Summer Fog and Live Fuel Moisture in Coastal Californian Shrubland Ecosystems
In semi-arid regions around the world, water availability has a significant impact on plant physiology, plant community dynamics and the functioning of ecosystems. While early research explored the relative effects of rainfall and groundwater on plant and ecosystem processes, recent studies have begun to elucidate the importance of fog on water-limited ecosystems. Along the southern California coast, shrub-dominated plant communities experience seasonal drought simultaneous with coastal fog during the summer months. These plant communities also experience periodic wildfire disturbances due to weather conditions and water limitation. This dissertation sought to address gaps in our knowledge of the effects of coastal fog on shrub water status and live fuel moisture including the potential for wildfires in two shrubland plant associations in Santa Barbara County, California. In the first chapter, I used stable isotopes of hydrogen and oxygen to track fog water uptake and a field manipulation study to exclude fog drip from Artemisia californica, a dominant species in the sage scrub association which occurs at low elevations along the California coast. I found that this species takes up fog water during the late summer when conditions are driest and fog is present. I also found that fog drip slightly increases water content in these shrubs, potentially buffering the negative effects of drought. I also detected fog water uptake in the plants that were excluded from fog drip, suggesting water uptake by plant organs other than roots. This led to an investigation of the potential for foliar water uptake of fog in five common native shrub species which is the basis for chapter two. Chapter Two consisted of a greenhouse experiment in which I exposed shrub leaves to artificial fog over several nights and compared the physiological responses to this treatment with control plants. In addition, one of the nights of exposure included isotopically labeled fog to determine if fog water was taken up by plant leaves. The results suggested that four of the five species tested undergo foliar uptake of fog. Of the species that took up fog water, Salvia leucophylla (a drought deciduous species) was affected the most, with treatment individuals having higher water potential and greater photosynthetic capacity than controls. Chapter Three investigated fog water use in a field setting for the same five shrub species from both the chaparral and sage scrub associations. I also attempted to determine the relative influence of fog on live fuel moisture, a metric of plant flammability. The results from this study suggested that all five study species take up fog water during the summer drought, although the lower elevation sage scrub species take up fog in the late summer months due to higher fog deposition compared to the chaparral association. Additionally, the live fuel moisture patterns of the two drought deciduous species, A. californica and S. leucophylla, were significantly affected by fog deposition over the dry summer months. This dissertation established previously unknown fog water uptake in several dominant shrub species, explored the mechanisms of fog water uptake, and illustrated the importance of fog water availability for plant function and live fuel moisture loss during the summer drought. Overall, these studies established the effects of fog water on dominant shrub species from the chaparral and sage scrub associations of the southern California coast including the potential for fog to influence live fuel moisture drawdown patterns in lower elevation sites.