Fog presence and ecosystem responses in a managed coast redwood forest

Fog inundation along California's Coast Range creates microclimates that support coast redwood (Sequoia sempervirens (D. Don) Endl.) forests during the summer drought period. With changes in land use and climate, the coast redwood ecosystem is more susceptible to increased drought stress. Thus, understanding the role of fog in relieving drought stress is important to manage the remaining coast redwood forests. Fog presence, other climatic conditions, soil moisture, and sap flow were monitored at the Caspar Creek Experimental Watersheds in northwestern California over the 2020, 2021, and 2022 fog seasons (1 May – 30 Sep). Observations were recorded at shoulder and ridge topographic positions in harvested and unharvested third-growth forest to examine 1) temporal and spatial distribution of fog, 2) soil moisture responses to fog drip, and 3) the influence of fog on transpiration and streamflow. Fog presence was found to vary across the landscape with no significant relationship to harvesting. Leaf wetness as a result of fog was higher at the shoulder position than at the ridge of the hillslope. At all study sites, fog events tended to result in small increases in soil moisture or reduced withdrawals of soil moisture, albeit variably throughout the fog season. All sites displayed lower transpiration rates during fog periods and streamflow recession rates were similarly reduced. Overall, this research suggests that the occurrence of fog is ecologically and hydrologically important at the Caspar Creek Experimental Watersheds, and those effects are influenced by topography but not apparently by forest density.