Unraveling the Controls on Snow Disappearance in Montane Forests Using
Multi-Site Lidar Observations
Abstract
Snow disappearance date (SDD) has a substantial impact on the
ecohydrological dynamics of montane forests, by affecting soil moisture,
ecosystem water availability, and fire risk. The forest canopy modulates
SDD through competing processes, such as intercepting snowfall and
enhancing longwave radiation (LWR) versus reducing near surface
shortwave radiation (SWR) and wind speed. Limited ground-based
observations of snow presence and absence have restricted our ability to
unravel the dominant processes affecting SDD over mountains with complex
forest structure. We apply a lidar-derived method to estimate fractional
snow cover area (fSCA) at two relatively warm sites in the Sierra Nevada
and two colder sites in the Rocky Mountain. Our analyses show that warm
sites and lower elevations are characterized by higher LWR and canopy
snow interception leading to less snow retention under dense forest
canopy. In contrast, snow retention in colder forests can be longer in
open or under canopy depending on interactions between vegetation
structure and topography. These colder climates have greater under
canopy snow retention on north-facing slopes and under low vegetation
density areas, but greater snow retention in open areas at lower
elevations and south-facing slopes. We develop a new conceptual model to
incorporate the role of topography and vegetation structure into
existing climate-based frameworks. The inferences into the interacting
energy and mass controls, derived from our lidar datasets give
opportunities to improve hydrological modeling and provide targeted
forest management recommendations.