Extending the Concept of Soil Water Characteristic to Derive Xylem Water Characteristics of Trees.

The application of Richards equation to simulate xylem water flow of trees based on three-dimensional graphs representing tree architecture needs water characteristic curves of the xylem porous medium.

A general concept to derive xylem water characteristic curves is presented. The concept is based on ideas to describe soil water retention and unsaturated hydraulic conductivity curves.  It uses the elastic modulus of the xylem to determine the xylem water content at the air entry value and to define the relationship between xylem water content and xylem potential.  A capillary bundle approach is used to calculate relative xylem conductivity.

Examples of xylem water characteristic are given for beech trees. Advantages and shortcomings of the newly derived concept to simulate sap flow in individual trees are discussed.