The inXuence of summertime fog and overcast clouds on the growth of a coastal Californian pine: a tree-ring study

Abstract The coast of California is home to numerous rare, endemic conifers and other plants that are limited in distribution by drought sensitivity and the summer-dry cli-mate that prevails across most of the state. Ecologists have long assumed that some coastal plant populations survived the early Pleistocene transition to a warmer and drier envi-ronment because they beneWt from frequent fog and stratus clouds that provide water and shade during the rainless summer. One such population is that of Torrey pine (Pinus torreyana ssp. Insularis) on Santa Rosa Island in Channel Islands National Park. Here we report that the tree-ring width record from this population indicates strong growth sensitivities to summer fog drip and cloud shading. We quantiWed the eVects of summer cloud cover by comparing ring-width indices to coastal airport cloud-frequency records (1944–2004). For the Wrst time observed, summer-time cloud frequency correlated positively with ring-width indices, regardless of whether the eVect of rainfall was Wrst removed from the ring-width record. The eVect of ground-level fog was strongest in July early mornings (03:00 PST, R2 = 0.262, P < 0.0002). The eVect of clouds high enough to provide shade but not fog water was also strongest in July, but climbed steadily throughout the day before becoming strongest in late afternoon (16:00–18:00 PST, R2 = 0.148, P < 0.004). Correlations were substantially stronger in years with higher soil moisture, suggesting that growth response to summer clouds is strongly aVected by pre-summer rainfall. A change in the height and/or timing of coastal cloud formation with climate change would likely aVect this and other populations of California’s coastal vegetation