Radiation and snowmelt dynamics in mountain forests

Utilising extensive field observations and physically-based simulations of forest-snow processes, the impacts of needleleaf forest-cover on radiation and snowmelt dynamics were investigated in an eastern Rocky Mountain headwater catchment. At low-elevation pine forest sites, the sparse canopy-cover allowed for substantial shortwave transmittance to snow, giving topography-influenced snow radiation balances and snowmelt timing. By comparison, the denser high-elevation spruce cover minimised shortwave radiation to snow, resulting in snowmelt dominated by longwave radiation gains, and close synchronisation in melt timing across opposing mountain slopes. Field observations were used to direct and evaluate physically-based simulation models describing radiation-snow exchanges in needleleaf forests. This included the estimation of shortwave irradiance transfer through sparse needleleaf canopies with explicit account for differing shortwave transmittance properties of trunks, crowns, and gaps within highly structured mountain pine stands. Improved representation of sub-canopy longwave irradiance to mountain snow was also made through the determination of added longwave emissions from shortwave heated canopies. From model simulations, forest-cover effects on radiation to snow were found to vary substantially with both topography and seasonal meteorological conditions. In general, forest-cover increased radiation during the mid-winter by reducing longwave losses from snow. However, with greater shortwave irradiance into the spring, forest-cover effects on radiation to snow became increasing influenced by topography, with greater radiation under more open canopies on south-facing slopes and under more closed canopies on north-facing slopes. Drawing upon past field investigations and modelling exercises, a physically-based simulation model was constructed to represent snow accumulation and melt processes in needleleaf forest environments. By means of an objective evaluation, the model well represented differences in snow accumulation and melt in paired forest and clearing sites of varying location and climate. The model was subsequently applied to examine forest-cover impacts on mountain snowmelt, revealing that forest-cover removal substantially increased total snowmelt and sizeably expanded the spring melt period through a de-synchronisation of melt contributions from south-facing and north-facing landscapes. These results demonstrate the potential for altering the magnitude and timing of mountain snowmelt through topographic-specific changes in mountain forest-cover