Regulation of water loss in populations of Populus trichocarpa: the role of stomatal control in preventing xylem cavitation

Variations in resistance to drought-induced xylem cavitation, xylem air-entry points, stomatal behavior, and hydraulic conductivity were measured in four populations of Populus trichocarpa Torr. & A. Gray collected along an east-west humidity and temperature gradient in Washington State, USA. Xylem air-entry points were less negative in trees from moist environments (-0.71 and -1.32 MPa in the Hoh and Nisqually populations, respectively) than in trees from dry environments (-1.55 and -1.67 MPa in the Palouse and Yakima populations, respectively). Xylem cavitation in response to experimental drought was consistent with air-injection measures of xylem air-entry points for a given population. Populations vulnerable to cavitation also exhibited higher stem specific hydraulic conductivities and limited stomatal control compared with resistant populations. Populations exhibiting vulnerability to cavitation and limited stomatal control desiccated more rapidly during drought compared with resistant populations. This study provides evidence of interpopulation variation in resistance to drought-induced xylem cavitation, stomatal behavior, and hydraulic conductivity within Populus trichocarpa.