2006-2011 Mission Kearney Foundation of Soil Science: Understanding and Managing Soil-Ecosystem
Functions Across Spatial and Temporal Scales Final Report: 2007029, 1/1/2009-12/31/2009
Department of Land, Air and Water Resources, University of California Davis, CA 95616
*Principle Investigator
For more information contact Dr. Randy Southard ([email protected])
Fog Contributions to Pedogenesis and Hydrology in Pinus muricata Ecosystems on Santa Cruz Island
Randal Southard*, Julie Baker
Project Objectives
In the Mediterranean-type climates that occur over much of California, water is severely
limited during the summer months. Several researchers have studied fog as a supplemental
water source for vegetation in coastal areas (Dawson, 1998; Ingraham and Matthews, 1995).
The importance of fog as a water source in plant and ecosystem function varies depending
upon the relative availability of fog water and other water sources (rain or groundwater). In
contrast, the importance of fog water in soil formation in these locations is not known. This
research addressed four objectives related to the role of fog in soil-forming processes on Santa
Cruz Island:
(1) Characterize soil temperature and moisture regimes, determine depth of wetting of rain and
fog, and describe soil morphology and genesis on a lithosequence under pine forest.
(2) Identify precipitation inputs from rainfall versus fog drip for soils under Bishop pine (Pinus
muricata) canopy versus grassland.
(3) Characterize organic matter decomposition rates in combinations of soil climate/ hydrology
through litterbag decomposition experiments.
(4) Characterize isotope signatures of rainfall, fog, soil solution, and pedogenic phyllosilicates.
Approach and Procedures
We studied two pairs of soils on a litho-biosequence of chlorite schist and rhyolitic tuff/breccia
on Santa Cruz Island. Each pair contained a soil on a north-facing slope that was covered by
Bishop pine (Pinus muricata) canopy, and a soil on a south-facing slope that contained grass or
shrub vegetation. Various species of arboreal vegetation (Pinus radiata, Eucalyptus sp., Sequoia
sempervirens, Erica arborea) have been shown to intercept and collect fog water, as opposed to
open grass- or shrub-covered areas where no medium for interception is present (Ingraham and
Matthews, 1995; Dawson, 1998; Prada and da Silva, 2001). The canopy-covered and open sites
were chosen to provide a comparison of soil climate and pedogenic processes within and
between parent materials.
The study sites are located at the Santa Cruz Island Reserve. Santa Cruz Island, the
largest of the northern Channel Islands, is dominated by two east-west trending mountain ranges,
and harbors a wide range of parent materials and vegetation types (Figure 1). Local climate and
weather patterns produce summer fogs created by moisture-
Fog Contributions to Pedogenesis and Hydrology in Pinus muricata Ecosystems on Santa Cruz Island—Southard
Figure 1. Location of Santa Cruz Island (inset) off the California coast, with site locations, pine coverage, and selected geology (Dibblee, 2001).
Fog Contributions to Pedogenesis and Hydrology in Pinus muricata Ecosystems on Santa Cruz Island—Southard
laden marine air blown onto Santa Cruz Island by the prevailing northwest winds (Junak et al.,
1995). The study sites, known as Weather Station (WS) on the Santa Cruz Island schist, a
Mesozoic chlorite schist, and Sierra Blanca (SB) on the Blanca formation, a Miocene rhyolitic
tuff/breccia (Dibblee, 2001), were located about 2 km apart. The WS site was located at about
425 m elevation, whereas the SB site was at about 275 m elevation. Locations were chosen to
minimize differences in topography between the sites (Table 1). Two pedons were excavated at
each site location, with three additional auger holes at each pine site to characterize any soil
variability related to canopy cover and determine depth of fog wetting. Each profile was
described by morphologic horizon in the field, and samples collected from each horizon, air
dried, and sieved to separate the <2 mm fraction (Soil Survey Staff, 2004). Standard x-ray
diffraction and selective dissolution procedures were performed following the methods of the
Soil Survey Staff (2004), Whittig and Allardice (1986), and Jackson (1975).
