Shrub Abundance Impacts on Arctic Surface-Atmosphere Exchanges of Water and Carbon

Vegetation change in the Arctic has been observed in some regions as an increase in deciduous shrub abundance, productivity, and range. These changes will impact surface-atmosphere exchanges of water and carbon with the potential for both negative and positive feedbacks to the climate system. This study presents six seasons of eddy covariance measurements of carbon dioxide (CO2) and latent and sensible heat at three sites with 17, 45, and 64% dwarf birch cover within a few km of each other in Canada's Low Arctic. We used these seasonal fluxes to examine if tundra with more shrub vegetation had i) later spring snowmelt due to greater snow trapping through winter and consequently, reduced growing season atmospheric heating, ii) greater growing season evapotranspiration, and iii) greater productivity and net ecosystem uptake of CO2. Although snow depth was 2 to 3 times greater at the site with greater (and taller) shrub cover, snowmelt was usually very similar among sites. Mid-season latent heat fluxes were also similar among sites. This contrasted with the spring/early summer period when evapotranspiration was low and sensible heat flux was high at the site with greatest shrub cover. Seasonal sensible heat fluxes increased with increasing growing degree days at all sites but was consistently greater at the shrubbiest site. As expected, the greater leaf area at the shrubbiest site resulted in greatest mid-summer net uptake of CO2. But in some years, there was a substantial lag between snowmelt and the period where there was a daily net uptake of CO2. The impacts of shrub abundance on tundra surface-atmosphere exchanges of energy and carbon are likely site specific but these observations highlight the complexity involved in predicting the net climate feedback effect of current and future Arctic vegetation change.