Seasonal trends in energy, water, and carbon dioxide fluxes at a northern boreal wetland

Abstract

Micrometeorological measurements were made over a northern boreal fen near Thompson, Manitoba, Canada, as part of the Boreal Ecosystem-Atmosphere Study. The measurement period extended from the start of snowmelt until the early fall, at which time senescence was widespread throughout the fen. Data analysis concentrated on identifying seasonal trends in energy, water, and carbon dioxide fluxes and linking them to observed surface cover changes. Albedos (solar and photosynthetically active radiation (PAR)) showed large decreases over the melt period, reaching seasonal lows at the end of melt. Solar albedo increased in the summer in response to vegetation growth on the fen, while PAR albedo remained constant. Incoming and outgoing longwave flux seasonal trends were similar, so seasonal changes in net radiation were driven by the net solar flux. During the spring thaw, the melting of snow and ground ice was equal to about 28% of the daily total net radiation, while the soil heat flux accounted for about 5%. Bowen ratios at this time were above unity. Mean Bowen ratio decreased to 0.70 during the period between spring thaw and leaf-out. As the vascular vegetation cover developed, Bowen ratios decreased to seasonal lows of 0.10-0.20 near midsummer and then increased to above unity during senescence. The daily evaporative fraction (EF) was highest (≥0.80) during midsummer when the vascular vegetation was in full leaf and actively photosynthesizing, and EF decreased to a mean of 0.55 during senescence. Eddy correlation measurements of carbon dioxide flux showed the fen acting as a net sink for CO2 only while the vascular vegetation was actively photosynthesizing with a daily mean flux of -0.81 g CO2-C m-2 d-1 (standard error = 0.16). Before leafing and during senescence the fen was a net source of CO2. Integrated over the study period of 124 days, the fen experienced a net loss of 30.4 g CO2 m-2 to the atmosphere.