Stomatal and xylem plasticity, not growth rate, determines white spruce resilience to warmer and drier climates

In a warmer and drier climate, forest productivity will depend on trees' ability to maintain carbon uptake and hydraulic function. Whether fast-growing genotypes of boreal conifers are more vulnerable to combined climatic stress remains uncertain. Using a full-factorial field experiment, we investigated how progressive soil drying combined with extended warming affects growth, xylem development, and photosynthesis in two Picea glauca families with contrasting growth strategies. Rainout structures first reduced soil moisture from 25% to 18%, followed by a +5{degrees}C warming treatment applied using infrared heaters. During the warmest and driest period in August, air temperature reached 34.5{degrees}C in the warmed plots, while soil moisture declined to a low of 15% in the combined rainout and warming treatment. Contrary to expectations, both fast- and slow-growing white spruce families exhibited similar resilience to concurrent warming and soil drying. This finding challenges the prevailing theory that faster growth increases vulnerability to climatic stress. Despite an approximately 50% reduction in rainfall, pre-dawn water potential remained above -0.5 MPa across treatments, reflecting that seedlings were able to avoid hydraulic stress. Although the fast-growing family maintained greater height and diameter growth compared to the slow-growing family, both exhibited similar physiological and anatomical responses to warming. Warming decreased stomatal conductance, which increased intrinsic water-use efficiency. Latewood xylem traits related to hydraulic efficiency were also reduced under warming. Together, these coordinated stomatal and xylem adjustments decreased water loss and protected hydraulic function, enabling both families to maintain high photosynthesis and growth under simulated climate conditions. Overall, white spruce exhibits strong phenotypic plasticity, supporting intraspecific resilience to moderate warming and soil drying representative of projected 21st-century summer conditions for central and eastern Canada.