Mangrove carbon sequestration in the Florida Everglades
Jordan G. Barr, José D. Fuentes, and Jay C. Zieman
Department of Environmental Sciences, University of Virginia, Charlottesville,
VA
Dan Childers
Department of Biological Sciences, Florida International University, Miami, FL
Mangrove forests represent one of the most geochemically and biologically active
biomes (Twilley et al. 1992), yet at present no unified modeling framework exists
to investigate processes governing carbon sequestration. Limited information is
available to learn how mangrove forests respond to climate forcings and
anthropogenic perturbations such as fresh water input dynamics. This research
addresses the hypothesis that the carbon sequestration capability of the riverine
mangrove ecosystem in south Florida is governed by rates of fresh water flows
into the bay and abiotic forcings such as environmental irradiance, and temporal
and spatial salinity gradients. The hypothesis is first evaluated through the
application of coupled atmospheric-biospheric modeling systems.
We are developing a coupled atmospheric-biospheric modeling system to
investigate trace gas exchange between the mangrove forest and overlying
atmosphere. The model consists of a plant canopy radiative transfer module to
describe solar and terrestrial irradiance disposition inside the forest, a module to
quantify turbulent transport through the canopy, a plant biochemistry module to
estimate carbon assimilation rates, and a component to evaluate soil respiration
rates. The plant biochemistry module is based on the theory developed for
terrestrial ecosystems (Baldocchi and Meyers 1998, Gu et al. 1999), but it
incorporates physiological characteristics determined by our research group for
riverine mangroves in Shark River Slough adjacent to Long-Term Ecological
Research (FCE-LTER) site SRS-6. As input to the biochemical module, local
physiological characteristics are considered including Rubisco, light limited
carboxylation rates, nighttime respiration rates, and stomatal conductance to water
vapor diffusion for red (Rhizophora mangle), white (Languncularia racemosa),
and black (Avicennia germinans) mangroves. To reflect the vertical gradients in
both mangrove physiology and environmental state variables, the model
represents the mangrove forest canopy as a conglomerate of layers. The net
carbon exchange between the forest and overlying atmosphere is taken as the
integral of the differences between photosynthetic gains and the respiratory
losses.
36
AN
B
o
gsv
TLeaf ( C)
A
100
32
0
2000
28
3
D
C
2
1000
1
0
7
9
11
13
15
Local time (hours)
17 7
9
11
13
15
VPDLeaf (kPa)
-2 -1
PAR (mol m s )
gsv , AN * 10
200
0
17
Local time (hours)
Figure 1. (A) Diurnal trends in net photosynthesis (AN, in mol (CO2) m-2 s-1),
stomatal conductance to water vapor (gsv, mmol (H2O) m-2 s-1). Figures B, C,
and D illustrate the diurnal variations in foliage temperature,
photosynthetically active irradiance (PAR), and vapor pressure deficit
between mangrove leaves and atmosphere, respectively. Measurements were
made on red mangrove leaves during 24 July 2001 at Key Largo, Florida.
Though the biophysical principles are essentially the same as those applied to
terrestrial forest ecosystems, mangrove forests exhibit unique physiological
attributes. For example, mangrove leaves exposed to direct sunlight throughout
the day achieve maximum photosynthetic rates and stomatal conductance around
10:00 h local time. These optimum physiological responses occur much earlier
than the time when the governing environmental variables attain their maximum
daily values (Figure 1). As a working hypothesis, we propose that mangroves
exhibit unique physiological responses to the local environment due to two
coupled processes. First, as the atmospheric evaporative demand increases (as
reflected in the high vapor pressure deficits, Figure 1D) mangroves need to reduce
the stomatal conductance (Figure 1A) to conserve water. Mangroves must
maintain low salt concentrations by either excluding salt in their xylem, as is the
case with red mangroves, or ridding themselves of salt, as is the case with black
mangroves. Second, mangrove leaves need to cope with exceedingly high
radiational loadings. Because of the unique mangrove-water relations, evaporative
cooling may not be the most effective mechanism to protect foliage from
excessive radiational heating. Instead, the modulation of energy loading on the
foliage may be accomplished through inclining leaf angle to reduce light
interception. As an example, Figure 1A shows that leaf-level photosynthesis
declines throughout the day in concert with increasing leaf temperature (Figure
1B).
To account for these unique mangrove-water relations, we have developed new
stomatal conductance algorithms specific to the mangrove biome. Also, a new
algorithm has been established to study the mangrove physiological responses to
temperature. These mangrove-environment interactions are necessary to
successfully investigate processes such as net carbon ecosystem exchange. In the
presentation, we will provide evidence to support the conclusion that coupled
atmospheric-biospheric modeling systems are important tools to assess the
impacts of the Florida Everglades restoration project on the mangrove ecosystem.
Acknowledgements
Support for this research was provided by the National Science Foundation
through the Long-Term Ecological Research program, NASA through an Earth
Science Fellowship to JGB, the Barley Scholar Program at the University of
Virginia, and the Key Largo Research Center of the Everglades National Park.
References
Baldocchi, D.D., T.P. Meyers, 1998. On using eco-physiological,
micrometeorological and biogeochemical theory to evaluate carbon dioxide, water
vapor, and trace gas fluxes over vegetation: a perspective. Agricultural and Forest
Meteorology. 90, 1-25.
Gu, L, H.H. Shugart, J.D. Fuentes, T.A. Black, S.R. Shewchuk, 1999.
Micrometeorology, biophysical exchanges and NEE decomposition in a two-story
boreal forest - development and test of an integrated model. Agricultural and
Forest Meteorology. 94, 123-148.
Twilley RR, RH Chen, and T Hargis, 1992. Carbon sinks in mangroves and their
implications to carbon budget of tropical coastal ecosystems. Water, Air and Soil
Pollution. 64, 265-288.