Union College '03
Mosses are non-vascular plants that lack the ability to control water loss through stomata. Instead, the dimensions of their canopy influences the thickness of the boundary layer above them (i.e. the unstirred layer adjacent to the plant surface) and controls evaporation rates. To explore how canopy size, shape and surface roughness affects evaporation in mosses, boundary layer properties were evaluated in fourteen 4 to 23 cm diameter cushions of Leucobryum glaucum (white cushion moss). Specimens were placed in a laminar flow wind tunnel at seven wind speeds ranging from 0.7 m/s to 4.3 m/s. Evaporation rates of water during 3-16 min intervals were used to estimate conductance to H2O loss (ga, cm/s). Both canopy size and wind speed influenced ga. At a wind speed of 0.8 m/s, ga ranged from 0.04 cm/s to 0.22 cm/s, while at 2.0 m/s ga increased to between 0.07 and 0.68 cm/s. At all wind speeds larger canopies had a smaller ga than smaller canopies. The relationships among moss canopy dimensions and ga in the lab was used to parameterize a whole-plant water balance model. The output of this model was compared to evaporation rates under field conditions. In the field, boundary layers associated with the forest floor reduce the influence of canopy dimensions on evaporation. Therefore, under field conditions, water balance is strongly influenced by the ability of the canopy to store water.