Limits to water transport in Juniperus osteosperma and Pinus edulis: implications for drought tolerance and regulation of transpiration

Pinyon–Juniper communities are found throughout the high deserts of the south-western United States, predominantly at elevations between 1500 and 2000 m (Johnston 1994). The Pinyon–Juniper habitat is semiarid, receiving c. 40 cm of yearly precipitation. The actual rainfall over a local elevational gradient, however, can vary more than twofold (West 1988). Juniper dominates in the lower, drier sites and with increasing elevation (and precipitation) community structure shifts to dominance by Pinyon (Woodbury 1947; Woodin & Lindsey 1954; Padien & Lajtha 1992; Lajtha & Getz 1993). As a result, Juniper is generally accepted as the more drought-tolerant species, intermixing with desert flora at the lower end while Pinyon gives way to montane conifers at the higher altitudes. This study addresses the vulnerability to xylem cavitation of Juniperus osteosperma Engelm. (Cupressaceae) (Utah juniper) and Pinus edulis (Torr.) Little (Pinaceae) (Colorado Pinyon Pine), and its relationship to the relative drought tolerance of these two species. Characterizing cavitation in xylem conduits is potentially important for understanding the water stress response of a plant. Thermodynamics of water movement require that if a plant is to continue extracting water from a drying soil, the water potential of the xylem sap must be below that of the soil and decrease in concert with the soil as it dries. As a result, xylem water is typically under tension (absolute negative pressure) and considerably so in xeric environments during drought. Below the vapour pressure of water (2·3 kPa at 20 °C), xylem sap is in a metastable state and at a certain critical pressure an ‘air seed’ can be pulled into the xylem conduit across a pit membrane (Zimmermann 1983; Crombie, Hipkins & Milburn 1985; Sperry & Tyree 1990; Cochard, Cruziat & Tyree 1992), which provides a nucleation site for the formation of water vapour, resulting in a gas-filled conduit. This process is termed xylem cavitation. Functional Ecology 1998 12, 906–911