ORIGINALARTICLE
Significance of summer fog and overcastfor drought stress and ecologicalfunctioning of coastal Californiaendemic plant species
Douglas T. Fischer1*, Christopher J. Still2,3 and A. Park Williams2
INTRODUCTION
Fog has often been identified as a crucial water source for
plants in a variety of ecosystems world-wide, including a
number of those identified as global biodiversity hotspots
(Hamilton et al., 1995; Olson & Dinerstein, 1998). Biodiversity
hotspots where fog is thought to play a particularly important
role in species distributions include tropical montane cloud
1Geography Department, California State
University Northridge, Northridge,2Department of Geography and 3Institute of
Computational Earth System Science,
University of California, Santa Barbara, Santa
Barbara, CA, USA
*Correspondence: Douglas T. Fischer,
Geography Department, California State
University Northridge, Northridge, CA 91330-
8249, USA.
E-mail: [email protected]
ABSTRACT
Aim 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.
Location 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.
Methods 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.
Results 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.
Main conclusions 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.
Keywords
California, Channel Islands, cloud shading, drought stress, evapotranspiration,
fog, Pinus muricata, range limits, Santa Cruz Island, water balance.
Journal of Biogeography (J. Biogeogr.) (2008)
ª 2008 The Authors www.blackwellpublishing.com/jbi 1Journal compilation ª 2008 Blackwell Publishing Ltd doi:10.1111/j.1365-2699.2008.02025.x
forests (65–75 million ha in the Neotropics alone; Kappelle,
2004) and the more than 25% of the world’s coastal areas that
receive regular fog (T. Dawson, personal communication,
2007). Foggy coasts with remarkable levels of species diversity
and endemism include mediterranean-climate regions of Chile,
south-western Australia, southern Africa and California.
Global climate change and land-cover change are already
altering fog patterns in fog-dependent ecosystems, with as yet
unknown effects on the range dynamics of rare species
(Pounds et al., 1999, 2006; Still et al., 1999). In California,
one of the areas of highest endemic and relict species richness
is within the ‘fog belt’ along the immediate coast, which is
subject to persistent summer fog and overcast conditions
(Raven & Axelrod, 1978).
Coastal fog and overcast conditions both result from low
stratus cloud formation. The distinction is that ‘overcast’ is a
cloud layer above the surface that obscures all (or most of) the
sky, while ‘fog’ is in contact with the surface and reduces
visibility to below 1 km (National Weather Service, 2007).
Crucially, the same stratus cloud that is fog at one site on a
coastal mountain can manifest as an overcast layer for sites at
lower elevations (Fig. 1). Both fog and overcast can reduce the
severity of plant drought stress during the summer rainless
season, though via different mechanisms. Previous studies in
the California fog belt have shown that stratus clouds can
enhance the availability of water to the ecosystem during the
rainless summer through fog drip, and some have demon-
strated uptake of fog water by endemic plants (Azevedo &
Morgan, 1974; Jacobs et al., 1985; Ingraham & Matthews,
1995; Dawson, 1998; Burgess & Dawson, 2004; Cole, 2005;
Corbin et al., 2005; Kennedy & Sousa, 2006). In areas of
frequent fog and favourable canopy structure, such as coast
redwood forests, fog drip has been shown to contribute
hundreds of millimetres of water to the soil surface annually
(e.g., Jacobs et al., 1985; Dawson, 1998).
The other mechanism by which stratus clouds can poten-
tially ameliorate plant water stress is by reducing ecosystem
water loss. This effect has received less attention from
researchers (but see Burgess & Dawson, 2004). Stratus clouds
can retard water loss by absorbing and reflecting large amounts
of solar radiation, thus greatly reducing net radiation at the
surface. This cloud shading leads to lower air and leaf
temperatures, increased relative humidities and smaller vapour
pressure deficits. The overall effect of these changes in
microclimate is to substantially reduce atmospheric evapora-
tive demand and potentially decrease ecosystem water losses
during the dry season. These reductions in water stress are even
thought to have been consistent enough over long time periods
to have allowed the evolution of larger leaf sizes in insular
species within the fog belt (Hochberg, 1980). Many of the relict
and endemic species throughout California’s fog belt are quite
sensitive to drought (Raven & Axelrod, 1978), and so any
change in summer water stress, whether from rainfall, fog drip
or cloud shading, is likely to affect species range limits and
long-term population viability.
Fog climatology
Like most of California, the coastal fog belt is characterized by
a mediterranean climate, with cool, rainy winters and warm,
rain-free summers. The annual summer drought ranges from 3
to 7 months, with a longer duration further south. Summer
cloud cover is most common along the immediate coast, with
both frequency and duration decreasing further inland. The
tops of the summertime marine stratus clouds along the coast
of California are most commonly below 400–500 m, which
roughly corresponds to the base of the coastal inversion
(Leipper, 1994). As a result of summertime inversion base
height dynamics, coastal sites above 400 m or so experience a
decreasing frequency of fog, sites below c. 200 m experience an
increasing percentage of stratus cloud events as overcast rather
than as fog, and sites at elevations of roughly 200–400 m
experience the most frequent fog. Coastal mountains generally
restrict the inland movement of the marine stratus layer to
areas within a few to 20 km of the immediate coast (with the
exception of some larger drainages). During stratus cloud
events, these clouds tend to form and/or intensify overnight
and then ‘burn off’ during the next day, usually evaporating
progressively from inland areas towards the coast (Marotz &
Lahey, 1975; Filonczuk et al., 1995; Fischer & Still, 2007).
Fog belt biogeography
Dozens of plant species are endemic to the fog belt. An
accepted explanation for these narrow, coastal distributions is
that widespread drought-sensitive species suffered range col-
lapse as the interior of California became drier and more
climatically variable following uplift of the Coast Ranges
during the Pliocene and Quaternary (Raven & Axelrod, 1978).
Figure 1 Conceptual model of differences in spatial distribution
of coastal cloud shading vs. fog drip. In southern California
summer fog drip is generally concentrated on west-facing slopes at
elevations of 200–400 m a.s.l. (with significant variability related
to funnelling of clouds by local topography). Cloud shading
decreases inland from the coast and on ridges that extend above
the typical heights of persistent stratus clouds. Drought-sensitive
pines at the study site are most abundant in areas at high enough
elevation to receive extensive fog drip, and close enough to the
coast to receive extensive cloud shading. Other foggy ecosystems
are likely to have different spatial separation between the two
processes of cloud shading vs. fog drip.
D. T. Fischer et al.
2 Journal of Biogeographyª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd
During this transition, summer rainfall was also eliminated
across most of California, with tremendous implications for
drought-sensitive species. One well-documented example of a
drought-sensitive species with such a relict distribution is the
coast redwood (Sequoia sempervirens D. Don). Many studies
have investigated the role of fog drip in maintaining stands of
coast redwood and other coastal species, particularly by
ameliorating the annual drought resulting from rainless
summers (Leyton & Armitage, 1968; Azevedo & Morgan,
1974; Ingraham & Matthews, 1995; Dawson, 1998; Burgess &
Dawson, 2004; Cole, 2005; Corbin et al., 2005). More recently
arrived or evolved species, such as Monterey pine (Pinus
radiata D. Don), may have spread in small pockets of equable
habitat along the coast during the Pliocene (Millar, 1986,
1999), expanding and contracting in response to repeated large
climate shifts throughout the Quaternary (Heusser, 1995).
After the Last Glacial Maximum, the southern range limits of
many California coastal species retreated tens to hundreds of
kilometres north as climate warmed (Chaney & Mason, 1930,
1933; Mason, 1934; Raven & Axelrod, 1978; Millar, 1999). All
of these fog-belt endemic species appear variously threatened
by the projected warming and drying of the climate in the
coming decades (IPCC, 2007). Also, regional climate model
simulations project changes to the upwelling regime along
California’s coast with climate warming (Snyder et al., 2003), a
phenomenon that has already been documented in other
eastern boundary coastal upwelling systems (Bakun, 1990;
Schwing & Mendelssohn, 1997; Mendelssohn & Schwing, 2002;
Snyder et al., 2003). Because fog formation is closely linked to
upwelling, these projections suggest alterations in coastal fog
dynamics. Changes in fog patterns, in addition to overall
drying and warming, may lead to large changes in the location
of suitable habitat for coastal endemic species.
