Controls on moss evaporation in a boreal black spruce forest

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Abstract
Mosses are an important component of the boreal forest, but little is known about their contribution to ecosystem carbon, water, and energy exchange. We studied the role of mosses in boreal forest evapotranspiration by conducting two experiments in a black spruce forest in Fairbanks, Alaska. Moss evaporation was measured using lysimeters filled with Hylocomium splendens or Sphagnum capillifolium. Microclimate and moisture content were varied by placing the lysimeters in different habitats (dense forest, open forest, bog), and by manipulating the water supply (no water, natural rainfall, water added). Moss evaporation rates between 1 June and 8 September averaged 0.3, 0.9, and 1.5 mm day−1 in the dense forest (Hylocomium), open forest (Hylocomium and Sphagnum), and bog (Sphagnum) respectively. Assuming a total forest evapotranspiration rate of 2 mm day−1, this study shows that moss evaporation contributes considerably to boreal black spruce forest evapotranspiration. Moss evaporation rates depended strongly on the openness of the forest and to a lesser degree on the density of the vascular plant canopy and on moss species. The strong influence of habitat suggests that microclimate is the primary factor determining moss evaporation rates. Hylocomium evaporation reacted strongly to experimental water additions, indicating that precipitation frequency is an important factor in addition to microclimate for this species. The large moss evaporation rates in this study suggest a potential cooling effect of mosses, of Sphagnum in particular.