Rainwater uptake in conifer twigs: five experiments tell a story of absorption, storage, and transport

While evidence supports the idea that a portion of the many raindrops that fall onto a forest canopy may be directly absorbed by the twigs they land on, we do not know how much is absorbed, how it enters the twig, or what internal path it might take on its way to the xylem. Using a diverse series of five experiments encompassing isotopic labelling, fluorescent tracers, rehydration kinetics, synchrotron-based X-ray tomographic microscopy, and thermal imaging, we follow the fate of rainwater from initial contact with the twig to its distribution to adjacent tissues. We provide conclusive, multi-pronged evidence of surface water-absorption into the xylem of 1-year-old conifer twigs with incomplete bark development. Additionally, we demonstrate a surface capillary phase, mixed apoplastic and symplastic internal routes, and the strong influence of intercellular airspace as a hydraulic capacitor across multiple tissues. We show that twigs are capable of rapid, large-volume water absorption, which may help trees take advantage of crown-wetting events and support the repair of hydraulic damage from frost and drought. Forecasting the impacts of climatic stress on different tree species will benefit from understanding the importance, and tissue-level specifics, of this critical water-acquisition pathway. Our work tells a detailed story of rain absorption by twigs and lays a foundation for future trait-based research into among-species differences in absorption capacity.