Effects of needleleaf forest cover on radiation and snowmelt dynamics in the Canadian Rocky Mountains

Abstract: Radiation is the main energy source for snowpack warming and melt in mountain needleleaf forests, and runoff from these forests is the main contributor to spring river flows in western North America. Utilizing extensive field obser-vations, the effect of needleleaf forest cover on radiation and snowmelt timing was quantified at pine and spruce forest sites and nearby clearings of varying slope and aspect in an eastern Canadian Rocky Mountain headwater basin. Compared with open clearing sites, shortwave radiation was much reduced under forest cover, resulting in smaller differences in melt timing between forested slopes relative to open slopes with different aspects. In contrast, longwave radiation to snow was substantially enhanced under forest cover, especially at the dense spruce forest sites where longwave radiation dominated total energy for snowmelt. In both pine and spruce environments, forest cover acted to substantially reduce total radiation to snow and delay snowmelt timing on south-facing slopes while increasing total radiation and advancing snowmelt timing on north-facing slopes. Results strongly suggest that impacts on radiation to snow and snowmelt timing from changes in mountain forest cover will depend much on the slope and aspect at which changes occur. Résume ́ : Le rayonnement est la principale source d’énergie qui réchauffe et fait fondre la couche de neige dans les forêts alpines de conifères ou ̀ le ruissellement contribue dans une large mesure au débit printanier des rivières dans l’ouest de l’Amérique du Nord. A ` l’aide de nombreuses observations sur le terrain, dans un bassin de tête de la partie est des monta-gnes Rocheuses canadiennes, l’effet du couvert de forêt de conifères sur le rayonnement et sur le moment de la fonte d