Forest ecosystems are affected by stress induced changes in various ways. Environmental factors that affect trees negatively can be distinguished between biotic and abiotic factors. Abiotic factors are non-living such as drought, storm, frost, etc. Biotic factors are of living kind such as fungi or insects. Tree species react to stress in terms of activating their repair process and/or long-term adaptation of their morphology and metabolism. Depending on the strength of stress events this can lead to resistance and repair or severe damages and even plant death. However, with regard to water or nutrient supply, tree species respond very differently. Especially for coniferous tree species bark beetle infestations are a consequence of primary damage in form of drought and unfavourale conditions for trees. Therefor it is crucial to analyse the predisposition of forests to stress events. Long-term temporal statistics of Landsat data will be analysed for change of forest state and linked to temporal climate records and soil-moisture data. Especially longterm and repeated drought periods result in lower vitality of forests prolonging for several years after the drought event. The preliminary results of a case study will be presented and an outlook for further research will be given.