Abstract: Hydraulic vulnerability, the sensitivity of xylem embolism to water stress, is a major determinant trait associated with conifer mortality. This study investigated branch P50 (50% hydraulic conductivity loss) of adult and juvenile ponderosa pines across multiple locations in the southwestern United States (SWUS), and examined relationships between P50 and climatic conditions, soil properties and forest management. Juvenile ponderosa pines had significantly more negative P50 values than adults on average. Both age groups exhibited a latitudinal pattern in P50 associated with regional climate conditions. Across the SWUS, juvenile P50 was positively correlated with growing season precipitation and temperature, and negatively correlated with soil pH and clay content. In some cases, significant differences in juvenile P50 between SWUS locations were associated with growing season moisture deficit, whereas other significant differences were less attributable to climate and may attest to potential genetic variation between populations. Climate variation, soil properties and population-level acclimation and/or adaptation may all influence the hydraulic vulnerability of juvenile ponderosa pines in the SWUS, promoting differences in stress tolerance of ponderosa pine forests across the region.

Key words: Drought, Vegetation response, Xylem embolism, Water stress tolerance, Hydraulic failure