Flygbildsanalys av trädskiktets status efter brand : en metodstudie

Sammanfattning: During the past years fire has been identified as an important factor for nature conservation in the forest landscape. To accurately quantify the size of a burnt area is obviously important, but also to quantify the degree of fire damage to the tree canopy. Such data are important for assessing the conservation value, and to support planning and management of the burned area in the future. I have examined how to estimate the status of tree crowns after fire from interpretation of aerial photographs. This is possible because fire changes the structure of tree crowns and this influences the colours in aerial photographs by spectral reflection. Included in the study are four areas, three of which burned in 2005 and one in 2006. Aerial photographs were taken of all areas in the summer of 2006. I did a field inventory of the burnt areas in 2006, to get a reference material for comparison with the interpretation of aerial photos. Study plots consisting of solitary trees (on seed-tree cut areas) or plots with a radius of 10 m (in closed forest), were chosen subjectively to get references for low to high effects from fire in the tree crowns. The fire effects were estimated in the field inventory as percentage loss of needles. Also variables which describe the stand structure in the plots (height, tree composition, basal area) were measured. In the field, the border of each burned area was also mapped with GPS. Information from the field inventory was compared with interpretation of the aerial photographs. Criteria for the degree of fire effects reflected in the aerial photograph were identified and it included scales for tone of grey, the shape of the tree crowns and the shape and density of shadows from the trees. With the support of these criteria, it was possible to classify the fire effects on the tree stands using remote sensing. A test of was performed with three experienced interpreters of aerial photography, using the field data as reference material. The results show that the method gives a high accuracy in classification of fire effects. However, it was more difficult to correctly estimate the degree of fire damage on solitary trees than on plots of closed forest. The ability to identify fire borders differed between the interpreters also differed with the adjacent land cover. Deviation from actual fire border was larger when the burned area was adjacent to closed forest. I conclude that interpretation of aerial photography can be a quick and effective alternative for inventory after fire, particularly when there is a large spatial variation in fire effects, which is difficult to cover from the ground. Photographs taken from 1000 m and 2000 m both yielded useful pictures for estimating fire borders and tree canopy status after fire. The best results are however achieved if the photos are taken in the same year as the fire, when fire-killed needles are still attached in the tree crowns.