Study goal
The goal of the current study was to investigate the separate
impacts of fog drip and cloud shading in maintaining the
ranges of coastal endemic species. Our first challenge in
meeting this goal was to quantify the degree to which shading
by clouds, independent of any fog drip, significantly reduced
summer ecosystem drought stress. We also quantified the
addition of water from fog drip and the resulting spatial
patterns in water availability. The second challenge was to
quantify the long-term effects of both cloud shading and fog
drip on the range dynamics of a fog-belt endemic species. We
utilized a combination of intensive field sampling, water
balance modelling and remote sensing to quantify these effects
in one part of the fog belt, on Santa Cruz Island off the coast of
southern California. Our study focused on a stand of the
drought-sensitive, fog-belt endemic species bishop pine (Pinus
muricata D. Don). This tree grows in a number of scattered
groves along the coast of central California, never more than
c. 30 km from the coast (Fig. 2a; Griffin & Critchfield, 1972;
Lanner, 1999). In the northern part of the range of bishop
pine, where rain and fog are greater and evapotranspiration is
generally lower, stands are larger and more numerous, and
trees are generally taller and grow more densely. Fossil records
of this and related species are likewise limited to the coast
(Axelrod, 1967; Raven & Axelrod, 1978; Millar, 1986). Our
study site on Santa Cruz Island focused on the southern-most
large stand of bishop pine.
Both the Materials and Methods and the Results sections are
organized by time-scale. We first discuss short-term monitor-
ing of water availability across our transect. These short-term
efforts to characterize spatial patterns combine both remote
sensing and extensive field measurements over 1–5 years. We
then describe the water balance modelling experiments we
conducted using 102 years of weather data to assess likely
long-term biogeographical impacts of these spatial patterns.
MATERIALS AND METHODS
Site description
Santa Cruz Island (SCI), the largest of the California Channel
Islands, is located c. 40 km south of Santa Barbara (Fig. 2a).
The island is 38 km long and is bisected by the rift valley of the
SCI fault. Steep east–west ridges rise both north and south of
the fault (Fig. 2b). Bishop pines grow in three stands on the
island, with approximate stand boundaries shown by white
polygons in Fig. 2c. These stands form the southern end of the
main distribution for this species and each stand is surrounded
by more xeric shrubland. (There is one small outlying stand in
northern Baja; Lanner, 1999). Within the SCI stand bound-
aries, pine densities are far from uniform. Our study was
confined to the large western stand. In this stand, the highest
pine densities occur near the centre (50–80% canopy cover).
Towards the eastern and western edges of the stand, pine
coverage decreases until there are only a few scattered trees in a
matrix of chaparral shrubs.
We installed a network of 12 weather stations spanning and
bracketing the western stand (Fig. 2b). The pines and the
weather stations are located primarily on north-facing slopes,
and site elevations range from 80–400 m. The stations are
numbered 1–12 from coast to inland and span a 7-km gradient
from the western-most to eastern-most pines within this stand
(Fig. 2b). Sites 1 and 10 were installed in the late 1990s, while
the remaining sites were installed between December 2003 and
April 2004.
Rainfall on SCI is highly variable (Fig. 3). Rainfall is also
highly seasonal, averaging more than 80 mm during each
month of the December–March rainy season (79% of the
average annual total of 509 mm falls during this period).
Rainfall averages <10 mm during each month of the May–
September dry season (4% of the average annual total falls
during these months). Rainfall is also highly variable for any
given month from year to year. On SCI, the standard deviation
of rainfall for a given month approximately equals the mean
rainfall for that month (except for February, which has a mean
rainfall of 116 mm and a standard deviation of 98 mm). The
interannual variability of annual rainfall is exceptionally high,
Fog and overcast conditions in coastal California
Journal of Biogeography 3ª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd
similar to that of arctic and desert ecosystems that have much
lower total mean annual rainfall (Knapp & Smith, 2001).
Patterns in cloud cover
We evaluated the degree of cloud shading from fog and
overcast by analysing shortwave radiometer (pyranometer)
data from the weather stations along our transect (Licor
Li-200X, Li-190SB and Hobo S-LIA; LI-COR Environmental,
Lincoln, NE, USA and Onset Computer Corp., Bourne, MA,
USA). We then used remote sensing data to spatially extend
cloud cover information across SCI. Quarter-hourly and
average daily radiometer data from each station were used to
generate idealized ‘clear sky’ curves for each station. These
‘clear-sky’ radiation curves were manually generated as the
sum of two to three annual sine curves to fit the seasonal
pattern of observed clear days for each site (Fig. 4a). The
difference between modelled clear sky radiation and observed
radiation could then be attributed largely to cloud shading.
(While aerosols other than cloud droplets could affect surface
irradiance, their role is deemed insignificant in this area
compared with the frequent, thick stratus cloud layers.) We
calculated a daily ‘sunniness index’ for each station as 0–100%
of the expected clear-sky radiation for that station for that
date. On clear days, the sunniness index was 100% while on
cloudy days it frequently dipped below 30%.
Evapotranspiration modelling
Irrigation managers have standardized methods for calculating
evaporative demand as potential evapotranspiration (PET).
The PET is the amount of water that would evaporate from
(a)
(b)
(c)
Figure 2 Maps of Santa Cruz Island (SCI). (a) Location map has stars showing areas with stands of bishop pines (Pinus muricata D. Don; data
from Griffin & Critchfield, 1972). Note large gaps in range. (b) Elevation map of Santa Cruz Island (low elevations are dark, high elevations
light) shows our weather stations deployed on an east–west transect throughout the island’s main, western stand of bishop pines. Stations
are numbered 1–12, west to east. The island lies 30 km off the coast of southern California, is 38 km long, and reaches 753 m a.s.l. just
north of site 11. Site 10 is 437 m a.s.l. (c) Shading shows spatial variability in summer cloud cover, which is represented here as the average
percentage of summer mornings (3 July–30 September at 10:30 am Pacific Standard Time) that are cloudy for 5 years (2000–04) (from
MODIS satellite images; data from Williams, 2006). White polygons indicate approximate stand boundaries of bishop pine. In the western
stand, pine density is highest in the centre of the stand, with only a few scattered trees towards the eastern and western edges.
D. T. Fischer et al.
4 Journal of Biogeographyª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd
plants and soils under given conditions of temperature,
humidity, radiation and wind assuming unlimited water
availability. Many different PET formulae of varying complex-
ity have been developed over several decades. The most
rigorously tested are derivatives of Penman–Monteith (P-M),
which take into account temperature, relative humidity, solar
radiation, wind speed impacts on aerodynamic conductance
and vegetation properties that influence surface conductance
(McKenney & Rosenberg, 1993). These models are meant to be
run at hourly or daily time steps. The preferred model, when
high-quality weather data are available, is the latest P-M
derivative (Snyder & Eching, 2004; Allen et al., 2005). This
model (see also the Appendix) has been coded into a
downloadable, user-friendly spreadsheet (Snyder & Eching,
2004). We used this spreadsheet to estimate hourly reference
evapotranspiration from static site parameters and hourly
wind speed, solar radiation, temperature and dew point data.
The P-M equation we used calculates ‘reference ET’ (ETref)
for a ‘tall canopy’, similar to alfalfa (Snyder & Eching, 2004).
Since most vegetation transpires at different rates from fields of
alfalfa, a ‘crop coefficient’ is used to scale ETref up or down to
estimate PET for a specific vegetation type (Dunne & Leopold,
1978; Costello et al., 2000). Pines transpire significantly less
water than alfalfa, even if they are well watered (due to lower
hydraulic and stomatal conductivity and lower leaf-level
photosynthetic capacity; Larcher, 2003). Careful measurements
Figure 3 Monthly (bottom, grey line) and water year rainfall
measured at the main ranch on Santa Cruz Island, 1904–2005.
