Abstract
Vegetation change, permafrost degradation and their interactions affect greenhouse gas fluxes, hydrology and surface energy balance in Arctic ecosystems. The Arctic shows an overall “greening” trend (i.e. increased plant biomass and productivity) attributed to expansion of shrub vegetation. However, Arctic shrub dynamics show strong spatial variability and locally “browning” may be observed. Mechanistic understanding of greening and browning trends is necessary to accurately assess the response of Arctic vegetation to a changing climate. In this context, the Siberian Arctic is an understudied region. Between 2010 and 2019, increased browning (as derived from the MODIS Enhanced Vegetation Index) was observed in the Eastern Siberian Indigirka Lowlands. To support interpretation of local greening and browning dynamics, we quantified changes in land cover and transition probabilities in a representative tundra site in the Indigirka Lowlands using a timeseries of three very high resolution (VHR) (0.5 m) satellite images acquired between 2010 and 2019. Using spatiotemporal Potts model regularization, we substantially reduced classification errors related to optical and phenological inconsistencies in the image material. VHR images show that recent browning was associated with declines in shrub, lichen and tussock vegetation and increases in open water, sedge and especially Sphagnum vegetation. Observed formation and expansion of small open water bodies in shrub dominated vegetation suggests abrupt thaw of ice-rich permafrost. Transitions from open water to sedge and Sphagnum, indicate aquatic succession upon disturbance. The overall shift towards open water and wetland vegetation suggests a wetting trend, likely associated with permafrost degradation. Landsat data confirmed widespread expansion of surface water throughout the Indigirka Lowlands. However, the increase in the area of small water bodies observed in VHR data was not visible in Landsat-derived surface water data, which suggests that VHR data is essential for early detection of small-scale disturbances and associated vegetation change in permafrost ecosystems.
Highlights
The Arctic is warming twice as fast as the global average, resulting in release of greenhouse gases (GHG) from thawing permafrost (Meredith et al, 2019; Schuur et al, 2015) and widespread vegetation change (Pearson et al, 2013)
The Russian Arctic is an underrepresented region in the context of ground studies and high-resolution remote sensing of Arctic shrub expansion in English scientific literature (Bjorkman et al, 2020; Elmendorf et al, 2012; Martin et al, 2017), despite harboring approximately half of the total circumarctic area of shrub and tussock-shrub dominated tundra ecosystems (Raynolds et al, 2019)
Dendrochronological studies in the Russian Arctic have shown a positive response of dwarf shrubs to climate warming (Blok et al, 2011b; Forbes et al, 2010), supported by several remote sensing studies that report increased NDVI and shrub expansion (Forbes et al, 2010; Frost et al, 2014; Lin et al, 2012; Nitze and Grosse, 2016)
Summary
The Arctic is warming twice as fast as the global average, resulting in release of greenhouse gases (GHG) from thawing permafrost (Meredith et al, 2019; Schuur et al, 2015) and widespread vegetation change (Pearson et al, 2013). Shrubs in particular have been observed to expand across the Arctic, resulting in an overall greening trend (i.e. an increase in Normalized Difference Vegetation Index, NDVI) as evident from remote sensing studies (Bhatt et al, 2017; Epstein et al, 2013; Frost et al, 2020; Myers-Smith et al, 2015; Tape et al, 2006). Dendrochronological studies in the Russian Arctic have shown a positive response of dwarf shrubs to climate warming (Blok et al, 2011b; Forbes et al, 2010), supported by several remote sensing studies that report increased NDVI and shrub expansion (Forbes et al, 2010; Frost et al, 2014; Lin et al, 2012; Nitze and Grosse, 2016). Mechanistic understanding of the drivers and impacts of browning is necessary to predict the response of this understudied region to a warming climate
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