Arctic browning: Impacts of extreme climatic events on heathland ecosystem CO₂ fluxes

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 (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.

Modelling past and future peatland carbon dynamics across the pan‐Arctic

“…In contrast, it is interesting to note that limited warming diminishes soil moisture in the upper layers through evapotranspiration, but is not sufficient to degrade the underlying permafrost and replenish soil moisture. This results in water stress conditions in some areas leading to vegetation damage and stunted growth affecting overall net productivity (Parmentier et al, 2018; Treharne, Bjerke, Tømmervik, Stendardi, & Phoenix, 2019). This issue needs to be explored further to evaluate the impact on productivity in the employed model tools.…”

A 20-Year MODIS-Based Snow Cover Dataset for Svalbard and Its Link to Phenological Timing and Sea Ice Variability

“…Moreover, a study of maximum NDVI (Normalized Difference Vegetation Index), which is an indicator of vegetation productivity, over central parts of Svalbard from 1986-2015 reported on a general greening trend which was linked to trends of increasing temperature over the study period [57]. However, it was shown that the rate of greening slowed during the second half of the study period, which has subsequently been attributed to more frequent occurrences of extreme winter warming events [58,59], but we do not rule out the possibility that the slower rates of greening since the start of the millennium may also have links to timing of spring snow disappearance, which can be affected by winter warming events through changes in the snowpack accumulation.…”

The Atmospheric Imaging Mission for Northern Regions: AIM-North

“…As permafrost thaws, microbes metabolize carbon and it is estimated that $5-15% (Schuur et al 2015) of the carbon will be released to the atmosphere as CO 2 or CH 4 by $2100, but the timing, spatial distribution and CO 2 :CH 4 ratio of future emissions is highly uncertain (Schneider von Diemling et al 2012;Schuur et al 2015;AMAP 2015a). This uncertainty is coupled with changes to CO 2 uptake related to changes in vegetation density (greening or browning) in the Arctic, which have also been reported as the North warms (Lucht et al 2002, Piao et al 2006Pearson et al 2013;Bhatt et al 2013;Fraser et al 2014;Edwards and Treitz 2017;Treharne et al 2019;USGCRP 2018). It remains important to reduce uncertainties and better understand these feedbacks on future climate change.…”