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Ecological site QX197X01X501

Mangrove Swamp

Last updated: 6/12/2025
Accessed: 12/05/2025

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General information

Provisional. A provisional ecological site description has undergone quality control and quality assurance review. It contains a working state and transition model and enough information to identify the ecological site.

MLRA notes

Major Land Resource Area (MLRA): 197X–Volcanic Islands of American Samoa

This MLRA consists of the islands of Tutuila, Aunuu, Ofu, Olosega, and Tau. The islands have extremely steep, highly dissected mountains, small valleys, and a narrow coastal plain. More than half of the area has slopes of more than 70 percent. The highest elevations are 3,056 feet (931 meters) on Tau and 2,142 feet (653 meters) on Tutuila. The islands consist of Pleistocene-age, basic igneous rocks, mainly basalt with some andesite and trachyte (USDA-NRCS, 2006).

The climate is moist, warm, and humid. Average annual rainfall ranges from 125 inches (3,175 millimeters) to more than 250 inches (6,350 millimeters). Rainfall varies over short distances due to topography. The driest months are June through September; the wettest months are December through March. Average annual temperature is 81 degrees F (27 degrees C). Relative humidity is 73 to 90 percent throughout the year. Prevailing winds are easterly trade winds. Cyclones occasionally strike the area (USDA-NRCS, 2006).

Soils are Mollisols, Andisols, Entisols, Oxisols, and Histosols. Soil moisture regimes are udic or perudic; the soil temperature regime is isohyperthermic. Natural vegetation is mostly tropical hardwood forest (USDA-NRCS, 2006).

Classification relationships

This ecological site occurs within Major Land Resource Area (MLRA) 197 – Volcanic Islands of American Samoa.

Ecological site concept

This ecological site occurs on the islands of Tutuila and Aunuu in American Samoa. It occurs on nearly level (0 to 1 percent) coastal plains at elevations ranging from 0 to 20 feet (0 to 6 meters) elevation. It is most easily accessible along the Pala Lagoon shore at Pago Pago on Tutuila (USDA-SCS, 1984).

Soils are in the Histosols order. They consist of mucky peat that formed from organic matter derived from mangrove roots and litter deposited along the seashore of coastal plains. Soil temperature regimes are isohyperthermic; soil moisture regimes are aquic. Average annual rainfall ranges from 125 to 225 inches (3130 to 5630 millimeters). Water runoff is very slow and ponded in spots, and flooding by seawater occurs twice daily at a duration of 12 hours daily. Effective rooting depth is 40 to 60 inches (100 to 150 centimeters) (USDA-SCS, 1984).

This ecological site is forest consisting of shorter, more salt-resistant Rhizophora mangle facing the sea and taller (to 53 feet or 15 meters) Bruguiera gymnorrhiza on the landward side. The forest floor is bare except for mangrove seedlings. The landward forest, being protected from salt spray, supports many epiphytes (USDA-SCS, 1984; Whistler, 2002).

Associated sites

QX197X01X502	Sandy Littoral Forest Sandy Littoral Forest occurs on coastlines and on bars adjacent to Mangrove Swamp. Soils are somewhat excessively to excessively drained, in contrast with very poorly drained Mangrove Swamp soils.
QX197X01X504	Alluvial Valley Forest Alluvial Valley Forest occurs on valley floors and coastal plains, where it may adjoin Mangrove Swamp. Alluvial Valley Forest differs from Mangrove Swamp in being somewhat poorly drained rather than very poorly drained, having fresh water rather than salt water, and by undergoing occasional, brief flooding rather than twice-daily tidal flooding.
QX197X01X506	Tuff or Rock Subsurface Forest Tuff or Rock Subsurface Forest adjoins Mangrove Swamp where uplands and mountain slopes descend to partially-protected areas of the coast that are open to seawater. It occurs on slopes up to 1500 feet in elevation and has well drained soils, in contrast with Mangrove Swamp, which ascends to 20 feet elevation and has very poorly drained soils.

