Fog Presence and Ecosystem Responses in a Managed Coast Redwood Forest

Abstract

Fog inundation along California’s Coast Range creates microclimates that support coast redwood (Sequoia sempervirens, D. Don) forests during the summer drought period. With changes in land use and climate, the coast redwood ecosystem is susceptible to increased drought stress. Thus, understanding the role of fog in relieving drought stress is important to properly manage the remaining coast redwood forests. Fog presence and ecosystem responses (e.g., climate, soil moisture, sap flow) were monitored at the Caspar Creek Experimental Watersheds in northwestern California over the 2020 fog season (Jun – Sept). 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 events, and 3) the influence of fog on transpiration. Fog presence was found to vary across the landscape with no significant relationship to harvesting. Fog deposition was higher at the shoulder position than at the ridge of the hillslope. Small increases in soil and litter moisture were observed at all study sites in response to fog events, with high temporal variation through the season. Surficial soil moisture was highest near the boles of S. sempervirens trees at the shoulder hillslope positions, regardless of harvest condition. All sites displayed lower transpiration rates during fog periods, but the greatest reduction was at the harvested sites. Overall, this research suggests that the distribution of fog and its ecological effects at the Caspar Creek Experimental Watersheds is primarily driven by topography and species composition rather than forest density.