In most bryophytes, the thickness of boundary layers (i.e., unstirred layers) that surrounds plant surfaces governs rates of water loss. Since bryophytes lack short-term control of this process, architectural features of canopies that influence boundary layer thickness affect the water balance of bryophytes. Using field samples (9.5 cm diameter cushions) from 12 species (n=3 per species) of mosses and liverworts, we evaluated the relationship between canopy structure and boundary layer properties. Canopy structure was characterized using a contact surface probe that measures canopy depth at spatial scales ranging from 0.08 to 3 cm at 186 points per sample. Variance in depth measurements at different spatial scales was used to estimate surface roughness (SR). Boundary layer properties were measured by evaporation of ethanol from samples in a laminar flow wind tunnel (0.8 to 3.5 m s-1) and applied to characterize mass transfer of cushions using principles of dynamic similarity (i.e., using dimensionless representations of conductance and flow). All bryophyte cushions exhibited the characteristics of turbulent as opposed to laminar boundary layers that results from the generation of turbulent flow over rough bryophyte surfaces. Bryophyte canopies with higher SR (at 0.8 cm sample scale) had greater conductances at all windspeeds. However when compared to species with high SR, species with low SR had proportionally greater increases in conductance at higher windspeeds, a pattern associated with increased turbulence. Thus, canopies with low surface roughness retain laminar boundary layers at low windspeeds, and maintain low rates of evaporation. However, at high windspeeds these species experience greater turbulence causing increased evaporation rates which approach those found in species with high SR.

Key words: boundary layer, bryophyte, conductance, functional morphology, mass transfer, surface roughness