Abstract: How do dramatic changes in water availability influence tree anatomical structure and physiological performance? The long-running Pfynwald experiment, in the driest part of Switzerland, began in 2003 with the irrigation of plots in a 120 yo Pinus sylvestris forest. In 2014, irrigation was stopped in half of the plots. Accessing 30 tree crowns with canopy scaffolds in March 2022, we measured winter embolism fractions and >50 anatomical traits of leaves and twigs with both synchrotron-based X-ray microtomography and traditional histology. We also separately exposed leaves and twigs to experimental rainfall to assess tree capacity for canopy water absorption. We found significant multivariate differences in anatomical traits among control, irrigated, and formerly irrigated trees which occupied a larger portion of the trait space than either irrigated or control trees. In pairwise comparisons, leaves and twigs of formerly irrigated trees were still not significantly different from irrigated trees, but had key areas of overlap with trees growing in control plots, despite significant anatomical differences. Likewise, both the ability of leaves to absorb water and their embolism frequency was intermediate in trees that had not experienced irrigation for 7 years. The capacity of leaves to absorb rainfall was greatest in control trees, which also had the lowest winter embolism frequency. The ability of twigs to absorb water was similar among all treatments, and ~ 3X greater than water absorption through leaves, signifying that twigs are a substantial and consistent source of above-ground water, regardless of shifting soil water-availability. In general, the transfusion tissue and cell-size traits of irrigated trees reverted to control-type, while many tissue-level size related traits remained indistinguishable from continually irrigated trees. Interestingly, the formerly irrigated trees were more extreme than either of the other treatments in having the shortest leaves, thinnest epidermis, and least xylem and phloem - occupying a distinct corner of the trait-space. Our results suggest that the ecological memory of water availability is highly persistent in this dry-forest conifer, shaping tree structure and function for many years. This legacy of past water availability implies an important contribution of climatic history to contemporary tree performance, with periods of wet or dry years driving long-term changes in organs and tissues developed at a much later date.
