Forest Canopy Hydraulics

Abstract

Water and carbon cycles are strongly coordinated and water availability is a primary limiting factor in many terrestrial ecosystems. Photosynthesis requires sufficient water supply to leaves and constraints on delivery at any point in the hydraulic continuum can lead to stomatal closure and reduced photosynthesis. Thus, maximizing water transport enhances assimilation and can provide plants with a competitive advantage. Unregulated water transport, however, can lead to excessive gradients in xylem tension that result in the development of air or vapor bubbles (i.e. embolisms) that block xylem water transport, potentially leading to permanent loss of function of the xylem. As such there can be a tradeoff between maximizing water transport and minimizing the risk of xylem embolism. This tradeoff has led to the development of a variety of hydraulic mechanisms to maximize efficiency and reduce vulnerability. Although several of these were first described centuries ago (such as stomatal control of transpiration), research in this field continues to uncover previously unrecognized processes employed by plants for maintaining hydraulic safety and/or efficiency. The hydraulic traits described in this chapter include xylem structural characteristics that enhance resistance to embolism such as pit and cell wall architecture; a continuum of strategies for constraining xylem tension to avoid embolism including isohydric and anisohydric control of leaf water potential; and safety and recovery mechanisms such as the capacitive discharge of stored water, hydraulic “circuit breakers” and the ability to repair xylem embolisms. Each of these will be discussed in terms of the variation in their use by contrasting tree types, their variability across organs and species, and their plasticity across environmental gradients. Beyond providing information about the means by which trees currently compete and survive, understanding the hydraulic mechanisms described in this chapter may provide insight into ways that trees are affected by, and the degree to which they may acclimate to rapidly changing climatic conditions.