Since 1995, the University of the Virgin Islands' Cooperative Extension Service
(CES) has been
working with UVI's Conservation Data Center
(CDC) and the Nature Conservancy to
complete a Rapid Ecological Assessment (REA) of the Virgin Islands. CES
has helped develop
a classification system for VI Vegetation Zones. These vegetation zones are
then delineated
(outlined) on aerial photographs of the islands. The delineations and classifications are digitized,
and computer maps are generated. The mapping of St. Croix's vegetation zones is nearly
completed. Maps will be available to government agencies, architects, builders, and the general
public through the UVI-CDC office on St. Thomas (contact: Dayle Barry, CDC Coordinator,
[340] 693-1030).
Basic Classification of St. Thomas
Vegetation
Formations
V.I. Moist Forest
Upland evergreen moist forest is
confined to the
summits and upper northern aspects of peaks (north-facing slopes) on the north of St.
Thomas (Crown
Mountain, Mountaintop). Evergreen species contribute
about 75% of the total percent tree cover. A few taller
trees (to 25 m) may emerge from a continuous canopy
about 15 m high. Lower shrubs, ferns, vines and
epiphytes may be abundant. Orchids, wild anthuriums,
and wild pepperomias grow as epiphytes anchored to
rocks or trees. On St. Thomas, most of this forest
formation has been disturbed by human activities and
hurricanes.
Gallery moist forest lines
ravines or guts (except guts in the driest parts of St. Thomas), draining
large upland areas. The island's tallest trees may be found in this zone. Most of the trees are
evergreen, but some of the tallest trees (25-30 m tall) may be deciduous. Shrubs, epiphytes and
herbs may be present. Unfortunately, this forest formation has been cleared in many areas, even
though it is illegal to clear vegetation within 30' of the center of a gut.
V.I. Semi-permanently, Seasonally or Tidally Flooded
Evergreen
Forest, Woodland or Shrubland
Mangroves are semi-permanently,
seasonally or tidally flooded evergreen vegetation formations
that grow near the sea. Mangrove trees are halophytes, plants that thrive in and are especially
adapted to salty conditions. In the Virgin Islands there are three species of mangrove trees: red
mangrove (Rhizophora mangle), black mangrove (Avicennia germinans),
and white mangrove
(Laguncularia racemosa ); another tree, buttonwood (Conocarpus erectus) is
often associated
with the mangrove formation. Red mangrove grows at the water's edge and in the tidal zone.
Black mangrove and white mangrove grow further inland in areas where flooding occurs only
during high tides. On the moister north and west shores of St. Thomas, where large watersheds
drain into moist basins, mangrove areas are classified as forests. Mangrove forests have the
tallest trees and the canopy formed by the crowns of these trees is generally closed (unless
damaged by hurricanes). In the drier parts of the islands mangrove areas may be classified
as
mangrove woodland where mangrove trees are more sparse or mangrove shrubland where
mangrove trees are smaller and grow as shrubs. Mangroves are protected under V.I. law
prohibiting the pruning, cutting, removing or disturbing of mangroves, unless a permit has been
obtained from the Department of Planning and Natural Resources. (Virgin
Islands Code Title 12,
Chapter 2, §103)
V.I. Dry Forest
Semi-deciduous dry
forest is the dominant forest formation on St. Thomas, covering at least
50% of the remaining undeveloped land. Evergreen and deciduous species generally contribute
25-75% of the total tree cover. There are distinctly different forest types in this category, varying
in structure, composition and human modification. Found on north hillsides, upper southern
elevations, mixed with drought-deciduous forest on many lower south slopes, in basins without
large watersheds, and along smaller or drier guts and ravines. The continuous canopy is about
8-11 m high with some taller trees reaching 15-20 m. A shorter form of this forest formation may
have a main canopy about 6-7 m.
Shrubs and herbs can vary.
Drought-deciduous dry forest
is a
common forest formation on St.
