Optimal and competitive stomatal behaviour

Abstract

Stomatal conductance links plant water use and carbon uptake. Its responses to environmental conditions play a key role in determining global water and carbon cycles and their coupling. Empirical and mechanistic models based on observational studies and physiological experiments are helpful in describing how plants regulate stomata while theoretical models aim to give a top-down explanation on why stomata are regulated in such a way. In this thesis, I focused on stomatal response to drought and optimised it based on theoretical modelling. Stochastic rainfall is a major component of this modelling study as it is an important determinant in the dynamics of soil water content in nature. In Chapter 2, I hypothesized that plants should adapt to the local rainfall regime, in the sense that they regulate stomata to maximise their long-term average net carbon gain in the given rainfall regime. Then, I derived the optimal stomatal strategy analytically based on mathematical optimisation. In Chapter 3, I further studied the impacts of water competition on stomatal regulation under the assumption that all water is shared among every plant. Following the theory of evolutionarily stable strategy (ESS), I identified the ESS stomatal strategy, which, if adopted by the dominant resident species in any given rainfall regime, cannot be invaded by any invader species with alternative strategy. The first main achievement of this study is that I successfully derived the optimal and the ESS functional relationships between stomatal conductance and soil water content. It is worth mentioning that there was no presumed functional form for either of them. Also, to account for the stochasticity of rainfall, these two optimal functions result from long-term optimisation (actually, they were optimised over an infinite period of time), which should be in sharp contrast with the short-term optima presented in the previous theoretical studies. The second main achievement is that I clearly illustrated in Chapter 3 that the optimisation and the competition models of stomatal regulation stem from fundamentally different logics but are of equal theoretical importance. In addition, in a model considering plant gas exchange only, I Proved that water competition should drive plants to use water as quickly as possible""a typical example of 'the tragedy of the commons'. Then, to restrict plant water use, I introduced xylem cavitation and refilling into the model and showed the great impact of the assumptions on xylem refilling on the resulting ESS stomatal strategy. Finally, based on the optimal or the ESS stomatal strategy, I derived various predictions on plant mean annual productivity, water use, plant hydraulic strategies, and so on. As shown in Chapter 2 to 4, these predictions are in reasonable agreement with many observational studies, which should help to justify my models.

Date of Award	2017
Original language	English