Influence of Forest Canopy Structure and Wind Flow on Patterns of
Sub-canopy Snow Accumulation in Montane Needleleaf Forests
Abstract
Vegetation structure is considered one of the most important factors
shaping the spatial variation of snow accumulation under forest
canopies. However, fine scale relationships between canopy density, snow
interception, wind redistribution and sub-canopy accumulation are poorly
understood and difficult to observe, and their influence governing
stand-scale snow distributions that determine snow covered area
depletion during melt is largely unknown. In this study, fine-scale
observations of forest structure and sub-canopy snow accumulation were
analyzed over two mid-winter snowfalls to a sub-alpine forest in Marmot
Creek Research Basin, Canadian Rockies, Alberta, to identify the impact
of snow-canopy interactions on spatial patterns of sub-canopy snow
accumulation. High spatial resolution (5 cm and 25 cm) snow accumulation
estimates and canopy structure metrics were calculated from the
combination of repeated UAV-lidar observations with snow and
photographic surveys, utilizing novel resampling methods including voxel
ray sampling of lidar (VoxRS) to improve metric robustness and reduce
bias. Over 50% of the spatial variance in forest snow accumulation was
found at length scales less than 2 m, supporting the role of local scale
canopy structure in governing variation in subcanopy snow accumulation.
Additionally, subcanopy snow accumulation showed significant angular
spread in relationships with overhead canopy structure; the vertical
asymmetry coinciding with local windflow directions during snowfall.
Detailed angular analysis showed nontrivial snow-vegetation
relationships that likely reflect multiple snowfall-vegetation
processes, including unloading and entrainment of intercepted snowfall
during wind gusts and funneling of entrained particles by downwind
vegetation. These fine-scale findings suggest several emergent processes
which may influence snow accumulation at the scale of forest stands,
with novel considerations for representing SWE distributions under dense
evergreen canopies under varying environmental and canopy conditions.
Similar studies over a broad range of conditions and forests will help
refine and generalize the effects observed here for further snow
hydrology and forestry applications.