QX197X01X502

Sandy Littoral Forest

Sandy Littoral Forest occurs on coastlines and on bars adjacent to Mangrove Swamp. Soils are somewhat excessively to excessively drained, in contrast with very poorly drained Mangrove Swamp soils.

QX197X01X504

Alluvial Valley Forest

Alluvial Valley Forest occurs on valley floors and coastal plains, where it may adjoin Mangrove Swamp. Alluvial Valley Forest differs from Mangrove Swamp in being somewhat poorly drained rather than very poorly drained, having fresh water rather than salt water, and by undergoing occasional, brief flooding rather than twice-daily tidal flooding.

QX197X01X506

Tuff or Rock Subsurface Forest

Tuff or Rock Subsurface Forest adjoins Mangrove Swamp where uplands and mountain slopes descend to partially-protected areas of the coast that are open to seawater. It occurs on slopes up to 1500 feet in elevation and has well drained soils, in contrast with Mangrove Swamp, which ascends to 20 feet elevation and has very poorly drained soils.

Similar sites

QX197X01X001	Coastal Marsh Like Mangrove Swamp, Coastal Marsh occurs on very poorly drained soils to elevations no greater than 20 feet. However, Coastal Marsh is protected from seawater, making its waters fresh rather than saline, undergoes occasional to frequent flooding rather than twice-daily tidal flooding, and is not accessible to mangrove seeds and seedlings.

QX197X01X001

Coastal Marsh

Like Mangrove Swamp, Coastal Marsh occurs on very poorly drained soils to elevations no greater than 20 feet. However, Coastal Marsh is protected from seawater, making its waters fresh rather than saline, undergoes occasional to frequent flooding rather than twice-daily tidal flooding, and is not accessible to mangrove seeds and seedlings.

Table 1. Dominant plant species

Tree	(1) Rhizophora mangle (2) Bruguiera gymnorhiza
Shrub	Not specified
Herbaceous	Not specified

Legacy ID

F197XY501AS

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Physiographic features

This ecological site occurs on organic matter deposited in mangrove swamps. These mangrove swamps are inundated by ocean tides twice daily. Ponding can occur in low spots (USDA-SCS, 1984).

Table 2. Representative physiographic features

Landforms	(1) Island > Mangrove swamp
Runoff class	Very low
Flooding frequency	Frequent
Elevation	20 ft
Slope	1%
Water table depth	Not specified
Aspect	Aspect is not a significant factor

Climatic features

The area is characterized by abundant rain and warm, humid days and nights. Average annual precipitation in this ecological site ranges from 96 to 183 inches (2440 to 4650 millimeters). Mean annual air temperature is 80F (27C). The driest period is June through September (winter), and the wettest is December through March (summer), although heavy showers and long, rainy periods can occur in any month. June, July, and August are the coolest months, and January, February, and March are the warmest. Daytime temperatures typically reach the upper 80s in summer and the middle 80s F in winter, while nighttime temperatures are in the middle 70s in summer and low 70s in winter.

The prevailing winds throughout the year are the easterly trade winds. They tend to be more directly from the east in December through March and mostly from the east-southeast and southeast during the rest of the year. The trade winds are less prevalent in summer than in winter. About 25 to 30 thunderstorms occur in an average year, mainly during the rainy season. The area lies across the path of tropical disturbances, including cyclones, that come usually from the north, but occasionally from east or west (USDA-SCS, 1984; USDI-FWS, 1982).

