Calculation of chloroplastic CO2 concentrations
Chloroplastic CO2 concentrations (c c) were calculated based on δ13C signatures of the moss (δ13Cmoss), using equations published by Flanagan & Farquhar (2014), with modifications to account for the lack of stomatal resistance in Sphagnum :
\begin{equation} {\mathrm{}{{}_{\mathrm{\ }}^{\mathrm{13}}\mathrm{C}}_{\mathrm{\text{moss}}}\mathrm{\ =\ 1000\ }\frac{\mathrm{\text{\ \ }}{\mathrm{\delta}_{\mathrm{\ }}^{\mathrm{13}}\mathrm{C}}_{\mathrm{\text{atm}}}\mathrm{-\ }\mathrm{\delta}{{}_{\mathrm{\ }}^{\mathrm{13}}\mathrm{C}}_{\mathrm{\text{moss}}}}{\mathrm{1000\ +\ \delta}{{}_{\mathrm{\ }}^{\mathrm{13}}\mathrm{C}}_{\mathrm{\text{moss}}}}\mathrm{\backslash n}}{\mathrm{}{{}_{\mathrm{\ }}^{\mathrm{13}}\mathrm{C}}_{\mathrm{\text{moss}}}\mathrm{\ =\ }\mathrm{a}_{\mathrm{m}}\left(\frac{\mathrm{c}_{\mathrm{a}\mathrm{\ }}\mathrm{-}\mathrm{\ }\mathrm{c}_{\mathrm{c}}}{\mathrm{c}_{\mathrm{a}}}\right)\mathrm{+}\mathrm{\ }\mathrm{b}\left(\frac{\mathrm{c}_{\mathrm{c}}}{\mathrm{c}_{\mathrm{a}}}\right)\mathrm{-}\mathrm{\text{\ \ }}\mathrm{f}\mathrm{\ }\left(\frac{\mathrm{\Gamma}^{\mathrm{*}}}{\mathrm{c}_{\mathrm{a}}}\right)\backslash n}\nonumber \\ \end{equation}
Here, δ13Catm is the δ13C signature of atmospheric CO2(-8.5‰, Graven et al ., 2017), amthe fractionation during CO2 diffusion through water to the chloroplast (1.8‰, Farquhar et al ., 1989), b the discrimination during carboxylation by Rubisco (29‰, Farquhar et al ., 1989), f the fractionation during photorespiration (16.2‰, Evans & von Caemmerer, 2013) and Γ * the CO2 compensation point (µ mol mol-1) in the absence of dark respiration (calculated from the temperature response: Γ * = 42.7 + 1.68 (T -25) + 0.0012 (T -25)2, whereT is temperature in °C; Brooks & Farquhar, 1985). When applying this equation we assumed, following Evans & von Caemmerer (2013), that no fractionation occurred during day respiration.