Earlier Snowmelt Decreases Bioclimatic Variation Across Elevation in the Mountains - a Test of Hopkin’s 1920 Bioclimatic Law One Century Later

Abstract:

The Bioclimatic Law proposed in 1920 by A.D. Hopkins to explain variation across elevation for the timing of plant and animal life history events may no longer be valid due to 100 years of human-caused climate change. Our research addresses the question of whether elevation leads to persistent phenological shifts in a warmed mountain world where snowfree dates are earlier? Across ~1,000 m elevation gradient in a Colorado, USA mountain watershed, we determined how earlier snowmelt affected the elevation-induced shift in the timing of first leaf expansion and first flower open for three life zones, the montane, subalpine and alpine. Our study included four years (2017-2020) with two years (2018-2019) in which we experimentally accelerated snowmelt timing. The considerably earlier snowmelt in 2018 compared to 2017 did not alter the elevation-induced shift for first leaf expansion, though it eliminated the shift for first flowering. The further advance of snowmelt by ~10 days in the experiment in 2018 led to even earlier leafing and flowering from 2 to 41 days with a greater shift at the highest elevations. As a result, experimentally earlier snowmelt in an early melt year shrunk the window of bioclimatic variation across elevation by 40% for first leaf expansion. Similarly, ecosystem function, including the timing of peak greenness and declining soil water content following snowmelt, varied less across elevation in the earlier melt years and due to the experimental advance of snowmelt. In mountain watersheds, the increasing synchrony in plant life history events and ecosystem function across elevation due to earlier snowmelt could greatly alter water resources and mountain biodiversity this century.