Surface vegetation controls on evapotranspiration from a sub-humid Western Boreal Plain wetland

Wetlands in the Western Boreal Plain (WBP) of North Central Alberta exist within a moisture-deficit regime where evapotranspiration (ET) is the dominant hydrologic flux. As such these systems are extremely susceptible to the slightest climatic variability that may upset the balance between precipitation (P) and ET. Wetland ET is predominantly controlled by vegetation composition but may also vary due to moisture regimes and microclimatic factors. To address this variability in moisture regimes, ET was examined in a typical moraine-wetland-pond system of the WBP during the 2005 and 2006 snow-free seasons. Closed dynamic chamber measurements were used to gather data on plant community-scale actual evapotranspiration (ET) in an undisturbed natural bog with varying degrees of canopy cover surrounding a shallow groundwater-fed pond. For the purposes of scaling plant community ET contributions to those of the wetland, potential ET (PETEQ) was measured using a Priestley-Taylor energy balance approach at three separate wetland sites with varying aspects surrounding the central pond, along with actual evapotranspiration using a roving eddy covariance (EC) tower. Growing season peak ET rates ranged from 0.2 mm/h to 0·6 mm/h depending on the location, vegetation composition and time period. Sphagnum contributions were the greatest early in the growing season, reaching peaks of 0.6 mm/h, while lichen sites exhibited the greatest late season rates at 0·4 mm/h. Thus, Sphagnum and other nonvascular wetland plant species control ET differently throughout the growing season and as such should be considered an integral part of the moisture and water balances within wetland environments at the sub-landscape unit scale.