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.

Inhalt

General Features of the Upper Timberline.- Reasons for Occurrence of Timberlines and Their Experimental Investigation.- Some Milestones in the History of Ecophysiological Research Concerning Alpine Timberline.- 1. Natural Regeneration of Tree Stands at Timberline.- 1.1 Seed Production. Frequency of Seed Years, Quantity, and Quality of Seed.- 1.2 Dispersal of Seed.- 1.3 Maturation of Seed.- 1.4 Germination.- 1.5 Vegetative Propagation.- 2. Growth of Trees at Timberline.- 2.1 Height Growth.- 2.2 Growth of Leaves.- 2.3 Diameter Growth.- 2.4 Root Growth and Mycorrhizae.- 3. Dry Matter Production of Trees at Timberline.- 3.1 Photosynthesis.- 3.1.1 Dependence of Photosynthesis on External Factors and Their Significance for CO2-uptake of Trees at Timberline.- 3.1.1.1 Light.- 3.1.1.2 Temperature.- 3.1.1.3 Wind.- 3.1.1.4 CO2-Content of the Atmosphere.- 3.1.1.5 Soil Temperature.- 3.1.1.6 Atmospheric and Soil Moisture.- 3.1.2 Annual Course of Net Photosynthesis of Trees at Timberline.- 3.1.2.1 Seedlings of Pinus cembra Covered by Winter Snow.- 3.1.2.2 Trees of Pinus cembra Exposed Above the Winter Snow-Pack.- 3.1.2.3 Larix decidua.- 3.1.2.4 Other Published Investigations.- 3.1.3 Photosynthesis with Respect to Altitude.- 3.2 Dark Respiration with Respect to Altitude and Timberline.- 3.3 Carbon Balance of Trees at Timberline.- 3.3.1 Dark Respiration.- 3.3.2 Winter Respiration.- 3.3.3 Root Respiration.- 3.3.4 Stem Respiration.- 3.4 Net Carbon Gain and Biomass Increment at Elevations up to Timberline.- 3.4.1 Reduction in Dry Matter Production from Valley Floor to Timberline.- 3.4.2 Primary Production of Trees in the Timberline Ecotone.- 3.5 Primary Production and Nitrogen Nutrition.- 4. Water Relations of Trees at Timberline.- 4.1 Water Relations in Summer.- 4.1.1 Precipitation and Soil Moisture.- 4.1.2 Evaporation.- 4.1.3 Transpiration of Trees.- 4.1.4 Water Balance in Trees.- 4.2 Water Relations in Winter.- 5. Climatic Resistance and Damage of Trees at Timberline.- 5.1 Frost Damage.- 5.2 Ultraviolet and High Intensity Radiation Damage.- 5.3 Heat Damage.- 5.4 Mechanical Damage.- 5.4.1 Wind.- 5.4.2 Snow.- 5.5 Winter Desiccation or "Frost-drought" Damage.- 5.5.1 Progression of Desiccation.- 5.5.2 Causes of Desiccation Damage.- 5.5.3 Experimental Evidence for the Michaelis Hypothesis.- 5.5.4 An Analysis of Dynamic Processes at Timberline.- 6. Synopsis.- 6.1 Carbon Budget.- 6.2 The Course of Developmental Phases.- 6.3 Climatic Resistance.- References.- Taxonomic Index.