Contrasting radiation and soil heat fluxes in Arctic shrub and wet sedge tundra

Abstract

<p><strong>Abstract.</strong> Vegetation changes, such as shrub encroachment and wetland expansion, have been observed in many Arctic tundra regions. These changes feed back to permafrost and climate. Permafrost can be protected by soil shading through vegetation as it reduces the amount of solar energy available for thawing. Regional climate can be affected by a reduction in surface albedo as more energy is available for atmospheric and soil heating. Here, we compared the shortwave radiation budget of two common Arctic tundra vegetation types dominated by dwarf shrubs (<i>Betula nana</i>) and wet sedges (<i>Eriophorum angustifolium</i>) in North-East Siberia. We measured time series of the shortwave and longwave radiation budget above the canopy and transmitted radiation below the canopy. Additionally, we quantified soil temperature and heat flux as well as active layer thickness. The mean growing season albedo of dwarf shrubs was 0.15 ± 0.01, for sedges it was higher (0.17 ± 0.02). Dwarf shrub transmittance was 0.36 ± 0.07 on average, and sedge transmittance was 0.28 ± 0.08. The standing dead leaves contributed strongly to the soil shading of wet sedges. Despite a lower albedo and less soil shading, the soil below dwarf shrubs conducted less heat resulting in a 17 cm shallower active layer as compared to sedges. This result was supported by additional, spatially distributed measurements of both vegetation types. Clouds were a major influencing factor for albedo and transmittance, particularly in sedge vegetation. Cloud cover reduced the albedo by 0.01 in dwarf shrubs and by 0.03 in sedges, while transmittance was increased by 0.08 and 0.10 in dwarf shrubs and sedges, respectively. Our results suggest that the observed deeper active layer below wet sedges is not primarily a result of the summer canopy radiation budget. Soil properties, such as soil albedo, moisture, and thermal conductivity, may be more influential, at least in our comparison between dwarf shrub vegetation on relatively dry patches and sedge vegetation with higher soil moisture.</p>

Seasonal changes and vertical distribution of root standing biomass of graminoids and shrubs at a Siberian tundra site

“…S1). We observed that in graminoid vegetation the snowmelt was later than in shrub vegetation (Juszak et al 2016) and soil temperature at 5 cm depth at the time of snowmelt was lower than in shrub vegetation (unpublished data from another study at the same site). The earlier snowmelt and higher soil temperature in the very early growing season in the shrub-dominated vegetation can also be in favor of the earlier shoot and root growth of the shrubs, which might explain the difference in shrub root growth between the vegetation types.…”

Drivers of shortwave radiation fluxes in Arctic tundra across scales

“…While we simulated clear-sky conditions, a higher fraction of diffuse light generally lowers the albedo (Eugster et al, 2000), an effect observed more strongly in wet sedge tundra as compared to dwarf shrub tundra (Juszak et al, 2016). Cloud cover also increases wet sedge transmittance more strongly than dwarf shrub transmittance (Juszak et al, 2016). Therefore changes in irradiance conditions will likely have different effects on the spatial variability of the radiation budget, depending on vegetation type.…”

“…The area is dominated by continuous permafrost of several hundred meters depth (Romanovskii et al, 2004) with an active layer thickness of 12 to 50 cm (Mi et al, 2014). The Kytalyk field site is described in more detail in van der Molen et al (2007); Nauta et al (2015) and Juszak et al (2016). The size of our study area is 1.4 km × 2 km (Figure 1b).…”

Trends in normalized difference vegetation index (NDVI) associated with urban development in northern West Siberia

“…Establishing woody species may revert this process. Thus, although Lloyd et al (2003) found that woody species are more readily established on disturbed microsites with better drainage and drier warmer soils, Juszak et al (2016) showed poor soil thermal conductivity below grown dwarf shrub microsites, which results in thinner active soil layer and worse rooting conditions. Frost et al (2013) studies in our NWS area strongly support the envisaged transformation of the soil conditions.…”