The water year runs from 1 October to 30 September. Note the
extreme variability in both monthly and annual rainfall. Of
1224 months, only 717 had any recorded rainfall. Fifty-eight
individual months had more than 50% of mean annual
precipitation of 509 mm (dashed line). More than 10% of the
century’s total rain came in the rainiest 13 months.
(a)
(b) (c)
Figure 4 Effects of cloud on solar radiation and evapotranspiration. (a) Temporal variability in short-wave radiation (SWR, 0.4–1.1 lm)
at site 7, which has the most complete meteorological record (December 2003–December 2005). Mean daily SWR (from quarter-hourly
measurements, light grey line) shows many days with dramatic reductions from modelled clear-sky conditions (top line). These reductions are
large, particularly around the summer solstice, even when examined through a relatively long 14-day moving average. (b) Temperature,
humidity, SWR and reference evapotranspiration (ETref; Snyder & Eching, 2004) are strongly affected by coastal stratus and fog events.
Environmental data are shown for a sequence of three contrasting days at site 7 on Santa Cruz Island: a clear day, a partly foggy day and
a mostly overcast/foggy day. Note the sharp reduction in solar radiation and ETref with the onset of fog late on 8 June followed by uniformly
high relative humidity and decreases in temperature variation (raised minimum and lowered maximum). Radiation, humidity and
temperature plus wind speed are the main drivers of reference evapotranspiration (ETref). (c) Differences in cloudiness account for
most of the difference in ETref between sites 1 and 12 (over the period 17 March–12 July 2005, during which driving data are available
at both sites). Daily SWR at both sites was expressed as a percentage of the expected clear-sky value (the ‘sunniness index’). This percent-
age at site 12 minus that at site 1 is plotted on the x-axis. Only a few days had substantially more cloudiness at site 12 than 1 (the days
with negative cloud gradient values). The y-axis shows the percentage reduction in daily ETref at site 1 compared with ETref at site 12.
Fog and overcast conditions in coastal California
Journal of Biogeography 5ª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd
of another maritime pine species (Pinus pinaster Ait.), when
well watered, showed transpiration rates to be 60% of P-M
ETref (Granier & Loustau, 1994). We have adopted this value
to estimate PET for bishop pines (and the other plants in the
surrounding chaparral community, all of which have similar or
lower water requirements; Costello et al., 2000). In the
remainder of this paper, we will use ETref to refer to the
values predicted by the P-M equations and PET to mean ETref
values that have been scaled down to 60% to estimate
evapotranspiration for bishop pine stands. (The precise value
used to scale PET affects patterns of water availability across
the transect in absolute but not relative terms).
The dense network of weather stations across our transect
allowed us to quantify the gradient in evaporative demand. We
calculated hourly PET for each of the sites that had a
functional datalogger for the warm season (March–October) of
2005. We chose this time period because it brackets the period
of maximum drought stress. It also includes the period of
maximum tree growth rates as observed by dendrometers
installed across the transect; in 2005 and 2006, the maximum
growth rates occurred between March and July, with variations
related to the timing of final rainfall events of the preceding
rainy season (Williams, 2006).
Water balance modelling
In order to extend our analysis across more years than those
for which we had weather data with high spatial and temporal
resolution, we used 102 years of monthly temperature and
rainfall records to drive a simple water balance model. This
model accounts for interactions among rainfall, PET and soil
moisture storage, and it estimates both actual evapotranspi-
ration (AET) and Deficit, which is a measure of drought stress.
Running this water balance model for the last century provided
insights into how cloud patterns may have driven spatio-
temporal differences in water availability and drought stress.
The water balance model we used was modified from
Thornthwaite & Mather (1955, 1957), and was driven by
monthly precipitation and estimated monthly PET. All other
terms were calculated from these two quantities as follows (in
units of mm of water per unit area; see the Appendix).
Incoming precipitation left the system via run-off or evapo-
transpiration with some intermediate soil moisture storage. On
a monthly basis, a small percentage (3%) of precipitation was
diverted immediately to run-off as storm flow (Dunne &
Leopold, 1978). This low percentage diverted to monthly
storm flow was, if anything, a conservative underestimate that
increased the relative importance of rain water compared with
fog drip in the model. It was a similar value to that used in an
earlier calibrated run-off study in coastal California (Fischer
et al., 1996), and model results were not sensitive to this
parameter up to storm flow values of at least 20%. The rest of
the precipitation became available to increase AET up to the
evaporative demand limit of PET.
In wet months, excess precipitation (after AET equalled
PET) recharged stored soil moisture until field capacity was
reached. After that, any remaining precipitation was diverted
to run-off (above and beyond storm flow). We estimated a
maximum soil moisture storage of 500 mm of water in the
rooting zone (equivalent to a functional rooting depth of 1 m
with 50% soil porosity), based on soil textures, rooting depths
estimated from soil pits and road cuts, and standard tables
(Dunne & Leopold, 1978).
In dry months, when precipitation was less than PET, AET
equalled precipitation plus withdrawn soil moisture. Soil
moisture was made available for AET as a negative exponential,
so that it became increasingly difficult to draw moisture from
the soil as it dried (Thornthwaite & Mather, 1955, 1957;
Dunne & Leopold, 1978; Fig. 5b). This negative exponential
roughly captured the increasingly negative soil water potential
that occurs as soil water content is decreased. If there was not
sufficient plant-available soil moisture, the shortfall between
PET and AET was termed Deficit. Note that in the water
balance model we initially made no allowance for additions of
water to the soil by fog drip, so that the only water input was
from rainfall.
Many water balance studies include additional terms that we
omitted. In particular, canopy interception is generally less
important in this ecosystem than in many others as the rain
comes in a relatively few, intense storms. Groundwater flow
and delayed run-off were deemed unimportant for determin-
ing plant-available water on the relatively steep slopes that
typify SCI and the Californian coast ranges.
Constructing long-term time-series data
The water balance model required monthly inputs of precip-
itation and PET. Rainfall at the main ranch on SCI has been
measured continuously since 1904 (Fig. 2, Laughrin, 2005).
However, rainfall can vary significantly over short distances in
the coast ranges of California due to the rugged topography.
To account for this spatial variation in rainfall, we regressed
monthly totals at sites 1 and 10 against the long-term record
from the main ranch (n = 76 for the period from 1998 to 2005,
with gaps). Site 10 received the same amount of rainfall as the
ranch (100.4%, R2 = 0.92). Site 1 received 70% as much
rainfall as the ranch (R2 = 0.93). For Site 12, we regressed daily
rainfall totals at Site 12 to daily totals at Site 10 (as a proxy for
the ranch). Site 12 received 92% as much rainfall as Site 10
(n = 99 rainy days, 6 April 2004 to 11 November 2005,
R2 = 0.96). For the model runs below we used rainfall
estimated for each site (i.e. ranch rainfall multiplied by 0.70,
1.0 or 0.92 for Sites 1, 10 and 12, respectively).
There are no long-term records of ETref for our site, and
weather records sufficient to calculate P-M ETref extend back
only to 1998 (Wall, 2005). Fortunately, there are many
alternative formulae for calculating ETref, including the
Blaney–Criddle (B-C) formulation, which requires only mean
monthly temperature and latitude (Brouwer & Heibloem,
1986). We calculated monthly B-C ETref using mean monthly
temperatures from Santa Barbara, 40 km north across the
Santa Barbara Channel, from 1898 to 2005 (National Weather
D. T. Fischer et al.
6 Journal of Biogeographyª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd
Service, 2005). We also calculated hourly P-M ETref from 1998
through 2005 (with some gaps) for sites 1 and 10 and for 2005
at site 12. When we regressed monthly B-C ETref against the
monthly sum of hourly P-M ETref, the agreement was very
good (Fig. 5c). We then used these regressions to hindcast
monthly ETref (and PET) time series for sites 1, 10 and 12 back
to 1904.
Representing fog drip at each site was more difficult. We
collected fog drip at each site along the transect 2004–05 and
found consistent spatial patterns related to local topography,
generally increasing with elevation (Fischer & Still, 2007).
