Northern Guam geospatial information server

Forests of northern Guam

There are several distinct types of forests in northern Guam. They can be broadly classified as strand forest, limestone forest, scrub forest, ravine forest, and broken forest.

Strand forest is the natural forest growing on inland sands flats just above the barren or scrub-dominated beach slope or rocky coastal outcrops. It grows on loose, sandy soils adjacent to coastlines and consists of woody and herbaceous plants that are well-suited to the elevated salt levels of coastal areas. A few species that grow here are true halophytes (plants that depend on the saline environment and are not normally found inland) but most are salt-tolerant plants that can survive certain amounts of increased salinity in the ground and seaspray in the air. Plants that are sensitive to salt are entirely absent from this type of forest. Typical flora include hunik (Heliotropium foertherianum); nger (Allophylus timorensis); nanaso (Scaevola taccada); and shrubby Suriana maritima, Sophora tomentosa, and Clerodendrum inerme. Taller trees include binalo (Thespesia populnea), puting (Barringtonia asiatica), nonak (Hernandia nymphaeifolia), kafu (Pandanus tectorius), and coconut. Chopak (Mammea odorata) forms dense stands, especially along the northeastern coast, and ironwood (Casuarina equisetifolia) forms locally pure groves. The herb layer may contain endemic påode’do’ (Hedyotis foetida var. mariannensis), with its delicate but foul-smelling flowers.

Limestone forest in its primary state is a native and ancient forest composed of mixed native species growing on a raised limestone substrate. It initially started as a modified strand forest developed from a natural inland progression of tree species that first colonized the coast. In the process, some have evolved into distinct species, endemic to Guam or the Marianas. Primary limestone forest occurs on cliffs and coastal terraces in remote locations in the northwest (from Hilaan north to Ritidian), north (Jinapsan and Tarague) and northeast (from Pati Point to Pagat area). Secondary limestone forest is similar to primary limestone forest, but it has been degraded by human, animal, and natural disturbance. As a result, it has a more open canopy and a higher density of non-native species than the primary limestone forest. Secondary limestone forests covers parts of the Northwest Field. The overstory of limestone forests is typically above 10 m tall and may include seeded breadfruit or dugdug (Artocarpus mariannensis), nunu (Ficus prolixa), yogga’ (Elaeocarpus joga), umumu (Pisonia grandis), pengua (Macaranga thompsonii), chopak (Mammea odorata), ifit (Intsia bijuga), niyoron (Cordia subcordata), pahong (Pandanus dubius), faniok (Merrilliodendron megacarpum), and fai’a (Tristiropsis obtusangula). In the understory, common trees include the native cycad fadang (Cycas micronesica), a’abang (Eugenia reinwardtiana), fago’ (Neisosperma oppositifolia), paipai (Guamia mariannae), and mapuñao (Aglaia mariannensis). The herb layer is not dense and may include scattered beds of succulent herbs, such as Procris pedunculata and Elatostema calcareum, growing on rock outcrops.

Scrub forest is a disturbed, yet diverse, brush-type forest that develops following the removal of primary or degradation of secondary forest. Plants forming this habitat on Guam derive from a variety of sources. Some arise from remnant native forest seedbank and wind-borne delivery; others have seeds brought in by birds, feral pigs, and deer. Pioneering and invasive species encroach from adjacent communities. The result is a botanical mosaic consisting of native flora and various naturalized or invasive species. Indigenous trees include Pipturus argenteus, Macaranga thompsonii, Neisosperma oppositifolia, Pandanus tectorius, Hibiscus tiliaceous, and others. Papaya (Carica papaya) is among the first species to establish in the clearings and can be a useful gauge of the age of the clearings. Ladda (Morinda citrifolia), lemonchina (Triphasia trifolia), and tentånchina (Cestrum diurnum) are all common in the understory, along with åhgao (Premna serratifolia) and custard apple (Annona reticulata).

Ravine forest usually occurs in river valleys and other topographic depressions. Common in southern Guam, in the north it can only be recognized in valleys that formed on argillaceous limestone between Hagåtña and Mangilao. These forests are highly degraded and contain many non-native species including betel-nut palm (Areca cathecu) and palma brava (Heterospate elata), in addition to native species such as Ficus prolixa, Glochidion mariannensis, Hibiscus tiliaceus, Pandanus tectorius, and Premna serratifolia.

Broken forest is similar to the limestone and ravine forests but is much dissected by small, open or scrubby fields. It contains both native and introduced vegetation and has much lower and more open canopy than mature forests. This type of forest is a result of human disturbance and occurs in northern and central Guam.

Text above is 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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