Chapter 51
Ecological Subregions of the United States



Puerto Rico

One Section has been delineated in this Province:

For the present time, this Section includes only the main island of Puerto Rico, which has an area of about 3,700 mi2 (9,600 km2).

Section M411A--Dry-Humid Mountains

Geomorphology. This Section is an island of volcanic and tectonic origin situated in the Caribbean Sea at approximately 18o N and 66o W. Landforms consist of an east-to-west oriented band of mountains in the south-central part of the island, foothills along both sides of the mountains, and coastal lowlands. Small areas of coastal valleys are on the eastern and western coasts. Elevation averages less than 500 ft (150 m) in the coastal lowlands, 1,000 ft (300 m) in the foothills, and 3,000 ft (900 m) in the mountains. The highest mountain summit on the island, Cerro de Punta, is 4,389 ft (1,338 m) in elevation.

Lithology and Stratigraphy. Reserved.

Soil Taxa. Predominant soils are Tropepts, Humults, and Paleudults. Eutropepts and Tropohumults are on steep side slopes in east-central and west-central areas. Tropaquepts, Fluvaquents, and Hapludolls are on flood plains and poorly drained areas. Troposaprists and Fluvaquents are in swamps and marshes. Rendolls and Tropudalfs are major soils in the limestone karst area. These soils have an isohyperthermic or isothermic temperature regime and kaolinitic or mixed mineralogy.

Potential Natural Vegetation. The general vegetation type is tropical (or subtropical, depending on definition) in temperature regime and dry to hydric in moisture regime. The potential vegetation of Puerto Rico, nearby islands, and the U.S. Virgin Islands has been described according to life zones of the Holdridge system of environmental classification. Six life zones have been recognized that are highly correlated with seasonal moisture regimes.

A small area (less than 0.1 percent) of tropical lower montane rain forest is present above the tropical rain forest. Annual precipitation may average 175 in (4,500 mm) and relative humidity averages above 98 percent.

The tropical rain forest (0.1 percent) occurs near the eastern coast, only on the windward side of the Luquillo Mountains, where precipitation exceeds 150 in (3,800 mm) annually. Species are similar to the tropical wet forest, but palms and epiphytes ("air plants") dominate.

The tropical lower montane wet forest occurs on only about 1 percent of Puerto Rico, beginning at an elevation of about 3,300 ft (1,000 m) and extending to the summits

Epiphytes and palms are prevalent in the tropical rain forest.}

Common vegetation consists of palo colorado (swamp cyrilla), nemoca, caimitillo, and caimitillo verde. This zone contains the main nesting sites of the nearly extinct Puerto Rican parrot, which nests in hollow trunks of cyrilla trees.

The tropical wet forest occupies 22 percent of the island and occurs in areas of annual precipitation ranging from 80 to 160 in (2,000 to 4,000 mm). It occupies most of the mountains of high altitude, above 2,000 ft (600 m). Most trees are evergreen and over 150 species occur. Vegetation common to this zone includes helecho gigante (common tree-fern), tabonuco, motillo, and ausubo (bulletwood). Other main species include palma de sierra (sierra palm) and spiny tree-fern. Many epiphytic ferns, bromeliads, and orchids are common.

The tropical moist forest covers over half of the island (58 percent), except for the southern coast and high interior mountains, and is delineated by a zone of annual precipitation ranging from 40 to 80 in (1,000 to 2,000 mm). Many trees in this zone are drought deciduous, as a result of periods of low rainfall in January through April. Characteristic species include mangle (black, red, and white mangroves), palma real (Puerto Rico royalpalm), roble blanco (white cedar), tulipan africano (African tulip tree), bucayo gigante (mountain immortelle), guama (sweetpea), cedro hembra (Spanish-cedar), algarrobo (West Indian locust), flamboyan (flamboyant-tree), and jaguey blanco (shortleaf fig).

The tropical dry forest occupies 18 percent of Puerto Rico and occurs where rainfall ranges from 24 to 40 in (600 to 1,000 mm) annually. This forest is prevalent along the southwest coast and on most neighboring islands, including the Virgin Islands. Vegetation is mostly drought deciduous. Palms are absent, but cacti, thorny legumes, grasses, and short trees with flattened crowns are common. Indicator species include almacigo (turpentine tree), bayahonda (mesquite), sebucan (dildo), tachuelo (fustic), ucar (oxhorn bucida), guayacan (lignumvitae), guayacan blanco (holywood lignumvitae), zarcilla (tantan), tamarindo (tamarind), tamarindo silvestre (steel acacia), aroma (sweet acacia), and quenepa (Spanish lime).

Fauna. Reserved. \null \vskip 0.10in

Climate. Mean annual precipitation ranges from about 25 to 150 in (600 to 3,800 mm). Annual temperature is about 75 to 77 oF (24 to 25 oC) and ranges from about 73 oF (23 oC) in January to 80 oF (27 oC) in July. The growing season lasts for 365 days.

Surface Water Characteristics. Reserved.

Disturbance Regimes. Reserved.

Land Use. Reserved.

Cultural Ecology. Reserved.

Compiled by Southern Region.

Cacti and thorny legumes are characteristic vegetation of the tropical dry forest along the southwestern coast and on some islands.