Vertical variation in swelling properties of Norway spruce bark depending on tree age and bark moisture content

In forest ecosystems, interception of rainwater on foliar and woody surfaces and the subsequent partitioning into stemflow is largely controlled by physical and hydrological properties of bark. Few forest ecohydrology studies have explored the role of bark properties (e.g., thickness, density) on bark water storage capacity and stemflow production. Even fewer have explored how different phases of water (e.g., liquid, vapor) may affect bark through bark swelling properties across the stem and how the degree of swelling affects tradeoffs between bark water storage and stemflow generation. Thus, the objective of this study was to analyze changes in a bark swelling index (BSI) vertically along stems of Picea abies (Norway spruce) after exposure to both water vapor and liquid water, as a function of tree age and bark moisture content. We found that tree age influenced BSI and bark moisture content, wherein BSI was ∼ 6.5% lower in older trees (70 years) compared to younger trees (35 and 50 years), and average moisture content was 10.4–13.2% lower. BSI increased when bark was exposed to hygroscopic water vapor and reached maximum swelling after 1 day of water saturation. BSI also increased from the base of the tree to 20–30% of total tree height, beyond which BSI remained relatively stable across all age classes. Enhanced understanding of bark swelling mechanisms as a result of stem position, age, and moisture content and exposure provide stronger foundations for understanding canopy hydrologic partitioning and the fate of rainwater moving through forest canopies.