Fog in the California redwood forest: ecosystem inputs and use by plants

Fog has been viewed as an important source of moisture in many coastal ecos ystems, yet its importance for the plants which inhabit these ecosystems is virtually unknown. Here, I report the results of a 3-year investigation of fog inputs and the use of fog water by plants inhabiting: the heavily fog inundated coastal redwood (Sequoia sempervirens) forests of northern Califo rnia. During the study period, 34%, on average, of the annual hydrologic in put was from fog drip off the redwood trees themselves (interception input) . When trees were absent, the average annual input from fog was only 17%, d emonstrating that the trees significantly influence the magnitude of fog wa ter input to the ecosystem. Stable hydrogen and oxygen isotope analyses of water from fog, rain, soil water, and xylem water extracted from the domina nt plant species were used to characterize the water sources used by the pl ants. All isotopic mixing model was employed to then quantify how much fog water each plant used each month during the 3-year study. In summer, when f og was most frequent, similar to 19% of the water within S. sempervirens, a nd similar to 66% of the water within the understory plants came from fog a fter it had dripped from tree foliage into the soil; for S. sempervirens, t his fog water input comprised 13-45% of its annual transpiration. For all p lants, there was a significant reliance on fog as a water source, especiall y in summer when rainfall was absent. Dependence on fog as a moisture sourc e was highest in the year when rainfall was lowest but fog inputs normal. I nterestingly, during the mild Fl Nino year of 1993, when the ratio of rainf all to fog water input was significantly higher and fog inputs were lower, both the proportion and coefficient of variation in how much fog water was used by plants increased. An explanation for this is that while fog inputs were lower than normal in this Fl Nino year, they came at a time when plant demand for water was highest (summer). Therefore, proportional use of fog water by plants increased. The results presented suggest that fog, as a met eorological factor, plays an important role in the water relations of the p lants and in the hydrology of the forest. These results demonstrate the imp ortance of understanding the impacts of climatic factors and their oscillat ions on the biota. The results have important implications for ecologists, hydrologists, and forest managers interested in fog-inundated ecosystems an d the plants which inhabit them.