Effect of Larch Forest Density on Snow Surface Energy Balance

Abstract

It is established that the density of a larch forest strongly influences the snowmelt energy under its canopy. In the spring thaw of 1994, 1995, and 1996, the surface snowmelt at three different sites located at the southern foot of Mt. Iwate, Japan, was measured. These sites were surrounded by the same tree species with similar tree height, but with different forest densities [open (OP) site; sparse larch forest (SLF): 411 larch trees per hectare; dense larch forest (DLF): 1433 larch trees per hectare]. It was observed that at most periods under analysis, the snow surface albedo under the canopy decreased as the forest density increased. With this in mind, a simple empirical model for the balance of snow surface energy at a forested site shows that variations in below-canopy snow albedo, due to forest density, are important for estimating the snowmelt energy, especially at high forest densities. The importance of such variations lies in the fact that their inclusion is necessary to estimate accurately the net all-wave radiation in DLF?net all-wave radiation being the most important energy component that affects snowmelt. Taking into account variations in the weather conditions, net all-wave radiation does not usually decrease as forest density increases, because when forest density increases, there is an associated increase in longwave radiation that is counterbalanced by a decrease in below-canopy snow albedo. By contrast, net all-wave radiation does increase slightly when forest density increases. Variation in net all-wave radiation under the larch forest canopy depends upon the variation in below-canopy incident short-wave radiation and below-canopy snow albedo. Variations in below-canopy snow albedo, which are due to variations in forest density, affect the snowmelt energy as follows: (i) when the initial below-canopy snow albedo is low, the snowmelt energy decreases significantly as forest density increases, because the decrease in below-canopy incident shortwave radiation is more than the decrease of below-canopy snow albedo, both of which are due to increases in forest density. By contrast, (ii) when the initial below-canopy snow albedo is high, the snowmelt energy changes only slightly as forest density increases, because the slight increase in net all-wave radiation that occurs when forest density increases is caused by the fact that the decrease in below-canopy incident shortwave radiation due to increases in forest density is cancelled out by the decrease in below-canopy snow albedo that occurs in response to increases in forest density.