Ecological and biogeographic impacts of fog and stratus clouds on coastal vegetation, Santa Cruz Island, California

Fog drip is a crucial water source for plants in many ecosystems, including a number of global biodiversity hotspots. In the California hotspots, dozens of rare, drought-sensitive plant species are endemic to coastal areas where the sole summer moisture source is fog. Low clouds that provide some water to these semi-arid ecosystems through fog drip can also sharply reduce evaporative water losses by providing shade. The goal of this dissertation is to quantify the relative hydrologic importance of cloud shading versus fog drip in the southern part of the California fog belt.; The study area is on Santa Cruz Island (SCI), the largest island in Channel Islands National Park, off the coast of southern California. It is near the southern range limit of the study species, Bishop pine (Pinus muricata D. Don), a tree species endemic to the coasts of California and Baja California. Spatial and temporal variability in fog drip and cloud cover were monitored across a coastal--inland elevation transect spanning the western-most stand of Bishop pine on SCI.; Twenty-one fog collectors of a new design were deployed at 12 sites across the stand, with weather stations at 7 of these sites. With significant spatial variability, fog drip was greatest at elevations of a few hundred meters, nearer the coast, and in topographic funnels. Plant physiological measurements and isotopic analysis of ecosystem water pools suggest fog water is taken up by trees, reducing summer drought stress. With episodic summer fog drip, soil water pools appear to be very dynamic on short time scales in both availability (water potential) and stable isotope composition.; Cloud cover decreases rapidly from the coast inland. Cloud shading appears to be as important as fog drip in enhancing ecosystem water availability near the coast, but that importance decreases rapidly with distance inland. Over multiple decades, fog and overcast appear to dramatically reduce frequency of severe, stand-killing droughts, thereby helping to maintain species ranges.; Understanding patterns of both shading and fog drip and their respective impacts on ecosystem water budgets will be necessary to anticipate climate change-induced range shifts in fog-dependent biodiversity hotspots.