Spatiotemporal Dynamics of Snow-Free Frost Exposure in the Western Cordillera Seasonal Snow Zones

In the mountain ecosystems of the Western Cordillera, seasonal snowpack provides insulation for soils and vegetation against freezing temperatures. While temperatures are warming overall, concurrent declines in snowpack are removing this protective thermal blanket. This may lead to events where the ground surface becomes more vulnerable to frost damage during synoptic-scale cold events, such as arctic air incursions, even as average temperatures rise. The net impact of these competing effects on ground-level frost exposure remains poorly understood. This thesis addresses this gap by analyzing the spatiotemporal dynamics of snow-free frost days (SFFDs), defined as days with maximum air temperatures below 0°C and minimal snow cover (≤20 mm Snow Water Equivalent), across the Western Cordillera. Using a quality-controlled dataset of daily temperature and snowpack records from 414 Snow Telemetry (SNOTEL) sites over a 31-year period (1990-2020), this study quantifies long-term trends in SFFD frequency, identifies the primary drivers of these trends, and assesses ecological impacts using a satellite-derived vegetation index (NDVI). The analysis reveals that SFFDs are a relatively rare phenomenon with occurrences concentrated in the fall when the snowpack is still developing and its insulating capacity is inconsistent. Results show a statistically significant network-wide decrease in SFFD frequency over the study period. This decline is mainly driven by increasing temperatures that have thus far outweighed the impact of a less persistent snowpack at most sites. The analysis suggests that grassland sites in areas with historically reliable snow cover (low-frost exposure) maintain higher productivity relative to sites with intermittent snow cover (high-frost exposure), and this gap widens during colder years, highlighting the importance of the snowpack’s insulating capacity on this ecosystem. This analysis did not find evidence that conifer ecosystems were affected by changes in SFFDs.