Coastal redwoods (Sequoia sempervirens) are valued for their superlative height, as a source of decay-resistant timber, and a long-lived stock of carbon. Their habitat is characterized by consistent summer fog cover. Fog both reduces the demand for water and supplies water to redwood trees. It is known that redwoods increase their use of fog water during droughts, but it is not well known how access to fog water varies over space within redwood forests, and how spatial variation in fog water access affects redwood sensitivity to regional droughts. In this study, we used high-resolution (2-meter) airborne remote sensing data to study the effect of fog cover, topography, and forest structure on redwood sensitivity to drought during the recent 2011-2016 California drought. The Carnegie Airborne Observatory (CAO) collected hyperspectral and LiDAR data over Big Basin State Park, Muir Woods National Monument, and Jackson State Demonstration Forest in 2012 and 2016. We used hyperspectral data to map the distribution of redwood trees in the three forests, and calculate their canopy water content in 2012 and 2016, and change in canopy water content between the two periods. We used satellite imagery to map fog cover and CAO LiDAR data to map topography.
Fog cover varied significantly within and across the three forests. Redwood trees showed a distinct spectral reflectance signature relative to the other species found in redwood forests. The distribution of mapped redwoods within forests was related to intra-site variability in fog cover. Water content of mapped redwood trees varied in relation to topographic position and fog cover, and was lowest in areas with low fog cover and at elevations above the fog base. Fine-scale variation in redwood access to fog water during droughts has important implications for the future of redwood forests in a changing climate.