Summertime fog and its impacts on the water relations of adult and sapling trees in a coastal pine forest

Background/Question/Methods

Following extreme drought in southern California in recent years (2007-2009), widespread tree mortality became evident in a Bishop pine (Pinus muricata) forest on Santa Cruz Island, which harbors one of the few populations of this endemic and relictual species in California. Our ability to predict shifts in the distribution of this and other coastal tree species in a drier and possibly less foggy future requires a mechanistic understanding of how this species responds to changes in available moisture, especially summertime fog water inputs that can offset droughty conditions. In this research, we addressed the following questions: Is the water status of Bishop pines affected by fog water inputs? If so, do adult and sapling trees respond differently? In summer 2011, we measured the water relations of saplings and adults throughout the summertime before, during, and after fog events at two sites along a coastal-inland moisture gradient. Predawn xylem pressure potential (XPP) measurements were used to quantify the physiological response of trees to fog events throughout the rainless summer. To quantify fog events in terms of potential plant-available water, we measured fog-drip inputs and shallow soil moisture (0-10 cm) after fog events. In December 2012, estimates of turgor loss point (TLP) were derived from pressure-volume curves to quantify drought tolerance for adults and saplings at both sites.

Results/Conclusions

We found that water status correlated negatively with the summertime dry-down length and positively with fog water inputs for both adult and sapling trees (p<0.01); however, the magnitude of these effects differed between age classes, where saplings were more affected by the dry-down and fog water inputs than adults at each of the sites. In addition, water status of Bishop pines improved with increasing shallow soil moisture from fog-drip, and this relationship was stronger for saplings (R²=0.60, p<0.05) than for adults (R²=0.45, p=0.06). While all trees maintained turgor throughout the dry season, we found that trees at the drier inland site were more drought tolerant compared to trees at the more coastal site (inland TLP: -1.82 +/- 0.39 MPa; coastal TLP: -1.13 +/- 0.29 MPa; p<0.001). Our results demonstrate that the water status of Bishop pines declines during the dry season. However, fog events improve the water status of these trees, and the mechanism is likely through increased use of shallow soil moisture. These results will contribute to more robust predictions of how fog-influenced coastal forests may respond to a warmer and drier climate.