Geographic Consequences of Glaze in Forests around Idrija and Postojna

Abstract

The study discusses the occurrence and the consequences of glaze which in the days from 17th to 19th November, 1975, broke down and turned over about 342.331 m3 of trees growing on the northern slopes of Snežnik and Javornik mountains, and above all in the region of Hrušica, Nanos, Trnovski gozd, and of the Karstic plateaus around the town of Idria (Vojsko, Sebrelje, Ledine). During the days with glaze, temperatures in the whole area of Slovenia were above the freezing point; they were below 0° C in the Alps only and on the northern slopes of the High karstic plateaus between Snežnik and Trnovski gozd (at an altitude between 800 and 1150 m above sea level). The phenomenon took place in a period when in greater altitudes warm and humid air currents bringing rain were already prevalent. Northern slopes were also cooled by a permanent stream of cold air coming from the north. Because of this the light rain was changing into ice on the northern slopes of these plateaus. The catastrophe was increased by the fact that in ali this area wide spread forests are prevalent containing mainly deciduous trees (Abieti - Fagetum dinaricum) which are not resistant against glaze. The damage was especially great in very steep slopes, in surfaces covered with a thick layer of weathered material, in periglacial fine scree, in dolomite, sandstone, and in slates; it was considerably smaller in limestone area. The catastrophe on the steep northern slopes was also caused by the fact that here trees grow regularly a little inclined away from the slope because of its steepnes and because of the sliding of the ground and of the snow. The crowns of these trees are especially wide at their external sides. The same sides were also most exposed to the accumulation of glaze caused by northern winds.