed, oak (Quercus robur) dominated nature reserves, representing one of the few landscapes in northern Europe still holding high densities of old oaks. Choosing sites within the boundaries of nature reserves minimized the influence of forest management, such as removal of dead trees. The region has an annual precipitation of 500-700 mm and a mean temperature of between -2 and -4° C in January and around 16° C in July.

Core Samples

Core samples were taken from 72 dead and 72 living oaks, using a Swedish increment borer. Each tree was cored twice at breast height, the cores taken perpendicularly to each other and in general 30 cm into the tree. The cored dead trees were randomly chosen out of all dead oaks within the sites. A pairwise design was used for selecting the control trees, wherein the living oak growing closest to the sampled dead one, having approximately the same circumference, was selected. Furthermore, the resemblance of the surroundings was taken into consideration when choosing the control tree, particularly when several oaks within the same size range were obtainable. Results from this part of the study are found under the link “Results –Core samples”, found in the menu at the left.

Environmental variables

All dead, standing oaks with a diameter of >25 cm were localized and sampled at the three sites, and a grid system was used for selecting living trees to act as a control. In total, 216 dead and 335 living oaks were sampled for environmental variables. Variables investigated included circumference (DBH), depth of bark crevices, crown silhouette, soil type, soil moisture, cover of bare rock, sun exposure and the presence/absence of holes and shelf fungus. Results from this part of the study are presented under the link “Results –Environmental variables”, found in the menu at the left.


Responsible for this page: Agneta Johansson
Last updated: 05/15/09