Trade-off between Growth Rate and Water Use Efficiency in Southwestern Ponderosa Pine Provenances

We investigated growth and a suite of physiological and structural traits of ponderosa pine (Pinus ponderosa Lawson & C. Lawson var. scopulorum Engelm.) seedlings from different provenances using a field common garden study. Twenty-one provenances from a range of elevations across Arizona and New Mexico were planted in 2018 at a field site in the core of the species range in northern Arizona. We measured stem growth rate for three years (2019–2021), leaf carbon isotope discrimination (Δ13C), and leaf nitrogen concentration in 2020 on all prove­ nances, and leaf-level gas exchange, instantaneous water use efficiency, predawn and midday water potentials, soil-to-leaf hydraulic conductance and specific leaf area on nine provenances in 2020. Provenances differed significantly in stem growth rate, Δ13C, and specific leaf area, and several traits were correlated with provenance environmental and climatic characteristics. Seedlings with higher growth rate had greater Δ13C suggesting a trade-off between growth rate and water use efficiency. Provenances from warmer sites had lower growth rate and Δ13C (higher water use efficiency) than provenances from cooler sites during the driest year. Growth rate of provenances in the driest year was positively associated with stomatal conductance, net photosynthetic rate, and soil-to-leaf hydraulic conductance and negatively associated with instantaneous water use efficiency indicating the importance of maintaining water uptake and gas exchange to growth during drought under dry conditions. Our results enhance understanding of physiological mechanisms of establishment for planted ponderosa pine seedlings and show a trade-off between growth rate and water use efficiency of provenances from different thermal environments that should be useful in selection of seed sources for reforestation.