Bijan Seyednasrollah, Mukesh Kumar

Net radiation reaching the forest floor is influenced by vegetation density. Previous studies have confirmed that in mid-latitude conifer forests in Greenville, ME (45.5°N), net radiation decreases and then subsequently increases with increasing vegetation density, for clear sky conditions. This leads to existence of a net radiation minimum at an intermediate vegetation density. With increasing cloud cover, the minimum radiation shifts toward lower densities, sometimes resulting in a monotonically increasing radiation with vegetation density. The net radiation trend, however, is expected to change with location of forests, affecting the magnitude and temporality of individual radiation components. This research explores the variability of net radiation on snow-covered forest floor for different vegetation densities along a latitudinal transect. We especially investigate how the magnitude of minimum/maximum radiation and vegetation density at which they are expressed, changes with site location. To evaluate these, net radiation is calculated using the Forest Radiation Model at six different locations in white spruce (Picea glauca) forests across North America, with latitudes ranging from 45 to 66°N. Results show that the variation of net radiation with vegetation density significantly varies between different latitudes. In higher latitude forests, the magnitude of net radiation is generally smaller, and the minimum radiation is exhibited at relatively sparser vegetation densities, for clear sky conditions. For interspersed cloudy sky conditions, net radiation non-monotonically varies with latitude across the sites, depending on the seasonal sky cloudiness and air temperature. Net radiation on north-facing hillslopes is less sensitive to latitudinal location than on south-facing sites.