The first change in tree growth with increasing proximity to the upper or polar timberline is a decrease in height due to heat deficiency. The intensity of climatic stress influencing tree growth at these timberlines depends not only on altitude and latitude but also on the timberline type, i.e., whether the forest remains closed to its limit or scatters into tree islands and solitary trees. Within the transitional belt between closed forest and the upper limit of the tree species, exposure to wind and radiation is the most decisive site-controlling factor, influencing temperature conditions, depth and duration of snow cover, soil moisture, evapo-transpiration, etc. Trees suffer from frost-drought, frost, ice-particle abrasion, creeping snow and snow blight infections. Studies on the distribution of climatic damage and its influence on sites with extreme wind exposure at upper timberline in the Colorado Front Range support the theory that injury by frost and frost-drought is promoted when too cool a growing season prevents the full maturation of new shoots and needles. Iceparticle abrasion may cause mechanical damage and, over very long periods, shape the tree physiognomy.
Under severe climatic stress the trees display crippled growth forms differing greatly from normal ones within the closed forest. Deformations range from stunting of still upright trees to flagging and mat-growth. Spruces show the greatest variety in developing growth forms in adaptation to site conditions.
Further, spruces, subalpine fir, larches and mountain birch can regenerate and propagate vegetatively by layering. Thus they can survive climatic deteriorations which would prevent sexual regeneration for even hundreds of years. Climatic stress not only shapes the tree physiognomy but also influences the distribution pattern within the forest-tundra ecotone. This paper refers only to some strip-like patterns at upper timberline in the Rocky Mountains ('tree-hedges', 'ribbon forest'), where they should be regarded as typical features of the timberline at very wind-exposed sites.