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Résumé :
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Wind shelter for trees in subalpine afforestations on extremely windy sites "Pru dal Vent" (Alp Grüm, GR) At the alpine timberline, the growth of trees is hindered by many environmental factors. Besides low temperatures, relatively short vegetation periods, mechanical forces and biotic diseases, extreme wind can also reduce the growth of young trees or even kill them through abrasion. In "Pru dal Vent" (windy pasture), an extremely windy area on the south side of the Bernina pass, in the Grisons, Switzerland (10° 2' 2" E, 46° 22' 44" N, 2210 m NN), wind speeds are a key factor affecting the growth and survival of young trees. Young Swiss stone pines and European larches (Pinus cembra; Larix decidua) were planted in "Pru dal Vent" to test the effectiveness of different windbreaks against abrasion caused by very high winds, in addition to the high-altitude afforestation experiments at Stillberg Alp near Davos. From 1971 to 1995 experiments with windbreak walls of various sizes up to 2.5 m high were built. Their permeability varied, but all were treated with agents to reduce transpiration. A meteorological station recorded the weather data. The anti-transpirants appeared to have no positive effects. The lee-sides of the windbreaks accumulated extremely hard snow covers, which led to multiple fractures occurring in the young stems. As a result, most trees died. In 1994 small windbreak devices, one for each tree, were put up, designed to be somewhat similar to those found in the luff of most of the few naturally growing trees in the surroundings. Again different shapes and forms of the shelters were tested including: small stone cairns, wooden shields of different shapes, and small topless boxes. The best results were observed with the stone cairns und with two small boards arranged in the shape of a V. Since the small windbreaks were put up in 1994, the majority (roughly 96% of L. decidua and 70% of P. cembra) of the experimental trees behind them have survived. Some have even grown well. Thus this method using small shelters rather than larger windbreaks seems to be an effective way of protecting young trees).
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