Table 3. Representative climatic features

Frost-free period (characteristic range)	365 days
Freeze-free period (characteristic range)	365 days
Precipitation total (characteristic range)	96-183 in
Frost-free period (actual range)	365 days
Freeze-free period (actual range)	365 days
Precipitation total (actual range)	96-183 in
Frost-free period (average)	365 days
Freeze-free period (average)	365 days
Precipitation total (average)	159 in

Characteristic range

Actual range

Bar

Line

Figure 1. Monthly precipitation range

Characteristic range

Actual range

Bar

Line

Figure 2. Monthly minimum temperature range

Characteristic range

Actual range

Bar

Line

Figure 3. Monthly maximum temperature range

Bar

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Figure 4. Monthly average minimum and maximum temperature

Figure 5. Annual precipitation pattern

Figure 6. Annual average temperature pattern

Climate stations used

(1) PAGO PAGO WSO AP [AQW00061705], AS

Soil features

NGERUNGOR VARIANT

The one soil in this ecological site, Ngerungor variant mucky peat, is a very poorly drained Histosol that formed from mangrove litter and roots. The soil temperature regime is isohypothermic (very warm) with a mean temperature of 80F (27C). The soil moisture regime is aquic. The soils are flooded by ocean tides twice daily to a depth of about 12 inches (30 centimeters). The water table varies twice daily from 0 to 12 inches (0 to 30 centimeters) below the soil surface. Scattered depressions may remain ponded to various depths between high tides (USDA-SCS, 1984).

Table 4. Representative soil features

Parent material	(1) Organic material
Surface texture	(1) Mucky peat
Drainage class	Very poorly drained
Permeability class	Rapid
Depth to restrictive layer	72 in
Soil depth	72 in
Surface fragment cover <=3"	Not specified
Surface fragment cover >3"	Not specified
Available water capacity (0-40in)	10 in
Calcium carbonate equivalent (0-40in)	Not specified
Electrical conductivity (0-40in)	Not specified
Sodium adsorption ratio (0-40in)	Not specified
Soil reaction (1:1 water) (0-40in)	5.6 – 7.3
Subsurface fragment volume <=3" (0-40in)	Not specified
Subsurface fragment volume >3" (0-40in)	Not specified

Ecological dynamics

Mangrove swamps occur primarily on coastal areas protected by reefs, headlands, or offshore islands; they may occur in river mouths where fresh water enters ocean. They establish on mudflats, estuaries, and sheltered lagoons where sedimentation is occurring in places to which their seeds or sprouted seedlings can reach by floating in the water (Forestry Program, Division of Community and Natural Resources, 2010; Sene, 2020; Whistler, 2002).

The main natural disturbance is strong storms that can damage and kill mangrove trees. The main human disturbances are wood harvesting and filling to claim areas for building construction (Forestry Program, Division of Community and Natural Resources, 2010; Sene, 2020; Whistler, 2002).

State and transition model

Custom diagram

Standard diagram

Figure 7. State and Transition Model (STM) for F197XY501AS (Mangrove Swamp).

More interactive model formats are also available. View Interactive Models

More interactive model formats are also available. View Interactive Models
Click on state and transition labels to scroll to the respective text

Ecosystem states

1. Reference State

2. Landfilled State

T1A	-	State 1 Reference transitions to State 2 Landfilled by intentional filling of the swamp to produce construction sites.

State 1 submodel, plant communities

1.1. Red mangrove – Oriental mangrove

1.2. Red mangrove/seashore paspalum

P1.1A	-	This phase change occurs due to heavy damage and tree mortality from storms or overharvesting of wood.
P1.2A	-	This community phase reverts to phase 1.1 with gradual regrowth of mangroves, particularly Oriental mangroves, when given adequate time to recover after disturbance.

State 2 submodel, plant communities

2.1. Urban Development

State 1
Reference State

This state consists of two community phases dominated by dense stands of mangrove trees occurring saline water at the coast. Zonation by mangrove species is caused by different salt tolerance between species. Shorter statured red mangrove (Rhizophora mangle) is dominant on outer areas nearer the ocean; taller Oriental mangrove (Bruguiera gymnorhiza) is dominant in more sheltered and less saline parts of the forest away from the ocean (Forestry Program, Division of Community and Natural Resources, 2010; Sene, 2020; Whistler, 2002). Storms or intensive wood harvest that destroy small to large patches of the forest may be recolonized initially by red mangrove. Oriental mangrove gradually recovers dominance over red mangrove in areas where it originally dominated because red mangrove is not shade tolerant (Forestry Program, Division of Community and Natural Resources, 2010; Sene, 2020; Whistler, 2002).

