U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2004–3126

August 2004

Belowground Dynamics in Mangrove Ecosystems

Mangrove Ecosystems

Mangrove ecosystems are tropical/subtropical communities 

of primarily tree species that grow in the intertidal zone. These 

tidal communities are important coastal ecosystems that are valued for a variety of ecological and societal goods and services 

(fig. 1). Mangrove wetlands are important filters of materials 

moving between the land and sea, trapping sediment, nutrients, 

and pollutants in runoff from uplands and preventing their direct 

introduction into sensitive marine ecosystems such as seagrass 

beds and coral reefs. Mangroves serve as nursery grounds and 

refuge for a variety of organisms and are consequently vital 

to the biological productivity of coastal waters. Furthermore, 

because mangroves are highly resilient to disturbances such as 

hurricanes, they represent a self-sustaining, protective barrier 

for human populations living in the coastal zone. Mangrove 

ecosystems also contribute to shoreline stabilization through 

consolidation of unstable mineral sediments and peat formation. 

In order to help conserve mangrove ecoystems, scientists with 

the United States Geological Survey (USGS) at the National 

Wetlands Research Center are working to more fully understand 

the dynamics that impact these vital ecosystems.

Sea-Level Rise, Peat Formation, and Habitat Stability

Early ecologists referred to mangroves as “landbuilders” 

based on observations that mangrove shorelines built seaward. 

Geologists, however, have criticized this viewpoint, offering 

evidence that mangroves do not actually build land, but instead 

respond to inputs of mineral sediment which creates the land 

upon which mangroves grow. Actually, mangrove soils develop 

through a combination of two processes: mineral sediment 

deposition and organic matter accumulation (fig. 2), but the 

relative contribution of these processes varies with geomorphology and other factors. 

The mangrove islands of the Mesoamerican Barrier Reef 

System, specifically located in Belize, are isolated from inputs 

of terrigenous sediment and are thus dependent upon accumulation of organic matter for soil formation. Because mangrove 

soils are waterlogged and nutrient availability is low, decomposition of mangrove roots and other plant tissues is extremely 

slow (Middleton and McKee, 2001). These conditions lead to a 

slow buildup of organic material called peat that raises the soil 

surface a few millimeters per year over long periods of time. 

Mangrove islands are underlaid by peat deposits that may be 

Figure 1. Mangroves are important coastal ecosystems valued for a variety of ecological and societal goods and services.

Shoreline Stabilization Linkages to Seagrass and Coral Reefs

Nursery Grounds Productivity

Wildlife Habitat Aesthetics Tourism Storm Barrier

Silviculture/

Aquaculture Filter
10 m in depth. These peat deposits are evidence that as sea level 

rose, mangroves have kept pace by slow deposition of plant 

parts (McKee and Faulkner, 2000). Mangrove peat is composed primarily of refractory roots (fig. 3), rather than leaf or 

wood material that decays more quickly or is removed by tides 

(Middleton and McKee, 2001).

Scientists are currently measuring surface and subsurface 

processes controlling soil formation and elevations in Florida, 

Belize, and other locations. They use surface elevation tables 

and marker horizons to quantify rates of elevation change and 

vertical accretion, respectively (fig. 4). 

Mangrove Roots: The Unseen Half

Mangroves have two general types of roots: (1) aerial roots, 

which occur above the soil surface and provide a pathway for 

oxygen entry to (2) belowground roots that grow in the anoxic 

(without oxygen) soil. An intriguing growth pattern by mangrove roots was recently uncovered by USGS scientists working 

in Belize. Peat-based soils on mangrove islands were characterized by old root channels filled with living and highly branched 

roots of red (Rhizophora mangle) and black (Avicennia germinans) mangrove (fig. 5). Experiments indicated that root 

proliferation inside old, decaying roots may improve recovery 

of nutrients from decomposing tissues and may explain, in part, 

how mangroves persist and grow in nutrient-poor environments 

(McKee, 2001).

In spite of the importance of belowground processes to ecosystem productivity, nutrient dynamics, and ability to accommodate sea-level rise, very little is known about production and 

turnover of mangrove roots. Some information exists about root 

morphological and physiological responses to flooding, anoxia, 

sulfide, and nutrients (McKee and Mendelssohn, 1987; McKee, 

1993; McKee, 1996), but few studies have examined belowground biomass or production rates in mangrove ecosystems.

Current work conducted by USGS scientists in Belizean 

mangrove forests is showing that root production rates vary 

across a tree height gradient from shoreline (tall trees) to interior (dwarf trees). Phosphorus additions to stunted red mangrove 

increased root production rates almost tenfold, whereas nitrogen 

enrichment had no effect on root production at any location. 

A relative measure of root production may be made by using 

in-growth bags filled with a standardized, root-free substrate. 

The bags are buried for a period of time to allow roots to grow, 

then retrieved and washed, and root biomass is measured. This 

technique is being used to compare relative rates of mangrove 

root production in Louisiana, Florida, Belize, Panama, Australia, and New Zealand. Information about root production aids 

in understanding controls on elevation change in mangrove 

systems occurring in different geomorphic settings. A greater 

understanding of mangrove systems will help to ensure that 

the most beneficial enviroments for their health are protected 

around the world. 

Figure 2. A coring device is used to extract soil from a mangrove ecosystem to help USGS scientists reconstruct the vegetation history of the site. The extracted core shows mangrove 

peat deposited beneath a sand layer. 

Figure 3. Contribution of roots to soil formation and vertical 

accretion in a mangrove ecosystem in Belize.

Unwashed Core

Washed Core
Figure 4. Measurement of soil elevation 

change in a mangrove ecosystem in Belize 

using a rod surface elevation table.

Figure 5. A mangrove root channel 

formed by a refractory, dead root and 

containing living, highly branched roots 

that have grown inside. Root proliferation 

in such microsites may be a strategy by 

mangroves to capture limiting nutrients 

from decomposing tissues.
References Cited

McKee, K.L. and Mendelssohn, I.A., 1987, Root metabolism in 

the black mangrove, Avicennia germinans (L.) L.: Response 

to hypoxia: Environmental and Experimental Botany, v. 27, 

p.147-156.

McKee, K.L., 1996, Growth and physiological responses of 

mangrove seedlings to root zone anoxia: Tree Physiology, v. 

16, p. 883-889.

McKee, K.L., 1993, Soil physicochemical patterns and mangrove species distribution- reciprocal effects?: Journal of 

Ecology, v. 81, p. 477-487.

McKee, K.L., 2001, Root proliferation in decaying roots and 

old root channels—a nutrient conservation mechanism in 

oligotrophic mangrove forests?: Journal of Ecology, v. 89, no. 

5, p. 876-887.

McKee, K.L. and Faulkner, P.L., 2000, Mangrove peat analysis 

and reconstruction of vegetation history at the Pelican Cays, 

Belize: Atoll Research Bulletin, v. 468, p. 46-58.

Middleton, B.A. and McKee, K.L., 2001, Degradation of mangrove tissues and implications for peat formation in Belizean 

island forests: Journal of Ecology, v. 89, no. 5, p. 818-828.

For more information, contact

Karen L. McKee

U.S. Geological Survey

National Wetlands Research Center

700 Cajundome Blvd.

Lafayette, LA 70506

337-266-8500

http://www.nwrc.usgs.gov