Analysis of drought mortality with a coupled xylem and phloem transport and leaf gas exchange model

Background/Question/Methods

Mortality of plants as a consequence of drought events has become a major focus of attention recently with the rise of severe mortality episodes around the globe. One central element of uncertainty in improving existing process-based models is given by the lack of detailed understanding of the physiological processes, and their interconnections, leading to mortality.  Here we present a physiological whole-tree level model of water and carbon related processes; stomatal control, photosynthesis and respiration, xylem water and phloem transport. The above mentioned processes are described to be driven by environmental conditions, underlying physical restrictions (such as viscosity, diffusion resistance and water potential equilibrium), tree structural properties and biological control mechanisms. All of the processes are described in terms of water potential and sugar concentration, which are intimately connected to each other due to the requirement of water potential equilibrium. Our steady state model allows exploring the possible biological parameter space in which all species must exist and which must constrain the range of processes that may lead to hydraulic failure, carbon starvation and phloem transport failure. The mode of carbon starvation was further divided into stomatal and non-stomatal related decreases in photosynthesis.

Results/Conclusions

The simulations separated the parameter combinations leading to each type of failure. Approximately half of all parameter combinations fell into the hydraulic mode of failure and half into the carbon starvation mode of failure. Xylem embolism versus stomatal sensitivity to water potential was predicted to be important in separating these two modes of failure. Plants with sensitive stomata were predicted to die of C starvation unless they also happen to have extremely vulnerable xylem. Conversely, plants with vulnerable xylem were predicted to die of hydraulic failure unless they also happen to have extremely sensitive stomata. Phloem transport was found to become practically non-functional at water potentials less than approximately -5 MPa due to excessive phloem sap viscosity build-up, assuming a pure sucrose solution. Direct phloem failure occurred in very few circumstances as hydraulic failure and carbon starvation had typically occurred already at higher water potentials. However, in addition to outright failure in phloem transport, phloem transport characteristics were important in determining the within plant sugar concentrations, which affected the carbon related processes, and thus the likelihood of carbon starvation.