Scaling Up Bark Hydrology: Tree Size and Functional Traits Shape Water Storage in a Tropical Cloud Forest

Bark has long been overlooked as a component of forest water cycling, particularly in fog-dependent tropical montane cloud forests (TMCFs). Here, we quantified the water absorption capacity of bark in 17 tree species from a Brazilian TMCF and evaluated how functional traits and tree size influence bark water uptake (BWU) at the whole-tree scale. Bark samples were saturated in the lab, and field data were used to estimate water uptake per tree. While BD and BWSC showed limited interspecific variation, BWU varied greatly, mainly due to bark volume (Vb). A multiple regression model explained 89.5% of the variation in BWU. Sensitivity and bootstrapping analyses confirmed that, although Vb was the dominant predictor, functional bark traits (BD and BWSC) also contributed significantly to water uptake. These findings highlight that BWU is shaped by both structural and functional traits, underscoring a dual control on bark hydrology. Critically endangered (Araucaria angustifolia) and ecologically dominant (Senna multijuga) species exhibited the highest water storage per tree. Although the absolute values represent the potential maxima under laboratory conditions, the comparative data provide insights into species-level contributions to forest water dynamics. Notably, bark water uptake may help maintain moisture availability during dry spells or fog-free intervals, particularly in ecosystems where rainfall is intermittent and canopy-dwelling organisms depend on sustained humidity. Our results suggest that bark traits should be more explicitly considered in hydrological models and conservation planning for TMCFs under changing climate regimes.