Community 1.1
Red mangrove – Oriental mangrove

The dominant trees in this community are Oriental mangrove (Bruguiera gymnorhiza), which can grow to about 50 feet (16 meters) tall, red mangrove (Rhizophora mangle (syn. samoensis), which is a shrub or small tree, and, in a few locations in American Samoa, puzzlenut tree (Xylocarpus moluccensis). Areas dominated by red mangrove support no epiphytes. However, epiphytes are common under Oriental mangrove in inner portions of the forest where they are protected from salt spray. Among these epiphytes are bird’s nest fern (Asplenium nidus), haresfoot ferns (Davallia spp.), musk fern (Microsorum grossum), orchids (Dendrobia spp.), and the vine fue selela (Hoya australis). Ground cover under Oriental mangrove consists entirely of Oriental mangrove seedling, which eventually die off in the shade unless a disturbance of the overstory allows them to grow (Forestry Program, Division of Community and Natural Resources, 2010; Sene, 2020; Stemmermann, 1981; Whistler, 2002).

Dominant plant species

red mangrove (Rhizophora mangle), tree
Oriental mangrove (Bruguiera gymnorhiza), tree

Community 1.2
Red mangrove/seashore paspalum

This community phase is dominated by red mangrove or red mangrove with seedlings and saplings of Oriental mangrove that have been released by overstory removal. In places, the introduced grass species seashore paspalum (Paspalum vaginatum) persists in sunny openings. Epiphytes are uncommon until Oriental mangrove regrows enough to provide sheltered growing locations (Forestry Program, Division of Community and Natural Resources, 2010; Sene, 2020; Stemmermann, 1981; Whistler, 2002).

Dominant plant species

red mangrove (Rhizophora mangle), tree
seashore paspalum (Paspalum vaginatum), grass

Pathway P1.1A
Community 1.1 to 1.2

This phase change occurs due to heavy damage and tree mortality from storms or overharvesting of wood.

Pathway P1.2A
Community 1.2 to 1.1

This community phase reverts to phase 1.1 with gradual regrowth of mangroves, particularly Oriental mangroves, when given adequate time to recover after disturbance.

State 2
Landfilled State

This state consists of filled land occupied by buildings.

Community 2.1
Urban Development

The hydrologic conditions of mangrove swamp have been obliterated. Vegetation consists of weedy or intentionally planted species (Forestry Program, Division of Community and Natural Resources, 2010; Sene, 2020; Stemmermann, 1981; Whistler, 2002).

Transition T1A
State 1 to 2

The Reference State (1) Reference transitions to the Landfilled State (2) by intentional filling of the swamp to produce construction sites.

Additional community tables

Supporting information

Other references

Annotated References for F197XY501AS Mangrove Swamp

Dixon JB and Schulze DG, eds. 2002. Soil Mineralogy with Environmental Applications. Volume 7. Soil Science Society of America. Available online at: https://acsess.onlinelibrary.wiley.com/doi/book/10.2136/sssabookser7 Exhaustive treatment of basics of soil mineralogy and implications for environmental management.

Federal Geographic Data Committee. 2019. Wetlands and Deepwater Habitats Classification Map Code Diagram. Tree diagram outlining translations of NWI map codes.

Forestry Program, Disvision of Community and Natural Resources. 2010. American Samoa Forest Assessment and Resource Strategy. American Samoa Community College, Pago Pago, American Samoa. Some discussion of disturbances, threats, and invasive species.

Kirch PV. 2000. On the Road of the Winds: An Archaeological History of the Pacific Islands Before European Contact. Berkeley: University of California Press. General discussion of effects of prehistoric Polynesians on native vegetation.

Mueller-Dombois D and FR Fosberg. 1998. Vegetation of the Tropical Pacific Islands. Springer-Verlag, New York. General account of tropical Pacific Island vegetation, with section on Hawaii. Discussion of likely effect of stoniness on soil moistue storage in dry habitats.