Temperature and throughfall precipitation data were used to construct a simple water
budget for each site according to the Thornthwaite model (Thornthwaite, 1948; Thornthwaite
and Mather, 1955; Thornthwaite and Mather, 1957). Potential evapotranspiration (PE) was
calculated using monthly averages of measured ambient temperature data, Ta, in the
Thornthwaite equation, PE = 1.6 (10 Ta / I)a.
A litterbag experiment was performed over a period of three years in which both pine and
grass litter were placed at two site pairs. Each pair consisted of a forested site, which received
fog drip and had an isomesic soil temperature regime (mean annual soil temperature (MAST) of
14 °C, <6 °C change between seasons), and a grass site, which did not receive fog drip and had a
thermic (MAST 19 °C) or hyperthermic (MAST 22 °C) soil temperature regime.
Table 1. Santa Cruz Island site locations.
We measured stable isotope ( D and 18
O) values of throughfall, soil solution, and
pedogenic phyllosilicates at each site. Fog events occurred dominantly during summer months,
and were generally enriched in isotope values compared to rain events, which occurred mainly
Fog Contributions to Pedogenesis and Hydrology in Pinus muricata Ecosystems on Santa Cruz Island—Southard
during winter months. The soil <0.2 m fraction, believed to be authigenic (Tabor et al., 2002;
Baker, 2010), was analyzed for stable 18
O isotope composition by heating and oxidation with
BrF5 following the methods of Clayton and Mayeda (1963). Hydrogen isotope ratios were
determined by reduction of the sample to hydrogen gas using a zinc catalyst and subsequent
measurement on the mass spectrometer following the methods of Kendall and Coplen (1985).
Phyllosilicate samples from the A and Bt horizons of each soil were analyzed for elemental
composition on a Cameca SX 100 microprobe (Cameca, Gennevilliers, France).
Results and Discussion
Objective 1
Soils at the SB site were classified as coarse-loamy over clayey, mixed, semiactive,
isomesic Ultic Paleustalfs under pine canopy, and fine-loamy, mixed, active, thermic Ultic
Argixerolls under grass vegetation (Baker, 2010). At the WS site, pine forest soils were coarse-
loamy, mixed, superactive, isomesic Typic Haplustalfs, and grassland soils were fine-loamy,
mixed, superactive, hyperthermic Typic Haplustalfs (Baker, 2010). Both sites were
characterized by mountainous terrain with steep slopes (30-60%) and exposed ridge tops. Slopes
with north aspects tended to have pine and oak vegetation, while south-facing slopes were
dominated by grasses and shrubs. Vegetation at the pine sites was dominated by Bishop pine
(Pinus muricata), an endemic island species, while the grass sites contained a mixture of forbs
(Eriogonum arborescens and Eriogonum grande), perennial grasses (Nassella sp.), and annual
grasses (Bromus sp., Hordeum sp., Poa sp., Vulpia sp.).
Soils formed under pine canopy tended to be deeper, have greater total clay, lower pH
and base saturation than soils formed under grass or shrub vegetation (Table 2). Higher Feo/Fed
ratios indicate that iron oxides in the pine soils may be less crystalline than in grass soils,
corresponding to less drying of pine soils due to summer fog drip. On a total profile basis, total
pedogenic iron (Fed) and Feo/Fed ratios indicate that the pine soils are more weathered than the
grass soils, and that the schist soils are more weathered that the rhyolitic tuff/breccia soils (Table
3). Chlorite was not detected in the WS soils; either chlorite has completely weathered to
vermiculite in the silt and sand fractions, and to kaolinite and smectite in the fine clay fraction, or
it was not initially present. The apparent ratio of kaolinite to smectite, based on XRD peak
intensity, is highest in the upper horizons and decreases with depth. A similar pattern is found in
WS grass soils. The SB pine soils are dominated by quartz and feldspar in the silt and sand
fractions, and kaolinite in the fine clay fraction, with smectite in the lower horizons of the pedon.
The SB grass soils are dominated by smectite in the fine clay fraction, with minor amounts of
kaolinite. Smectites in both pine soils have higher d-spacings than in the grass soils; reduced
drying due to fog inputs and lower temperatures may produce more weathered, lower charge, or
less crystalline phyllosilicates than at the grass sites. Fog appears to control soil microclimates
on the island with profound effects on soil forming processes.
Fog Contributions to Pedogenesis and Hydrology in Pinus muricata Ecosystems on Santa Cruz Island—Southard
Table 2. Selected morphological and chemical properties of Santa Cruz Island soils.
Fog Contributions to Pedogenesis and Hydrology in Pinus muricata Ecosystems on Santa Cruz Island—Southard
Table 3. Selective dissolution of selected Santa Cruz Island soils.
Objective 2
Yearly average precipitation (rain and fog drip) measured as throughfall by a rain gauge
under pine canopy from 2006-2008 at the Weather Station pine site was 52.1 cm yr-1
, while the
Sierra Blanca pine site received 38.1 cm yr-1
for the same period (Table 4). The range of
precipitation at the WS pine site was 39.1 cm (2008) to 68.2 cm (2006), while the range at the
SB pine site was 24.7 cm (2007) to 50.9 cm (2008). By comparison, the WS grass site received
estimated rainfall (no fog) averaging 43.1 cm yr-1
with a range of 31.1 cm (2008) to 62.1 cm
(2006). The SB grass site received estimated rainfall averaging 35.3 cm yr-1
for the years 2006-
2008 with a low of 20.5 cm (2007) and high of 48.0 cm (2008). Monthly throughfall means
showed high variability at both sites, with the standard deviation approaching or exceeding
monthly totals in about half the year (Table 4). Fischer et al (2009) also observed high spatial
two near-average years and two years of below-normal precipitation compared to the long-term
record (Laughrin, 2009).
At the WS pine site, data from the 2007 and 2008 summers show that fog events
routinely increase soil moisture in the surface 5 cm, and large fog events can infiltrate beyond 10
cm (Figure 2). Field observations show many very fine and fine roots at these depths at both
sites, perhaps allowing the pine trees to maximize water uptake during summer fog events. At
the SB pine site (Figure 3), the relationship between fog events and soil moisture is not as
obvious, perhaps because of the smaller recorded volumes of throughfall. During many fog
events, the lowest probe, at 25 cm, actually shows a greater increase in moisture than the middle
Fog Contributions to Pedogenesis and Hydrology in Pinus muricata Ecosystems on Santa Cruz Island—Southard
two probes, at 15 and 20 cm. This may be due to the change in texture with depth; the surface
textures are sandy loams, which have a lower water holding capacity than the clay loams and
clays beneath them. Although the sandier soils should have a higher water potential than the
clayier soils with the addition of a similar amount of water, the moisture probes measured
volumetric water content, not water potential. Even though the water contents measured by the
ECH2O probes at the SB site only included the top of the estimated moisture control section, we
classified the soil moisture regime as ustic rather than udic as suggested by the nearby Theta
probes, given the spatial variability of soil properties and precipitation.
Although our throughfall data shows greater precipitation at the WS site than the SB site,
both the soil moisture probes indicate that the SB soils retain water at potentials above wilting
point for longer than the WS soils. This confirms Fischer et al. (2009) finding that an elevational
belt of maximum fog precipitation exists on Santa Cruz Island from about 200-400 m. The WS
soils, at 425 m, are above this belt and may experience a decreased frequency of fog events
compared to the SB soils, although there is high spatial variability due to topography and wind
conditions. We had only one rain gauge at each site due to budget constraints, so it is possible
that throughfall was underestimated at the SB site and is actually closer to Fischer and Still’s
(2007) reported amounts for site 7.
Table 4. Monthly average throughfall (rain and fog) collected under pine canopy.
Throughfall was collected from June 2005 to July 2009.
Fog Contributions to Pedogenesis and Hydrology in Pinus muricata Ecosystems on Santa Cruz Island—Southard
Figure 2. Soil moisture content at the Weather Station pine site, as measured by ECH2O probes.
Figure 3. Soil moisture at the Sierra Blanca pine site, as measured by ECH2O Missing data from Jun-Oct 2008 are due to data logger malfunction.
Fog Contributions to Pedogenesis and Hydrology in Pinus muricata Ecosystems on Santa Cruz Island—Southard
Potential evapotranspiration (PE) at each grass site reached a maximum at about twice
the value and a minimum at about half the value of its pine pair, corresponding to the greater
temperature fluctuations at the grass sites (Figure 4). Although winter rainfall at each site pair is
equal, summer precipitation at the grass sites was zero, creating a much larger water deficit (PE-
AE). Both of the grass sites show an estimated AE approaching zero by July, while water
utilization continues throughout the dry season at the pine sites until recharge begins in
November. According to this model, differences in available water for sites only about a
hundred meters apart indicate a severe deficit of water during the summer months at the grass
sites, but the addition of intercepted fog water at the pine sites provides an additional 10 mm (SB)
to 20 mm (WS) of water per month. The grass sites, which have lower AWC than the pine soils
due to depth, also have a water surplus at the end of the wet season. Modeled water balances for
the pine soils do not show a water surplus, but the Thornthwaite model does not account for any
reduction in solar radiation caused by fog or overcast. Fischer et al. (2009) found a 29%
reduction in water deficit (PE-AE), at the WS pine site (site 10 in their study), so it is possible
that the decreased PE due to fog could create a water surplus near the end of the wet season,
which would have implications for leaching and mineral weathering.
Although the water budget models show no surplus at the pine sites, the soils under pine
canopy are deep with well-developed argillic horizons, have low pH, and lack carbonates and
soluble salts (Baker, 2010). It is possible that much of the leaching that must have occurred to
create these soils took place during a prior climate with cooler and wetter conditions. Because
our sampling period captured only average and below-average precipitation years, it is also
possible that substantial leaching occurs during above-average precipitation years. The absence
of carbonates or soluble salts, which could accumulate without leaching, seems to confirm that
some leaching is occurring under the current climate.
Fog Contributions to Pedogenesis and Hydrology in Pinus muricata Ecosystems on Santa Cruz Island—Southard
Figure 4a. Weather Station pine site (WSP).
Figure 4b. Sierra Blanca pine site (SBP).
Fog Contributions to Pedogenesis and Hydrology in Pinus muricata Ecosystems on Santa Cruz Island—Southard
Figure 4c. Weather Station grass site (WSG).
Figure 4d. Sierra Blanca grass site (SBG). Figure 4. Water balance month numbers correspond to calendar months, starting with January. Estimated available water-holding capacity used in the calculation of actual evapotranspiration was 150 mm for the pine soils and 75 mm for the grass soils.
Fog Contributions to Pedogenesis and Hydrology in Pinus muricata Ecosystems on Santa Cruz Island—Southard
Objective 3
Decomposition rates of both pine and grass litter were highest at the pine sites,
suggesting that moisture is the most limiting factor in this environment (Figures 5 and 6). The
additional moisture received as fog drip at the pine sites accelerates decomposition processes
even though the pine sites are cooler than the grass sites. Decomposition rates of both litter types
decreased with time at the pine sites, while rates for both litters remained relatively constant at
the grass sites over the duration of the experiment. Grass litter decomposed faster than pine litter
at the grass sites, suggesting that litter composition and/or microbial community composition and
size regulate decomposition at the grass sites. Pine litter C:N ratios decreased with time, as did
C:N ratios of grass litter at pine sites (Figures 7 and 8). The C:N ratios of grass litter at grass
sites increased initially, then began to decrease over time. The C:N ratios of both litter types
were lowest at the pine sites throughout the experiment, consistent with the higher
decomposition rates at those sites. Litterbag decomposition rates for both pine and grass litter
were similar at the pine sites, indicating that climate, and specifically available moisture, not
organic matter composition, is the most limiting factor in this environment. Mineral weathering
may be accelerated under pine canopies where moisture from fog drip creates environments
favorable for increased decomposition and biological activity. Climate change scenarios that
decrease precipitation or alter the temporal distribution of fog drip may limit nutrient cycling and
availability, while scenarios that increase precipitation may accelerate organic matter
decomposition and deplete soil C stocks.
The additional water received as fog drip by soils under pine canopies may contribute to
an increase in leaching and mineral weathering compared to grass soils. Pine forest soils are
deeper, have more total clay, and lower pH and base saturation than grass soils (Baker, 2010).
