PS 29-149 - Regional variation in hydraulic vulnerability of juvenile ponderosa pines in the southwestern United States

Abstract: Juvenile ponderosa pines (Pinus ponderosa) are vulnerable to water stress imparted by soil moisture limitation and low soil water potentials (Ψ_p: MPa), but it is unclear if their tolerance to water stress varies between populations growing in different environments. We measured the hydraulic vulnerability of juvenile (20–30 years old) ponderosa pines in six climatically varying locations (northern and central Arizona, southern and northern New Mexico, southern Colorado, and southern Nevada) of the southwestern United States (SWUS). We evaluated hydraulic vulnerability using the P<sub>50 value of stem conduits. P<sub>50 is the pressure (MPa) at which 50% of xylem conductance is lost, and lower P<sub>50 values correspond to lower tree hydraulic vulnerability to soil moisture limitation. We then contrasted the climate and environmental conditions of our sampling sites from 1990–2020 using site characterization data, gridded meteorological estimates (DayMET), soil estimates (POLARIS), and soil water balance modeling (SOILWAT2). Our objectives were to determine if hydraulic vulnerability differed between juvenile ponderosa pines growing in climatically different locations, and to determine if climate and environmental conditions were associated with these differences.

Juvenile ponderosa pines in southern Nevada had more negative P<sub>50 values (-5.05 ± 1.16 MPa) than juveniles in southern Colorado (-4.26 ± 0.79 MPa), southern New Mexico (-3.72 ± 0.86 MPa), and central Arizona (-4.06 ± 0.96 MPa; ANOVA, p < 0.05). Compared to all other SWUS locations, southern Nevada also had significantly higher warm season (April – September) meteorological water deficit, lower cool season (October – March) and warm season Ψ_p, and lower soil water holding capacity (p < 0.05). Thus, lower Ψ_p, imparted by climate and physical soil characteristics, was associated with the higher stress tolerance found in juvenile ponderosa pines growing in southern Nevada. Although future research is required to determine the reason for their higher stress tolerance – which could include acclimation, adaptation, and/or environment – our results provide new evidence for physiological differences at early stages of tree development between ponderosa pine populations in the SWUS.