Shifted energy fluxes, increased Bowen ratios, and reduced thaw depths linked with drainage-induced changes in permafrost ecosystem structure

Abstract

Abstract. Hydrologic conditions are a key factor in Arctic ecosystems, with strong influences on ecosystem structure and related effects on biogeophysical and biogeochemical processes. With systematic changes in water availability expected for large parts of the northern high-latitude region in the coming centuries, knowledge on shifts in ecosystem functionality triggered by altered water levels is crucial for reducing uncertainties in climate change predictions. Here, we present findings from paired ecosystem observations in northeast Siberia comprising a drained and a control site. At the drainage site, the water table has been artificially lowered by up to 30 cm in summer for more than a decade. This sustained primary disturbance in hydrologic conditions has triggered a suite of secondary shifts in ecosystem properties, including vegetation community structure, snow cover dynamics, and radiation budget, all of which influence the net effects of drainage. Reduced thermal conductivity in dry organic soils was identified as the dominating drainage effect on energy budget and soil thermal regime. Through this effect, reduced heat transfer into deeper soil layers leads to shallower thaw depths, initially leading to a stabilization of organic permafrost soils, while the long-term effects on permafrost temperature trends still need to be assessed. At the same time, more energy is transferred back into the atmosphere as sensible heat in the drained area, which may trigger a warming of the lower atmospheric surface layer.

Answer to Anonymous Referee #2

“…Field work was conducted at an observation site within the floodplain of the Kolyma River (68.78 • N, 161.33 • E, 6 m above 25 sea level) situated about 15km south of the town of Chersky in Northeast Siberia (Kittler et al, 2016;Kwon et al, 2017). The site is classified as wet tussock tundra underlain by continuous permafrost, with very flat topography.…”

“…Both programs were running on-site on a personal computer, using the local time zone (Magadan time, UTC +12 h). The mean local solar noon is UTC + 13 h. Within the context of this study, datasets within the period 1 st June to 15 th September 2014 were 10 analysed.…”

“…If a positive peak was followed or preceded by a negative one or vice versa, both were combined into a single "peak event" as long as the magnitude of the second peak was lower than one quarter that of the main peak. 10 We termed the second event class down-up events. Down-up events had the same basic properties as single peak events, but in contrast they consisted of one sharp negative and positive peak each, which were of similar magnitude ( Fig.…”

“…For the stability of atmospheric stratification (zL −1 , with z as measurement height and L as Obukhov length), no general pattern for the conditions before, during and after a peak event could be found. In 43 % of all events (15/35) Summarising, since the majority of events were detected during the night, it could be expected that a large number of cases 10 would be subject to systematically falling temperatures, and associated increases in relative humidity. On the other hand, the high percentage of peak events that are characterised by an increase and subsequent decrease in wind speed and friction velocity indicates that turbulence intensity in the atmospheric surface layer is a major influence factor.…”

“…be detected at smaller scales with soil chambers (e.g. Kwon et al, 2017), usually are too small as individual emissions, or not coordinated enough spatially across the relatively large footprint area (approx. 4000 m 2 at neutral 30 stratification) to be detected by an eddy-covariance system.…”