U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2007–3018

March 2007

Physiological Ecology and Ecohydrology of 

Coastal Forested Wetlands

The form, function, and productivity 

of wetland communities are influenced 

strongly by the hydrologic regime of 

an area. Wetland ecosystems persist by 

depending upon surpluses of rainfall, 

evapotranspiration, soil moisture, and 

frequency and amplitude of water-level 

fluctuations. Yet, wetland vegetation can 

also influence ecosystem water economy 

through conservative water- and carbonuse strategies at several organizational 

scales.

Scientists have described leaf-level 

water-use efficiency in coastal mangrove 

forests as being among the highest of any 

ecosystem. These forested wetlands occur 

in intertidal areas and often persist under 

flooded saline conditions. Are these same 

strategies used by other types of coastal 

forested wetlands? Do conservative 

water-use strategies reflect a consequence 

of salt balance more than efficiency in 

water use per se? At what organizational 

scales do these strategies manifest? These 

are just a few of the questions being 

answered by physiological and landscape 

ecologists at the U.S. Geological Survey 

National Wetlands Research Center 

(NWRC).

Determining how vegetation 

interacts with water flow through 

wetlands is important for defining 

how atmospheric carbon and water 

vapor exchange will occur within 

forested wetlands as sea level rises or 

as hydrologic restoration projects are 

implemented. Studies are also important 

for learning more about ecosystem 

stress. At NWRC, we are evaluating 

water use, describing carbon cycling, 

and identifying stress at several scales 

by studying leaf-level atmospheric 

gas exchange [carbon dioxide (CO2) 

and water vapor (H2O)], quantifying 

individual tree water use, developing 

stand-level water budgets, and assessing 

ecosystem-level soil CO2 exchange 

within mangroves and tidal freshwater 

swamp forests.

Experimental Studies

Leaf-level Gas Exchange

Studies at NWRC coordinate 

field and greenhouse investigations to 

determine how individual seedlings 

or saplings respond to combinations 

of flooding and salinity, which are 

common stressors to coastal wetlands. 

Observed leaf-level changes in net 

assimilation (atmospheric carbon 

fixation), transpiration (water loss), 

and stomatal conductance (CO2 plus 

water vapor exchange) on field sites 

often lead to hypotheses designed to 

identify environmental controls (fig. 1A). 

Greenhouse studies are then constructed 

to manipulate factors that cannot be 

controlled under field conditions and that 

allow for more intensive investigations of 

leaf gas exchange at targeted stress levels. 

Parameters such as CO2 and light can 

also be altered as plants are exposed to 

different stressors (fig. 1B).

Individual Tree Water Use

One of the best ways to quantify 

individual tree water use in natural field 

settings is through the use of sap-flow 

techniques. Water flowing through 

the outer portion of a tree’s stem is 

determined with a heat dissipation 

process (Granier, 1985). Two probes are 

inserted into the outer portion of a tree’s 

stem, embedded with thermocouples, 

and heated at the downstream probe; 

temperature differences decrease during 

the day as stem water transport increases. 

Figure 1. A, Leaf gas exchange 

of a mangrove tree sapling in 

Everglades National Park, Fla., 

being evaluated at a constant 

light level with an infrared 

gas analyzer (IRGA); B, Leaf 

gas exchange for a mangrove 

tree sapling being measured 

across a range of light levels in 

a greenhouse study designed to 

vary hydroperiod while keeping 

salinity and fertility constant.

A B
Soil Respiration

Leaf gas exchange drives CO2

uptake by ecosystems, but CO2 is lost 

from ecosystems through aerobic soil 

respiration (microbes) and belowground 

plant respiration. To understand 

ecosystem-level CO2 exchange and 

how exchange is altered by different 

hydrologic and salinity regimes, it is 

useful to measure soil CO2 efflux along 

with leaf-level processes. Several studies, 

both greenhouse- and field-based, 

have been conducted to determine how 

Through an empirical formula, water 

flow at the location of probe insertion 

can be determined with a fair degree of 

accuracy and, with appropriate scaling 

metrics (e.g., sapwood area, total leaf 

area, etc.), can allow for an estimate 

of water volume and flow within an 

individual tree. These studies have been 

useful in confirming that conservative 

water-use strategies documented for 

seedlings and saplings at the leaf level 

also apply to mature mangrove trees in 

the field (fig. 2). 

