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


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 climate 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 environment because they benefit 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 quantified the effects of summer cloud cover by comparing ring-width indices to coastal airport cloud-frequency records (1944–2004). For the first time observed, summertime cloud frequency correlated positively with ring-width indices, regardless of whether the effect of rainfall was first removed from the ring-width record. The effect of ground-level fog was strongest in July early mornings (03:00 PST, R 2 = 0.262, P < 0.0002). The effect 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, R 2 = 0.148, P < 0.004). Correlations were substantially stronger in years with higher soil moisture, suggesting that growth response to summer clouds is strongly affected by pre-summer rainfall. A change in the height and/or timing of coastal cloud formation with climate change would likely affect this and other populations of California’s coastal vegetation.

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This work was supported by the Andrew W. Mellon Foundation, UCSB, and a NASA Earth System Science Graduate Student Fellowship. Weather station data from 1997 through 2005 were provided by J.B. Wall at CSU Northridge and collected on the University of California Natural Reserve System’s Santa Cruz Island Reserve, on property owned and managed by The Nature Conservancy. Tree cores were collected from the Channel Islands National Park. Thanks to K. McEachern, S. Chaney, C. Cowan, and CINP staff for their knowledge and assistance in conducting the field work. Thanks to D. Roberts, J. Michaelsen, J. Schimel, and R. Perroy for their helpful comments. Field work could not have been performed without the help of C. Ebert, D. Nelson, L. Merrill, and W. Amselem. Rainfall data were provided by L. Laughrin at the UC Natural Reserve System. Oxnard Airport cloud-height data were provided by J. Johnstone. All sampling and experiments complied with the current laws of the USA.

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Correspondence to A. Park Williams.

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Communicated by Todd Dawson.

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Williams, A.P., Still, C.J., Fischer, D.T. et al. The influence of summertime fog and overcast clouds on the growth of a coastal Californian pine: a tree-ring study. Oecologia 156, 601–611 (2008).

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  • Fog
  • Cloud
  • Pinus torreyana
  • Channel Islands
  • Tree rings