Southern Guam geospatial information server

Environment of Southern Guam

Despite its small size and high development and urbanization, Guam supports a diverse range of terrestrial, freshwater, and marine environments.

Terrestrial environments support a diverse flora of over 600 species of plants, including more than 100 species of trees. Their distribution is influenced by sharply contrasting soil types between predominantly limestone terrain in Southern Guam and predominantly volcanic terrain in southern Guam. The land surface of Southern Guam is covered by several distinct natural vegetation and man-made landscapes. That includes strand scrub and strand forest (salt-tolerant plants that grow in coastal areas), limestone forest (primary and secondary forest of native species), ravine forest (present in some valleys on argillaceous terrain), scrub forest (secondary vegetation that is a mixture of native and introduced species), and savanna vegetation (forms on volcanic inlier terrain only). Due to development, large parts of Guam are now classified as urban vegetation (parks, lawns) and urban buildup areas (buildings, roads, paved places). The Southern Guam is dominated by secondary scrub and urban vegetation, though limestone forests and other natural vegetation persists in protected and isolated coastal areas.

A major factor that determines the current condition of terrestrial environments on Guam is natural and anthropogenic disturbance. Windthrow (uprooting and breaking of trees by wind) and defoliation (loss of leaves) by typhoons periodically damage forests and keep vegetation patterns in a state of flux due to changing light, moisture, and nutrient levels. Anthropogenic disturbance has more permanent effects. A large part of the land area of Southern Guam changed recently from forest to secondary vegetation and urban vegetation and buildups. Terrestrial environments are extremely vulnerable to clearing for construction and expansion of residential, commercial, and military facilities. For that reason, an ever-increasing proportion of Guam’s natural land cover is being replaced by disturbed vegetation, managed landscapes, and urban areas.

Freshwater environments are places dominated by fresh water. Surrounded by the vast, salty ocean, Guam is quite limited in freshwater resources. Freshwater environments are comparatively small, fragile, and extremely important.,This is particularly so in Southern Guam, where limestone geology precludes development of surface water flow and accumulations. Groundwater from the aquifer contained within Southern Guam provides the majority of water for the island’s human population.

The freshwater environments of Guam can be classified as wetlands (which are saturated lands, such as marshes and swamps), lentic environments (which are stagnant bodies, such as ponds and artificial impoundments), lotic environments (which are flowing waters, such as streams and rivers), and groundwater (which encompasses subterranean water bodies).

The distribution of fresh water on Guam is controlled by local geology and topography. Surface fresh waters are common in the south, where the land is dominated by volcanic bedrock and clays. Surface water is rare in the north, where the terrain is made of highly porous limestone. As a consequence, southern Guam has numerous rivers but limited groundwater areas, whereas Southern Guam has no rivers, but is underlain by plenty of groundwater.

Southern Guam has only a few permanent and intermittent streams flow over clay-rich limestone in the southern part of the Southern Guam plateau, and intermittent and ephemeral streams exist on the volcanic flanks of Mt. Santa Rosa. Surface waters are absent elsewhere. Wetlands are limited to freshwater marshes on clayey limestone and limited swamp vegetation around Pago River. In the subsurface, the aquifer represents a complex freshwater ecosystem in which different habitats can be recognized: freshwater zone, brackish water zone, salty groundwater beneath the aquifer, large water-saturated cavities, small water-saturated interstices, open-surface freshwater pools and streams inside caves, coastal springs discharging fresh water into the ocean, etc.

Coastal and marine environments in Southern Guam are rich and complex. They include shorelines where the ocean laps against the land and the shallow waters that surround the island. Those areas are ecologically extremely valuable as they provide food, shelter, and breeding grounds for countless marine species. They are also vital for the well-being of the people of Guam, as they are focal points of the tourism industry, fishing and other commercial activities, and recreation. In addition, coastal and nearshore marine environments provide irreplaceable services by buffering the land and the ocean from each other’s influence: they protect the sea by filtering and retaining pollutants and excessive nutrients from land, and protect the land by dissipating wave energy and absorbing the power of typhoons coming from the sea.

The most remarkable and extensive of Guam’s nearshore environments are coral reefs. They are underwater structures of calcium carbonate secreted by corals, algae, and other marine organisms. Reefs grow best in shallow, warm, clear, and somewhat-agitated waters. During the last ice age, between 100,000 and 15,000 years ago, the sea level was significantly lower than it is now. Today’s reefs were dry coastal plateaus. As the climate warmed, distant polar ice melted, and global sea level rose. Coastal lands were flooded and provided new substrate for corals and other reef-building organisms to create the reefs we see today.

Coral reefs are incredibly complex ecosystems with many distinct environmental zones and habitats. They surround the entire island and get progressively deeper with distance away from the shore. Offshore, the bottom drops off, reefs’ outer portions grade into deep seafloor, and coastal environments change into open ocean.

Texts above are slightly modified from "Environments of Guam" by Danko Taborosi, David R. Burdick, Claudine M. Camacho, Frank Camacho, published by BessPress, 2013.


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