Fog in the California Redwood forest:

Ecosystem inputs and use by plants

T.E. Dawson

Objectives

• Is fog an important source of moisture for the plants that inhabit the ecosystem– Redwoods use 600±145 L/day (45 m tree)– Greatest demand during summer when rain is sparse, but fog

is common– During summer, deep soil water may become unavailable for

shallow rooted species– How much is actually used, not just stored?

• Areas with redwoods and without– Trees significance in influencing the magnitude of fog water

input into the ecosystem

Importance of Fog

• Ecosystem nutrient balance or aspects of biochemistry

• Reduce plant moisture stress by reducing canopy transpiration or evaporation from habitat

• Improve plant water status by direct absorption

• When trees are removed water input from fog drip deceases and so does streamflow

• Higher water input/soil moisture around tree canopies

Fog formation

• Heaviest from midnight to early or mid-morning

• Interaction between warm air and cold water (up-welling, or currents)

• Causes condensation---thus fog

• Doesn’t require condensation nuclei like clouds

• Enriched in the heavier 2

H and 18O isotopes relative to water source (ocean)

• Heavier then rain because rains come from storm systems that have moved great distances, which causes them to become depleted in 2H and 18O

Methods

• Fog and rain samples– Total input

• Rain, fog, fog drip off trees– Local meteoric water line

• 2H=7.718O+9.6• Provided a mixing line that

was more useful for interpretation local variations

• Plant and soil samples• Plant water use

– Whole tree transpiration– Sapflow sensors

• Different size trees

Mixing Models

• Proportion of fog water (Pf) used by plants

– Two compartment mixing model (Brunel et al)

• Assumes water comes from 2 sources– Fog and rain

– Deep soil/groundwater

• Weighted values-not all sources are equally available

ResultsInterception off trees always higher by 18-40%

-stripping fog-solar radiation, wind velocities

Forested areas have greater input

Redwoods: 8-43%

Understory: 6-100%

Rooting patterns, water demand, direct absorption through leaves, funnel water

Water use patterns

• Fraction of fog within xylem that was used

• 13-45% of all water used annually– Smaller trees used more

• Obtain ~19% from fog– Water use is higher in summer when trees

take up and use more fog water– More important source than isotopic

information implies

El niño: ratio of rainfall to fog water input higher (less fog), Pf and coefficient of variation increased -plant demand for water was highest in summer when there was no rain, and fog inputs did occur

Dry: Less rain in winter, so more dependence on fog in summer

VS.

Intact forests increase annual income of water-if moisture inputs decline, so do nutrient inputs,

decomposition and mineral cycling-therefore, tree loss = more drought prone,

warmer, open ecosystem-plants will experience more water stress

Saguaro Cactus:How important are they?

(Wolf and Martinez del Rio)

Saguaro cactus• Succulent CAM• 4000 or more liters of water• 300 flowers, 50-60% fruit• A ton of pollen, nectar, fruit—to

attract pollinators and seed dispersers

• Produce fruit during driest months (June-July)

• Fruit : water and sugar• Seeds: protein, lipids and

carbs 13C = -13.1±0.2‰

– Most common C3= -24.9 ±0.2‰ • D = 48.4±1.6 ‰

– Surface water=-37.3 to -23.5‰

Desert Nesting Birds

Mourning Dove• Gained only nutrients

(35% total C) for ~3 weeks in July

• No correlation between 13C and D

White-winged Dove• Saguaro fruit = >60% of

diet between June and mid-Sep.

13C and D linearly and positively correlated—fruit was important for C and H2O

Differences in foraging modes!!

Importance for community of Sonoran Desert birds…

• Determining proportion of diet that is represented by two isotopic sources:

isotopic composition of 2 sources

tissue= p(1 + ) + (1-p)(2 + ) Isotopic discrimination factor( tissue- diet) fraction of diet incorporated into focal tissue

• Blood plasma– Stable C3 resource signal in bird plasma during periods when they saguaro fruit was not available– Thus- = +3.3‰– Reflects isotopic composition of C incorporated recently

13C of plasma during summer showed distinct swell—coresponds to Saguaro input into ecosystem

• without: 13C= -21.6‰• with: 13C= -16‰• 43% of avian

community’s carbon derived from fruit

• Lasted for ~ 6 weeks

Individual species

• Granivorous and frugivorous• Insectivorous (7 of 16 that get 25% diet from

cactus)

Deuterium D of fruit water is enriched

75-100‰• White-winged Doves

– When using fruit, body water pools became enriched

– Strong correlation between C and D

• Body water discrimination factor– Enriched above fruit

• Evaporation• But…the presence or absence

of a correlation between 13C and D can be used to determine whether birds feed on seeds, or pulp as well