Quantification of fog input and use by trees of an oak woodland on Santa Catalina Island

The proportion of summer water input resulting from fog and the impact of canopy dieback on fog water input was measured during 2005 in order to understand the contribution of summer fog to oak woodlands of Santa Catalina Island. The amount of summer fog drip was quantified using specially designed fog drip collectors. The relative abundance of the stable isotopes of oxygen in collected fog and rain water, as well as soil and stem water, were analyzed to identify the extent to which oak trees use fog drip in their transpirational stream. In the summer of 2005, fog drip contributed up to 29% of water found in the upper soil layers of the oak woodland of Santa Catalina Island but oxygen isotope ratios in stem water suggest that oak trees are primarily using water from a deeper water source. Fog drip measurements indicate that the oak canopy actually inhibits fog from reaching soil underneath the trees; however, fog may contribute additional water to areas with no canopy. Recent observations on Santa Catalina have noted a significant decline in oak woodlands and an increase in the spread of grasslands. The results of this study indicate that as canopy thinning and oak mortality continue, more water may be available for the eventual conversion of these native woodlands to grasslands.