How does a warm and low-snow winter impact the snow cover dynamics in a humid and discontinuous boreal forest? An observational study in eastern Canada

Abstract. In the boreal forest, winter temperatures are projected to increase substantially by 2100, resulting in a reduction in snow cover thickness and duration. These changes are likely to affect hydrological processes such as snowmelt, the soil thermal regime, and snow metamorphism. The exact impact of future changes is difficult to pinpoint in the boreal forest, due to its complex structure, and the fact that snow dynamics under the canopy are very different from those in the gaps. Although the effects of warmer winters on snow-related processes are well documented, their interactions to influence the spring runoff in evergreen forest remain poorly understood. In this observational study, we assess the influence of a low-snow and warm winter on snowmelt dynamics, soil freezing, snowpack properties, and spring streamflow in a humid and discontinuous boreal catchment of eastern Canada (47.29° N, 71.17° W, 850 m ASL). We monitored the soil and snow thermal regimes and sampled physical properties of the snowpack under the canopy and in two forest gaps during an exceptionally low-snow and warm winter, plausibly representative of future winters, and during a winter with conditions close to normal. We observe that snowmelt was earlier but slower, top soil layers were cooler, and gradient metamorphism was enhanced during the low-snow and warm winter. However, we observe that snowmelt duration increased in forest gaps, that soil freezing was enhanced only under the canopy, and that snow permeability increased more strongly under the canopy than in either gap. Overall, we observe that the spring streamflow discharge was significantly reduced in the warmest year due to a slower melt and low precipitation in April and May. Our results, based solely on field observations, highlight the complex effects of warmer winters on snow hydrology in discontinuous boreal forests.