Fog in the California redwood forest: ecosystem inputs and use by plants

Abstract

Fog has been viewed as an important source of moisture in many coastal ecosystems, yet its importance for the plants which inhabit these ecosystems is virtually unknown. Here, I report the results of a 3-year investigation of fog inputs and the use of fog water by plants inhabiting the heavily fog inundated coastal redwood (Sequoia sempervirens) forests of northern California. During the study period, 34%, on average, of the annual hydrologic input was from fog drip off the redwood trees themselves (interception input). When trees were absent, the average annual input from fog was only 17%, demonstrating that the trees significantly influence the magnitude of fog water input to the ecosystem. Stable hydrogen and oxygen isotope analyses of water from fog, rain, soil water, and xylem water extracted from the dominant plant species were used to characterize the water sources used by the plants. An isotopic mixing model was employed to then quantify how much fog water each plant used each month during the 3-year study. In summer, when fog was most frequent, ∼19% of the water within S.sempervirens, and ∼66% of the water within the understory plants came from fog after it had dripped from tree foliage into the soil; for S.sempervirens, this fog water input comprised 13-45% of its annual transpiration. For all plants, there was a significant reliance on fog as a water source, especially in summer when rainfall was absent. Dependence on fog as a moisture source was highest in the year when rainfall was lowest but fog inputs normal. Interestingly, during the mild El Niño year of 1993, when the ratio of rainfall to fog water input was significantly higher and fog inputs were lower, both the proportion and coefficient of variation in how much fog water was used by plants increased. An explanation for this is that while fog inputs were lower than normal in this El Niño year, they came at a time when plant demand for water was highest (summer). Therefore, proportional use of fog water by plants increased. The results presented suggest that fog, as a meteorological factor, plays an important role in the water relations of the plants and in the hydrology of the forest. These results demonstrate the importance of understanding the impacts of climatic factors and their oscillations on the biota. The results have important implications for ecologists, hydrologists, and forest managers interested in fog-inundated ecosystems and the plants which inhabit them.

Fog Water and Ecosystem Function: Heterogeneity in a California Redwood Forest

“…The amount and timing of water delivery affects species distribution, productivity, and nutrient cycling. Although the spatial and temporal distribution of water is largely climatically controlled, the form in which water reaches terrestrial ecosystems-as rain, snow, fog, rime, or dew-can strongly influence its effect on an ecosystem through variation in timing, distribution, accessibility, and chemistry (Azevedo and Morgan 1974;Weathers and Likens 1997;Dawson 1998;Weathers andothers 1986, 2000;Vitousek 2004). …”

Regional And Watershed-Scale Coherence In the Stable-Oxygen and Carbon Isotope Ratio Time Series In Tree Rings Of Coast Redwood (Sequoia sempervirens)

“…et Ingraham and Matthews 1995;Dawson 1998) or that modify isotopic fractionation within the tree (e.g. humidity effects on leaf evaporative enrichment, Lipp et al 1996;Roden and Ehleringer 1999b).…”

“…By frequently saturating the canopy atmosphere, fog has strong effects on redwood transpiration rates, as shown by short-term physiological monitoring (Burgess and Dawson 2004;Ewing et al 2009;Johnstone and Dawson 2010). Dawson (1998) estimated that mature redwood forests obtain between 22-46% of their yearly water input from fog drip, which can further affect tree water balance (Azevedo and Morgan 1974;Huntley et al 1997;Simonin et al 2009). Redwood tree crowns and forest canopies can effectively strip moisture from fog (Azevedo and Morgan 1974;Burgess and Dawson 2004) where it can either drip into the surrounding soils or be retained and taken up directly through plant leaves (Burgess and Dawson 2004).…”

“…In turn, if wintertime precipitation inputs increase or decline, then the fractional importance of fog as a water source will be oppositely affected. During a drier-than-normal year, Dawson (1998) found that redwood trees became more dependent on fog water inputs as other water sources became exhausted.…”

“…The stable-oxygen and hydrogen isotope compositions (d 18 O and d 2 H) of meteoric water, fog and tree sap are valuable tools for assessing hydrologic inputs as fog, which is isotopically distinct from rainfall (Gonfiantini and Longinelli 1962;Aravena et al 1989;Ingraham and Matthews 1995;Dawson 1998). Through a series of known fractionation events, d 18 O (and d 2 H) variation derived from hydrologic inputs is incorporated into the organic matter that makes up tree rings (Yapp and Epstein 1982;White et al 1994;Roden and Ehleringer 1999b;Roden et al 2000;Treydte et al 2006;Gessler et al 2009).…”