Temporal patterns were more difficult, as the summer of 2005
had much more fog drip than 2004. To put these 2 years of
spatially explicit data into a longer temporal context, we
employed nearby airport weather records. Williams (2006)
developed an index of summer fogginess based on the number
of hours per day (between midnight and 10 am) when weather
observers at Oxnard and Santa Barbara airports reported cloud
bases of different heights. These airports are 59 km east-north-
east and 46 km north of site 10, respectively, across the Santa
Barbara Channel and within 11 m of sea level. Summer stratus
cloud layers tend to be relatively level across the Santa Barbara
Channel so that months with many hours of low cloud on the
mainland generally also have many hours of low cloud on SCI.
The frequency with which cloud bases are observed below the
altitude of a given site is related to the amount of fog drip,
because fog drip can only occur at sites higher than the local
cloud base. The fogginess index based on these airports is not a
perfect proxy for fog drip at our sites on SCI for several
reasons, but it does allow at least some insight into how our
recent fog drip records fit into the long-term picture for the
local region. Further, fog collection and solar radiation data
collected at site 7, as well as tree-ring widths on Santa Rosa
Island, are significantly and positively correlated with this
fogginess index (Williams et al., 2008). The fogginess index is
available for 49 years during which sufficient observations
were recorded (1944–2005 with some gaps). For cloud bases of
400 m and lower, the fogginess index varied from 3.3 to 8.3 h
per day. The summers of 2005 and 2004 were 32% and 10%
foggier than average, respectively. To estimate ‘typical’ fog drip
inputs, we adjusted downward the cumulative 2005 and 2004
summer fog collection values at each site by 32% and 10%,
respectively. The results from both years were nearly identical
(± 10 mm) at each site (data from Fischer & Still, 2007; typical
values are listed in Table 1).
There was no strong correlation between the fog index and
either of our long-term data sets (rainfall and PET), so
building a continuous fog time series was problematic. Instead,
we added the same ‘typical’ fog drip values to the water balance
in every year. We added that annual fog drip in the model as
extra monthly precipitation from June to September (distrib-
uted as 10%, 40%, 40%, 10%, respectively) to match observed
seasonal patterns of fog drip (Goodman, 1985; Estberg, 2001;
Ruiz, 2005; Fischer & Still, 2007). Given the limited data
(a)
(b)
(c)
Figure 5 (a) Water balance calculations (following Thornthwaite & Mather, 1955) from averages of 1904–2005 rainfall (R) and potential
evapotranspiration (PET). Note that rainfall is restricted to the late autumn to early spring period. Stored soil moisture (SM) peaks in
early spring and declines over the dry season. Note that SM represents an amount of water remaining in the soil, while other variables
are fluxes of water per month. SM is distinct from plant-available soil moisture, as described in the text. Deficit is the difference between PET
(black squares) and actual evapotranspiration (AET, grey circles), and is not plotted explicitly. (b) Water balance SM as a function of
cumulative deficit. In wet soils near the field capacity (500 mm in this case), SM drawdown to meet evaporative demand is nearly
linear 1 : 1. As the soil dries, SM becomes proportionally less available. When the soil dries to 50% of field capacity it only releases 50%
of the monthly deficit applied to it. At 20% (100 mm) it releases only 20% of the deficit applied. (c) Monthly reference evapotrans-
piration (ETref) from the Santa Barbara temperature record (Blaney–Criddle, grey lines) vs. summed hourly ETref calculated from site
temperature, humidity, radiation and wind speed (Penman–Monteith, black lines) when available driving data exist at sites 1, 10 and 12.
The Blaney–Criddle ETref captures the seasonal trend and many of the excursions (R2 = 0.92, 0.93 and 0.96, respectively). Note the
overlapping scales on the y-axis, and the increase in ETref from site 1 (coastal) to site 12 (7 km further inland).
Fog and overcast conditions in coastal California
Journal of Biogeography 7ª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd
available and large uncertainties involved, more complex
modelling of temporal variability in fog drip did not seem
justified and would be likely to introduce additional errors in
our analysis.
Long-term modelling experiments
We ran the water balance model for the three sites (sites 1,10
and 12 from Fig. 2a) for the period 1904–2005 to look for
temporal and spatial patterns in cumulative annual Deficit. All
runs used the observed spatial patterns in rainfall across the
stand. There were four runs in total. Run A was the reference
case, with which the other runs were compared. For this run,
no fog drip was added at any site, and the effect of cloud
shading on PET was held constant at all three sites (using
modelled PET from the least cloudy, inland site – site 12).
Thus, differences in Deficit across sites for this run were due
solely to rainfall variations as described in the previous section.
For model runs B and C, fog drip and cloud-driven PET
variations were modelled separately to quantify their individ-
ual impacts, while run D incorporated both fog drip and PET
variations to simulate their combined impacts on water
balance. Specifically, run B incorporated reduced PET at sites
1 and 10 as observed in the weather record (due to increased
relative humidity and cloud shading at these more coastal
sites) but did not incorporate fog drip at any site. Run C used
PET from site 12 (inland) at each site but also included
‘typical’ summer fog drip amounts at each site. Run D
included both fog drip and PET variations across the three sites
(results are summarized in Table 1).
RESULTS
Spatial and temporal patterns of cloud cover
We wanted to know whether differences in frequency and
intensity (opacity) of cloud shading across the transect were
significant, and if so whether they appreciably reduced water
loss through evapotranspiration. Cloud cover varies seasonally.
The radiation record from site 7 (near the centre of the stand
and having the most complete record) shows that the
frequency and thickness of fog and cloud cover varied
substantially over time (Fig. 4a). There were periods of
pronounced reductions in mean daily solar radiation
(W m)2), particularly around the June solstice (locally referred
to as the ‘June gloom’). These radiation reductions were large
and persistent, even when viewed over a 14-day running
average.
Summer cloud shading decreases from the coast inland. This
gradient of decreasing cloud shading from the coast is
illustrated in Fig. 2c by a composite of morning MODIS
satellite images across five summers (averaged across 2000–04).
The resulting image shows the percentage of days that are
cloudy at 10:30 am Pacific Standard Time (PST) (Williams,
2006). At the western coast, c. 50% of days were cloudy at
10:30 am, while in the centre of the stand that figure decreased
to 35% and to <30% near the last few pines at the eastern
(inland) edge of the stand (Fig. 2c). This metric serves as a
proxy for overall cloud shading because clouds and fog tend to
form or intensify overnight, and burn off towards the coast the
next morning, so that the spatial cloud patterns captured by
morning MODIS images are representative of mean daily
cloud patterns (Fischer & Still, 2007). The radiometer records
for summers 2004–05 from our weather stations across the
western stand show the same pattern of a steady decrease in
Table 1 Site characteristics and model results. Model run
(a) takes into account differences in rainfall between the sites and
serves as a reference point for subsequent model runs. Deficit is
the difference between potential evapotranspiration (PET) and
actual evapotranspiration (AET). Model run (b) shows that
reduced coastal PET decreases both average and maximum
drought stress by about a third compared to just 7 km further
inland. Run (c) shows that the average amount of fog drip received
at higher-elevation sites is sufficient to reduce drought severity by
19–36% (compared to run a). Run (d) includes both fog drip and
PET reduction. The combined effect is a 36–56% reduction in
average deficit. At site 10 (which receives both persistent cloud and
significant fog drip) only 1989–90 had deficits > 400 mm, while
such severe droughts were common at site 1 (little fog drip) and
site 12 (little cloud cover).
