Physiological Ecology and Ecohydrology of Coastal Forested Wetlands

The form, function, and productivity Determining how vegetation common stressors to coastal wetlands. of wetland communities are influenced interacts with water flow through Observed leaf-level changes in net strongly by the hydrologic regime of wetlands is important for defining assimilation (atmospheric carbon an area. Wetland ecosystems persist by how atmospheric carbon and water fixation), transpiration (water loss), depending upon surpluses of rainfall, vapor exchange will occur within and stomatal conductance (CO2 plus evapotranspiration, soil moisture, and forested wetlands as sea level rises or water vapor exchange) on field sites frequency and amplitude of water-level as hydrologic restoration projects are often lead to hypotheses designed to fluctuations. Yet, wetland vegetation can implemented. Studies are also important identify environmental controls (fig. 1A). also influence ecosystem water economy for learning more about ecosystem Greenhouse studies are then constructed through conservative water- and carbon- stress. At NWRC, we are evaluating to manipulate factors that cannot be use strategies at several organizational water use, describing carbon cycling, controlled under field conditions and that scales. and identifying stress at several scales allow for more intensive investigations of by studying leaf-level atmospheric leaf gas exchange at targeted stress levels. Scientists have described leaf-level gas exchange [carbon dioxide (CO2) Parameters such as CO2 and light can water-use efficiency in coastal mangrove and water vapor (H2O)], quantifying forests as being among the highest of any individual tree water use, developing also be altered as plants are exposed to ecosystem. These forested wetlands occur stand-level water budgets, and assessing different stressors (fig. 1B). in intertidal areas and often persist under ecosystem-level soil CO2 exchange flooded saline conditions. Are these same within mangroves and tidal freshwater Individual Tree Water Use strategies used by other types of coastal swamp forests. One of the best ways to quantify forested wetlands? Do conservative individual tree water use in natural field water-use strategies reflect a consequence settings is through the use of sap-flow of salt balance more than efficiency in Experimental Studies techniques. Water flowing through water use per se? At what organizational the outer portion of a tree’s stem is scales do these strategies manifest? These determined with a heat dissipation are just a few of the questions being Leaf-level Gas Exchange process (Granier, 1985). Two probes are answered by physiological and landscape Studies at NWRC coordinate inserted into the outer portion of a tree’s ecologists at the U.S. Geological Survey field and greenhouse investigations to stem, embedded with thermocouples, National Wetlands Research Center determine how individual seedlings (NWRC). and heated at the downstream probe; or saplings respond to combinations temperature differences decrease during of flooding and salinity, which are the day as stem water transport increases.

Figure 1. A, Leaf gas exchange A B 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.

U.S. Department of the Interior Fact Sheet 2007–3018 U.S. Geological Survey March 2007 Through an empirical formula, water Soil Respiration salinity, tides, and permanent flooding flow at the location of probe insertion interact to affect soil CO efflux from Leaf gas exchange drives CO 2 can be determined with a fair degree of 2 tidal freshwater swamp forests (fig. 3). uptake by ecosystems, but CO is lost accuracy and, with appropriate scaling 2 from ecosystems through aerobic soil metrics (e.g., sapwood area, total leaf respiration (microbes) and belowground area, etc.), can allow for an estimate plant respiration. To understand of water volume and flow within an ecosystem-level CO exchange and individual tree. These studies have been 2 how exchange is altered by different useful in confirming that conservative hydrologic and salinity regimes, it is water-use strategies documented for useful to measure soil CO efflux along seedlings and saplings at the leaf level 2 with leaf-level processes. Several studies, also apply to mature mangrove trees in both greenhouse- and field-based, the field (fig. 2). have been conducted to determine how

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. 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.

state (e.g., vapor pressure deficit, light Accordingly, additional studies need to Modeling regime). This approach has indicated be conducted to determine the ultimate that stand water use in south Florida role of coastal forested wetlands in Stand Water Use mangroves, for example, is extremely affecting water budgets as hydrologic conservative overall and is affected little regimes change. Our first step in this Sap flow investigations can be by relative site flood frequency, even process is embedding empirical functions expanded to quantify water flow at though sap flow is reduced by flooding that describe stand water use as sub- multiple depths into the tree; not all at the individual tree level. Additional routines within existing landscape sapwood is as effective as the outer studies by scientists at NWRC will ecological simulation models (e.g., Doyle portions at conducting water. In fact, attempt to verify these results from and others, 2003). It is also important water flow is approximately 60 percent different mangrove locations and to design landscape-level studies; we less at a depth of 4 cm into the tree than determine whether stand water use is as are currently developing a surface water at a depth of 2 cm in mangrove trees conservative in tidal freshwater swamps hydrologic budget for a marsh-mangrove in south Florida (Krauss and others, and other coastal wetland forests. complex on Ten Thousand Islands NWR, 2007). Once absolute rates of sap flow Fla., to help empirically identify the role are determined and depth profiles are Landscape Assessments of coastal wetlands in regional water constructed, stand water use can be budgeting (fig. 4). Coastal land managers calculated as a function of species To date, studies have suggested convergence among seedling, sapling, and scientists can then begin to determine distribution; tree diameter distribution; exactly how coastal forested wetlands environmental stress regime (e.g., and individual tree responses in order to justify the extrapolation of many respond to altered hydroperiods on a flooded versus drained); and daily, large scale. monthly, and annual meteorological water use functions to the landscape. A

B

C

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.

Granier, A., 1985, A new method of References sap flow measurements in tree stems: For more information, contact Annals de Science Forestiere 42: Doyle, T.W., Girod, G.F., and Books, 193–200. Ken W. Krauss, Ph.D. M.A., 2003, Modeling mangrove forest U.S. Geological Survey migration along the southwest coast National Wetlands Research Center of Florida under climate change, in Krauss, K.W., Young, P.J., Chambers, 700 Cajundome Blvd. Ning, Z.H., Turner, R.E., Doyle, T.W., J.L., Doyle, T.W., and Twilley, R.R., Lafayette, LA 70506 and Abdollahi, K., eds., Integrated 2007, Sap flow characteristics of 337-266-8882 assessment of climate change: Baton neotropical mangroves in flooded and [email protected] Rouge, La, Gulf Coast Regional drained soils: Tree Physiology 27: Climate Change (GCRCC) Assessment 775–783. Program, p. 211– 221.