Terrestrial biosphere models may overestimate Arctic CO₂ assimilation if they do not account for decreased quantum yield and convexity at low temperature

Abstract

Summary How terrestrial biosphere models (TBMs) represent leaf photosynthesis and its sensitivity to temperature are two critical components of understanding and predicting the response of the Arctic carbon cycle to global change. We measured the effect of temperature on the response of photosynthesis to irradiance in six Arctic plant species and determined the quantum yield of CO2 fixation (ϕnormalCO2) and the convexity factor (θ). We also determined leaf absorptance (α) from measured reflectance to calculate ϕnormalCO2 on an absorbed light basis (ϕCO2.normala) and enabled comparison with nine TBMs. The mean ϕCO2.normala was 0.045 mol CO2 mol−1 absorbed quanta at 25°C and closely agreed with the mean TBM parameterisation (0.044), but as temperature decreased measured ϕCO2.normala diverged from TBMs. At 5°C measured ϕCO2.normala was markedly reduced (0.025) and 60% lower than TBM estimates. The θ also showed a significant reduction between 25°C and 5°C. At 5°C θ was 38% lower than the common model parameterisation of 0.7. These data show that TBMs are not accounting for observed reductions in ϕCO2.normala and θ that can occur at low temperature. Ignoring these reductions in ϕCO2.normala and θ could lead to a marked (45%) overestimation of CO2 assimilation at subsaturating irradiance and low temperature.

Ecosystem photosynthesis in land-surface models: a first-principles approach

“…It reproduces observed variation in Vcmax, Jmax and stomatal conductance for CO2 (gs: Table 1) along environmental gradients (Wang et al, 2017;Bloomfield et al, 2019;Smith et al, 2019;Dong et al, 2020;Wang et al, 2020). It also includes the measured effect of low temperatures on the intrinsic quantum efficiency of photosynthesis (φ0: Table 1) (Singsaas et al, 2001;Rogers et al, 2017Rogers et al, , 2019. The parameters in the P model are either approximately constant and known from independent physiological studies, or estimated from analyses of independent data (c* and β: see Table 1).…”

Ecosystem Photosynthesis in Land‐Surface Models: A First‐Principles Approach Incorporating Acclimation

“…It reproduces observed variation in V cmax , J max, and stomatal conductance for CO 2 ( g s: Table 1) along environmental gradients (Bloomfield et al., 2019; Dong et al., 2020; Smith et al., 2019; Wang et al., 2017, 2020). It also includes the measured effect of low temperatures on the intrinsic quantum efficiency of photosynthesis (φ 0 ( T ): Table 1) (Rogers et al., 2017, 2019; Singsaas et al., 2001). The parameters in the P model are either approximately constant and known from independent physiological studies, or estimated from analyses of independent data ( c* and β: see Table 1).…”

Rapid estimation of photosynthetic leaf traits of tropical plants in diverse environmental conditions using reflectance spectroscopy

“…Before beginning each measurement, a single leaf was placed in the leaf chamber under stable conditions (irradiance, CO 2 , temperature, flow rate) and allowed to acclimate to these conditions for a minimum of 20 minutes. Measurements of the response of photosynthesis to irradiance and carbon dioxide concentration followed an established protocol [ 47 ]. The response to irradiance was measured by sequentially lowering the irradiance as follows: 1800, 1400, 1200, 1000, 800, 600, 400, 300, 200, 120, 80, 50, 30, 20, 10 and finally 0 μmol m -2 s -1 .…”