P-model v1.0: An optimality-based light use efficiency model for simulating ecosystem gross primary production

Abstract

Abstract. Terrestrial photosynthesis is the basis for vegetation growth and drives the land carbon cycle. Accurately simulating gross primary production (GPP, ecosystem-level apparent photosynthesis) is key for satellite monitoring and Earth System Model predictions under climate change. While robust models exist for describing leaf-level photosynthesis, predictions diverge due to uncertain photosynthetic traits and parameters which vary on multiple spatial and temporal scales. Here, we describe and evaluate a gross primary production (GPP, photosynthesis per unit ground area) model, the P-model, that combines the Farquhar-von Caemmerer-Berry model for C3 photosynthesis with an optimality principle for the carbon assimilation-transpiration trade-off, and predicts a multi-day average light use efficiency (LUE) for any climate and C3 vegetation type. The model is forced here with satellite data for the fraction of absorbed photosynthetically active radiation and site-specific meteorological data and is evaluated against GPP estimates from a globally distributed network of ecosystem flux measurements. Although the P-model requires relatively few inputs and prescribed parameters, the R2 for predicted versus observed GPP based on the full model setup is 0.75 (8-day mean, 131 sites) – better than some state-of-the-art satellite data-driven light use efficiency models. The R2 is reduced to 0.69 when not accounting for the reduction in quantum yield at low temperatures and effects of low soil moisture on LUE. The R2 for the P-model-predicted LUE is 0.37 (means by site) and 0.53 (means by vegetation type). The P-model provides a simple but powerful method for predicting – rather than prescribing – light use efficiency and simulating terrestrial photosythesis across a wide range of conditions. The model is available as an R package (rpmodel).

When and where soil is important to modify the carbon and water economy of leaves

“…An improved formulation of the cost factor 'a' can derive from recent studies that formalized the hydraulic cost/risk of delivering water from soil to leaves as an explicit set of equations that can be integrated into existing ESMs (e.g. Wolf et al, 2016 PNAS;Sperry et al, 2017 PCE;Stocker et al, 2020). As such it can include the impact of soil silt content on soil and leaf hydraulic potentials, transpiration rates, the cost of water uptake and transport by plants and, ultimately, the leaf C i /C a ratio (e.g.…”

“…This is why it has been considered to be a value optimized by natural selection (Prentice et al, 2014). The C i /C a ratio is also a pivotal physiological variable for the coupling of transpiration and photosynthesis from vegetated surfaces in relation to climate in Earth System Models (ESMs) (Wang et al, 2014;Stocker et al, 2020). The impact of soil environment on biophysical-biochemical interactions and C i /C a variation is expected to be substantial but has yet to be quantified.…”

An optimality‐based model explains seasonal variation in C3 plant photosynthetic capacity

“…Under natural conditions, however, Φ PSII,max,dark is only relevant when leaves begin photosynthesizing at dawn. Therefore, we replaced Φ PSII,max,dark by the maximum quantum yield of photosystem II for a typical light-adapted leaf Φ PSII,max,light , which is a function of temperature (T; °C; Bernacchi, Pimentel, & Long, 2003;Stocker, Wang, et al, 2019;Wang et al, 2020):…”

“…Although efforts have been made to develop water stress functions for V cmax,25C (Keenan, Sabate, & Gracia, 2010), most are empirical and thus are not incorporated into our mechanistic model to avoid over-tuning. Recent studies have indicated that parameterizing the cost ratio β (Equation 5), which varies over time as a result of varying water potential between soil and leaves, is a potential pathway to incorporate the soil moisture effect into the optimality model (Lavergne et al, 2020;Stocker, Wang, et al, 2019).…”

Strong overestimation of water‐use efficiency responses to rising CO2 in tree‐ring studies

“…f is the fractionation associated with photorespiration (≈12‰) and Γ * is the CO 2 compensation point in the absence of mitochondrial respiration (Bernacchi, Singsaas, Pimentel, Portis, & Long, 2001). Γ * was computed using the rpmodel R package (Stocker et al, 2019) using the average summer maximum temperature, which is representative of the temperature of the photosynthetically active period. Since no climate data were available before 1866 in our study area, 1783-1865 Γ * values were estimated using the average summer maximum temperature of the 1866-1900 period.…”