Fog and dew impacts the hydrological balance in a montane chaparral community

Abstract The dry climate of southern California chaparral can cause substantive water stress on ecosystems that limits productivity throughout the year. From the mid-20th century, fog-days have happened less frequently and with decreasing levels of thickness in southern California (Witiw & LaDochy 2008), yet summer fog and higher winter precipitation have been found as key factors that explain the compositional diversity of the maritime chaparral community (Vasey et al. 2014). While the meteorological impact of fog lessens surface radiation in chaparral, the seasonal influence of year-round fog events on chaparral species remained uncertain. In particular, the effect of fog and dew on chaparral water supply and water stress has not been well understood. This includes for chamise (Adenostoma fasciculatum) and redshank (A. sparsifolium), two common, co-occurring species in semi-arid montane chaparral communities of southern California. Evaluating evapotranspiration, precipitation, and fog and dew from an eddy covariance tower system we estimated seasonal daily contributions of fog and dew water in a watershed at Sky Oaks Biological Field Station Reserve, California, USA, from May 2019 to April 2020. A leaf wetness sensor (LWS) measured the amount of water deposited during fog conditions or dew events, showing the highest deposition and longest duration in spring, and the lightest and shortest fog and dew presence during summer. The fog and dew water deposition typically peaked between 00:00 and 06:00 and totaled 97.3 mm y− 1 during the study period. Despite the low fog and dew deposition rates in the summer (20.5 mm) it was the only season when the amount of fog and dew water was greater than precipitation (10.3 mm). Based on stable isotope measurements, chamise absorbed more water from atmospheric sources (rain and fog + dew) than redshank across seasons, yet redshank had a higher foliar uptake capacity for liquid water than chamise in laboratory tests. This study suggests that the importance of fog and dew as a water source is species dependent, and that fog and dew water input may influence community structure and overall water balance in semi-arid shrublands. The intensity and frequency of fog and dew occurrence under future climate conditions may therefore be important to the future structure and functioning of this chaparral ecosystems.