MODEL COMPUTATIONS ON THE CRITICAL COMBINATION OF SNOW LOADING AND WINDSPEED FOR SNOW DAMAGE OF SCOTS PINE, NORWAY SPRUCE AND BIRCH SP AT STAND EDGE

Model computations were made on the critical combination of snow loadi ng and windspeed for snow damage of Scots pine(Pinus sylvestris L.), N orway spruce(Picea abies Karst.) and birch sp. (Betula sp.) at the new ly formed stand edge with varying tree height and stem taper using the model developed by H. Peltola, S. Kellomaki and H. Vaisanen (1996, HW IND: A Mechanistic Model for Wind and Snow Damage of Scotts Pine, Norw ay Spruce and Birch sp.) for the mechanism of wind and snow damage. In the computations, the total turning moment arising from the wind and snow load and from the bending of stem and crown was calculated along with the breaking stress of the stem and root anchorage. Windspeed var iation within the crown and the vertical distribution of snow, stem an d crown weight were also taken into account. According to computations , the critical combination of snow and wind loading for stem breakage and uprooting of trees was caused mainly by accumulation of snow on tr ee crowns, rather than by wind, which did, however, increase the risk of damage. The risk of damage increased along with stem taper decrease or tree height increase for all tree species studied. However, Scots pine and Norway spruce were found much more susceptible to snow damage than birch, which (being leafless) had much less crown area for snow attachment and wind loading. The trees most likely to suffer stem brea kage were slightly tapering Scots pines and Norway spruces with tapers of 1:120 for varying tree heights of 12-20 m under short-term snow lo ading of 60kgm(-2), i.e. they would have suffered stem breakage under windspeeds of less than 9ms(-1) above the tree canopy top. Respectivel y, even Scots pine and Norway spruce with tapers of 1:100 were at risk of stem breakage through sustained snow loading of 60 kgm(-2). In add ition, even snow loads of 20-40 kgm(-2) were found big enough to cause stem breakage of these trees with stem tapers of 1:120 during sustain ed snow loading. Correspondingly, similar pines and spruces with stem tapers of 1:120 were found to even more liable to be uprooted during c onditions of unfrozen soil than of having their stem broken by short-t erm snow loading of 20-60 kgm(-2), i.e. less windspeed was needed to c ause uprooting. However, pines and spruces with tapers of 1:80 were no t at risk for stem breakage and uprooting. This was because snow would have more probably been dislodged from the tree crowns by windspeeds greater than 9ms(-1) which are needed to worsen the damage. Nor would very slender birch without leaves have suffered stem breakage or uproo ting under any circumstances with windspeeds of less than 9ms(-1). (C) 1997 Elsevier Science B.V.