A

Figure 2. A, Individual trees in a B

mangrove forest stand in Rookery 

Bay, Fla., with xylem sap flow 

sensors embedded; B, Sap flow of 

individual trees of three mangrove 

species while stands were flooded 

versus drained. Flood state had 

a significant effect on individual 

trees of all three species in this 

evaluation.

salinity, tides, and permanent flooding 

interact to affect soil CO2 efflux from 

tidal freshwater swamp forests (fig. 3).
Figure 3. A tidal freshwater swamp forest in Savannah National Wildlife Refuge, Ga. Insets depict measurements being taken by a soil CO2

efflux chamber and infrared gas analyzer (IRGA) during greenhouse and field studies.

Modeling

Stand Water Use

Sap flow investigations can be 

expanded to quantify water flow at 

multiple depths into the tree; not all 

sapwood is as effective as the outer 

portions at conducting water. In fact, 

water flow is approximately 60 percent 

less at a depth of 4 cm into the tree than 

at a depth of 2 cm in mangrove trees 

in south Florida (Krauss and others, 

2007). Once absolute rates of sap flow 

are determined and depth profiles are 

constructed, stand water use can be 

calculated as a function of species 

distribution; tree diameter distribution; 

environmental stress regime (e.g., 

flooded versus drained); and daily, 

monthly, and annual meteorological 

state (e.g., vapor pressure deficit, light 

regime). This approach has indicated 

that stand water use in south Florida 

mangroves, for example, is extremely 

conservative overall and is affected little 

by relative site flood frequency, even 

though sap flow is reduced by flooding 

at the individual tree level. Additional 

studies by scientists at NWRC will 

attempt to verify these results from 

different mangrove locations and 

determine whether stand water use is as 

conservative in tidal freshwater swamps 

and other coastal wetland forests.

Landscape Assessments

To date, studies have suggested 

convergence among seedling, sapling, 

and individual tree responses in order 

to justify the extrapolation of many 

water use functions to the landscape. 

Accordingly, additional studies need to 

be conducted to determine the ultimate 

role of coastal forested wetlands in 

affecting water budgets as hydrologic 

regimes change. Our first step in this 

process is embedding empirical functions 

that describe stand water use as subroutines within existing landscape 

ecological simulation models (e.g., Doyle 

and others, 2003). It is also important 

to design landscape-level studies; we 

are currently developing a surface water 

hydrologic budget for a marsh-mangrove 

complex on Ten Thousand Islands NWR, 

Fla., to help empirically identify the role 

of coastal wetlands in regional water 

budgeting (fig. 4). Coastal land managers 

and scientists can then begin to determine 

exactly how coastal forested wetlands 

respond to altered hydroperiods on a 

large scale.
Figure 4. An aerial image of a marsh-mangrove wetland complex at 

Ten Thousand Islands National Wildlife Refuge, Fla., with depictions 

of parameter station locations. A, weather station used to model 

evapotranspiration; B, surface and ground water stage recorders; C, tipping 

bucket rain gage.

For more information, contact

Ken W. Krauss, Ph.D.

U.S. Geological Survey

National Wetlands Research Center

700 Cajundome Blvd.

Lafayette, LA 70506

337-266-8882

kkrauss@usgs.gov

References

Doyle, T.W., Girod, G.F., and Books, 

M.A., 2003, Modeling mangrove forest 

migration along the southwest coast 

of Florida under climate change, in

Ning, Z.H., Turner, R.E., Doyle, T.W., 

and Abdollahi, K., eds., Integrated 

assessment of climate change: Baton 

Rouge, La, Gulf Coast Regional 

Climate Change (GCRCC) Assessment 

Program, p. 211– 221.

Granier, A., 1985, A new method of 

sap flow measurements in tree stems: 

Annals de Science Forestiere 42: 

193–200.

Krauss, K.W., Young, P.J., Chambers, 

J.L., Doyle, T.W., and Twilley, R.R., 

2007, Sap flow characteristics of 

neotropical mangroves in flooded and 

drained soils: Tree Physiology 27: 

775–783.

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B

C