Abstract

Stomata play a central role in regulating the exchange of carbon and water vapor between ecosystems and the atmosphere. Their function is represented by land surface models (LSMs) by conductance models. The Functionally Assembled Terrestrial Ecosystem Simulator (FATES) is a dynamic vegetation demography model that can simulate both detailed plant demographic and ecophysiological dynamics. To evaluate the effect of stomatal conductance model representation on forest water and carbon fluxes in FATES, we implemented an optimality-based stomatal conductance model—the Medlyn (MED) model, that simulates the relationship between photosynthesis (A) and stomatal conductance to water vapor (gsw) as an alternative to the FATES default Ball-Woodrow-Berry (BWB) model. To evaluate how the behavior of FATES is affected by stomatal model choice, we conducted a model sensitivity analysis to explore the response of gsw to synthetic climate forcing variables including atmospheric CO2 concentration, air temperature, radiation, and vapor pressure deficit (VPD). We found that modeled gsw values varied greatly between the BWB and MED formulations due to the different default stomatal slope parameters (g1). After harmonizing g1 and holding the same stomatal intercept parameter (g0) for both model formulations, we found that the divergence in modeled gsw was limited to conditions when the VPD exceeded 1.5 kPa. We then evaluated model simulation results against measurements from a wet evergreen forest in Panama. Results showed that both the MED and BWB model formulations were able to capture the magnitude and diurnal change of measured gsw and A but underestimated both by about 30 % when the soil was predicted to be very dry. Our study suggests that the parameterization of stomatal conductance models and current model response to drought are the critical areas for improving model simulation of CO2 and water fluxes in tropical forests.

Highlights

  • 30 Global climate change has resulted in significant modifications to Earth’s ecosystems through changing weather patterns, an increased frequency and severity of extreme drought, floods, and heatwaves, which has resulted in increased risk for terrestrial vegetation (Pachauri et al, 2014; Reichstein et al, 2013; Gatti et al, 2021)

  • 80 In this study, we explored the impact of stomatal behavior under simulated and realistic environmental conditions in the Functionally Assembled Terrestrial Ecosystem Simulator (FATES) model (Koven et al, 2020)

  • Using FATES and the MED and BWB representations 90 we addressed two main questions: (1) How do projected leaf-level and canopy-level CO2 and water vapor fluxes differ between the BWB and MED formulations in response to key meteorological forcing variables? (2) How do the two model outputs of stomatal conductance and photosynthesis compare to leaf-level gas exchange data collected through a dry season in a tropical forest?

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Summary

Introduction

30 Global climate change has resulted in significant modifications to Earth’s ecosystems through changing weather patterns, an increased frequency and severity of extreme drought, floods, and heatwaves, which has resulted in increased risk for terrestrial vegetation (Pachauri et al, 2014; Reichstein et al, 2013; Gatti et al, 2021). The exchange of water vapor and carbon dioxide between plants and the atmosphere is dominated by transport through stomata (Hetherington and Woodward, 2003; Kala et al, 2016). 35 biochemical and biophysical processes that are currently not represented in land surface models (LSMs) (Lawson et al, 2014; Buckley and Mott, 2013; Blatt, 2000; Davies et al, 2002). A range of much simpler, largely empirical, formulations that describe the responses of stomata to their environment have been successfully used by LSMs for many years. Most of them require only two parameters i.e. the intercept parameter (g0), which is the conductance when photosynthesis (A) is zero, and slope parameter (g1) that describe the relationship between stomatal

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