Impact of a tree's hydraulic strategy on its survival in a global climate change context

Abstract

Global climate models predict less rain for many regions with Mediterranean climates such as the South West of Western Australia, together with an increasing frequency of extreme events (e.g. exceptionally warm and dry years or days). In this context it is important to understand how trees in natural environments will respond to changing conditions. Over millions of years, evolution has favoured different strategies to enable these plants to extract water at a rate sufficient to sustain themselves even in these relatively arid regions where summer drought is a regular occurrence. These strategies involve mechanisms occurring at the cell scale, such as stomatal regulation, increased vessel size to improve the conductance for water flow or increased elasticity of cell walls to increase water capacitance of sapwood. These mechanisms all have interrelated effects and there are important trade-offs between them. For example, increasing the conductance of vessels by increasing the size of pit pores between vessel elements also leads to a greater susceptibility to embolism and hence an increased risk of complete loss of conductivity in the case of a particularly extreme drought. Growing new roots to facilitate water uptake requires a significant investment of carbon that must be obtained by photosynthesis at the expense of more transpired water. We need a model to understand the importance of cell-level changes for the adaptation of trees to their environment. At the interface between the microscopic scale of plant cells and the macroscopic scale of the vegetation layer used in climate models, a model of the hydraulics of a single individual tree can be used to test different strategies for water uptake, transport and transpiration. In this paper we will describe how we developed such a model (Figure 1), how we used the model to analyse some overall strategies adopted by trees and discuss them in the light of drought tolerance. Results show that optimizations for water use occurs at different scales and are interrelated. At long time scale, trees can adapt the allocation of carbon to different parts of their architecture (roots, trunk, and canopy) to keep their stomata open longer. Within a day, stomatal control prevents hydraulic failures and adjusted tissue water capacitance allows the tree to answer rapid changes in physical conditions.