Xylem cavitation and the distribution of Sonoran Desert vegetation

Plants replace evaporative water losses with water drawn through the xylem under negative pressure. Water in the xylem is therefore susceptible to cavitation: the nucleation of vaporization. Cavitation results in a gas-filled or embolized conduit that is nonfunctional in transport. This dissertation examines the mechanism of water transport, the occurrence of cavitation induced by water stress and freezing, and its implications for the ecophysiology of Sonoran desert plants. In recent years, the widely accepted cohesion-tension theory has been challenged by evidence against the existence of significant negative xylem pressure. Using centrifugal force to generate negative xylem pressure, I found that several species sustained negative pressures as predicted by the cohesion tension theory. Furthermore, cavitation occurred at species specific xylem pressures in accordance with a previously postulated air-seeding mechanism. Seasonal studies showed that a riparian species limited transpiration to avoid inducing complete xylem cavitation. In contrast, two desert shrubs halted gas exchange with large safety margins from the xylem pressures leading to complete cavitation. This suggested that stem cavitation did not limit transpiration in upland species. Comparative studies showed that the critical pressures leading to complete cavitation decreased with decreasing minimum xylem pressure across habitats. In addition, there was a weak tradeoff between resistance to cavitation and conducting efficiency. These results suggested that the interaction between vulnerability to cavitation and the phenological and morphological characters determining xylem pressure was important in determining species distribution. Xylem pressure and anatomy also determine resistance to freezing-induced cavitation. Although predicted to cavitate completely following freezing, L. tridentata exhibited no cavitation after freezing to -11°C or higher. Below -11°C cavitation increased until it was complete between -16 and -20°C. The mechanism responsible is unknown but the northern limit of L. tridentata predicted using long-term minimum temperatures of -16° to -20°C corresponded to L. tridentata's distribution over much of its range. These studies confirmed our understanding the mechanism of xylem water transport and showed that cavitation is important in determining the distribution and performance of desert plants.