Abstract

The percentage of respiratory and photorespiratory CO₂ refixed in leaves (P_r) represents part of the CO₂ used in photosynthesis. The importance of P_r as well as differences between species and functional types are still not well investigated. In this study, we examine how P_r differs between six temperate and boreal woody species: Betula pendula, Quercus robur, Larix decidua, Pinus sylvestris, Picea abies, and Vaccinium vitis-idaea. The study covers early and late successional species, deciduous broadleaves, deciduous conifers, evergreen conifers, and evergreen broadleaves. We investigated whether some species or functional types had higher refixation percentages than others, whether leaf traits could predict higher P_r, and whether these traits and their impact on P_r changed during growing seasons. A/Ci-curves, measured early, mid and late season, were used to estimate and compare P_r, mesophyll resistance (r_m), and stomatal resistance (r_s) to CO₂ diffusion. Additionally, light images and transmission electron microscope images were used to approximate the fraction of intercellular airspace and cell wall thickness. We found that evergreens, especially late successional species, refixed a significantly higher amount of CO₂ than the other species throughout the entire growing season. In addition, r_m, r_s and leaf mass per area, traits that typically are higher in evergreen species, were also significantly, positively correlated with P_r. We suggest that this is due to higher r_m decreasing diffusion of (photo) respiratory CO₂ out of the leaf. Cell wall thickness had a positive effect on P_r and r_m, while the fraction of intercellular airspace had no effect. Both were significantly different between evergreen conifers and other types. Our findings suggest that species with a higher r_m use a greater fraction of mitochondria-derived CO₂, especially when stomatal conductance is low. This should be taken into account when modeling the overall CO₂ fertilization effect for terrestrial ecosystems dominated by high-r_m species.

This article appeared on the Tree Physiology website at https://academic.oup.com/treephys/advance-article/doi/10.1093/treephys/tpab016/6140907?login=true