The 3D-CMCC-FEM computes the Autotrophic Respiration (AR) mechanistically, as the the sum of maintenance respiration (MR) of all living tissues and daily growth (GR) - the so called 'growth-and-maintenance-respiration paradigm (GMRP)' (Amthor 2000) - on daily time step.
MR is computed daily through the standard Arrhenius Q10 relationship modified to take into account short- and long-term acclimation responses (i.e. ‘Type I’ and ‘Type II’ acclimation). The formulations included into the model to consider acclimation on maintenance respiration comprise the Q10 modification that more closely matches both the instantaneous and the longer-time responses of maintenance respiration. Maintenance respiration rate is related also to the tissue nitrogen content within each ‘live’ structural pool. The C:N stoichiometry is constant across ages and depends on species.
Type-I or short-term acclimation. Comparison of Q10 respiration rate function at varying temperature with no modification (fixed Q10 ) Rno_ Tx (Q10 = 2.0) versus modified (variable Q10) RT x (Q10 = 3.22–0.046Tx as in Smith & Dukes, 2012; for both Rrefx = 0.218 gC·gN ·day, Tref = 20 °C, Collalti et al., 2016). (b) Type-II or long-term acclimation. Comparison of daily leaf temperature respiration responses including only Type I (RT, blue line) and Type-I + Type-II acclimation (RMaccl , red line). Temperature refers to 10-days average temperature (Collalti et al. 2018, James).
Growth respiration (GR) is considered varying with tree age from 0.30 to 0.25 (30 to 25%) of the new daily C allocated to each pool (excluding non-structural carbohydrates, NSC, i.e. labile carbon and starch, undistinguished) and it does not change with climate cause there is little evidence that the specific costs of tissue biosynthesis is temperature dependent. An in between value (28%). When AR exceeds GPP, respiration is fuelled by NSC.
Overall at the end of the day AR = MR + GR.
Model results for autotrophic respiration (R, gC m−2 year−1) performed with varying τ (coloured lines). The beginning of simulations correspond to 1950 (stand age 30 years); the end of simulations correspond to 2100 (stand
age 180 years). The dark‐pointed red line can be considered as a mechanistic representation of Waring et al.'s (1998) fixed NPP:GPP ratio (τ = 1 year–1), while the dark pink line approximates Reich et al.'s (2006) scaling relationship between R and biomass (τ = 0.1 year–1). Orange dotted lines represent Amthor's (2000) (A00) ‘allowable’ range for the NPP:GPP ratio (0.65–0.2). The red dots give the average measured values (Wu et al., 2013) at the site for R. Vertical bars represent the standard deviation with horizontal bars representing the period 2006–2010 (stand age~85–90 years). The shaded area represents the overall uncertainty of model results (Collalti et al. 2020, GCB).