Combining multiple observations to improve our understanding of forest-snow interactions and inform forest management

Abstract

Many regions of the world rely on snowmelt from seasonal snowpacks for water resources. In the Northern Hemisphere, half of the seasonal snow zone overlaps with forests which interact with both snowfall and snow on the ground to influence total snow storage and water avaibility. This dissertation expands observations of forest-snow interactions to inform global modeling of canopy-snow interception in Chapter 2 and inform forest management in Washington State in Chapters 3 and 4. In previous work, we identified the need for global meteorological variable thresholds that determine canopy-snow unloading to reduce the need for site specific parameter calibration of unloading models. Thus, in Chapter 2 we create a dataset of canopy-snow unloading observations from timelapse photography at three different sites and combine them with meteorological data to determine what variable thresholds determine snow unloading across climates. Our results identify four primary unloading regimes that vary the thresholds for wind and temperature dependent unloading by climate and shortwave radiation. Furthermore, our unique dataset can be used for future model development and evaluation. In Chapter 3, we used aerial snow-on lidar from NCALM over 63 km2 of the Eastern Cascades of Washington State, USA, to quantify the effects of forest cover and topography on snow depth in this climate transition zone. We found that topographic position and canopy cover matter for snow depth in the Eastern Cascades, where the most snow at all elevations is found in topographically shaded gaps. We also found that forest cover is particularly important in predicting snow depth in warmer, low elevation terrain. In Chapter 4, we collect, process, and analyze drone lidar data before and after forest treatments on Cle Elum Ridge over a 3 km2 area in this low elevation zone of the Eastern Cascades to determine how forest treatments for fire resiliency impact hydrologic resiliency. We found that forest treatments on north-facing aspects increase snow storage in this climate transition zone compared to treatments with greater solar exposure which do not significantly affect snow depth. Thus, forest thinning and especially canopy-gap creation, on north-facing slopes provides a viable path forward for managing forests for both fire and hydrologic resilience in this climate zone.