Observational evidence suggests the vernal window extends under climate warming

Abstract:

Climate warming results in shorter snow seasons and earlier springs. The gap between the last day of snow on the ground and the first signs of spring on the trees –the vernal window– is a crucial period for ecosystems’ transitioning from the dormant season into the growing season. However, our understanding of climate warming impacts on the vernal window is limited. With increasing temperature, if the onset of spring advances at a faster rate than snowmelt, the vernal window shortens. But if snowmelt accelerates faster than the advancement of spring, the vernal window is extended.

The Spruce and Peatland Responses Under Changing Environments (SPRUCE) experiment can provide clues on how the vernal window may change under climate warming. The SPRUCE experiment consists of ten chambers with five temperature treatments (up to +9°C). We tracked three different plant functional types in each chamber including a deciduous needleleaf tree, an evergreen needleleaf tree, and a mixed layer of shrubs. Using digital cameras and ground observation data, we investigated the inter-annual variability of the snow disappearance timing vis-a-vis the spring onset.

Our results from digital cameras showed that the timings of both spring onset and snow disappearance advanced, as temperature increases. However, the advances in the snow disappearance timing were considerably stronger than the shifts in spring onset for all plant types at warmer chambers (up to 10 weeks for snow and 1-2 weeks for spring onset). While ground observations suggest species-specific differences in phenological sensitivities to warming, the results affirmed the findings from digital cameras on the extended vernal window across all species. Our study provides strong evidence on the lengthening of the vernal window under climate warming. This disruption in the winter to spring transition may result in ecological and hydrological mismatches between the ecosystem’s demand and availability of water and nutrient resources.