Fog as a potential source of nitrogen for coastal redwood forest ecosystems

Fog water contains a range of important nutrients and ions that can potentially influence terrestrial forest ecosystems. Nitrogen (N) is present in fog, sometimes in high concentrations and often in much higher concentrations than normally found in rainwater. A defining feature of the redwood forest in coastal California is the presence of fog in the summer months. In this Mediterranean climate region, the fog provides water in a time when there is typically no rainfall. However, it is not known whether redwood trees can access N directly from fog. The goal of this project is to better understand the links between the water and N cycles of coastal forest ecosystems. In this project, we hypothesize that fog water provides redwood trees with N during the summer months when they would otherwise not be taking up nutrients. We are using natural abundance techniques to determine the source of nutrients for redwood trees throughout the year. Our results show that inorganic N entering the forest floor via fog is greatest at the forest edge closest to the ocean (p<0.05) where the redwood canopy has the greatest potential for interception, compared to the interior of the forest. Similarly, we have found that rain water N inputs to the forest floor are greatest at the forest edge (p<0.05), but the magnitude of this difference is not as strong. While δ15NO3 values were not significantly different between rain and fog water samples (average δ15N = -1.2 + 0.68 per mil), δ15NH4 values were significantly greater in fog (10.04 + 1.56 per mil) compared to rain water (1.21 + 0.88 per mil) and throughfall (3.37 + 0.69 per mil). δ15N values of foliage from forest edge trees were significantly greater compared to interior trees (p<0.05), suggesting that the forest edge trees may be obtaining a greater proportion of their N directly from fog compared to the interior trees.