Arctic shrub effects on NDVI, summer albedo and soil shading

Abstract

The influence of Arctic vegetation on albedo, latent and sensible heat fluxes, and active layer thickness is a crucial link between boundary layer climate and permafrost in the context of climate change. Shrubs have been observed to lower the albedo as compared to lichen or graminoid-tundra. Despite its importance, the quantification of the effect of shrubification on summer albedo has not been addressed in much detail. We manipulated shrub density and height in an Arctic dwarf birch (Betula nana) shrub canopy to test the effect on shortwave radiative fluxes and on the normalized difference vegetation index (NDVI), a proxy for vegetation productivity used in satellite-based studies. Additionally, we parametrised and validated the 3D radiative transfer model DART to simulate the amount of solar radiation reflected and transmitted by an Arctic shrub canopy. We compared results of model runs of different complexities to measured data from North-East Siberia. We achieved comparably good results with simple turbid medium approaches, including both leaf and branch optical property media, and detailed object based model parameterisations. It was important to explicitly parameterise branches as they accounted for up to 71% of the total canopy absorption and thus contributed significantly to soil shading. Increasing leaf biomass resulted in a significant increase of the NDVI, decrease of transmitted photosynthetically active radiation, and repartitioning of the absorption of shortwave radiation by the canopy components. However, experimental and modelling results show that canopy broadband nadir reflectance and albedo are not significantly decreasing with increasing shrub biomass. We conclude that the leaf to branch ratio, canopy background, and vegetation type replaced by shrubs need to be considered when predicting feedbacks of shrubification to summer albedo, permafrost thaw, and climate warming. leaf biomass resulted in a significant increase of the NDVI, decrease of transmitted photosynthetically active radiation, and repartitioning of the absorption of shortwave radiation by the canopy components. However, experimental and modelling results show that canopy broadband nadir reflectance and albedo are not significantly decreasing with increasing shrub biomass. We conclude that the leaf to branch ratio, canopy background, and vegetation type replaced by shrubs need to be considered when predicting feedbacks of shrubification to summer albedo, permafrost thaw, and climate warming.

Woody plant encroachment intensifies under climate change across tundra and savanna biomes

“…Woody encroachment can alter ecosystem function and structure, ecosystem services (Wangai, Burkhard, & Müller, 2016), ecohydrology (Huxman et al, 2005) and, ultimately, biome persistence (Moncrieff, Scheiter, Bond, & Higgins, 2014;Scheiter & Higgins, 2009;Skre, Baxter, Crawford, Callaghan, & Fedorkov, 2002). Tundra vegetation regulates surface reflectance and carbon stores (Juszak, Erb, Maximov, & Schaepman-Strub, 2014;Williamson, Barrio, Hik, & Gamon, 2016), with greater release of soil carbon predicted with warming, permafrost thaw and vegetation change (Natali et al, 2011;Schuur et al, 2009).…”

“…Woody encroachment can alter ecosystem function and structure, ecosystem services (Wangai, Burkhard, & Müller, ), ecohydrology (Huxman et al, ) and, ultimately, biome persistence (Moncrieff, Scheiter, Bond, & Higgins, ; Scheiter & Higgins, ; Skre, Baxter, Crawford, Callaghan, & Fedorkov, ). Tundra vegetation regulates surface reflectance and carbon stores (Juszak, Erb, Maximov, & Schaepman‐Strub, ; Williamson, Barrio, Hik, & Gamon, ), with greater release of soil carbon predicted with warming, permafrost thaw and vegetation change (Natali et al, ; Schuur et al, ). Similarly, savannas store large amounts of carbon and are essential in supporting millions of people through ecosystem services, farming and tourism (Lehmann & Parr, ; Scurlock & Hall, ), which could be directly affected by woody encroachment (Table ).…”

Shrub growth and expansion in the Arctic tundra: an assessment of controlling factors using an evidence-based approach

“…The Arctic tundra biome provides essential regulatory effects to global climate, in particular albedo (Juszak et al 2014), storage of organic carbon in its living biomass (Nauta et al 2014), and permafrost dynamics (Blok et al 2010). Over at least the last three decades, changes in vegetation composition have occurred that have significant consequences for the regulatory capability of tundra environments.…”

The role of summer precipitation and summer temperature in establishment and growth of dwarf shrub Betula nana in northeast Siberian tundra

“…The observed shrub expansion could reduce tundra albedo, leading to additional Electronic supplementary material The online version of this article (doi:10.1007/s00300-015-1847-0) contains supplementary material, which is available to authorized users. regional warming (Sturm et al 2005;Blok et al 2011b;Loranty et al 2011;Juszak et al 2014).…”