Site characteristics Site 1 Site 10 Site 12
Distance inland (km) 2.1 7.1 9.2
Elevation (m) 61 437 387
Avg. annual PET (mm) 681 766 916
Avg. annual rain (mm) 356 509 468
(as % of Ranch rain) 70% 100% 92%
Est. typical fog drip (mm) 10 139 178
(a) Reference case (holding PET constant, without fog drip)
Avg. AET (mm) 344 468 439
Avg. deficit (mm) 572 448 477
Max. deficit (mm) 809 753 768
(b) Effect of DPET only (without fog drip)
Avg. AET (mm) 339 449 439
Avg. deficit (mm) 342 318 477
Max. deficit (mm) 561 593 768
Reduction of Avg. deficit by DPET 40% 29% –%
Reduction of Max. deficit by DPET 31% 21% –%
(c) Effect of fog drip only (holding PET constant)
Avg. AET (mm) 354 597 609
Avg. deficit (mm) 562 319 307
Max. deficit (mm) 798 607 582
Reduction of Avg. deficit by fog drip 2% 29% 36%
Reduction of Max. deficit by fog drip 1% 19% 24%
(d) Effects of DPET and typical fog drip
Avg. AET (with fog drip) (mm) 349 570 609
Avg. deficit (with fog drip) (mm) 332 197 307
max deficit (with fog drip) (mm) 551 441 582
Reduction of Avg. deficit by DPET
plus fog drip
42% 56% 36%
Reduction of Max. deficit by DPET
plus fog drip
32% 41% 24%
No. annual deficits > 400 mm
(1904–2005)
21
years
2
years
20
years
D. T. Fischer et al.
8 Journal of Biogeographyª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd
cloud frequency and opacity from west to east for all hours of
the day (not shown).
Spatial and temporal patterns of evapotranspiration
Calculated values of ETref across the transect showed that,
with small exceptions related to local aspect, ETref increased
consistently from the coast to inland. Throughout the 2005
warm season (March–October), hourly calculations of ETref at
each site showed that seasonal mean ETref was 25–30% lower
at the coastal sites than at the inland sites.
We tested the role of clouds in reducing ETref at individual
weather stations. For example, clouds significantly reduced
ETref at site 7 for three adjacent days in June 2005 (Fig. 4b).
Hourly shortwave radiation observations show that 7 June was
clear all day, while 8 June was cloudy in the afternoon, and 9
June was cloudy for most of the day. Temperature varied
diurnally by 5�C on 7 June and early on 8 June, but then
temperature dropped sharply as the cloud layer arrived in the
late afternoon. Temperature variation was much more muted
for the rest of the period. Similarly, relative humidity showed a
moderate daily cycle until the arrival of the cloud layer, at
which point relative humidity rose to 100% and remained
unchanged for the rest of the period. These three variables,
plus wind speed, are used in the P-M ETref formulation
(Snyder & Eching, 2004; Allen et al., 2005).
The reduction in daily total ETref from 7 June (4.5 mm) to
9 June (1.5 mm) was 66%. This was due to the combined
effects of reduced solar radiation, reduced midday tempera-
tures and increased relative humidity, all of which were driven
by increased clouds (wind speeds averaged c. 3 m s)1 on both
days). Reduced short-wave radiation not only limits the energy
required for vaporization of water inside the leaf and at the soil
surface, but it reduces surrounding air temperature and vapour
pressure deficit. To test the relative impact of each weather
component on ETref, we substituted hourly 9 June variables
into the 7 June calculations one at a time. Low solar radiation
(from 9 June) reduced ETref by 40% (holding all other
variables at 7 June values). Increased relative humidity reduced
ETref by 22%. Substituting temperature from 9 June (lower at
midday, but higher at night) reduced ETref by 2%. Therefore,
the primary driver of changes in ETref at a given site, under the
relatively narrow ranges of temperature and relative humidity
that typify our sites and the fog belt in general, is variation in
solar radiation. Aerosols other than cloud droplets also reduce
incoming solar radiation, but in this area of frequent thick
stratus cloud cover their effects are comparatively small.
The spatial gradient in ETref across the transect is also
driven primarily by variations in radiation. Across a landscape,
radiation variations are strongly driven by aspect, but among
our sites, primarily on north-facing slopes, we hypothesized
that the strongest driver of radiation differences (and therefore
differences in ETref) was the observed gradient in cloud
shading. To test this hypothesis, we compared the daily
coastal/inland gradient in ETref with the daily cloud-driven
gradient in solar radiation (comparing sites 1 and 12, at
opposite ends of the transect). We used the longest period of
continuous hourly data, 17 March–12 July 2005 (which also
generally coincided with the period of maximum tree growth,
mid-March to mid-July; Williams, 2006). We calculated a daily
cloud shading gradient as the sunniness index (0–100%) at site
12 (inland) minus site 1 (coastal). We also calculated a daily
ETref gradient as the percentage difference between ETref at
site 12 minus site 1 (Fig. 4c). The cloud shading gradient
explained most of the difference in daily ETref. Mean daily
ETref over this period was 4.9 mm at site 12, and 4.0 mm at
site 1. Departures from the linear regression (of cloud shading
gradient vs. ETref difference) are primarily on days with
unusual altitude effects (e.g. a strong temperature inversion
between Site 1 at 61 m and Site 12 at 387 m, or rain only at the
higher site). The small positive intercept of the regression
indicates a slightly higher ETref at site 12 even when cloud
shading is equal, resulting from the slightly greater continen-
tality of this site. In summary, ETref is significantly lower near
the coast than just a short distance inland. Despite differences
in elevation and continentality, most of that difference can be
attributed to the observed differences in solar radiation
resulting from increases in frequency and intensity of cloud
shading near the coast.
Spatial and temporal patterns of fog drip
Fog water was collected and volumes were logged at each site
with harp-style passive fog collectors (Fischer & Still, 2007).
There were large spatial differences in the amounts of fog water
collected during the 2005 dry season (10 May to 16 October;
Fig. 6a). Site 1, at the lowest elevation, received a negligible
amount, while sites 10 and 12 received substantial fog water
inputs (Fig. 1). At site 10 we calibrated the fog collector against
a throughfall collector placed immediately under a mature pine
canopy. The regression estimated 30 mm of throughfall per
litre of collected fog water, R2 = 0.74 (Fischer & Still, 2007).
This calibration allowed us to estimate how much fog drip a
mature pine might collect at each site, irrespective of current
pine abundance at each site. Under areas of similarly dense
pine canopy, the fog water collected at site 10 could generate of
the order of 175 mm of throughfall, or 34% of the mean
annual rainfall at this site, using the above calibration.
Assuming a similar relationship between throughfall and the
fog collector at site 12 (which collected more fog water in
fewer, larger events in 2004 and 2005; Fischer & Still, 2007),
throughfall from fog drip might approach 225 mm. It should
be noted that the long narrow leaves of conifers are more
effective at collecting fog drip than the leaves of many other
plants (Goodman, 1985).
Previous work has shown that fog drip can penetrate into
the soil and increase soil water potential (Cole, 2005). We
installed paired sets of soil water potential sensors from depths
of 2–15 cm both inside and outside the tree canopy at site 7
(Fig. 6b). The two soil profiles are c. 10 m apart. The soil
sensors outside the canopy had gone off scale as the soil dried
(i.e. below )1.5 MPa) within 2 weeks after the last rain of the
Fog and overcast conditions in coastal California
Journal of Biogeography 9ª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd
season on 9 May 2005. By contrast, under the tree canopy,
repeated fog drip events throughout the summer kept the
upper soil moist into mid-August. We dug a soil pit under a
canopy on an adjacent slope on 2 September 2005, and found
the soil quite wet to the touch at the bottom of the soil profile
(50 cm depth). This was 4 months after the last rain.
Water balance for an ‘average’ year
Modelling plant water availability during the growing season is
more complicated than just assessing annual rainfall and PET.
Other important factors include the timing of rainfall and
seasonal changes in soil moisture storage. For areas with highly
variable rainfall, year-to-year changes in soil moisture must
also be considered. Given the extreme variability of rainfall
throughout southern California both within and among years
(Fig. 3), it is important to consider a broad suite of both wet
and dry years in order to estimate how observed differences in
quantities like PET actually affect availability of water to
plants. Years with very high rainfall in a few storms may not
provide as much summer moisture as years with moderate
rainfall that occurs more evenly, or that falls later in the season.
Water availability in low-rainfall years is strongly dependent
on how wet the previous year was. Also, in very wet years,
changes in PET have less of an impact on plant drought stress.
Tracking this temporal variability required a running water
balance in which monthly soil moisture carried over from
1 month to the next across years.
