Abstract
Extreme winter events that damage vegetation are considered an important climatic cause of arctic browning—a reversal of the greening trend of the region—and possibly reduce the carbon uptake of northern ecosystems. Confirmation of a reduction in CO2 uptake due to winter damage, however, remains elusive due to a lack of flux measurements from affected ecosystems. In this study, we report eddy covariance fluxes of CO2 from a peatland in northern Norway and show that vegetation CO2 uptake was delayed and reduced in the summer of 2014 following an extreme winter event earlier that year. Strong frost in the absence of a protective snow cover—its combined intensity unprecedented in the local climate record—caused severe dieback of the dwarf shrub species Calluna vulgaris and Empetrum nigrum. Similar vegetation damage was reported at the time along ~1000 km of coastal Norway, showing the widespread impact of this event. Our results indicate that gross primary production (GPP) exhibited a delayed response to temperature following snowmelt. From snowmelt up to the peak of summer, this reduced carbon uptake by 14 (0–24) g C m−2 (~12% of GPP in that period)—similar to the effect of interannual variations in summer weather. Concurrently, remotely-sensed NDVI dropped to the lowest level in more than a decade. However, bulk photosynthesis was eventually stimulated by the warm and sunny summer, raising total GPP. Species other than the vulnerable shrubs were probably resilient to the extreme winter event. The warm summer also increased ecosystem respiration, which limited net carbon uptake. This study shows that damage from a single extreme winter event can have an ecosystem-wide impact on CO2 uptake, and highlights the importance of including winter-induced shrub damage in terrestrial ecosystem models to accurately predict trends in vegetation productivity and carbon sequestration in the Arctic and sub-Arctic.
Highlights
The frequency of extreme winter warming events is increasing in the Arctic (Vikhamar-Schuler et al 2016, Graham et al 2017), and these episodes are capable of causing widespread and severe plant damage (Bjerke et al 2017)
Impact of the 2013/2014 winter on summer CO2 exchange This study shows that the severe frost drought event of January and February 2014, unprecedented in the climate record on Andøya, led to the strong dieback of the shrub species Calluna vulgaris and Empetrum nigrum
A period of colder weather may have contributed to this delayed response, this pattern remained present when GPP rates at light saturation (GPPsat) was compared to accumulated degree days
Summary
The frequency of extreme winter warming events is increasing in the Arctic (Vikhamar-Schuler et al 2016, Graham et al 2017), and these episodes are capable of causing widespread and severe plant damage (Bjerke et al 2017). A partial melt and re-freeze of snow is damaging due to the formation of thick, hermetic ground ice (Bjerke et al 2015, Milner et al 2016) These extreme winter events may be an important driver of arctic and subarctic browning (Phoenix and Bjerke 2016)—reductions in greenness that have been observed by satellites (Bhatt et al 2013). The recent browning of the Arctic appeared as somewhat of a surprise, since satellite data had shown a greening of the region until recently (Bhatt et al 2014) Field observations connected these past increases in remotely sensed greenness—expressed as NDVI (normalized difference vegetation index)—to an expansion of shrubs that responded to increases in summer warmth (Myers-Smith et al 2011, Elmendorf et al 2012). The widespread vegetation damage indicated by arctic browning implies a reduction in vegetation productivity, and possibly a reduction in the net uptake of CO2 by affected ecosystems
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