Supplementary material to "Reviews and syntheses: Carbon use efficiency from organisms to ecosystems – Definitions, theories, and empirical evidence"

Abstract

Estimating carbon-use and Carbon-storage efficiencies 1.1 Leaves Leaves are responsible for fixing atmospheric CO 2 , thereby representing the entry points of C into terrestrial ecosystems. By measuring net photosynthesis and respiration, CUE at the leaf level can be defined as the ratio of net to gross photosynthetic rates, CUE , (1) where the net photosynthetic rate (, also referred to as net CO 2 assimilation) is the difference between gross photosynthesis and the sum of photorespiration () and mitochondrial respiration (). Photorespiration releases CO 2 , and occurs when the photosynthetic enzyme Rubisco (which fixes CO 2) experiences non-saturating CO 2 conditions in the presence of O 2 , as is the case for most plants in our current atmosphere. We therefore include photorespiration costs in the term gross photosynthesis in Eq. (1), as done in other studies (Way and Sage, 2008), although we will not be able to account for that in our calculations due to data limitations. In leaves, mitochondrial respiration proceeds in both the dark and in the light, although respiration rates are often lower in the light than in the dark. As the phenomenon of light-suppression of respiration is poorly understood and leaf respiration in the light is difficult to measure (Tcherkez et al., 2017), we use dark respiration rates and assume that they represent respiration rates over a 24-hour period. Moreover, photorespiration is neglected in our calculations, because the compensation point was not reported in the dataset we used (Atkin et al., 2015), so that our estimates of leaf CUE are slightly overestimated. 1.2 Individual organisms (autotrophs and heterotrophs) When egestion is neglected, the balance of growth and respiration plus exudation defines the CUE (or GGE, Eq. (6) in the main text) of individual organisms, CUE 1 , (2) where includes all respiration components shown in Eq. (7) in the main text. The equalities in Eq. (2) show how CUE can be estimated from different combinations of observations: net biomass accretion (), C consumption from the resource pool (-organic C for heterotrophs or CO 2 for autotrophs), and respiration rate () (Geyer et al.