To calculate long-term effects of the observed spatial pattern
of cloud shading and fog drip, we used a running water balance
driven with monthly rainfall and PET time-series data from
1904 to 2005. Figure 5a shows the water balance for an
‘average’ year at site 10. Rainfall and PET are long-term site
averages from these time series. Rainfall averages
509 mm year)1 and is concentrated during the winter months,
with almost no rain from May to October. Rainfall surplus
(beyond evaporative demand) allows for recharge of soil
moisture from December to March. Run-off is a minor part of
the water balance in the ‘average’ year. PET averages
766 mm year)1; it is low during the winter and peaks in late
summer. AET peaks in spring as temperature and insolation
are rising, but before soil moisture becomes limiting. AET is
higher than rainfall from April to October, by an amount that
equals the drawdown of soil moisture for that month. Still, the
available soil moisture is not sufficient to match the evapo-
rative demand (Fig. 5b). Deficit is greatest in August, but
remains large from June to October.
Both cumulative annual AET and cumulative annual Deficit
have been shown to be ecologically meaningful predictors of
plant distributions for species, including other Californian
conifers (Stephenson, 1998). We tested AET from this running
water balance model as a predictor of tree growth based on a
tree-ring chronology developed on torrey pines (Pinus torre-
yana ssp. insularis Haller) on adjacent Santa Rosa Island, and
found AET to be a better predictor of tree growth than rainfall
(Williams, 2006; Williams et al., 2008). Annual Deficit appears
to correlate with mortality risk for bishop pines as well (see
Discussion).
We compared site 10, near the heart of the stand where pine
density is highest, with sites 1 and 12, which bracket the coastal
and inland ends of the stand as described previously. We
compared ‘average’ water balances for the three sites, holding
(a)
(b)
Figure 6 Fog drip inputs and the impact of fog drip on soil moisture. (a) Cumulative fog water collected at seven sites (using the
harp fog collector pictured) over the 2005 summer dry season shows that higher elevations (black lines, 300–440 m) receive more fog
than lower sites (60–200 m). Site 7 is in a topographic funnel south of the main transect and gets significantly more fog than the other sites.
(b) Soil water potential was logged hourly at site 7 in two profiles 10 m apart, just outside (O, grey lines) and inside (In, black lines)
the tree canopy (dotted lines are closer to the surface). Surface soils (0–15 cm) dried substantially prior to the last rains of the wet
season (28 April and 9 May). The soil outside was drier than the permanent wilting point ()1.5 MPa) within 2 weeks of the last rains.
Repeated fog drip events throughout the summer (grey bars) kept the soil under the tree canopy moist until late August. Daily fog drip
at site 7 is estimated from fog collection volumes (see text).
D. T. Fischer et al.
10 Journal of Biogeographyª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd
rainfall constant and varying only PET (Fig. 5c; variations in
PET reflect decreasing cloud shading from the coast inland).
Lower PET values towards the coast resulted in decreased AET
in winter. This allowed increased soil moisture recharge, which
increased soil moisture availability all year round, and
therefore increased dry season AET. Monthly Deficits were
smaller throughout the dry season, due to both decreased PET
and increased AET. This pattern illustrates how PET can affect
AET and Deficit, but it is important to remember that the use
of an ‘average’ year is problematic given the extreme interan-
nual variability of rainfall in California (McGuirk, 1982;
Schonher & Nicholson, 1989; Fig. 2).
Long-term running water balance
Figure 7 shows 10 years of running water balance estimates for
site 10, spanning two of the most extreme years of the century.
Water year 1983 (WY 1983, 1 October 1982 to 30 September
1983), associated with a very large El Nino–Southern Oscil-
lation (ENSO) event, was the fifth wettest year of the century.
Soil moisture reached the estimated rooting zone capacity of
500 mm, and AET remained high throughout the summer. In
contrast, WY 1989 and WY 1990 had the fifth lowest and
second lowest rainfall totals of the century, respectively.
Further, each water year during the period 1987–1991
experienced <80% of the long-term average of 510 mm
rainfall, resulting in a prolonged drought that was unprece-
dented in the historic record. (It should be noted that the
historic record up to the mid-1980s shows substantially fewer
and less intense droughts than inferred for the previous 300–
500 years based on tree-ring analyses; Michaelsen et al., 1987;
Haston & Michaelsen, 1994, 1997). The largest annual Deficits
of the century were in 1989 and 1990 (Fig. 7). (Water year
rainfall was analysed using water years ending 30 September so
as to keep entire rainy seasons together. Annual Deficit was
analysed using calendar years, as monthly Deficits often
continue from summer through the autumn.) Stored soil
moisture was extremely low in these years, especially WY 1990,
so that AET was essentially the same as monthly rainfall.
Modelled PET did not vary substantially over the 10-year
period (Fig. 7).
We also examined the long-term (1904–2005) running water
balance for three sites (1, 10 and 12) to quantify interactions
among rainfall, fog drip and PET in determining annual
Deficit, which we took as a proxy for drought stress and
mortality risk. As described previously, run A was the reference
case (no fog drip and cloud shading/PET held constant across
sites); run B included variations in cloud shading/PET but did
not include fog drip; run C included ‘typical’ amounts of
summer fog drip at each site but held cloud shading/PET
constant across sites; and run D included both fog drip and
variations in cloud shading/PET.
In run B (effects of cloud shading on PET only), average
Deficits decreased by 29% at site 10 and 40% at site 1 relative
to run A (control case; Table 1). At site 1, the reduction in PET
compared with site 10 was sufficient to make up for the 30%
lower rainfall received at the low elevation of site 1. Average
Deficit at site 1 across the entire record was 572 mm under run
A; reduction in Deficit from cloud shading under run B
regularly exceeded 200 mm at this site. This made Deficits at
site 1 comparable to site 10, even though the low elevation of
site 1 reduces rainfall by 30%. The reduction in Deficit at both
sites was greater in dry years than wet years, reducing overall
climatic variability. Another measure of variability we exam-
ined is the recurrence interval of severe droughts. At site 1, the
1987–91 drought resulted in the highest modelled Deficit of
561 mm. At site 12, this extreme Deficit was exceeded in
30 years out of 102, and by as much as 207 mm. This
comparison shows that observed changes in PET over a short
distance can have large impacts on climatic variability and
consistency of habitat suitability over time.
In run C (effects of typical fog drip only), the low elevation
of site 1 resulted in negligible fog drip, while sites 10 and 12
both received large amounts (139 and 178 mm, respectively)
resulting in reductions in Deficit of 29% and 36% at sites 10
and 12, respectively (Table 1, Fig. 8). The reductions in
average Deficit from fog drip are similar in magnitude
to those from reduced PET, but occur primarily at the
Figure 7 Running water balance for site 10, Santa Cruz Island 1983–92. This decade started with one of the rainier years on record during
the 1982–83 El Nino event. The most severe multi-year drought of the century began in 1987, culminating in the driest year of the
century in 1990. Potential evapotranspiration (PET) is relatively consistent from year to year. Actual evapotranspiration (AET) varies
much more, depending on both monthly rainfall (R) and soil moisture (SM). Deficit (Def) is the difference between PET (black
squares) and AET (grey circles). Annual Def is also shown in grey bars plotted against the right axis; it varies greatly from year to year.
Fog and overcast conditions in coastal California
Journal of Biogeography 11ª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd
higher-elevation stations (sites 10 and 12) rather than at the
more coastal ones. Including fog drip in the water balance
shifts the prediction of the most drought-prone habitat to low-
elevation coastal habitat, with both high-elevation sites being
more mesic.
In run D (combined effects of fog drip and cloud shading),
the impacts of fog drip and cloud shading were essentially
additive (despite minor differences related to timing and
interactions among variables). Annual Deficits were reduced
by about a third at higher elevation (sites 10 and 12) as a result
of higher fog drip. Annual Deficits were reduced by about a
third near the coast (sites 1 and 10) by reduced PET from
increased cloud cover. Sites 1 and 12 showed similar levels of
Deficit for most years. Site 10 (with both abundant fog drip
and low PET) had the lowest mean Deficit (a 56% reduction in
mean Deficit from run A), and the lowest frequency of
droughts exceeding any given threshold (Fig. 8; Table 1).
