Modeling the impacts of diffuse light fraction on photosynthesis in ORCHIDEE (v5453) land surface model

Yuan Zhang,Georg Wohlfahrt,M Altaf Arain,Ivan Mammarella,Philippe Ciais,Will Woodgate,Xiuzhi Chen,Olivier Roupsard,Lukas Siebicke,Tomomichi Kato,Nicolas Vuichard,Christof Ammann,Fabienne Maignan,Alessandro Cescatti,Leonardo Montagnani,Ana Bastos,Marek Urbaniak,T Andrew Black,Laurent Li,Francesco Primo Vaccari,Bogdan H Chojnicki ,Daniel S Goll ,María José Sanz ,Oliviér Boucher

doi:10.5194/gmd-13-5401-2020

Abstract

Abstract. Aerosol- and cloud-induced changes in diffuse light have important impacts on the global land carbon cycle, as they alter light distribution and photosynthesis in vegetation canopies. However, this effect remains poorly represented or evaluated in current land surface models. Here, we add a light partitioning module and a new canopy light transmission module to the ORCHIDEE (Organising Carbon and Hydrology In Dynamic Ecosystems) land surface model (trunk version, v5453) and use the revised model, ORCHIDEE_DF, to estimate the fraction of diffuse light and its effect on gross primary production (GPP) in a multilayer canopy. We evaluate the new parameterizations using flux observations from 159 eddy covariance sites over the globe. Our results show that, compared with the original model, ORCHIDEE_DF improves the GPP simulation under sunny conditions and captures the observed higher photosynthesis under cloudier conditions in most plant functional types (PFTs). Our results also indicate that the larger GPP under cloudy conditions compared with sunny conditions is mainly driven by increased diffuse light in the morning and in the afternoon as well as by a decreased vapor pressure deficit (VPD) and decreased air temperature at midday. The observations show that the strongest positive effects of diffuse light on photosynthesis are found in the range from 5 to 20 ∘C and at a VPD < 1 kPa. This effect is found to decrease when the VPD becomes too large or the temperature falls outside of the abovementioned range, which is likely due to the increasing stomatal resistance to leaf CO2 uptake. ORCHIDEE_DF underestimates the diffuse light effect at low temperature in all PFTs and overestimates this effect at high temperature and at a high VPD in grasslands and croplands. The new model has the potential to better investigate the impact of large-scale aerosol changes and long-term changes in cloudiness on the terrestrial carbon budget, both in the historical period and in the context of future air quality policies and/or climate engineering.

Highlights

Process-based land surface models (LSMs), which simulate the water and energy balance as well as biogeochemical processes on land, have been widely used to attribute past changes in carbon (C) fluxes (Piao et al, 2009; Sitch et al, 2015) and to project the future land C budget (Ciais et al, 2013)
We introduce a modified version of the LSM ORCHIDEE (Organising Carbon and Hydrology In Dynamic Ecosystems; Krinner et al, 2005), referred to as ORCHIDEE_DF, which uses a semiempirical method to calculate the fraction of diffuse light (Weiss and Norman, 1985) as well as a process-based multilayer canopy light transmission model to simulate the effects of diffuse light fraction on photosynthesis (Spitters, 1986)
We evaluated the gross primary production (GPP) simulated by ORCHIDEE_DF and the same version of the ORCHIDEE code without diffuse light using observations collected from 159 eddy covariance flux sites over 11 plant functional types (PFTs; Baldocchi et al, 2001)

Summary

Introduction

Process-based land surface models (LSMs), which simulate the water and energy balance as well as biogeochemical processes on land, have been widely used to attribute past changes in carbon (C) fluxes (Piao et al, 2009; Sitch et al, 2015) and to project the future land C budget (Ciais et al, 2013). In situ observations have found that, under the same light level, the increase in the diffuse light fraction can enhance light use efficiency and photosynthesis or gross primary production (GPP; Gu et al, 2003; Niyogi et al, 2004; Misson et al, 2005; Alton, 2007a; Knohl and Baldocchi, 2008; Mercado et al 2009; Oliphant et al, 2011; Kanniah et al, 2013; Williams et al, 2014; Cheng et al, 2015; Wang et al, 2018). The plant LAI (leaf area index – the area of leaves per unit land area) maximum may become acclimated to the cloudier seasons, which contributes to higher GPP (Williams et al, 2016)

Results

Discussion

Conclusion