Canopy structure affects temperature distributions and free convection in moss shoot systems

Nonvascular plants play important roles in exchange of water and heat at the soil-atmosphere interface. Differential evaporative cooling may cause temperature gradients within bryophyte canopies, influencing convective heat and mass transport. Understanding mechanisms that affect fluxes through moss layers should improve models of forest floor function.

A three-dimensional thermal imaging system measured temperature distributions within moss shoot systems that were used to explore relationships among canopy structure, temperature gradients, evaporation, and conductance to water vapor (g_s ). We studied five moss species under dark and light conditions in the lab. Also, these properties were measured in two species that differed in canopy structure during drying.

Differential evaporative cooling led to a 1.4 to 5.0°C range in shoot temperatures within canopies. Samples displayed -0.5 to -0.9°C/cm temperature gradients with cooler apical temperatures. Gradient magnitudes did not differ among species, but taller canopies expressed greater temperature differences. Light enhanced both the gradient and the temperature difference. Rates of evaporation were significantly related to canopy height in the light, but not in the dark, although g_s was positively associated with canopy height in both. Rayleigh (Ra) numbers characterize whether temperature gradients likely generate free convection. In tall canopies, Ra numbers exceeded the value indicative of free convection and turbulent flow. As plants dried, temperature gradients decreased.

When moss canopies are wet, cooler apical temperatures create thermal instabilities within the canopies that appear sufficient to enhance convective transport of water vapor and heat in tall canopies with low bulk density.