The timing of spring initiates an important period for resource availability for large trophic webs within ecosystems, including forage for grazing animals, flowers for pollinators, and the higher trophic levels that depend on these resources. Spring timing is highly variable across space, being influenced strongly by the departure of snow cover (i.e. snowmelt timing, in locations with a seasonal snowpack), climate, weather, elevation, and latitude. When spring timing occurs along a gradient (e.g. spring arriving later in higher elevations of mountainous terrain), the organisms that rely on spring resources often migrate to maintain an optimal position for spring resources – a phenomenon known as ‘surfing the green wave.’ While this behavior has been observed by tracking animals, there have been no studies to quantify the green wave as a movement of phenology across space and time. Furthermore, considering that snowmelt timing has moderate power to explain green-up timing for a given location, we ask the question: does snowmelt velocity predict green wave velocity? We adapt a method from the climate velocity literature to track the movement of retreating snowpack and advancing spring green-up phenology, and subsequently compare these velocities using methods from linear and circular statistics.
Results/Conclusions:
Here, we introduce the first known continental maps of snowmelt and green wave velocity for North America from 2001-2016 as derived from the MODIS-based Snowmelt Timing Maps and MCD12Q2 phenology dataset. We use dynamic animations of snowmelt and green wave velocity to demonstrate our method, and to highlight resource wave movement in the ecologically important Lamar Valley of Yellowstone National Park, USA. We show that both snowmelt and green wave velocities are influenced strongly by topography, including slope and aspect. Furthermore, we quantify the relationships between snowmelt and green wave velocities according to three variables: direction, speed, and distance traveled. We conclude that mountainous ecoregions, such as the western North American cordillera, have the highest correspondence between snowmelt and green wave velocities, compared to flatter regions such as the Great Plains and tundra. This work will be of interest to wildlife ecologists, biologists, and land managers who seek to conserve migratory animals and the ecosystems that support them.