Forest responses to drought and El Niño with a stomatal optimization model

Comparison between the unified stomatal optimization model (USO) and SOX. Red circles are the model predictions from the throughfall exclusion treatment (TFE) in the Amazon forest (Caxiuanã National Forest), black circles are the control treatment. The dotted lines are derived from linear regressions fitted to the data at high (greater than 10⁻⁴ mol m⁻² s⁻¹) and low (less than 10⁻⁴mol m⁻² s⁻¹) incident photosynthetically active radiation (I_PAR) levels. The dashed line is the 1 : 1 relationship.

Modelling tropical forest responses to drought and El Niño with a stomatal optimization model based on xylem hydraulics

Eller C. B., Rowland L., Oliveira R. S., Bittencourt P. R. L., Barros F. V., da Costa A. C. L., Meir P., Friend A. D., Mencuccini M., Sitch S., Cox P. (2018)

Cleiton B. Eller, Lucy Rowland, Rafael S. Oliveira, Paulo R. L. Bittencourt, Fernanda V. Barros,Antonio C. L. da Costa, Patrick Meir, Andrew D. Friend, Maurizio Mencuccini, Stephen Sitch, Peter Cox,

In Philosoph. Transact. Royal Soc. B, Biol. Sciences – https://doi.org/10.1098/rstb.2017.0315 –

https://royalsocietypublishing.org/doi/full/10.1098/rstb.2017.0315

Abstract

The current generation of dynamic global vegetation models (DGVMs) lacks a mechanistic representation of vegetation responses to soil drought, impairing their ability to accurately predict Earth system responses to future climate scenarios and climatic anomalies, such as El Niño events.

We propose a simple numerical approach to model plant responses to drought coupling stomatal optimality theory and plant hydraulics that can be used in dynamic global vegetation models (DGVMs).

The model is validated against stand-scale forest transpiration (E) observations from a long-term soil drought experiment and used to predict the response of three Amazonian forest sites to climatic anomalies during the twentieth century.

We show that our stomatal optimization model produces realistic stomatal responses to environmental conditions and can accurately simulate how tropical forest E responds to seasonal, and even long-term soil drought.

Our model predicts a stronger cumulative effect of climatic anomalies in Amazon forest sites exposed to soil drought during El Niño years than can be captured by alternative empirical drought representation schemes.

The contrasting responses between our model and empirical drought factors highlight the utility of hydraulically-based stomatal optimization models to represent vegetation responses to drought and climatic anomalies in DGVMs.

Elevated ozone concentrations and stomatal sluggishness

 

 

Technical Note: A simple theoretical model framework to describe plant stomatal sluggishness in response to elevated ozone concentrations

by Huntingford C., Oliver R. J., Mercado L. M., Sitch S. (2018)

Chris Huntingford¹, Rebecca J. Oliver¹, Lina M. Mercado^2,1, and Stephen Sitch²

¹ Centre for Ecology and Hydrology, Benson Lane, Wallingford, Oxfordshire, OX10 8BB, UK
² College of Life and Environmental Sciences, University of Exeter, Amory Building, Rennes Drive, Exeter, EX4 4RJ, UK

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in Biogeosciences discussion paper preprint in review – https://doi.org/10.5194/bg-2018-206 –

https://www.biogeosciences-discuss.net/bg-2018-206/

Abstract. 

Elevated levels of tropospheric Ozone [O₃] causes damage to terrestrial vegetation, affecting leaf stomatal functioning and reducing photosynthesis. Climatic impacts under future raised atmospheric Greenhouse Gas (GHG) concentrations will also impact on the Net Primary Productivity (NPP) of vegetation, which might for instance alter viability of some crops. Together, ozone damage and climate change may adjust the current ability of terrestrial vegetation to offset a significant fraction of carbon dioxide (CO₂) emissions.

Climate impacts on the land surface are well studied, but arguably large-scale modelling of raised surface level [O₃] effects is less advanced. To date most models representing ozone damage use either [O₃] concentration or, more recently, flux-uptake related reduction of stomatal opening, estimating suppressed land-atmosphere water and CO₂ fluxes. However there is evidence that for some species, [O₃] damage can also cause an inertial sluggishness of stomatal response to changing surface meteorological conditions.

In some circumstances e.g. droughts, this loss of stomata control can cause them to be more open than without ozone interference. The extent of this effect may be dependent on magnitude and cumulated time of exposure to raised [O₃], suggesting experiments to analyze this require operation over long timescales such as full growing seasons. To both aid model development and provide empiricists with a system on to which measurements can be mapped, we present a parameter-sparse framework specifically designed to capture sluggishness.

This contains a single time-delay parameter τ_O3, characterising the timescale for stomata to catch up with the level of opening they would have with- out damage. The larger the value of this parameter, the more sluggish the modelled stomatal response.

Through variation of τ_O3, we find it is possible to have qualitatively similar responses to factorial experiments with and without raised [O₃], when comparing to measurement timeseries presented in the literature.

This low-parameter approach lends itself to the inclusion of ozone-induced inertial effects being incorporated in the terrestrial vegetation component of Earth System Models (ESMs).

On stomatal conductance

Photo credit: Science Direct

Combining the [ABA] and net photosynthesis-based model equations of stomatal conductance

• by Chris Huntingford, D. Mark Smith, William J. Davies, Richard Falk, Stephen Sitch, Lina M. Mercado

in Ecological Modelling, Volume 300, 24 March 2015, Pages 81–88

Highlights

• Complete equation set of ABA soil moisture to stomata controls plus responses to other drivers.
• Our ABA-based ecosystem model reproduces empirical fitted dependences found elsewhere.
• Physiological understanding of vegetation–drought coupling is relevant to climate change.
• Argue root-plant hormonal signalling to be routinely included in next generation climate models.
• Discover remarkable balancing dependencies on stores and fluxes from water and carbon cycles.

Abstract

Stomatal conductance g_s is variously depicted as being dependent on environmental conditions ( Jarvis, 976), transpiration ( Monteith, 1995), net photosynthesis ( Leuning, 1995) or chemical signalling arriving in the xylem ( Tardieu and Davies, 1993). Accurate descriptions of g_s are being increasingly demanded in the large-scale land surface model components of General Circulation Models (GCMs) to predict future land-atmospheric fluxes of water vapour, heat and carbon dioxide. The JULES model, for instance, uses the net photosynthesis description combined with a relatively simple semi-linear dependence on soil moisture content that modulates the photosynthesis dependence ( Cox et al., 1998).

Dewar (2002) combines the Leuning (1995) and Tardieu and Davies (1993) models. We revisit that combination, and discuss whether the Vapour Pressure Deficit (VPD) implicit in both components is different or in common. Further, we show a potential re-arrangement of the combined equations reveals that this model for g_s can be considered as being dependent on only four variables: evaporative flux Jw, net photosynthesis a_n, soil moisture content θ and ambient CO₂ concentration c_a. Expressed this way, g_sis influenced by two relatively slowly varying stores of the hydrological and carbon cycles (soil water content and atmospheric CO₂) and two more rapidly fluctuating fluxes from both cycles (evaporation and net photosynthesis).

We consider how the modelling structure and its response to both canopy-level and soil environmental controls may make it suitable for inclusion in GCMs, and what this entails in terms of parameterisation.

Read the full article: Science Direct