Extreme climatic events are among the drivers of recent declines in plant biomass
and productivity observed across Arctic ecosystems, known as “Arctic browning.”
These events can cause landscape‐scale vegetation damage and so are likely to have
major impacts on ecosystem CO2 balance. However, there is little understanding of
the impacts on CO2 fluxes, especially across the growing season. Furthermore, while
widespread shoot mortality is commonly observed with browning events, recent
observations show that shoot stress responses are also common, and manifest as
high levels of persistent anthocyanin pigmentation. Whether or how this response
impacts ecosystem CO2 fluxes is not known. To address these research needs, a
growing season assessment of browning impacts following frost drought and
extreme winter warming (both extreme climatic events) on the key ecosystem CO2
fluxes Net Ecosystem Exchange (NEE), Gross Primary Productivity (GPP), ecosystem
respiration (Reco) and soil respiration (Rsoil) was carried out in widespread sub‐Arctic
dwarf shrub heathland, incorporating both mortality and stress responses. Browning
(mortality and stress responses combined) caused considerable site‐level reductions
in GPP and NEE (of up to 44%), with greatest impacts occurring at early and late
season. Furthermore, impacts on CO2 fluxes associated with stress often equalled or
exceeded those resulting from vegetation mortality. This demonstrates that extreme
events can have major impacts on ecosystem CO2 balance, considerably reducing
the carbon sink capacity of the ecosystem, even where vegetation is not killed.
Structural Equation Modelling and additional measurements, including decomposition
rates and leaf respiration, provided further insight into mechanisms underlying
impacts of mortality and stress on CO2 fluxes. The scale of reductions in ecosystem
CO2 uptake highlights the need for a process‐based understanding of Arctic browning
in order to predict how vegetation and CO2 balance will respond to continuing
climate change.
arctic, browning, Calluna vulgaris, climate change, dwarf shrub, extreme events, snow cover,
stress, winter
