Print PageIn recent decades, widespread increases in tree mortality rates have been observed in many forested regions across the globe, and these mortality events have been attributed to drought stress induced by regional warming. To date, study regions affected by widespread tree mortality have largely been limited to continental, montane climates. Much less is known about mortality events in other regions, such as coastal forests, which, due to the relatively unvarying nature of maritime climates and the presence of fog and cloud shading, have traditionally been thought to be buffered from large climate variations. However, following extreme drought in southern California between 2007 and 2009, widespread mortality became evident in a Bishop pine (Pinus muricata) forest on Santa Cruz Island, one of the California Channel Islands. Our ability to predict shifts in the distribution of this and other coastal tree species in a warmer, drier, and perhaps less foggy climate, requires a mechanistic understanding of how this species responds to changes in available moisture. In this research, we addressed the following questions: 1) Does summertime fog alleviate drought stress and reduce the risk of mortality of Bishop pine? 2) Does fog impact the water status of Bishop pines similarly across age classes? We used a decision tree approach to predict the probability of tree mortality at the stand scale using remotely sensed data of spatial cloud cover patterns and other environmental variables that control plant available water. To quantify the physiological response of trees from different age classes to fog events, we measured the xylem pressure potential and stomatal conductance of trees before, during, and after fog events between two sites that represent the ends of a coastal-inland moisture gradient.
Results/Conclusions
Based on our decision tree approach, we found an inverse correlation between fog water inputs and cloud shading and tree mortality at the stand scale. As expected, water stress in trees was greater at the dry, inland site (p< 0.001), and fog water input tended to have a disproportionate effect on alleviating water stress of trees at this site. While adults had a greater improvement in their water status with fog events (p<0.001) compared to saplings (p=0.05), each age class experienced relief from water stress immediately after a fog event. The outcome of this research will improve predictions of the distribution of coastal forests in Mediterranean climates based on the physiological mechanisms that determine the water status of the trees.
