The memory of past water abundance shapes trees 7 years later.

Premise: Tree structure and function are constrained by and acclimate to climatic conditions. Drought limits plant growth and carbon acquisition and can result in "legacy" effects that last beyond the period of water stress. Leaf and twig-level legacy effects of past water abundance, such as that experienced by trees that established under wetter conditions are unknown.

Methods: In an 18-year forest irrigation experiment, we explored the lasting structural impact of past water richness on leaves and twigs of Pinus sylvestris using synchrotron-based X-ray microscopy. We compared 47 anatomical traits among never-irrigated control trees, trees irrigated for 18 years, and formerly irrigated trees, 7 years after their return to control-level water availability in this naturally dry forest.

Results: We found that legacy effects induced by a period of experimental irrigation continue to shape the structure of new leaves and twigs long after a sharp decrease in water availability. Legacy effects shaping twigs were present but dissipating, while leaf traits displayed long-lasting effects on structural strategy, with extreme values for traits associated with high water stress and low productivity.

Conclusions: Mature trees acclimating to an increasingly dry world may be at a disadvantage compared to young trees that have known only chronic drought. However, these young trees may be capable of thriving in sites of drought-related forest decline especially if planted while larger individuals are still present to support tree establishment. Without a legacy of past water abundance, trees in future forests may be better equipped to cope with our rapidly changing climate.