Water Relations and Leaf Morphology of Juniperus occidentalis in the Northern Great Basin

Relationships between seasonal and diurnal leaf conductance, xylem sap potential, transpiration rates, osmotic potential, vapor pressure deficit, soil and air temperatures, soil water potential, and photosynthetic active radiation are quantified for Juniperus occidentalis Hook. Additionally, xeromorphic aspects of leaf morphology were examined with electron microscopy. Xylem sap potentials and leaf conductance ranged from -0.5 to -2.7 MPa and 0.02 to 0.13 cm s-1, respectively, during the two-year study. Leaf conductance is not clearly related to any single environmental variable, but is primarily influenced by vapor pressure deficit, photosynthetically active radiation, and factors such as soil and air temperatures, and soil water, which influence the resistance of water flow through the plant. Stomates usually closed when xylem sap potentials reached - 2.0 MPa. Stomates on mature leaves are distributed unequally on adaxial and abaxial surfaces, with none occurring on exposed surfaces. Drought avoidance mechanisms displayed by the plant were leaf morphological characteristics, low maximum leaf conductance, and reduced maximum leaf conductance under high evaporative conditions. Cold soil temperatures increased resistance of water flow through the soil-plant-atmosphere continuum. Xeromorphic leaf structure as well as seasonal osmotic adjustment provide primary mechanisms in the tolerance of Juniperus occidentalis to drought stress. For. Sci. 33(3):690-706