MODELING THE PHYSICAL-ENVIRONMENT OF TREE SEEDLINGS ON FOREST CLEARCUTS

A soil and atmospheric boundary layer model has been developed to predict soil temperature, soil moisture, air temperature and humidity on forest clearcuts. The model comprises a numerical solution to the one-dimensional transfer equations for energy and water in the air and soil near a surface. Heat and water transport in the soil are coupled using a linked source-term approach. Model predictions were compared with field observations from three forest clearcuts in northern Ontario. Soil temperatures on all sites were predicted with less than 1-degrees-C bias. It was necessary to increase the value for soil hydraulic conductivity in the model to obtain good agreement between predicted and observed soil water contents. During lapse conditions on two sand flat sites, air temperature at 20 cm was predicted with less than 0.1-degrees-C bias and water vapor pressure at 20 cm with less than 10 Pa bias. As a consequence, the 20-cm saturation deficit was predicted with less than 10 Pa bias. On a swamp site, however, bias in predicted air temperature was much larger, possibly because of heterogeneity of the surface. In most instances, uncertainty in observed or predicted values was of similar magnitude to the differences between predicted and observed values. The model is suitable for examining the effects of soil and surface properties and of boundary conditions on the physical environment of flat, homogeneous, unvegetated forest clearcuts.