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LESSON 1:
DOMINICA'S CLIMATE, GEOLOGICAL HISTORY AND PLANT COMMUNITIES

Mountainous Dominica, 29 miles long and 16 miles wide, is the wettest island in the Lesser Antilles . It is estimated that Dominica receives up to 10 times more rainfall than is observed for the general region. Rainfall has been recorded as high as 340 inches per year for interior areas, providing abundant freshwater for hundreds of streams and two large lakes. Yet, some sections along the island's west coast receive only about 50 inches annually, becoming dry enough at certain times of the year to create brush fire conditions. For information on the government, people and economy of Dominica go to: http://www.emulateme.com/dominica.htm

Why is there such variation in annual rainfall on Dominica?

Located in the tropics at 15° 12" N to 15°39'N and 61°14 to W 61°29 W Dominica is affected by prevailing easterly trade winds bringing moisture from the Atlantic Ocean. As the moist winds first strike the windward eastern coast of the island , they rise up the sides of Dominica's high volcanic mountains. With increasing altitude, the air cools, and the water vapor it holds condenses to create clouds and rain. This influence of high mountains, to produce precipitation from the air that crosses them, is called the orographic effect. Dominica has plenty of high mountains to capture rain. Four peaks are over 4000 ft. above sea level: Diablotin (4,747 ft.), Trois Pitons (4550 ft.), Watt (4017 ft.) and Macaque (4006 ft.), while seven others climb to over 2000 ft. None of Dominica's mountain summits are more than 5-miles from the sea.

Yet, on the leeward, western coast of the island, a rain shadow results in locations on the sides of mountains where dry winds (that have already dropped their moisture on the windward side) no longer have enough water vapor to yield heavy rains. This is where Dominica's lowest rainfall levels have been recorded.

There are also differences in rainfall levels during the year. Shifts in the direction of the prevailing easterly trade winds create a very rainy Wet Season (May-December) and a not as rainy Dry Season (January-April). Despite these seasonal variations, Dominica is still a very wet place, with a climate described as humid tropical marine.

As a general rule, Dominica's rainfall rises and temperature falls with increasing elevation, and temperatures are usually cooler in the interior than on the coast (Annual average temperature map). The highest temperatures around the island occur during the rainy season and the lowest in January at the start of the dry season. The yearly average temperature has been estimated at 79°F (26°C).

How did Dominica's mountainous terrain form?

Dominica is volcanic in origin, first emerging from the ocean floor 25 million years ago as a result of tectonic action between two crustal plates, the Caribbean and Atlantic Plates.

Eruption upon eruption has provided the igneous rock which has built present-day Dominica and its mountainous terrain, piece by piece. Much of the island's formation has been associated with eruptions that have taken place over the last 1-12 million years.

Dominica is one of the largest (289.8 sq. miles) and youngest islands in the Lesser Antilles, with volcanic activity still continuing in the form of hot springs, fumeroles and a boiling lake. A violent steam eruption occurred in Dominica in 1880, and tremors occurred in 1975 in the Morne Macaque area. Morne Macaque is a dormant volcano that, geologically, is one of the island's youngest mountains, and, at 4006 ft. the fourth tallest. It physically separates Dominica's two largest lakes discussed in lesson 2.

The dominant rock type for the whole island is of andesitic composition. Recent volcanic deposits are the parent materials to Dominica's soils. The heavy rain and high temperatures create tropical soils that are typically leached (removed) of minerals and susceptible to erosion. Yellow and brown colored soils are younger soils in early stages of weathering. Red soils are older, highly leached, and sometimes acidic. A grey podsolic soil forms in higher elevations with heavy rain.

What major plant communities have resulted on Dominica from the influence of its mountains, rainfall levels and prevailing winds?

Dominica's major plant communities are various forest types which form concentric bands around the mountain slopes due to the changing amount of wind and rain. The amount and distribution of the rain, combined with the drying effects of the wind, are important factors influencing the forest types. At lower altitudes, the trees must tolerate periodic droughts experienced during the dry season. At the higher altitudes, the constant trade winds have a drying effect on the trees despite the heavier rainfall. It is only in the middle altitudes where wind and rain are most favorable for tree growth of the rain forest type commonly associated with tropical regions. The orographic influence also affects the distribution of plant communities so that tropical dry forest can be found on the drier, leeward, western coast of the island. The wetter, windward, east coast of the island supports tropical wet forest, which grades into elfin forest at higher elevations.

