Comparison of carbon and water vapor exchange of forest and grassland in permafrost regions, Central Yakutia, Russia

Boreal grasslands have been largely neglected in carbon and water vapor flux models despite being originated by past global climate changes. Therefore in this study, meteorological conditions, water vapor and CO2 fluxes were measured by the eddy correlation technique simultaneously in a larch forest and alas ecosystem (grassland thermokarst depression) in Central Yakutia, eastern Siberia, during the growing season of 2006 (approximately 100 days, May 23rd–August 31st). The alas ecosystem was a carbon sink (−1.38 tC ha−1) but had a 60% lower carbon sequestration capacity than the surrounding larch forest (−3.44 tC ha−1) during the study period. Despite this large difference in carbon exchange, water loss from the alas ecosystem (118 mm) was only 13% lower than that from the forest ecosystem (136 mm). Water vapor flux measured in the alas was higher under similar environmental conditions when the source was the lake water than when the source was the grassland. This supports the theory that lake evaporation contributes significantly to the evaporation from the alas as indicated also by the lake water level constant decrease during the growing season. Mid-summer forest and alas mean evapotranspiration was 1.4 and 1.2 mm d−1 respectively. Mean daily canopy conductance was higher in the forest than in the alas (3.8 and 2.4 mm s−1, respectively) as expected due to differences in canopy architecture at each site. In this study a rough estimate of the NEE of grassland in Central Yakutia shows an underestimation of 0.9 × 10−3 Pg if this area is considered as forested, as most regional models do. Our results suggest that a more detail analysis of distinctive areas within the territory of eastern Siberia is needed in order to obtain a better understanding of carbon and water fluxes from this immense boreal region. Furthermore, if the present global warming evokes landscape change from forest to grassland, the carbon sink capacity of this boreal region could be significantly reduced.

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