Snow cover in forest stands
Appearance and disappearance of a snow cover and hence the accumulation, duration and melt of a snowpack in a forest is viewed from mainly two groups of influencing factors. The first includes meteorological parameters. Not only amount and intensity of snow precipitation, but also wind, airtemperature, radiation and humidity, influence the interception process and therefore the accumulation and melt of a snowpack in a forest. Man has virtually no possibility to influence or change these factors.
The second group of parameters describes and characterizes the forest site, including the stand itself, its ground vegetation, forest floor and soil surface. These parameters show an even more pronounced influence on the interception process. In contrast to the first group, stand characteristics can be changed through forest treatment practices. Hence, it is expected that within limits, snowpack management can be practiced. This has actually been done to improve water yield, but to our knowledge rarely to reduce avalanche danger in mountainous areas.
The interception process in a forest is a mechanism by which snow precipitation is caught on the plant's above ground surface, stored and again released. Snow interception loss is the quantity of precipitation water which - after it is caught - is lost to the atmosphere by sublimation or evaporation. Interception gains occur as a result of moisture condensation on the plant's surface. Interception is therefore the interaction between meteorological and forest stand parameters. It is the primary process which affects snow cover in forest stands particularly during the accumulation phase when snow interception loss occurs in the canopy.
The interception of energy by the tree crowns, on the other hand, is a process which is rnost irnportant during the melt phase of the snowpack in the forest. Radiation as well as heat exchange are much influenced by forest stand characteristics and result in variable melt rates of the snowpack.
Forests in the high country near timberline affect the occurence of avalanches most. This review is therefore restricted to snow interception of coniferous forest stands and to areas with no or short periods of snow cover disappearance in mid-winter. It surnmarizes information on snow cover in forest stands as related to climate and stand conditions.
Todays literature shows either information on the purely hydrologic aspects of snow cover or purely physical and structural information which rarely allow a hydrologic interpretation of the data. Therefore it is suggested to give careful consideration to studies which link the hydrologic with the physical-structural aspects of the snowpack in forest stands. There is a research need to establish the scientific background for forest treatrnent practices in the high country towards reducing avalanche hazargs in mountain forests.