In bryophytes, rates of net photosynthesis vary among populations. How this variation is shaped by shoot biochemical or structural traits has not been established, yet would be essential to develop useful models of forest floor function in the boreal zone. The objectives of this study were to characterize functional trait relationships in the widespread feathermoss Pleurozium schreberi, to develop an empirical model to predict net photosynthesis in this species, and to compare the performance of a surface model that incorporates the shoot system's average properties with a canopy model that accounts for the vertical distribution of light and shoot area. Maximal rates of net photosynthesis (Amax) and dark respiration (Rd) were measured (n = 25) using gas exchange at optimal water contents. Shoot system concentrations of chlorophyll (a b), carotenoids and nitrogen were measured in addition to the water content, surface roughness (Lr), canopy height, and the vertical distribution of shoot area and light within the canopy. The light extinction coefficient and transmission parameters were estimated from the latter. Amax ranged from 2.20 to 7.78 µmol CO2 m−2 s−1 with a mean value of 4.97. A linear multiple regression model using shoot area index (SAI) and chlorophyll concentration explained 55% of variation in Amax, and no other factors associated significantly with the residuals. The Monsi-Saeki canopy model was also fit to the data, which explained only 33% of variation in Amax. Residuals were related to Lr, and the full model improved to explain 53% of the variation. Given senescence and acclimation of shoots within the canopy, a more refined model will be necessary to add predictive power to the canopy model. Unlike vascular plants, the canopy models are not likely to be improved by considering the allocation of nitrogen because it does not associate with photosynthetic characteristics as it does in vascular plant leaves.
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Vol. 114 • No. 2