Sene DM. 2020. Forest Action Plan. Forestry Program, American Samoa Community College, Pago Pago, American Samoa. Discussions of forest types, disturbances, and succession.

Soil Survey Staff. 2014. Soil Taxonomy, Twelfth Edition. USDA – NRCS. Standard book of soil taxonomy; useful for terminology and interpretation of soils.

Space JC and T Flynn. 2000. Observations on invasive plant species in American Samoa. USDA Forest Service, Pacific Southwest Research Station, Honolulu. http://www.hear.org/pier/references/pierref000021.htm Recent, detailed report on invasive and potentially-invasive plants in American Samoa.

Stemmermann L. 1981. A Guide to Pacific Wetland Plants. US Army Corps of Engineers, Honolulu District. Guide to Pacific Islands wetlands types and plant species.

USDA-NRCS. 2011. Soil Survey Laboratory Information Manual. Soil Survey Investigations Report No. 45, Version 2.0. National Soil Survey Center, Lincoln, Nebraska. Provides additional insight for interpretation of soils information.

USDA-NRCS. 2006. Major Land Resource Regions. USDA Agriculture Handbook 296. http://soils.usda.gov/MLRAExplorer Description of MLRAs of Hawaii.

USDA-SCS. 1984. Soil Survey of Islands of American Samoa. Nakamura S, Chavez CL and MW Roybal, in cooperation with the Government of American Samoa. The latest NRCS soil survey for these islands. Some of the taxonomic names are outdated.

USDI-FWS. 1982. Woldlife and Wildlife Habitat of American Samoa. II. Accounts of Flora and Fauna. R Banks, editor. Washington DC. Discussion of vegetation as habitat types for local fauna. Good climate discussion.

USDI-FWS. 1979. Classification of Wetlands and Deepwater Habitats of the United States. Cowardin LM, V Carter, FC Golet, and ET LaRoe. Office of Biological Services. Washington, DC. The standard reference for defining wetlands in the US.

USDI-NPS. 2009. Natural History Guide to American Samoa, 3rd Edition. P. Craig, editor. National Park of Samoa, Department of Marine and Wildlife Resources-American Samoa, American Samoa Community College-Community and Natural Resources Division. General reference for terrestrial and marine resources of American Samoa, with color photographs.

Whistler WA. 2002. The Samoan Rainforest. Isle Botanica, Honolulu. A guide to the various vegetation types of the Samoan archipelago, with some reference to landscape and soils, and based on extensive fieldwork.

Whistler WA. 1995. Wayside Plants of the Islands: A Guide to the Lowland Flora of the Pacific Islands. Isle Botanica, Honolulu. Reference of common introduced plant species in lowland areas of the Pacific Islands including American Samoa; with color photographs.

Contributors

David Clausnitzer PhD
John Proctor
Ann Tan
Carolyn Auweloa
Daniel Bowman
Jennifer Fedinko
Amy Koch
Kendra Moseley
Sarah Quistberg
Jill Ficke-Beaton
Daniel Block
Brendan Brazee
Curtis Talbot

Approval

Kendra Moseley, 6/12/2025

Acknowledgments

Assistance, advice, review, and/or insights:

Michael Constantinides, NRCS-PIA
Jennifer Higashino, USFWS and NRCS
Mike Kolman, NRCS

Rangeland health reference sheet

Interpreting Indicators of Rangeland Health is a qualitative assessment protocol used to determine ecosystem condition based on benchmark characteristics described in the Reference Sheet. A suite of 17 (or more) indicators are typically considered in an assessment. The ecological site(s) representative of an assessment location must be known prior to applying the protocol and must be verified based on soils and climate. Current plant community cannot be used to identify the ecological site.

Author(s)/participant(s)
Contact for lead author
Date	06/13/2025
Approved by	Kendra Moseley
Approval date
Composition (Indicators 10 and 12) based on	Annual Production