In the European Alps the importance of forests as protection against ava­ lanches and soil erosion is becoming ever clearer with the continuing increase in population and development of tourism. The protective potential of the moun­ tain forests can currently only be partially realised because a considerable propor­ tion of high-altitude stands has been destroyed in historical times by man's extensive clearing ofthe forests. The forests still remaining are of limited effec­ tiveness, due to inadequate density of trees and over-maturity. Considerable efforts, however, are now being made in the Alps and other mountains of the globe to increase the high-altitude forested area through reforestation, to raise depressed timberlines, and to restore remaining protection forests using suit­ able silvicultural methods to their full protective value. This momentous task, if it is to be successful, must be planned on a sound foundation. An important prerequisite is the assembly of scientific facts con­ cerning the physical environment in the protection forest zone of mountains, and the course of various life processes of tree species occurring there. Since the introduction of practical field techniques it has been possible to investigate successfully the reaction of trees at various altitudes to recorded factors, and the extent to which they are adapted to the measured situations. Such ecophysio­ logical studies enable us to recognize the site requirements for individual tree species, and the reasons for the limits of their natural distribution.