Comparative Criteria for Models of the Vascular Transport Systems of Tall Trees

Abstract

During the past decade, several detailed models of plant vascular transport systems (i.e. xylem and phloem) have been presented in the literature, and many of them are currently capable of accurately modelling the hydraulic characteristics of trees, including tall trees. This marks a departure from earlier modelling exercises in plant water relations, when models were intended primarily to promote an understanding of the biophysical and physiological mechanisms of transport, but whose performance was comparatively poor when they were required to predict the behaviour of organisms spanning the logarithmic ranges from small seedlings to gigantic old trees. In addition, many of these modelling efforts have assumed, more or less explicitly, that a principle of optimality operates in the design of the transport systems, i.e. they have assumed criteria by which various aspects of the transport system should be optimised. Moreover, these models are characterised by very different approaches, structures and objectives, and differ significantly with regard to several other important characteristics. Because models formally organise our knowledge, we review them here, in the hope of highlighting the theoretical progress achieved so far and the challenges remaining in our understanding of the vascular transport systems of trees.