Variations in evapotranspiration fluxes across geomorphological units and plant functional types in a polygonal-structure Tundra in Barrow, Alaska

Abstract:

Although the landscape in tundra ecosystems is relatively flat, and the vegetation is typically shorter than 10 cm, micro-topographical changes within the polygonal structure produce spatial heterogeneity in the form of permafrost depth, soil temperature, soil moisture, and wind speed. Plants react to these conditions and form linkages with the landscape. For example, mosses occupy the wet troughs and lichens are more abundant in the drier high-centred polygons.

We conducted measurements in a polygonal-structure tundra site at Barrow, Alaska, to investigate the interconnections between evapotranspiration fluxes, geomorphology and plant cover, during two consecutive years. Fluxes were measured at three spatial and temporal scales: (1) Eddy covariance flux tower, (2) Continuous, fixed, surface clear chamber, and (3) Discontinuous measurements with mobile chambers in approximately 60 locations across the landscape.

Our results indicate that different environmental conditions (soil moisture, soil temperature, wind speed, and thaw depth) and plant community composition, driven by microtopographical features, have significant influences on soil greenhouse gas and energy fluxes. Among plant types, evapotranspiration fluxes from moss-covered and inundated areas were more than twice those from other plant types. Continuous chamber measurements were similar in trend and values to eddy-covariance measurements, implying on the high contribution of surface fluxes to atmospheric concentrations. However, wind direction influenced the upscaling of fluxes from chamber to tower, because maritime winds had different moisture content and temperature than terrestrial winds. Microclimate was also affected by microtopography, and wind speed was higher on polygon ridges, and lower in the more protected trough areas, affecting evapotranspiration fluxes. In addition, we observed a strong seasonal trend in fluxes. During peak summer, although 24-hour daylight occurs, our results indicated substantial diurnal variations, despite constant daylight conditions.

Information gathered in this research has advance our understanding of coupled processes in Arctic terrestrial ecosystems, and will be used to improve climate model predictions for this already rapidly changing ecosystem.