Ontogenetically stable hydraulic design in woody plants

1. An important component of plant water transport is the design of the vascular network, including the size and shape of water-conducting elements or xylem conduits. 2. For over 100 years, foresters and plant physiologists have recognized that these conduits are consistently smaller near branch tips compared with major branches and the main stem. Empirical data, however, have rarely been assembled to assess the whole-plant hydraulic architecture of woody plants as they age and grow. 3. In this paper, we analyse vessels of Fraxinus americana (White Ash) within a single tree. Vessels are measured from cross-sections that span 12 m in height and 18 years' growth. 4. We show that vessel radii are determined by distance from the top of the tree, as well as by stem size, independently of tree height or age. 5. The qualitative form for the scaling of vessel radii agrees remarkably well with simple power laws, suggesting the existence of an ontogenetically stable hydraulic design that scales in the same manner as a tree grows in height and diameter. 6. We discuss the implications of the present findings for optimal theories of hydraulic design.