Tree bark actively mediates atmospheric water storage in tropical cloud forests

Tree bark plays a crucial but underexplored role in water uptake under fog-rich conditions, particularly in tropical cloud forests where atmospheric moisture often surpasses rainfall as a hydrological input. This study evaluated bark water uptake in 17 tree species from a tropical montane cloud forest in southeastern Brazil. Samples were exposed to increasing levels of relative humidity (RH: 59 %, 75 %, 85 % and 99 %) and full saturation, under controlled conditions reflecting the humidity of fog-dominated ecosystems. Gravimetric analyses quantified bark moisture and bark uptake across humidity treatments, while contact angle quantified surface wettability, and X-ray diffraction assessed bark crystallinity structure. Results revealed pronounced interspecific differences: some species exhibited high moisture absorption under vapor-phase conditions but low water storage (e.g., Alchornea glandulosa, Campomanesia guaviroba), while others showed limited hygroscopicity but large water storage due to thick bark and large diameters (e.g., Araucaria angustifolia). Wettability showed limited predictive power. Despite initial hydrophobicity, most bark surfaces became more wettable over time, suggesting dynamic responses to moisture. Crystallinity indices (CrI) were positively correlated with vapor uptake, suggesting that cellulose-rich barks enhance moisture acquisition under high RH. Overall, bark traits modulate atmospheric water capture strategies, reflecting trade-offs between rapid responsiveness and storage capacity. These findings provide novel insights into bark ecohydrology and functional adaptations in cloud forest trees.