Thomas. At least 75% of the tree
cover consists of deciduous species
that shed their leaves during dry
seasons. Found mostly at lower
elevations (less than 900') on south
and southwest facing slopes, in
sections of the east end, in patches
on south and west aspects of the
north shores. Trees are smaller; the
main canopy averages 7-10 m with
a few taller trees to 15 m. Shrubs
may be sparse to abundant, and the
herb layer tends to die back during
dry periods.
V.I. Woodland
Evergreen woodland is
exemplified by the
naturally occurring palm groves of native tyre
palms (Coccothrinax alta) and sabal palms (Sabal
causiarum) found near the northern coastlines or
on steep northern slopes (to about 1000' above sea
level). These palm groves are probably the only
natural example of this formation on St. Thomas.
Other palm woodlands were originally cultivated
like the coconut groves in basin areas (Magens
Bay, Neltjeberg Bay).
V.I. Shrubland
Mixed evergreen / drought-deciduous
shrubland is a common vegetation formation on the
south shore and east end, usually at low elevations, occasionally extending up to 900' altitude or
more on the south slopes. This is a very diverse formation; height may range from 1-15 m.
Taller forms usually have a canopy layer of species that may attain greater heights under moister
conditions. Shorter forms are usually found on very exposed locations, such as east sides of
headlands on south shores. Century plants (Agave missionum) and cacti may be
common,
scattered or absent. Shrubs and vines may form interlocking masses, and epiphytic orchids
anchored to trees and rocks may also be found in these formations. Many of the evergreen plants
in this dry formation have small leathery leaves that can withstand the dryness, intense sun and
wind exposure. Leaf structure and composition may help these plants preserve moisture and
protect against foraging wildlife.
Thicket/scrub,
evergreen shrubland is divided into different
types of shrubland that grow as thickets generally less than 5 m
high. Evergreen plants that keep their leaves during the dry
season are most common, and there are also some deciduous
species that periodically shed their leaves during the dry months.
Many plants have thorns and spines. Stunted trees with multiple
branches may be common, and there may be a few taller trees
that emerge from the thicket.
Thicket and scrub, thorn and cactus, evergreen
subdesert
shrubland is a fairly tall shrubland dominated by succulent
species like century plant and pipe organ cactus (Pilosocereus
royenii), and thorny plants such as casha (Acacia spp.). This
formation is common in the drier south and east.
Thicket and scrub, drought-deciduous
shrubland occurs in
areas that have been disturbed by constant grazing of animals,
especially the abandoned pasture lands of east and south St.
Thomas. Aggressive weedy species such as tan-tan (Leucaena
leucocephala), maran bush (Croton spp.), lantana (Lantana
spp.), guinea grass (Panicum maximum) and the thorny casha (Acacia
tortuosa) take over and
form thickets of dense low growth less than 4 m high.
Thicket and scrub, coastal
shrubland is common in coastal areas like the mostly undisturbed
south end of Magens Bay. The height of the coastal thicket is generally 3-4 m, with occasional
emergent trees or palms.
Coastal hedge, evergreen
shrubland is a wind and salt adapted shrubland with a limited number
of plant species. Plant height is under 3 m and usually less than 1 m. This formation may occur
on beach berms or rocky coasts, especially those with exposure to the prevailing winds. The
sheering wind creates a hedge effect and plant growth is usually very dense. Drought tolerant
plants like epiphytic orchids and bromeliads cling to the coastal rocks amongst the coastal hedge.
Mixed evergreen drought-deciduous dwarf
shrubland is the
extreme form of the coastal hedge
formation. It only reaches a height
of 0.5 m or less. It only occurs on
some really dry areas of the east and
south, and some cays. Evergreen
species can form a carpet-like cover.
Turk's cap cactus (Melocactus
intortus) may be common amongst
the rocks.
For further information, contact:
Natural Resources & Environmental Management Program
Cooperative Extension Service
University of the Virgin Islands
St. Thomas: (340) 693-1084
St. Croix: (340) 778-9491
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Last Updated August 10, 1998 by Toni
Thomas