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