Natural Features & Ecosystems

Photograph of gumbo limbo trees and cactus in the subtropical dry forest of Buck Island
Gumbo limbo (or turpentine) trees and cactus, part of the recovering subtropical dry forest at Buck Island.

Digital Island (Sean Corsaut)

Subtropical Dry Forest

Subtropical dry forest covers much of the landmass of St. Croix, but this is a relatively recent development. Prior to the arrival of European colonists, much of the northern (windward) side of the island would have been covered in semi-evergreen rain forest, while the southern (leeward) side, receiving less rainfall, would have likely consisted of subtropical dry deciduous hardwood forests.

These forests consisted of species like water mampoo (Pisonia subcordata), dogwood (Piscidia carthaginensis), wild lime (Adelia ricinella), limber caper (Capparis flexuosa), ginger thomas (Tecoma stans), gumbo-limbo (Bursera simaruba), pigeonberry or bodywood (Bourreria succulenta), ironwood or lignumvitae (Guaicacum officinale), and white manjack. The only remnant of the semi-evergreen rainforest is located in the northwest mountainous region of the island; however, these mountains are not high enough in elevation to generate cloud forests as on Puerto Rico and Jamaica. Plants found in the rainforest include kapok (Ceiba pentandra), Jamaican caper (Capparis indica), black mampoo (Guapira fragrans), West Indian mahogany (Swietenia mahogoni), and spiceberry (Eugenia rhombea), with an understory that includes limeberry (Triphasia trifolia), hairy wild coffee (Psychotria pubescens), and painkiller (Morinda citrifolia).

Underwater photo of elkhorn coral at Buck Island
Elkhorn coral at Buck Island.

NPS photo (S. Pershern)

Coral Reefs

Buck Island’s Barrier Reef's underwater scene taxes human perceptions with the abundant variety of shape, patter, color, texture, and movement. It’s barrier reef ranks among the Caribbean’s best. It’s thick branching elkhorn corals push their sheer mass to 30-foot heights. Like fortress walls corals rise off the sea floor and dominate the underwater world. The irregular arc of reef surrounding Buck Island’s northern and eastern shore creates a lagoon between reef and island. Wide and shallow lagoon waters seldom exceed 12-feet deep, and the protecting reef moderates the wave action. In the calmer waters of the lagoon, brain corals grow larger, nearly reaching the surface. Seaward of the barrier reef, elkhorn and star coral patch reefs occur around the island, except to the southwest.

Coral Reef Tiny colonial animals called coral polyps build reefs by extracting calcium carbonate from seawater. Coral reefs are home to many animals and plants, forming what has been called the “rainforest of the sea”, the second most complex and biologically diverse ecosystem on Earth.

The reefs you see today at Buck Island were formed within the past 7,000 years. Elkhorn coral, Acropora palmata, is the major builder of the barrier reef surrounding the island. Coral polyps live in partnership with algae called zooxanthellae, which provide corals with their vivid colors and some of their food. Coral reefs live in nutrient poor, tropical waters within a narrow temperature range (70 - 85 °F). They grow slowly and are vulnerable to pollution, sedimentation, overfishing, warming of the seas, and boat damage.

Within the past few decades, coral diseases and hurricanes have significantly impacted Buck Island’s reef system. Its regrowth is now being closely monitored.

Geologists in the field discussing the rock formations on Buck
Geologists and NPS staff discussion geological features of Buck Island.


Buck Island’s geological story is long and complicated. St. Croix and the other islands of the Greater Antilles are actually the remains of an extinct volcanic ridge that began forming between two tectonic plates – the Caribbean and North American plates – nearly 145 million years ago (during the Cretaceous Period). The North American plate was subducted (forced down), which pushed up the earth’s oceanic crust and created the islands of the Greater Antilles. This period of mountain building stopped sometime between 54-38 million years ago, when this Greater Antilles Ridge collided with the North American continental plate. St. Croix and Buck Island were created at the southern end of this Greater Antilles Ridge.

Sedimentary layers, or layers of rock deposited by water, formed on top of older volcanic layers, beginning around the end of the Cretaceous Period, around 60 million years ago. The Cretaceous was a period of warmer climates that today, and had high eustatic sea levels. Movements of tectonic plates pushed these layers to the east, tilted and squeezed them up and over.

Last updated: April 24, 2018

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