摘要：Embolism, the blockage of water transport in the xylem by air, is an important consequence of low water availability for all plant species. Riparian plants, since they typically experience mesic conditions, are not water stress tolerant and hence are vulnerable to xylem cavitation, the formation of emboli. We have constructed a composite vulnerability curve for Populus fremontii (Fremont cottonwood); assessed native state embolism, critical xylem pressure potential (Ψcav), and safety margin; and determined predawn and midday leaf water potential (ΨL) within a central New Mexico cottonwood population. Our results indicate (1) that this population of P. fremontii is extremely vulnerable to cavitation, with complete xylem blockage occurring at −2.25 MPa, and (2) that native state embolism is between 19% and 42%. Ψcav was determined to be −1.36 MPa. Measurements of predawn ΨL were typically near −0.5 MPa while midday ΨL values averaged −1.7 MPa. Estimates of midday xylem pressure potential (Ψpx) were −1.1 MPa. These values suggest that these individuals maintain small safety margins (0.26 MPa) between Ψpx and Ψcav. This small safety margin may be detrimental under increased variation in water availability caused by anthropogenic alteration of river systems.
