Fog and its influence on the water relations of a California coastal pine forest

Background/Question/Methods

Most studies of drought-induced tree mortality have been limited to continental climates. Less is known about mortality events in coastal forests, where the maritime climate is thought to buffer large climate variations. However, following extreme drought in southern California in recent years (2007-2009), widespread mortality became evident in a Bishop pine (Pinus muricata) forest on Santa Cruz Island, California. Our ability to predict shifts in the distribution of this and other coastal tree species in a warmer, drier, and perhaps less foggy future requires a mechanistic understanding of how this species responds to changes in available moisture from fog water inputs. Moreover, if our predictions are limited to a single age class, we could misrepresent how whole populations may be affected by changes in moisture. In this research, we addressed the following questions: 1) Does summertime fog alleviate water stress of Bishop pines? 2) Does summertime fog affect the water stress of Bishop pines differently between sapling and adult trees? Xylem pressure potential (XPP) measurements were used to quantify the physiological response of trees to fog events throughout the rainless summer. We measured saplings and adults through fog events at two sites along a coastal-inland moisture gradient. To quantify fog events in terms of potential plant-available water, we measured fog-drip and shallow soil moisture (0-10 cm) after fog events.

Results/Conclusions

We found that adults maintained a lower level of water stress throughout the summer compared to saplings, with saplings more affected by the summer drydown (June XPP: -0.55, -0.54 MPa; September XPP: -0.96, -1.16 MPa, for adults and saplings, respectively). We also found that water stress of Bishop pines declined with increasing shallow soil moisture from fog-drip for both age classes, and this relationship was stronger for sapling (R²=0. 60, p<0.05) than for adult trees (n.s.). Three days of cumulative fog water prior to XPP measurements explained more of the variance in water stress for saplings at the drier inland site compared to when fog was not accounted for (fog, R²=0. 97, p<0.01; no fog, R²=0. 77, p<0.01). These results suggest that water stress in saplings is alleviated by summertime fog more than for adults, and that the mechanism is increased use of shallow soil moisture. The results will improve our ability to make mechanistically based predictions of how coastal forests may respond to a less foggy climate, and how different age classes may vary in their responses to fog variations.