Abstract
The Arctic is experiencing an increased frequency of extreme events which can cause landscape-scale vegetation damage. Extreme event-driven damage is an important driver of the decline in vegetation productivity (termed ‘Arctic browning’) which has become an increasingly important component of pan-Arctic vegetation change in recent years. A limited number of studies have demonstrated that event-driven damage can have major impacts on ecosystem CO2 balance, reducing ecosystem carbon sink strength. However, although there are many different extreme events that cause Arctic browning and different ecosystem types that are affected, there is no understanding of how impacts on CO2 fluxes might vary between these, or of whether commonalities in response exist that would simplify incorporation of extreme event-driven Arctic browning into models.To address this, the impacts of different extreme events (frost-drought, extreme winter warming, ground icing and a herbivore insect outbreak) on growing season CO2 fluxes of Net Ecosystem Exchange (NEE), Gross Primary Productivity (GPP) and ecosystem respiration (Reco) were assessed at five sites from the boreal to High Arctic (64°N-79°N) in mainland Norway and Svalbard. Event-driven browning had consistent, major impacts across contrasting sites and event drivers, causing site-level reductions of up to 81% of NEE, 51% of GPP and 37% of Reco. Furthermore, at sites where plot-level NDVI (greenness) data were obtained, strong linear relationships between NDVI and NEE were identified, indicating clear potential for impacts of browning on CO2 balance to be consistently, predictably related to loss of greenness across contrasting types of events and heathland ecosystems.This represents the first attempt to compare the consequences of browning driven by different extreme events on ecosystem CO2 balance, and provides an important step towards a better understanding of how ecosystem CO2 balance will respond to continuing climate change at high latitudes.
Highlights
Rising temperatures in Arctic regions are causing extreme events to occur more frequently (Jentsch et al 2007, Bokhorst et al 2009, Vikhamar-Schuler et al 2016)
There are many different extreme events that cause Arctic browning and different ecosystem types that are affected, there is no understanding of how impacts on CO2 fluxes might vary between these, or of whether commonalities in response exist that would simplify incorporation of extreme event-driven Arctic browning into models
Impacts of browning on C fluxes Significant linear declines in both GPP600 and NEE600 with increasing percentage browned were identified at all sites
Summary
Rising temperatures in Arctic regions are causing extreme events to occur more frequently (Jentsch et al 2007, Bokhorst et al 2009, Vikhamar-Schuler et al 2016). These can cause vegetation damage and mortality at landscape or even regional scales, as has been observed through plotlevel, regional and remote sensing studies (Beniston et al 2011; Bhatt et al 2013, Bjerke et al 2014, 2017, Treharne et al 2020). Ice encapsulation of plants through snow thaw-freeze and rain-on-snow events can cause severe damage to vegetation through a combination of hypoxia, CO2 accumulation and exposure to greater temperature variability (Hansen et al 2014, Milner et al 2016), though some species may be generally tolerant (Preece et al 2012, Preece and Phoenix 2013, 2014). Higher winter temperatures improve over-wintering egg survival and facilitate range expansion, increasing the incidence, intensity and duration of outbreaks (Wolf et al 2008, Johansson et al 2011)
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