DISCUSSION
Drought sensitivity of bishop pines
Bishop pine, like many rare and endemic species of the fog
belt, is sensitive to drought. The most severe 2-year drought of
the last century on SCI (and in most of CA) occurred in WY
1989–90 (Fig. 3; WY 1989 is 1 October 1988 to 30 September
1989). This drought was followed by widespread pine mortality
across the island over the next few years – estimated as 70–90%
of individuals (Walter & Taha, 1999). The previous large 2-
year drought of WY 1976–77 was also followed by widespread
(a)
(b)
(d)
(c)
Figure 8 Annual deficits for three sites on Santa Cruz Island under four model runs. Deficit is the difference between potential evapotrans-
piration (PET) and actual evapotranspiration (AET), with drier years extending towards the bottom of the graph. Run (a) is the reference
case, ignoring fog drip, and using PET from site 12 (inland) for each site. Differences in deficit result from lower and higher rainfall at sites
1 (coastal) and 10, respectively. Run (b) incorporates reduced PET from greater cloud cover at sites 1 and 10 along with observed rainfall
patterns, but still has no fog drip. Note the marked decrease in severity of both average and extreme droughts at both sites compared to (a).
Run (c) ignores PET, using site 12 PET at each site, but adds ‘typical’ summer fog drip at each site. The low elevation of site 1 results in
negligible fog drip, while sites 10 and 12 both receive large amounts resulting in large reductions in deficit. Run (d) includes both fog drip
and reduced PET. Annual deficits are reduced by about a third at higher elevation (sites 10 and 12) by higher fog drip. Summer deficits are
about a third lower near the coast (sites 1 and 10) due to reduced PET from cloud cover. The result is that sites 1 and 12 have similar rates
of drought frequency and severity, and site 10 (with both abundant fog drip and low PET) has the least drought.
D. T. Fischer et al.
12 Journal of Biogeographyª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd
pine mortality (M. Carroll, J.R. Haller, L. Laughrin, personal
communication, 2006). In both instances, the proximal cause
of the die-off is reported to be bark beetle infestation. Bark
beetles, fungi and other pathogens often cause mortality in
pines in the years following drought because the weakened
trees are unable to produce sufficient pitch and defensive
chemicals (Richardson, 1998). During severe droughts, even
relatively healthy pines may succumb to disease as large
concentrations of weakened trees provide abundant breeding
ground for pathogens (Richardson, 1998; Breshears et al.,
2005; White et al., 2006).
Range limits are necessarily determined by a balance
between recruitment and mortality. On the recruitment side,
seedlings are abundant throughout the stand in most years,
even following drought when mature trees are dying en masse
(Wehtje, 1991; Walter & Taha, 1999). Prior to the removal of
feral sheep in the 1990s, pine recruitment was somewhat
limited as most seedlings were eaten (Hobbs, 1980). However,
despite intensive sheep grazing, starting in the 1850s, a 1929
aerial photograph suggests that overall range limits of the
western stand that we studied (Fig. 2) were similar to the
present day (J. Howarth, personal communication). This
suggests that range limits are less influenced by recruitment
than mortality.
On the mortality side, the die-off of the early 1990s appears
to have been spatially selective. A 1983 photograph shows large
numbers of mature pines between sites 11 and 12 prior to the
1987–91 drought (M. Carroll, personal communication). In
2006, none of those trees remained alive, or even standing
dead, and all of the pines east (inland) of site 11 appear to have
established since 1990. Also, the entire area west (coastward) of
site 4 contains few, if any, pines that pre-date the last drought.
Around sites 7 and 10 older pines are fairly common, although
the oldest pines we could find to core at both sites date back to
just 1952. This suggests another die-off and recruitment pulse
following the severe drought of the late 1940s (Fig. 3).
Drought-mediated mortality appears to be the main driver
of range limits at least in the western stand, with some range
expansion in wet years, and range reduction (or density
reduction, especially near the edges) following droughts.
Direct effects of fog drip and cloud shading
Fog drip is taken up by coastal pines and contributes to tree
growth. Uptake of fog water has been demonstrated using
isotopic techniques (Fischer, 2007). Increased growth is
apparent in a tree-ring chronology developed from torrey
pines 15 km west of site 1 on adjacent Santa Rosa Island
(Williams et al., 2008). This chronology (1920–2004) was
compared with a rainfall-based water balance model that
explained most of the variability in tree growth (R2 = 0.60).
However, more than half of the remaining variability could be
explained using proxies for fog drip and cloud shading
(derived from nearby airport weather observations, 1944–
2005). Importantly, cloud shading was shown to significantly
increase growth even when looking only at clouds too high to
provide fog drip. In addition to statistical correlation, the
increased growth from fog drip can also be observed directly in
the form of numerous ‘false bands’ in tree cores from both
Santa Rosa Island and SCI. These bands occur when a tree
whose summer growth has slowed reverts to rapid growth in
the same season (Schulman, 1947). Since growth is so tightly
linked to water availability in these systems, and no rainfall was
recorded during these summers, we interpret these false bands
as indicating additions of fog water to the ecosystem The
presence of numerous false bands within the tree ring record
from SCI, combined with the strong correlation of annual
growth and a fog proxy, suggest that fog drip relieves drought
stress significantly across individual years and multiple decades
(Williams, 2006; Fischer, 2007; Williams et al., 2008).
Long-term effects
Higher PET due to reduced cloud shading at the inland edge of
the stand (site 12) is potentially significant as a range-limiting
factor for this drought-sensitive species. Given similar amounts
of soil water at the beginning of a drought, trees growing under
a higher PET regime will run out of water before trees growing
where PET is lower (see Results: ‘Water balance for an
‘‘average’’ year’).
Lower rainfall at the coastal end of the stand where
elevation decreases (Site 1) is also potentially range-limiting.
However, the results of run B show that reduced coastal PET
(attributable largely to increased cloud shading) essentially
compensated for the 30% lower rainfall (compared with site
10) leaving no large differences in deficits between those two
sites.
Both fog drip and cloud shading can sharply reduce drought
frequency and drought severity. Run B shows that lowered PET
from cloud shading not only reduced mean Deficit, but also
reduced climatic variability. Run C suggests the same is true of
fog drip, although more rigorously assessing the strength of
that effect will require more long-term data on fog drip. Run D
shows that cloud shading and fog drip were somewhat spatially
independent, and, importantly, that their effects may be
additive in only limited locations.
We have used Deficit as a proxy for drought stress, and, by
extension, as a proxy for mortality risk. Deficit is a useful proxy
for mortality risk, but the relationship is nonlinear. Mortality
in most years is very low, with only a few trees succumbing to
insects or disease. As drought becomes more severe, more and
more pines weaken and become susceptible to disease. As a
result, host/pathogen population dynamics and stand connec-
tivity patterns become important factors in the spread of
epidemic disease (Richardson, 1998; White et al., 2006). To
better assess mortality risk, then, it makes less sense to examine
average Deficit than to look at the return interval of extreme
events. The two most extreme annual Deficits at site 10 were
just over 400 mm (1989–90). This level of Deficit, associated
with the severe pine die-off of the early 1990s, seems an
appropriate threshold for analysis. At site 10, near the middle
of the stand, this level of Deficit was modelled in just those 2 of
Fog and overcast conditions in coastal California
Journal of Biogeography 13ª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd
the last 102 years. By contrast, at sites 1 and 12, modelled
Deficits of this severity (> 400 mm) occurred in 20 or more
years out of 102 (Table 1, Fig. 8d). This same pattern of
reduced drought severity at site 10 also holds when comparing
the frequency of severe 2-year droughts (measured as summed
monthly Deficit for two calendar years). There were at least 13
2-year droughts at both sites 1 and 12 that exceeded the driest
2-year drought at site 10. There were at least 23 3-year
droughts at both sites 1 and 12 that exceeded the driest 3-year
drought at site 10. The greatly reduced frequency of severe
drought at site 10 suggests that the middle of the stand
provides suitable habitat much more consistently over time
than do areas near the edges. This habitat consistency, or
reduced frequency of severe droughts, due to the combined
effects of fog drip and cloud shading, is a critical habitat
feature for long-term population viability.
