Mapping moth induced birch forest damage in northern Sweden, with MODIS satellite data

Abstract: Large synchronous outbreaks of herbivory geometrids is regularly occurring at 9-10 years intervals when they reach peak densities in the Fennoscandian birch forest, in the northern part of Scandinavia (Tenow 1972, Bylund 1995). Climate change is likely to increase the frequency, intensity and extent of the outbreak due to increasing temperatures in the area (Callaghan 2010, Heliasz et al. 2011, Wolf et al. 2008). The consequence is a detrimental effect to the birch forest since the forest might not have enough time to recover between outbreaks, which will potentially decrease the proliferation and distribution of birch forest (Tenow et al. 2001, 2003, Karlsson et al 2004). This will have an ecological cost by making the forest inhabitable and non-resourceful for the animals and people that depend on it (Helle 2001). However, the effects on the Birch forest are not well known, therefore it is important to continue studying the distribution of forest damage, to gain a better understanding of its dynamics and the underlying spatio-temporal factors controlling the synchronous outbreaks. The most current year of infestation in the study area of the surroundings of lake Torneträsk in northern Sweden was 2012. To map the distribution and dynamics of the geometrids of the birch forest, time series data of MODIS 16-day NDVI composites were analyzed. To facilitate the analysis, a tree cover map with high resolution was created based on Lidar data. The Lidar based forest cover map was created to mask the forest. The topographical distribution of infested forest at four altitudinal intervals with 100 meter equidistance in between was also studied. A method was developed in this study to separate infested from non-infested forest with a threshold value based on z-score, which was successful at showing the distribution of the geometrid outbreak in 2012. The size of the infested area was 80km², equal to 54.3% of the forest in the area. If the forest classified as “likely infested” would have been included, the ratio of infested forest would increase to 64.4%. This is a significant proportion of the forest that will certainly affect the forest in future years. The topographical distribution of the infestation over the study area was relatively evenly distributed, without displaying any range of altitude that was more prone to infestation.