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.