The Vernal Window Flow Path: a Cascade of Ecological Transitions Delineated at Scales from Points to Pixels

The "vernal window" is the period between the end of winter and the start of the growing season. In seasonally snow covered areas, this period is characterized by dramatic changes in ecosystem energy balance, hydrology, and biogeochemistry. Transitions at the beginning of the vernal window, such as warming air temperature, are occurring earlier due to climate change while the green-up that indicates the close of the window is responding more slowly. The result is a lengthening of the vernal window, with unclear ecosystem implications. Equally uncertain is how changes in the sequence, timing, or duration of transitions along the vernal window flow path affect subsequent processes and functions. We synthesized data collected throughout New Hampshire, USA to investigate the seasonal shift from winter to spring as manifested by changes in energy balance, physical properties, and biological phenomena across a variety of spatial and temporal scales. Data included observations from citizen science networks, terrestrial and aquatic sensors, remote sensing products, and meteorological model outputs and encompassed variables such as shortwave radiation; snow depth and water equivalent; soil temperature, moisture, and conductivity; and stream discharge, nitrate, and dissolved oxygen. For each variable, we developed algorithms to detect thresholds delineating the end of winter and the onset of spring. We also calculated lags between transitions and examined spatial patterns in thresholds and lags throughout the state. We found that transitions within the vernal window followed a predictable sequence, that there were quantifiable lags between transitions, and that the duration of lags varied with latitude, slope, aspect, and land cover. The sequence of biogeochemical changes and lags were sometimes surprising and indicate how integration of near-real time data collected a variety of scales can transform insights into ecosystem function during the critical vernal window period.