Cloud and fog interactions with coastal forests in the California Channel Islands

Background/Question/Methods

Coastal forests in California are frequently covered by clouds or immersed in fog in the rain-free summer. Scientists have long surmised that fog might provide critical water inputs to these forests, arguing, for example, that coastal trees should be well adapted to absorb moisture directly from the atmosphere through their foliage. However, there is little process-level research to support how or why plants should prefer foggy regions; also, there is very little work quantifying water delivered by fog drip along the California coast except for a few notable sites. Summertime fog and stratus cover in California’s Channel Islands should ameliorate summer drought stress and might enhance soil water budgets. However, without spatial datasets of summer cloudcover and fog, combined with detailed process studies, questions regarding the roles of these two phenomena in dictating plant distributions and soil water balance cannot be effectively addressed. The overall objective of this project was to better understand how cloudcover and fog influence forest metabolism, growth, and distribution. Across a range of sites we measured a wide variety of ecosystem processes and properties. We then related these to cloudcover and fog immersion maps created using satellite datasets and airport and radiosonde observations.

Results/Conclusions

Our results demonstrate that cloudcover and fog strongly modulate radiation, water, and carbon budgets, as well as forest distributions, in this semi-arid environment. We compiled a spatially continuous dataset of summertime cloudcover frequency of the Southern California bight using daily satellite imagery from the NOAA geostationary GOES-11 Imager. We also created map of summertime cloudcover frequency of this area using morning MODIS imagery from the morning Terra overpass. To assess the ability of our mapping approach to predict spatial and temporal fog inundation patterns, we compared our monthly average daytime fog maps for GOES pixels corresponding to stations where fog inputs were measured with fog collectors on Santa Cruz Island. We have also compared our cloudcover maps to measurements of various physical and biological variables in a Bishop pine forest. Measurements of summertime fog drip, pine sapflow and growth, and soil respiration are strongly related to variations in cloudcover and fog drip. Importantly, spatial variations in cloud cover and fog immersion drive large changes in modeled water budgets and correspond closely to patterns of tree growth and mortality.