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Significance of summer fog and overcast for drought stress and ecological functioning of coastal California endemic plant species

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Fog drip is a crucial water source for plants in many ecosystems, including a number of global biodiversity hotspots. In California, dozens of rare, drought-sensitive plant species are endemic to coastal areas where the dominant summer moisture source is fog. Low clouds that provide water to these semi-arid ecosystems through fog drip can also sharply reduce evaporative water losses by providing shade. We quantified the relative hydrological importance of cloud shading vs. fog drip. We then examined how both factors influence the range dynamics of an apparently fog-dependent plant species spanning a small-scale cloud gradient. The study area is on Santa Cruz Island off the coast of southern California. It is near the southern range limit of bishop pine (Pinus muricata D. Don), a tree endemic to the coasts of California and Baja, Mexico. We measured climate across a pine stand along a 7 km, coastal-inland elevation transect. Short-term (1-5 years) monitoring and remote sensing data revealed strong climatic gradients driven primarily by cloud cover. Long-term (102 years) effects of these gradients were estimated using a water balance model. We found that shade from persistent low clouds near the coast reduced annual drought stress by 22-40% compared with clearer conditions further inland. Fog drip at higher elevations provided sufficient extra water to reduce annual drought stress by 20-36%. Sites located at both high elevation and nearer the coast were subject to both effects. Together, these effects reduced average annual drought stress by 56% and dramatically reduced the frequency of severe drought over the last century. At lower elevation (without appreciable fog drip) and also near the inland edge of the stand (with less cloud shading) severe droughts episodically kill most pine recruits, thereby limiting the local range of this species. Persistent cloud shading can influence hydrology as much as fog drip in cloud-affected ecosystems. Understanding the patterns of both cloud shading and fog drip and their respective impacts on ecosystem water budgets is necessary to fully understand past species range shifts and to anticipate future climate change-induced range shifts in fog-dependent ecosystems.

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