LAUSR

Increasing dryness challenges trees' ability to maintain water transport to the leaves. Most plant hydraulics models use a static xylem response to water stress. Yet in reality, lower soil moisture and warmer temperatures during growing seasons feed back onto xylem development. In turn, adjustments to droughts in the newly built xylem influence future physiological responses to droughts. In this study, we investigate the annual variation of anatomical traits in branch xylem in response to different soil and atmospheric moisture conditions and tree stress levels, as indicated by seasonal predawn leaf water potential (ΨL,pd). We used a six-year field experiment in southwestern USA with three soil water treatments applied to Pinus edulis trees-ambient, drought (45% rain-reduction) and irrigation (15-35% annual water addition). All trees were also subject to a natural one-year acute drought (soil and atmospheric) that occurred during the experiment. The irrigated trees showed only moderate changes in anatomy-derived hydraulic traits compared to the ambient trees, suggesting a generally stable, well-balance xylem structure under un-stressed conditions. The artificial prolonged soil drought increased hydraulic efficiency but lowered xylem construction costs and decreased tracheid implosion safety ([t/b]2), suggesting that annual adjustments of xylem structure follow a safety-efficiency tradeoff. The combination of acute and prolonged drought resulted in vulnerable and inefficient new xylem, disrupting the stability of the anatomical tradeoff observed in the rest of the years. The xylem hydraulic traits showed no consistent direct link to ΨL,pd. In the future, changes in seasonality of soil and atmospheric moisture are likely to have a critical impact on the ability of Pinus edulis to acclimate their xylem to warmer climate. Furthermore, the increasing frequency of acute droughts might reduce hydraulic resilience of Pinus edulis by repeatedly creating vulnerable and less efficient anatomical structure.