Mapping insect damage in Finnish forests using remotely sensed imagery

The economic value of boreal forests in Europe are affected by disturbances that reduce productivity and in extreme cases can cause tree mortality. Repeated outbreaks of defoliating insect such as the geometrid moth and pine sawfly are responsive to climate change and threaten forest resources in Scandinavia. The signal of defoliator infestations can be detected from optical remotely sensed (RS) imagery, although improvements are necessary to characterize areas affected and to account for variability from differences in species and topographic position. Furthermore, the spatial and temporal resolution of different RS data products should be explored to determine if syntheses of available datasets prove useful in detecting outbreaks and how we can make the best use of newly available data products, such as Sentinel-2.
Recent studies show that the detection of forest damage/defoliation caused by pine sawflies with coarse-resolution RS data (MODIS) fails in heterogeneous landscapes which are typical for the managed forests of Finland (Olsson et al., 2016). Even in cases where the detection of damaged areas with MODIS data may be useful (i.e., more homogenous landscapes), the estimation of the degree of damage has not been successful (Eklundh et al., 2009). These results refer to the correspondence (or lack of it) between MODIS data and observations at the scale of pixels/forest stands. Despite these poor results at this level, one might hypothesize, that because of the generally moderate to high spatial correlation in pine sawfly densities the regionally derived MODIS products focusing on properly selected (large, homogenous) pine stands should correlate better with results from regional pine sawfly monitoring programs, and smaller resolution Landsat products should improve the correlations further.
The elaboration of factors that can attenuate the signal of defoliator outbreaks such as management activities and drought will be helpful in focusing the research only on those areas that are potentially affected. It is possible that through the creation of regionally derived products could be used to create an early warning system of defoliator outbreaks that could be used to target areas for mitigation or containment strategies to reduce their impact.