Moss shoot systems present a critical surface for the exchange of matter and energy with the environment. Functional properties of the shoot system are complex traits that arise from the structure and organization of leaves, branches, stems and their interaction. Most studies characterize and model moss function by averaging properties and processes across and into their surface. In effect, the shoot system is treated as a well-equilibrated unit with average properties, similar to the characterization of vascular plant canopies using “big leaf” models. However, properties that control the rate of shoot system carbon and water fluxes are highly variable both horizontally and vertically. Important among these are local incident energy and water contents. These interact to influence rates of evaporation and local temperatures, thereby affecting plant water and carbon dynamics. We hypothesize that differences in shoot structure that alter rates of evaporation will affect canopy thermal gradients and that drying will invert the gradient as dry distal shoots heat up. We have developed a 3D thermal imaging system that combines 3D laser scanning with thermal imaging to measure temperature distributions within moss shoot systems. Under laboratory conditions, we explored thermal gradients in wet and dry samples of the feather mosses Pleurozium schreberi and Rytidiadelphus triquestris.
Results/Conclusions
Species responded differently to the wet—dry condition. In R. triquestris, variation in vertical temperature gradients flipped in direction between the two conditions. When wet, evaporative cooling in distal shoots left canopy interiors relatively warm. However, when dry, distal shoots were the warmest part of the canopy. Such a “thermal inversion” may generate convective currents from the warm, dry canopy top, affecting water loss from moist canopy interiors. This was not the case in P. schreberi, which displayed a more dense shoot structure. Functionality of moss shoot systems arises from the interaction of structural elements of the shoot system together with characteristics that affect shoot water storage and transport. Figuring out how to deal with such complex traits presents a challenge to functional trait analyses in mosses.