Abstract
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 (R eco) and soil respiration (R soil) 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.
Highlights
The Arctic is warming twice as fast as the global average, with the most rapid temperature increases occurring during the winter months (AMAP, 2017; Richter‐Menge, Overland, Mathis, & Osborne, 2017)
We report the first detailed assessment of key ecosystem CO2 fluxes across the growing season in a sub‐Arctic heathland dominated by Calluna vulgaris in northern Norway, quantifying the impacts of both shoot mortality and visible stress following exposure to extreme winter conditions through direct comparison of affected and unaffected vegetation
It was hypothesized that (a) Net Ecosystem Exchange (NEE), Gross Primary Productivity (GPP) and Reco would be reduced both in plots dominated by a visible stress response and, to a greater degree, in those dominated by shoot mortality, compared to green control plots; (b) reductions in these fluxes would diminish throughout the growing season, most rapidly between early and peak season, and more rapidly in plots primarily exhibiting stress compared to those exhibiting shoot mortality; (c) reductions in plot‐level CO2 fluxes would be associated with reduced shoot growth and slower decomposition rates; (d) ecosystem CO2 fluxes would correlate negatively with overall browning throughout the growing season
Summary
The Arctic is warming twice as fast as the global average, with the most rapid temperature increases occurring during the winter months (AMAP, 2017; Richter‐Menge, Overland, Mathis, & Osborne, 2017). The severity of browning from extreme events can be considerable, as demonstrated when multiple extreme events in 2012, including frost drought and extreme winter warming, reduced Normalised Difference Vegetation Index (NDVI, a measure of greenness) to the lowest levels ever recorded across the Nordic Arctic Region (Bjerke et al, 2014) These large‐scale impacts reflect shoot mortality (Bokhorst et al, 2009, 2011), but likely the sub‐lethal stress response indicated by high, persistent anthocyanin pigmentation, that has received limited attention to date (Bjerke et al, 2017). It was hypothesized that (a) NEE, GPP and Reco would be reduced both in plots dominated by a visible stress response and, to a greater degree, in those dominated by shoot mortality, compared to green control plots; (b) reductions in these fluxes would diminish throughout the growing season (due to recovery and resprouting of evergreens and leaf‐out of herbaceous species), most rapidly between early and peak season, and more rapidly in plots primarily exhibiting stress compared to those exhibiting shoot mortality; (c) reductions in plot‐level CO2 fluxes would be associated with reduced shoot growth and slower decomposition rates; (d) ecosystem CO2 fluxes would correlate negatively with overall browning throughout the growing season
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