Cushion size, surface roughness, and the control of water balance and carbon flux in the cushion moss Leucobryum glaucum (Leucobryaceae)

We explored the size dependence of water balance and carbon flux in the cushion moss Leucobryum glaucum (Leucobryaceae). Conductance to water vapor (ga) was modeled empirically using 4-24 cm diameter cushions (N = 14) evaluated across wind speeds from 0.7 to 4.3 m/s in a wind tunnel. Model parameters included wind speed (u), kinematic viscosity (v), cushion diameter (Ld), and surface roughness (Lr). The model ga = -9.62(u/v)^1.21 · Ld^-0.35 · Lr-in^-1.85 (where Lr-in represents a dimensionless form of Lr; R^2 = 0.88) indicates negative relationships between ga and both Ld and Lr. These predictions were evaluated during a 5-d field experiment where water loss and net carbon exchange (estimated by (delta)F/Fmʹ) were monitored. In the field (N = 18, 4-34 cm diameter cushions), Lr, but not Ld, controlled rates of evaporation due to additional turbulence that reduced size dependence of cushions along the forest floor. However, the duration of positive net carbon gain varied from 1.4 to 4.4 d and was significantly longer in larger diameter cushions. Thus, under field conditions, size-dependent changes in surface-area-to-volume relationships influence the duration of net carbon gain more than differences in water flux and lead to a strong size dependence of water balance and carbon flux.