Surface Moisture Budget of Tundra and Boreal Ecosystems in High Latitudes: Variations and Drivers

Abstract:

The trajectory of land surface wetness is one of the most consequential unknowns in the Arctic climate system. Projections of the crucial net surface moisture flux, Precipitation minus Evapotranspiration (P - ET), vary widely among climate models. The present analysis is intended to (1) document the seasonal and interannual variations of the surface moisture fluxes and (2) clarify the drivers of P - ET variations over time in different Arctic vegetative types. The analysis is based on field measurement from sites in boreal and tundra ecosystems of Alaska. The surface moisture budget at boreal sites generally shows a moisture deficit in the late spring and early summer, followed by moisture surpluses from late summer into autumn, consistent with the seasonal increase in precipitation. Tundra sites show a similar but more interannually variable seasonal cycle of P-ET than the boreal sites. The net annual gain (P-ET) is larger at permafrost-free boreal sites than at the tundra and boreal sites underlain by permafrost. Interannual variability of the yearly net moisture gain is large at the boreal sites, ranging from about 3 cm to more than 30 cm at the Poker Flat black spruce supersite in interior Alaska. Multiple analysis methods including factor analysis and random forest models were performed to isolate the main drivers controlling variability in ET and other related variables. The primary drivers of variations in evapotranspiration shared between the boreal and tundra sites are radiative fluxes and sensible heat flux. Boreal sites also have a large dependency on relative humidity where tundra sites have a large dependency on air temperature and ground heat flux. These differences imply tundra sites are more temperature limited and boreal sites are more humidity limited.