Since many of the processes of mineral weathering are accelerated by microorganisms (Kostka et
al., 1999; Reith and McPhail, 2007), it is likely that the increased biological activity at the pine
sites occurs concurrently with increased mineral weathering. Although we did not measure
respiration, increased decomposition rates at the pine sites probably result in increased
production of CO2 and carbonic acid, leading to greater rates of mineral dissolution. It has been
widely reported in the literature that increased microbial and biological activity in the
rhizosphere contributes to greater mineral weathering and dissolution rates, especially in nutrient
limited environments (Hinsinger et al., 2005; Turpault et al., 2009; Uroz et al., 2009). Higher
total iron (Fed) and aluminum (Ald), measures of intensity of weathering (McFadden and
Hendricks, 1985), observed at the pine sites (Baker, 2010) indicate that mineral weathering has
progressed to a greater degree at the pine sites compared to the grass sites. In the Channel
Islands, fog appears to be linked to mineral weathering by creating environments favorable for
increased activity by microorganisms, which then accelerate weathering processes.
Fog Contributions to Pedogenesis and Hydrology in Pinus muricata Ecosystems on Santa Cruz Island—Southard
Figure 5. Mean decomposition rates and standard deviations of pine litter for Study 2. WS, Weather Station site; SB, Sierra Blanca site. Litterbags were installed in March 2006. Numbers in parentheses are mean decomposition rates for each sampling date.
Figure 6. Mean decomposition rates and standard deviations of grass litter for Study 2. WS, Weather Station site; SB, Sierra Blanca site. Litterbags were installed in March 2006. Numbers in parentheses are mean decomposition rates for each sampling date.
Fog Contributions to Pedogenesis and Hydrology in Pinus muricata Ecosystems on Santa Cruz Island—Southard
Figure 7. Mean C:N ratios and standard deviations of pine litter for Study 2. Ratio at time=0 is the initial composition of undecomposed plant material. WSP, Weather Station Pine; WSG Weather Station Grass; SBP, Sierra Blanca Pine; SBG Sierra Blanca Grass.
Figure 8. Mean C:N ratios and standard deviations of grass litter for Study 2. Ratio at time=0 is the initial composition of undecomposed plant material. WSP, Weather Station Pine; WSG Weather Station Grass; SBP, Sierra Blanca Pine; SBG Sierra Blanca Grass.
Fog Contributions to Pedogenesis and Hydrology in Pinus muricata Ecosystems on Santa Cruz Island—Southard
Objective 4
Stable oxygen (Figure 9) and hydrogen (Figure 10) isotope analyses of soil solution
samples indicate a depletion of the heavier isotopes in the rainwater compared to fog. Soil
solution samples from surface horizons measured during precipitation events show a shift in
isotope ratios to heavier values during fog events and lighter values during rain events, but
generally increase in value with depth as packets of water influenced by evaporation infiltrate
deeper into the profile. A plot of 18O versus D for the soil solution samples (Figure 11)
shows a decrease in slope from the Local soil water lines (LMWL), indicating that evaporation
has occurred, in particular at the grass sites.
Extracted soil solution isotope values indicate an approximation of piston flow of water
with each precipitation event (Figures 9 and 10). For example, the March 2006 rain event
brought lighter water into about the top 10 cm of the profile, while heavier water was pushed
down deeper into the profile. Similarly, the soil water profile from the May 2006 fog event
shows heavier water near the surface of the soil, while lighter water is pushed deeper into the
profile. Although evaporation may produce similar patterns of soil solution isotope ratios
(increase in values towards the bottom of the profile during wet periods, decrease in towards
the bottom of the profile during dry periods) (Hseih et al., 1998), samples were collected during
precipitation event to minimize evaporation effects. Soil solution in open areas appears to be
heavily influenced by evaporation, while soil solution under pine canopy did not show strong
evaporation effects, and the range of isotope values measured at the pine sites is much narrower
than those measured at the grass sites. Extracted soil solution from the grass sites shows an
enriched isotope signature over fog values, and plots off the meteoric water line, indicating that
fractionation during evaporation has occurred. In particular, the soil solution isotope
composition measured after rainfall events in December and January shows enriched values at
depth at the grass sites compared to the pine sites. This water, near the soil surface during the
warm summer months, likely indicates fractionation due to evaporation, then transport to deeper
horizons during subsequent rain events.
