RICE, STEVEN K.* AND DAVID COLLINS. Department of Biological Sciences, Union College, Schenectady, NY 12308. - Living within boundary layers: the functional significance of variation in bryophyte canopy structure.
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