RICE, STEVEN K.* and SCOTT ROBINSON. Department of Biological Sciences, Union College, Schenectady, NY 12308. - Canopy exchange and functional trade-offs in bryophytes.
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