Local soil water lines for each site were constructed from measured soil solution 18
O
and D values (Table 5). Because this study focused on the formation of pedogenic
phyllosilicates, the composition of the soil solution with which the minerals formed in
equilibrium, rather than of precipitation, likely gives a better estimation of the mineral-water
isotope relationship. Grass sites, with LSWL slopes higher than those of the pine sites, showed a
strong influence of evaporation, and pine sites showed minimal evaporation effects, so soil water
samples were separated by site to give the best estimate of meteoric water composition in
equilibrium with pedogenic phyllosilicates. Estimated temperatures of formation are similar for
both grass sites (WSG and SBG); these sites are similar in terms of soil depth, climate, and water
contributions (i.e., no fog). Although we did not measure soil solution chemistry, pedogenic
minerals may be expected to crystallize during summer months as the soil dries and solutes are
concentrated in the soil solution (Furquim et al., 2008; Ryan and Huertas, 2009). The calculated
temperatures of formation may reflect this temporal preference for mineral formation with
increased surface temperatures. In the subsurface, temperature fluctuations are smaller and
temperatures lower than at the surface during the summer.
Fog Contributions to Pedogenesis and Hydrology in Pinus muricata Ecosystems on Santa Cruz Island—Southard
Figure 9a. Soil solution δ18O composition with depth at Weather Station Grass site.
Figure 9b. Soil solution δ18O composition with depth at Sierra Blanca Grass site.
Fog Contributions to Pedogenesis and Hydrology in Pinus muricata Ecosystems on Santa Cruz Island—Southard
Figure 9c. Soil solution δ18O composition with depth at Weather Station Pine site.
Figure 9d. Soil solution δ18O composition with depth at Sierra Blanca Pine site.
Fog Contributions to Pedogenesis and Hydrology in Pinus muricata Ecosystems on Santa Cruz Island—Southard
Figure 10a. Soil solution δD composition at Weather Station Grass site.
Figure 10b. Soil solution δD composition at Sierra Blanca Grass site.
Fog Contributions to Pedogenesis and Hydrology in Pinus muricata Ecosystems on Santa Cruz Island—Southard
Figure 10c. Soil solution δD composition at Weather Station Pine site.
Figure 10d. Soil solution δD composition at Sierra Blanca Pine site.
Fog Contributions to Pedogenesis and Hydrology in Pinus muricata Ecosystems on Santa Cruz Island—Southard
Figure 11. Isotope values of soil solution samples at all sites. WSP, Weather Station Pine; WSG, Weather Station Grass; SBP, Sierra Blanca Pine; SBG, Sierra Blanca Grass. See Table 5 for LSWL equations for each site.
Table 5. Estimated temperatures of crystallization for smectite-kaolinite mixtures assuming
equilibrium with meteoric water. Also given are estimates of soil water δ18O values assuming
equilibrium between phyllosilicates and meteoric water in the soil profile.
Fog Contributions to Pedogenesis and Hydrology in Pinus muricata Ecosystems on Santa Cruz Island—Southard
At the pine sites (WSP and SBP), the estimated temperatures of formation do not follow
the pattern established at the grass sites. At the WSP site, surface horizon temperatures are lower
than subsurface temperatures, and both temperatures are higher than the WSG site, a surprising
result given the measured climatic conditions at each site. Although organic matter was removed
prior to isolation of the <0.2 m fraction, both WSP and SBP A horizons had high organic matter
contents, and it is possible that some organic matter was protected from oxidation in
microaggregates. The inclusion of this organic matter in the sample for isotope analysis may
have altered the measured D value, introducing error to the temperature estimates.
Nonexchangeable soil organic matter D values are typically more depleted than our measured
phyllosilicate values, with reported del values ranging from -244 to -180 (Seki et al, 2010) and -
167 to -75 (Ruppenthal et al, 2010). In particular, at the SBP site, which had the lowest
measured surface phyllosilicate D value, contamination by organic matter could result in a
lower estimated meteoric water composition in equilibrium with the phyllosilicates at
crystallization, underestimating the input of fog. At the SBP site, both surface and subsurface
temperatures are similar, and lower than surface temperatures at the SBG site. This may be a
reflection of the isomesic soil climate at the SBP site, where seasonal variations in soil
temperatures are minimal. Since the SBP Bt horizon sample was contaminated with excess Na,
resulting in an imbalance of Al in the calculated smectite formula, and SBP A horizon smectite
chemical composition used in the calculation of fractionation factors and temperature for the Bt
horizon, it is also possible that this substitution resulted in the similar estimated temperatures for
both horizons.
