Improved representation of plant physiology in the JULES-vn5.6 land surface model: photosynthesis, stomatal conductance and thermal acclimation
Authors: Oliver Rebecca J. , Mercado Lina M. , Clark Doug B. , Huntingford Chris , Taylor Christopher M. , Vidale Pier Luigi , McGuire Patrick C. , Todt Markus , Folwell Sonja , Shamsudheen Semeena Valiyaveetil , Medlyn Belinda E. 
Published: July 2022

Abstract. Carbon and water cycle dynamics of vegetation are controlled primarily by
photosynthesis and stomatal conductance (gs). Our goal is to improve the
representation of these key physiological processes within the JULES land
surface model, with a particular focus on refining the temperature
sensitivity of photosynthesis, impacting modelled carbon, energy and water
fluxes. We test (1) an implementation of the Farquhar et al. (1980) photosynthesis scheme and associated plant functional type-dependent
photosynthetic temperature response functions, (2) the optimality-based
gs scheme from Medlyn et al. (2011) and (3) the
Kattge and Knorr (2007) photosynthetic capacity thermal
acclimation scheme. New parameters for each model configuration are adopted
from recent large observational datasets that synthesise global experimental
data. These developments to JULES incorporate current physiological
understanding of vegetation behaviour into the model and enable users to
derive direct links between model parameters and ongoing measurement
campaigns that refine such parameter values. Replacement of the original
Collatz et al. (1991) C3 photosynthesis model with the Farquhar scheme
results in large changes in GPP for the current day, with ∼ 10 %
reduction in seasonal (June–August, JJA, and December–February, DJF) mean GPP
in tropical forests and ∼ 20 % increase in the northern
high-latitude forests in JJA. The optimality-based gs model decreases
the latent heat flux for the present day (∼ 10 %, with an
associated increase in sensible heat flux) across regions dominated by
needleleaf evergreen forest in the Northern Hemisphere summer. Thermal
acclimation of photosynthesis coupled with the Medlyn gs scheme reduced
tropical forest GPP by up to 5 % and increased GPP in the high-northern-latitude forests by between 2 % and 5 %. Evaluation of simulated carbon and
water fluxes by each model configuration against global data products shows
this latter configuration generates improvements in these key areas. Thermal
acclimation of photosynthesis coupled with the Medlyn gs scheme improved
modelled carbon fluxes in tropical and high-northern-latitude forests in
JJA and improved the simulation of evapotranspiration across much of the
Northern Hemisphere in JJA. Having established good model performance for
the contemporary period, we force this new version of JULES offline with a
future climate scenario corresponding to rising atmospheric greenhouse gases
(Shared Socioeconomic Pathway (SSP5), Representative Concentration Pathway 8.5 (RCP8.5)). In particular, these calculations allow for understanding of the effects of long-term warming. We find that the impact of thermal acclimation
coupled with the optimality-based gs model on simulated fluxes increases
latent heat flux (+50 %) by the year 2050 compared to the JULES model
configuration without acclimation. This new JULES configuration also
projects increased GPP across tropical (+10 %) and northern-latitude
regions (+30 %) by 2050. We conclude that thermal acclimation of
photosynthesis with the Farquhar photosynthesis scheme and the new
optimality-based gs scheme together improve the simulation of carbon and
water fluxes for the current day and have a large impact on modelled future
carbon cycle dynamics in a warming world.