Comparing these model results with reality, we observed
that pine density is much higher around site 10 (50–80%
cover) than near sites 1 and 12 (a few widely scattered young
trees in chaparral). Growth rates and longevity (from tree
cores) are also much higher at site 10 than at sites 1 and 12
(not shown). For a drought-sensitive species like bishop pine
this spatial pattern only makes sense when compared with
drought frequency under run D. Each of the other model runs
fails to produce a credible spatial pattern of moisture
availability; it is only by incorporating both cloud shading
effects (reduced PET near the coast) and fog drip (estimated
typical water additions each summer) that the model matches
well with the observed pattern of pine density.
CONCLUSIONS
Fog is recognized as an important component of the hydro-
logical cycle in many ecosystems that are of great conservation
concern (Olson & Dinerstein, 1998). The same low stratus
clouds that produce fog drip where they intercept vegetation
also influence ecosystem hydrology by cloud shading. Low
clouds shade plants, both where they intercept vegetation, and
at lower elevations beneath stratus clouds. This shading
reduces PET and water loss by reducing net radiation,
increasing relative humidity and decreasing air temperature.
We evaluated the potential ecological effects of cloud
shading and the resulting spatial differences in PET using a
water balance model. The model tracked monthly ecosystem
water availability over the last century at three sites across a
stand of endemic coastal pines. We compared tree mortality
risk among these sites by analysing the frequency and severity
of drought stress, and examined how cloud patterns mediate
drought stress and thereby influence range dynamics. We show
that for low-elevation sites within the California fog belt, the
effects of cloud shading can be much greater than those of fog
drip. Even high-elevation sites that receive substantial fog drip
may benefit as much from shading as from fog drip – this
would be particularly true for species whose leaves are less
efficient than conifers at collecting fog droplets. Either effect
alone can reduce annual drought stress by a third compared
with adjacent less cloudy areas, and together these effects can
reduce drought stress by more than half. The shading aspect of
coastal fog and its potential impact on vegetation growth and
range dynamics has not previously been quantified, but should
be considered in future fog-vegetation research.
Inferred range dynamics of the western stand of bishop pine
on SCI over the last century suggest that pines can disperse
outward from the centre of the stand in favourable years, but
that drought frequently kills off pines near the periphery of the
stand. Further, there are micro-refugia from severe drought
near the centre of the stand. This pattern was not predicted by
water balance modelling until the separate spatial patterns of
both fog drip and cloud shading were taken into account.
While this study focused on bishop pine, the same cloud-
influenced patterns of episodic drought at coastal and inland
range edges presumably affect many other species endemic to
the Californian fog belt. Similar large differences in moisture
availability over relatively small changes in distance and
altitude also exist in other semi-arid, fog-inundated ecosystems
(e.g. other mediterranean-climate coasts and the lomas of the
Atacama Desert; Rundel et al., 1991). Even tropical montane
cloud forests, which receive much more precipitation and are
much wetter, are drying in places (Pounds et al., 1999, 2006).
As global climate change and deforestation alter cloud patterns
in fog-dependent ecosystems around the world (e.g. Still et al.,
1999; Lawton et al., 2001), increasing attention and effort are
focusing on conserving their disproportionately large share of
global biodiversity and species endemism. Conservation efforts
in these ecosystems will be better able to anticipate range
dynamics of drought-sensitive species by considering the
spatially distinct hydrological impacts of both fog drip and
cloud shading.
ACKNOWLEDGEMENTS
This work was supported by the Andrew W. Mellon Foun-
dation and the University of California, Santa Barbara. The
fieldwork was performed at the University of California
Natural Reserve System’s Santa Cruz Island Reserve, on
property owned and managed by The Nature Conservancy.
We are grateful to J. B. Wall at CSU Northridge for long-term
weather data, and to L. Laughrin and staff at the reserve for
weather records, long-term perspective and field support.
K. McEachern, S. Chaney and staff at Channel Islands
National Park and Island Packers helped with extensive
transportation to the study sites. M. Carroll provided invalu-
able information and photographs from many years of careful
observation of the ecosystem. The manuscript was improved
by helpful comments from O. A. Chadwick, B. Tiffney, M.
Scholl, G. MacDonald and an anonymous referee. We thank
all the following for their assistance with fieldwork: R.
Richardson, E. Thorsos, T. Eckmann, M. Kelly, S. Battersby,
J. Bernstein, A. Hartshorn, K. Cover, W. Amselem, E. Mason,
R. Roff, N. Wilson, E. Edwards, J. Rumbly, J. Hecht, N. Nagle,
P. Dennison, J. Baker, K. Hertel, T. Laverne, all of the
Geography 194 students and especially C. Ebert.
D. T. Fischer et al.
14 Journal of Biogeographyª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd
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BIOSKETCHES
Douglas T. Fischer is an assistant professor of geography at
California State University Northridge. This paper is derived
from his dissertation work at University of California, Santa
Barbara. His research focuses on microsite- to regional-scale
controls on species range limits, climate change and develop-
ment of conservation strategies that incorporate long-term
environmental variability.
Christopher J. Still is an associate professor of geography at
the University of California, Santa Barbara. His research
interests include ecosystem ecology, climate change, earth
system science, isotope biogeochemistry and biogeography.
A. Park Williams is a PhD student at University of
California, Santa Barbara. His research focuses on how large-
scale climatic variability affects plant species distribution and
on using plants, and tree rings in particular, to infer details
about past climate.
Editor: Glen MacDonald
APPENDIX
The standardized reference evapotranspiration equation (Allen
et al., 2005) follows:
ETrs ¼0:408DðRn � GÞ þ c Cn
Tþ273 u2ðes � eaÞDþ cð1þ Cdu2Þ
;
where Rn = calculated net radiation at the crop surface
(MJ m)2 h)1); G = soil heat flux density at the soil surface
(MJ m)2 h)1); T = mean hourly air temperature at 2-m height
(�C); u2 = mean hourly wind speed at 2-m height (m s)1);
es = mean hourly saturation vapour pressure at 2-m height
(kPa); ea = mean hourly actual vapour pressure at 2-m height
(kPa); D = slope of the saturation vapour pressure–tempera-
ture curve (kPa �C)1); c = psychrometric constant (kPa �C)1);
Cn=numerator constant (66 K mm s3 Mg)1 h)1 for tall veg-
etation, hourly time step); Cd = denominator constant
(0.25 s m)1 for tall vegetation, hourly time step, during the
day, 1.7 at night). Units for the 0.408 coefficient are
m2 mm MJ)1. Snyder & Eching’s (2004) spreadsheet calculates
these terms (following procedures in Allen et al., 2005) using
site parameters and hourly measurements of air temperature,
dew point, wind speed and incoming solar radiation.
The relevant terms of the monthly water balance model are
presented below (modified from Thornthwaite & Mather,
1955, 1957, and excluding terms for calculating run-off). Soil
moisture storage is in mm (or mm3 mm)2) and all fluxes are
in mm month)1.
Term Symbol Calculation
Precipitation Pt Estimated as f(t), after
excluding stormflow
Potential
evapotranspiration
PETt Estimated as f(t), from
modelled ETref
Field soil moisture
capacity
FSMC Estimated as 500 mm
Accumulated
potential water loss
APWLt If (Pt ) PETt) < 0,
APWLt = APWL(t–1)
+ (Pt ) PETt); else APWLt
= FSMC · ln(SMt/FSMC)
Soil moisture SMt If (Pt ) PETt) < 0, SMt
= FSMC · e(APWLt/FMSC);
else SMt = minimum of
[FSMC or SM(t–1) + (Pt ) PETt)]
Actual
evapotranspiration
AETt AETt = minimum of
[PETt or Pt ) (SMt ) SMt–1)]
Deficit Dt PETt ) AETt
Fog and overcast conditions in coastal California
Journal of Biogeography 17ª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd