Forest ecosystems, especially forest ecosystems in mountain regions, are under the influence of complex stressors. Apart from the natural, mostly climatic factors, they include human influences, such as air pollution, tourism, accessibility of forests, and various types of management for silvicultural purposes, wildlife, and grazing. Air pollution stresses to the forest ecosystems of the Northern Tyrolean Limestone Alps are caused by local emittors and long-distance transport. Apart from the gaseous components SO 2 , NO x , and O 3 , they contain predominantly protons, nitrogen compounds, and heavy metals. The air pollution stress of the Northern Tyrolean Limestone Alps was described by using the results of all-Austrian investigations (Austrian Bioindicator Grid), air monitoring data from the Federal Province of the Tyrol, model calculations, and surveys from the area of Achenkirch. The monitoring data were evaluated in accordance with effect-related limiting values or Critical Loads. The evaluation of the air monitoring data for the area under investigation showed that the stress patterns differed significantly depending on the absolute altitude above sea level or the relative altitude above the valley floor. In valleys, effect-related limiting values for SO 2 and NO 2 , which were established to protect sensitive plant species, are exceeded in conglomerations, and ozone has been a permanent stressor especially at higher altitudes. The influence of accumulative pollutants (e.g. heavy metals) on the soil and the fact that these pollutants have increased and reached the sensitive zone of the timber line is particularly important in respect of soil-biological processes. While acid inputs can easily be buffered by the bedrock of the Northern Tyrolean Limestone Alps, the determined nitrogen inputs constitute a potential risk to forest ecosystems.