Transport Challenges in Tall Trees

Older physiological studies with tall trees tested predictions of the cohesion- tension model. In recent years, provocative proposals about the limits to tree height and size growth have sparked debate about the hydraulic and physiological constraints underlying vascular system design and function. Improved canopy access via cranes and rope-based techniques have extended the height range of trees in temperate and tropical forests suitable for well-replicated experiments. Studies of xylem transport in moderately tall trees now abound, but those of individuals close to historic maximum heights for a species, and of long distance transport in the phloem, are scarce. This chapter examines the constraints on transport imposed by extremes of height and path length, and synthesizes information on the mechanisms that may compensate for these constraints. It emphasizes issues of xylem transport, the scant relevant literature making one's treatment of phloem transport necessarily brief and speculative. Because stomatal regulation both affects, and is affected by, xylem transport and interacts with phloem transport via its influence on water potential and the production of photosynthate, the chapter also presents a new supply-loss model that links stomatal regulation to hydraulic conductivity of the xylem.