The upper forest line in the Julian Alps

Abstract

E/OGEOGHAPHIA - val. XI/I — 1987 Biageografia delle A/pi Sud—Orieni‘aIi The upper forest line in the Julian Alps FRANC LOVRENCAK Depzm‘/izem‘ of Geogmp/Jy, U/zz'versz'ty of Ljzzbljzma SUMMARY The forest line in the Julian Alps is the result of the complex interaction of several natural (climatic, orographic, edaphic, biotic) and antropogenic factors typical of the high mountains. They were responsib- le for the formation of different types of the forest line. We discern: the climatic (thermal and winds affected), the orographic and the antropogenic forest line. The course and the altitude of the natural and the actual forest line differ, because of the action of the physical geographical and social factors, even by several hundred metres. Two main species of trees (latch and spruce) grow along the forest line in the Julian Alps. The climatic (thermal) forest line is located at a higher altitude of 1900 m in the highest and the most massive central and northwestern part of the Julian Alps. The forest line is lower, because of the climatic conditions in the southwestern part of the Julian Alps, where it is at an altitude between 1550- 1600 rn above sea-level. In the higher and more massive interior of the alpine area the forest line climbs to altitudes of about 1700 in and even about 1800 m at the highest points. The altitude, the physiognomy and the structure of the forest line in the Julian Alps are determinated by several factors related both to physical ge- ographical as well as to social geographical conditions. The decisive role is played by the air temperatures and of particular importance are air temper- atures during the vegetation period. Some authors assert that the forest line is located at altitudes where the July isotherm line does not excede 10°C (Marek 1905, Plesnik 1971). The comparison of the interpolated values for the mean July temperature of 10°C in separate parts of the Julian Alps reveals that it follows in the border areas of the mountain system, the altitudes of just above 1700 m above sea—level and slightly above 1800 In in the central part of the mountain system. This is, approximately, the altitudinal location of the forest line. The growth of trees is affected also by other climatic factors, notably by precipitation. Precipitations are most abundant in the southwestern part of the Julian Alps. The higher parts of the Karim group receive over 3000 mm of precipitations in a year, but the situation is similar also in the central parts of the mountain system. Snow is quite important for tree growth because of its positive influences (protection of the soil from frost and winds), but also negative influences (mechanical damage to trees, retention of growth). The 113