Premise of the study: The young seedling life stage is critical for reforestation after disturbance and for species migration under climate change, yet little is known regarding their basic hydraulic function or vulnerability to drought. Here, we sought to characterize responses to desiccation including hydraulic vulnerability, xylem anatomical traits, and impacts on other stem tissues that contribute to hydraulic functioning. Methods: Larix occidentalis , Pseudotsuga menziesii , and Pinus ponderosa (all < 6 weeks-old) were imaged using X-ray computed microtomography during desiccation to assess seedling biomechanical responses with concurrently measured hydraulic conductivity ( k s ) and water potential (Y) to assess vulnerability to xylem embolism formation and other tissue damage. Key Results: In non-stressed samples for all species, pith and cortical cells appeared circular and well-hydrated, but they started to empty and deform with decreasing Y which resulted in cell tearing and eventual collapse. Despite the severity of this structural damage, the vascular cambium remained well-hydrated even under the most severe drought. There were significant differences between species in vulnerability to xylem embolism formation, with 78% xylem embolism in L. occidentalis by Ψ of -2.1 MPa, but only 47.7% and 62.1% in P. ponderosa and P. menziesii at -4.27 and -6.73 MPa, respectively. Conclusions: L. occidentalis seedlings appeared to be more susceptible to drought stress compared to the other two species, but all three maintained hydration of the vascular cambium under severe stress, which could facilitate hydraulic recovery by regrowth of xylem when stress is relieved. 

These are the seedling conductivity data along with the primary and secondary embolism data from Miller et al. 2020 AJB.