At high elevations a combination of environmental factors restricts the distribution of forest ecosystems. In addition to these natural limitations, high mountains are particularly prone to the deposition of air pollutants, which can lead to detrimental effects on the already struggling ecosystems. In the present chapter we review two typical examples of pollution impact on mountain forests. (1) The effects of high concentrations of SO2 on spruce forests of the Ore Mountains in Central Europe and (2) the effects of photo-oxidants (mainly O3) on mixed conifer forests in the San Bernardino Mountains in Southern California. Particular attention is paid to the potential interaction between natural stress factors and anthropogenic pollution impact. The development of oxidative stress and antioxidative defence systems play a key role in plant responses to adverse environmental conditions. Components of these systems have been used as stress markers, a task that is complicated due to their involvement in plant responses to both natural factors and pollution. We present a multivariate approach, which has been evaluated under various different field conditions as a step towards the distinction of the effects of different stress factors on forest trees in the field.