| Marine fog is a defining feature of the summertime climate of coastal California where the redwood forest ecosystem occurs. While fog provides critical water to this ecosystem when rainfall is largely absent, it is poorly understood how plants acquire fog water as a resource and if fog alleviates plant water deficit during the driest time of year. I investigated (1) how many species in the redwood forest acquire fog directly by foliar water uptake, (2) how foliar fog uptake influences the water balance of the dominant understory fern Polystichum munitum, (3) how geographical variation in P. munitum canopy cover and foliar uptake capacity affects fog drip interception along the redwood forest range, and (4) how fog influences the seasonal timing and intensity of drought stress for understory plants. For these studies I evaluated aspects of plant water relations among redwood forest species in the field and after artificial fog exposure in the glasshouse with particular emphasis on P. munitum. First, I found that 80% of dominant species investigated exhibit foliar fog uptake and become more hydrated following fog exposure. Second, I found that foliar fog uptake by P. munitum increases plant water status (water potential and content) above the soil water status. This causes water to leak from the fern roots into the soil, establishing an atmospheric-plant-soil gradient never before reported. Third, I found that P. munitum canopy cover increases towards the northern end of the redwood forest ecosystem, but foliar uptake capacity is highest in the center of the range This indicates that little fog drip interception and absorption can occur in southern forests where P. munitum leaf area and water absorption capacity is lower. Fourth, I found that fog eliminates drought stress in both tree saplings and ferns during the summer growing season when leaf wetting frequently occurs. Together, these findings demonstrate that redwood forest species readily acquire fog water directly through their leaves and thereby become hydrated even when soil water availability is low. With less fog exposure during the summer dry season, water limitation could reduce the growth potential and even survival of redwood forest plants. |
