Governing the exchange between cellular physiological processes and the surrounding atmospheric environment, terrestrial bryophyte canopies control fluxes of energy (e.g., light, heat) and mass (most importantly H2O, CO2, CH4). Measurements of mass transfer in wind-tunnels from small (10 cm diameter) cushions have demonstrated that unit increases in canopy roughness (i.e., average canopy depth) are related to exponential rates of mass transfer with a scaling factor n > 1.5. This suggests a potentially high physiological cost from water loss associated with increases in canopy depth, even though many species exhibit a rough-canopy growth form. We explored the magnitude of the scaling factor under field conditions using 14 cm diameter in situ cushions from forest floor bryophytes in mesic, temperate forests in New York state. By embedding the cushions within continuous carpets, exchange was influenced by length scales associated with substrate features in addition to intrinsic canopy properties. Mass transfer was determined gravimetrically in 1 hr sampling intervals. During sampling, wind velocity, humidity, and surface and air temperature were continuously monitored (10 s intervals) and recorded using a data logger. Under field conditions, scaling factors were much diminished (n = 0.7) and indicate a lower physiological cost than previously suggested in wind-tunnel studies where cushions experience high levels of through-flow initiated at cushion margins. Results from this study are used to parameterize a canopy exchange model for bryophytes that explores the physiological trade-offs associated with canopy structural variation and its influences on light penetration, heat loss, CO2 uptake, and nutrient transport in evaporation streams.

Key words: boundary layer, bryophyte, evaporation, mass transfer