Summer water use by California coastal prairie grasses: fog, drought, and community composition

Abstract

Plants in the Mediterranean climate region of California typically experience summer drought conditions, but correlations between zones of frequent coastal fog inundation and certain species’ distributions suggest that water inputs from fog may influence species composition in coastal habitats. We sampled the stable H and O isotope ratios of water in non-photosynthetic plant tissue from a variety of perennial grass species and soil in four sites in northern California in order to determine the proportion of water deriving from winter rains and fog during the summer. The relationship between H and O stable isotopes from our sample sites fell to the right of the local meteoric water line (LMWL) during the summer drought, providing evidence that evaporation of water from the soil had taken place prior to the uptake of water by vegetation. We developed a novel method to infer the isotope values of water before it was subjected to evaporation in which we used experimental data to calculate the slope of the δH versus δO line versus the LMWL. After accounting for evaporation, we then used a two-source mixing model to evaluate plant usage of fog water. The model indicated that 28–66% of the water taken up by plants via roots during the summer drought came from fog rather than residual soil water from winter rain. Fog use decreased as distance from the coast increased, and there were significant differences among species in the use of fog. Rather than consistent differences in fog use by species whose distributions are limited to the coast versus those with broader distributions, species responded individualistically to summer fog. We conclude that fogwater inputs can mitigate the summer drought in coastal California for many species, likely giving an advantage to species that can use it over species that cannot.

Appendix

Details of the experiment to estimate the evaporation line: the slope of the change in δ²H and δ¹⁸O values in water from LMWL as water evaporates

We calculated the slope of the deviation of δ²H and δ¹⁸O values in water from the LMWL as water evaporates by measuring isotope values in water in soil samples collected as soil moisture evaporated over the course of four separate days at TP (13 June 2002, 25 June 2002, 14 September 2003, and 9 October 2003) and one day at BMR (29 August 2003). On each day, surface soil (0–2 cm depth) was sampled once every 1–2 h in a single 1 m² area without vegetation cover. The stable H and O isotopic composition of the water in each soil sample (n=32) was analyzed using methods described above.

The slope of the relationship between δ²H and δ¹⁸O values of the water in the soil samples as evaporation took place was analyzed using linear regression between δ²H and δ¹⁸O. The resulting “evaporation line” was δ²H = 3.5×δ¹⁸O − 33.6 (F _1,31=143.7, P<0.0001; r ²=0.82; Fig. 1b; Slope SE=0.3; Intercept SE=1.6), and represents the pooled effect of evaporation on the isotopic content of water in soil over repeated episodes of evaporation at two different coastal California habitats. The slope of the resulting equation, 3.5, is comparable with estimates of the relationship between δ²H and δ¹⁸O in other systems, including slopes of 2–5 in an experimental soil column using dune soils, (Allison 1982), and 4.0 in a Mexican aquitard (Ortega-Guerrero et al. 1997). Our slope and those reported by Allison (1982) and Ortega Guerrero et al. (1997), all using non-saturated soils, are lower than slopes derived from saturated soils or free water bodies (Gonfiantini 1986; Barnes and Turner 1998).

Climatic conditions ranged significantly, from heavy fog cover and high humidity to clear conditions and low humidity, during sampling to determine the evaporation line. Temperature and relative humidity were measured on 14 September and 9 October 2003 at TP using a Hobo data logger (Onset Computer Corp., Bourne, MA, USA). Temperature ranged from 11.4°C to 25.2°C, while relative humidity ranged from 38.1% to 99.1%. Relative humidity, in particular, is known to influence the kinetic fractionation in non-saturated soils (Allison et al. 1983; Barnes and Turner 1998) and, therefore, the slope of the evaporation line. Variation in the slope of the evaporation line as we sampled sites further inland (where humidity is lower) could call into question our conclusion that coastal and inland sites varied in their use of fog if decreasing humidity resulted in a decrease in the slope of the evaporation line. This would cause a spurious pattern of lesser “apparent fog use” in inland sites. However, Allison (1982), examining the effect of humidity on the slope of the δ²H–δ¹⁸O relationship in sandy, non-saturated columns much like our soils, reported that decreasing humidity resulted in an increase in the slope of the evaporation line, though the effect was small. If we apply Allison’s scenario of increasing slopes as we move inland, then we would have found marginally greater differences in fog use between our coastal and inland sites than we found using a single slope.

To test what effect different evaporation conditions might have for our estimates of fog use by coastal prairie vegetation, we simulated shifts of the evaporation line slope to the 95% confidence intervals (slope=2.9–4.1). Increasing the evaporation line slope from 3.5 to 4.1 decreased mean summer fog use by vegetation (n=84 samples) from 38% to 24%; decreasing the evaporation line slope to 2.9 increased mean fog use to 49%. As a result, while use of a different evaporation line may change the quantitative estimate of plants’ fog use, it would likely not influence the conclusion that coastal prairie vegetation relies on summer fog to a considerable extent.