Higher global gross primary productivity under future climate with more advanced representations of photosynthesis

Abstract

Gross primary productivity (GPP) is the key determinant of land carbon uptake, but its representation in terrestrial biosphere models (TBMs) does not reflect our latest physiological understanding. We implemented three empirically well supported but often omitted mechanisms into the TBM CABLE-POP: photosynthetic temperature acclimation, explicit mesophyll conductance, and photosynthetic optimization through redistribution of leaf nitrogen. We used the RCP8.5 climate scenario to conduct factorial model simulations characterizing the individual and combined effects of the three mechanisms on projections of GPP. Simulated global GPP increased more strongly (up to 20% by 2070–2099) in more comprehensive representations of photosynthesis compared to the model lacking the three mechanisms. The experiments revealed non-additive interactions among the mechanisms as combined effects were stronger than the sum of the individual effects. The modeled responses are explained by changes in the photosynthetic sensitivity to temperature and CO 2 caused by the added mechanisms. Our results suggest that current TBMs underestimate GPP responses to future CO 2 and climate conditions.

Variable influence of photosynthetic thermal acclimation on future carbon uptake in Australian wooded ecosystems under climate change

“…Thus, our implementation may have mildly enhanced the thermal acclimation effect under RCP8.5 compared to simulations where J : V is acclimated to growth temperature as supported by Knauer et al. (2023).…”

“…Alternatively, "co-ordination of photosynthesis" influences the CO 2 fertilization effect by affecting the contribution of Vc max limited photosynthesis, which is more sensitive to CO 2 concentration (Haverd et al, 2020). Thus, our implementation may have mildly enhanced the thermal acclimation effect under RCP8.5 compared to simulations where J:V is acclimated to growth temperature as supported by Knauer et al (2023).…”

“…We integrated functions for the acclimation and adaptation of photosynthetic parameters developed by Kumarathunge et al (2019) into the CABLE-POP code (Knauer et al, 2023). These functions adjust parameters of the peaked Arrhenius equation (Equation 1, Johnson et al, 1942) that describes the temperature dependence of J max and Vc max within the Farquhar et al (1980) model of photosynthesis.…”

“…These improvements broaden the functions' application in LSMs and more accurately represent the temperature responses of V c max and J max compared with Kattge and Knorr (2007) according to Smith and Keenan (2020). Despite these improvements, we are unaware of any studies that have incorporated these novel functions in LSMs (except for Knauer et al., 2023 who used the implementation we report herein to conduct global‐scale simulations), and until now they have not been used to model future carbon uptake and storage at regional scales.…”

‘How to adapt forests?’—Exploring the role of leaf trait diversity for long‐term forest biomass under new climate normals

“…Test simulations of LPJmL‐FIT suggest a substantial increase in vegetation carbon (Figure S6), yet we see these projections as unrealistically high. Given the uncertainty surrounding CO 2 fertilization and its unrealistic strong influence on the model, we conducted simulations without it (Knauer et al., 2023; Kovenock et al., 2021). This allows to assess the isolated effects of climate variables such as temperature and precipitation on vegetation without CO 2 fertilization's confounding effects.…”