Adjustment of tree structure in response to the environment under hydraulic constraints

1. The hypothesis is tested that functional limitations, possibly related to the risk of disruptive xylem embolism, maintain the leaf water potential of trees within a safety range. This strategy would particularly apply to conifers. 2. It is also hypothesized that homeostasis in water transport is achieved by a combination of short-term stomatal regulation and the optimal allocation of resources between transpiring foliage, conductive sapwood and absorbing roots. 3. The implications of this hypothesis for responses of plant structure to the environment are explored from a theoretical viewpoint, focusing in particular on the effects of temperature, vapour pressure deficit and soil water availability and texture. 4. A comparison with published data sets confirms model predictions of greater allocation to transpiring foliage with increasing temperature, and lower allocation with increasing vapour pressure deficit and soil drought. Allocation to fine roots would respond more strongly to drought, and the effect would be magnified by coarse soil textures. 5. The applicability of the model to other taxa remains to be determined, and the extent to which the observed structural adjustment is the result of phenotypic plasticity or long-term adaptation is still unclear.