In addition, an estimation of meteoric water 18
O in equilibrium with the phyllosilicates
was calculated from temperature estimates and mixture fractionation equations, using the
relationship 1000 ln mineral-water = 18
Omineral - 18
Owater (Table 5). The estimated water 18
O
values are within the range of measured soil solution values, and show that pedogenic minerals
in A horizons formed in equilibrium with consistently depleted waters compared to minerals in
Bt horizons. This may indicate that the isotope signature of rain is overwhelming the signature
of fog, resulting in the depleted ratio near the surface of the pedon. The lower surface
temperatures associated with fog events at the pine sites may also result in lower than expected
water 18
O values, underestimating the input of fog drip. This type of distribution may also be
encountered where evaporation has enriched surface soil solution during the dry season,
followed by displacement by wet season precipitation to the subsurface. Water budget estimates
for these soils indicate that any dry season soil solution may remain in the profile in normal to
dry precipitation years (Baker, 2010). At the grass sites, this is the likely mechanism for water
infiltration into the pedon, and may indicate that phyllosilicate formation is occurring dominantly
during the wet season. The depth profiles of soil solution isotope composition at each site
indicate that pine sites are not substantially affected by evaporation, since subsurface values do
not exceed surface values collected during fog events. At the pine sites, the reversal of the
pattern of higher temperatures in the surface compared to the subsurface that is seen at the grass
sites may indicate that some phyllosilicate crystallization from fog water is occurring during the
dry season, when temperatures are higher.
The calculated temperatures of mineral formation and associated meteoric waters in
equilibrium with the soil minerals indicate that the contribution of fog to mineral formation in
the Channel Islands may not be sufficient to distinguish a difference in isotope signatures of
pedogenic phyllosilicates formed in equilibrium with rain or fog water. However, the higher
Fog Contributions to Pedogenesis and Hydrology in Pinus muricata Ecosystems on Santa Cruz Island—Southard
estimated temperature of formation in the pine surface horizons may indicate some fog
influence. These soils contained a mixture of 1:1 and 2:1 phyllosilicates; if precipitation of
these minerals is temporally separated, the seasonal variation in isotope composition of the
precipitation may result in phyllosilicates with fractionations other than anticipated (Stern et
al., 1997). Nonetheless, pedogenic phyllosilicates appear to be capable of recording
systematic differences in temperature and meteoric water composition between surface and
subsurface soil horizons. For future study, a refinement of this technique may be useful for
detecting seasonal variation in phyllosilicate formation corresponding to meteoric water and
soil solution composition.
Conclusions
Soil moisture sensors, water balance models, and isotope signatures of precipitation
(Baker, 2010) indicate that water from fog drip infiltrates into the pedon. The fog water,
available at a time when soil temperatures are at a maximum, also contributes to increased
organic matter decomposition rates (Baker, 2010), possibly accelerating nutrient turnover and
mineral weathering and contributing to soil fertility. Historically, it is likely that the combination
of pine forest and foggy climate have aided in the formation of deeper, clayier soils than the
adjacent grasslands. Increased water and increased organic and carbonic acid production may
accelerate mineral weathering, leading to the well-developed argillic horizons observed in the
soils under pine canopy. While a change in weather patterns is unlikely to alter the soils already
in place over the short term, organic matter decomposition and nutrient cycling rates may
decrease if fog drip is reduced or shifts in temporal distribution, also threatening the pine forest
stands. It is clear that fog drip is an important part of the hydrologic cycle of the island pine
stands, but fog may also affect soil processes in less direct ways that are equally important to
pine tree growth and survival.
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This research was funded by the Kearney Foundation of Soil Science: Understanding and Managing Soil-Ecosystem Functions Across Spatial and Temporal Scales, 2006-2011 Mission (http://kearney.ucdavis.edu). The Kearney Foundation is an endowed research program created to encourage and support research in the fields of soil, plant nutrition, and water science within the Division of Agriculture and Natural Resources of the University of California