Structural and Functional Analyses of Bryophyte Canopies

Abstract

Although not often discrete, the canopy (i.e., the organization of branches, shoot systems and their extent) remains the most definable and useful unit of function in bryophytes. Chambers used for gas exchange provide an integrated summary of canopy photosynthetic function. However, other techniques can provide more information on spatial variation in physiological process in both the horizontal and vertical planes. Three examples of such studies are presented here. First, variation in photosystem II (PSII) function has been evaluated, along a canopy surface, using an imaging chlorophyll fluorometer. We evaluated the quantum yield of PSII, ϕPSII, and calculated the relative rate of photosynthetic electron transport (RETR) on 7 cm diameter samples of ten Sphagnum species during drying. Spatial variation in RETR increased both during drying as well as in high light, which led to different relationships between mean RETR and its variation—across light gradients, the relationship was positive, but negative when RETR was reduced by tissue desiccation. Studies of photosynthetic function using chlorophyll fluorescence measurements need to match their sampling protocols to account for this difference. Further, combining a laser scanning approach that provides three-dimensional information on canopy structure with functional imaging allows assessment of function in three dimensions (3D) within the canopy. This is illustrated using a thermal imaging camera to measure temperature distribution within Pleurozium schreberi canopies under still conditions and with wind. This imaging system resolved 9 °C temperature differences within the canopy and localized shoot temperature relative to canopy height. Finally, computational canopy (i.e., virtual) models have been developed for bryophyte canopies, particularly ones with simple branching structure. A model of this type is shown here for the liverwort Bazzania trilobata and a light model implemented using a ray tracing algorithm. Output from this model followed the attenuation of light predicted by the Lambert-Beer Law and such a technique can be used to evaluate how branching architecture and density affect the dynamics of light capture in bryophytes. New approaches based on novel imaging technologies are in rapid development and present opportunities to further our understanding of function within bryophyte canopies.