Abstract:
Photosynthesis is a central process in the terrestrial carbon cycle and a key determinant of
predicting plant responses to climate change. Terrestrial biosphere models (TBMs) commonly
simulate the temperature response of C₃ photosynthesis using the peaked Arrhenius function,
which relies on parameters including Vcmax (maximum rate of Rubisco activity), Jmax (potential
electron transport rate), K25 (process rate at 25 °C), Ea (activation energy), ΔS (entropy), and Hd
(deactivation energy). To minimize overfitting, many models assume a fixed Hd value, though the
consequences of this assumption remain underexplored. We analysed a global dataset of plant
photosynthetic temperature responses to evaluate differences in parameter estimates and
photosynthesis predictions between models with fixed and estimated Hd. Maximum rate of
Rubisco activity at 25°C (K25) and Ea were generally higher with fixed Hd models, with ΔS showing
higher or similar values and strong correlations. Potential electron transport rate at 25°C (K25)
and Ea were also higher with fixed Hd, though Ea varied more, while ΔS showed higher values with
weaker alignment, indicating greater inconsistency, suggesting Jmax is less sensitive to model
structure compared to Vcmax, which exhibits consistent trends across parameters with fixed Hd
assumptions. Percentage decline in modelled photosynthesis between fixed and estimated Hd
assumptions was evaluated at optimum temperature (Topt) and ±5 °C from the optimum. Models
with fixed Hd consistently estimated higher Topt values and lower declines, suggesting that fixed
Hd models may overestimate thermal tolerance. These results highlight that while fixed Hd
assumptions may be acceptable for estimating K25, they risk misrepresenting temperature
responses of Ea, ΔS, and photosynthetic performance under warming. Incorporating variable Hd
improves model flexibility and enhances the realism of TBM projections under climate change.
