Stable isotopes reveal foliar uptake of fog water by Redwood forest plants

Summertime fog events wet the leaves of redwood forest plants along the coast of Northern California and provide a water subsidy during the drought season. Previous research demonstrated that the coastal redwood (Sequoia sempervirens) absorbs fog water directly through leaf surfaces and our study investigates if foliar uptake occurs in 9 other dominant redwood forest species ranging from canopy trees to understory herbaceous plants. In a greenhouse experiment, we prevented fog drip into the soil and compared the change in leaf water hydrogen isotope composition between plant canopies exposed to six hours of deuterated fog water and plants exposed only to ambient air overnight. The fog treatment significantly reduced plant water stress and affected the change in hydrogen isotope composition of leaf water overnight across plant species, suggesting that foliar uptake is a common physiological phenomenon. Using a 2-source mixing model, we calculated the fraction of leaf water derived from fog water after the fog exposure for each species. These results suggest the coastal redwood, Douglas-fir (Pseudostuga menziesii), sword fern (Polystichum munitum), salal (Gaultheria shallon), and Pacific huckleberry (Vaccinium ovatum) do foliar uptake and that fog water accounts for up to 10-20% of their internal leaf water following a fog event. Foliar uptake of fog water provides direct hydration to leaves and therefore improves plant physiological performance during the summer season in the redwood forest when soil water availability is low.