There are seven vegetation communities on Dominica, comprised of primarily woody plants, and defined by their distinctive species compositions and habitat parameters.

Key to Major Plant Communities On Dominica (source: Archibold Tropical Research Center Handbook)

I. Tree growth < 10 meters (33 ft.) high.

i. Trees with tiny often leathery leaves, often thorny
C. Dry Scrub Woodland.
ii. Trees normal-leaved, rarely thorny.
iii. Windswept seashore with few epiphytes.
B. Littoral Woodland.
iv. Windswept mountain summits and ridges with many epiphytes
G. Elfin Woodland.

II. Tree growth > 10 meters (33 ft.) high.

i. Deciduous trees forming > 25% of top layers
D. Deciduous Forest.
ii. Forest evergreen (< 25% of top layers.deciduous)
iii. Inundated with fresh water.
A. Swamp Forest.
iv. Non-inundated forest with continuos canopy > 20m high..
v. Trees in 3-4 layers; dominants > 30m; leaves mostly compound.
E. Rainforest.
vi. Trees in 2 layers; dominants < 30m; leaves mostly simple.
F. Montane Rainforest
Each plant community can be defined as follows, progressing from lower to higher elevations:

A. Swamp Forest:
Dominated by Pterocarpus officinalis, locally called "mangrove", but actually is a legume. Very local, found only along the Indian River near Portsmouth and near the beach on the north coast. Disturbed swamp forests are also found at the Cabrits Swamp with wetland genera such as Chrysobalanus, Annona, Montrichardia, Acrostichum, and sedges present.

B. Littoral Woodland:
A community restricted to a narrow coastal band on the Atlantic eastern (windward) coast, south to Scott's Head . The littoral woodlands have a hedge-like appearance. The strong, salt-laden winds from the Atlantic Ocean act to shear and stunt the trees and gnarl their tops. Only a few plant species are able to withstand the effects of the wind and salt such as the Sea grape (Coccoloba uvifera) and the White Cedar (Tabebuia pallida).

C. Dry Scrub Woodland:
A community dominating the Caribbean western (leeward) coast of the island between the seashore and the inland rain forests, especially well-developed on the Cabrits, Morne au Diable, and Pointe Ronde. Most of the trees in this area have very small leaves which they shed during the dry season. The presence of prickles or spines on the trunks, branches and leaves of the plants is typical of the trees and shrubs in this area. This feature helps the plants to live in this dry region by conserving water and protecting them from grazing animals.

D. Deciduous Forest:
This is considered a somewhat transitional community between the Dry Scrub Woodland and the Rain forest, containing both deciduous and evergreen trees. While it is best developed on the Caribbean side of the island, it also occurs to a lesser extent on the Atlantic side. It is mostly disturbed on Dominica, having been removed for cultivation of the land.

E. Rainforest:
The Rain forest is Dominica's dominant plant community. Dominica has the best display of rain forest in the Caribbean. It is found generally between 1,000-2,500 ft. elevation. Rain averages 175-300 in/yr. The trees have extensive buttresses and exposed and shallow root systems which protect the red and yellow soils it grows in from erosion. The canopy (at a height of about 100 ft.) is dense. There is a mid-layer of 40-80 ft. trees and a lower layer of small 15-40 ft. trees. The limited light reaching the forest floor makes it sparsely vegetated, and is composed of suppressed tree seedlings, ferns, and monocots. The Rain forest is also characterized by many woody vines (lianas) and epiphytes.

F. Montane Rainforest:
This community is transitional between the Rain forest and the Elfin Woodland of the mountain summits. The forest is found on the steeper slopes at higher elevations (generally above 2,500 ft.) and is best developed in the southern part of Dominica in Morne Trois Pitons National Park. There are fewer trees with compound leaves in this community than in the rain forest. There are generally only two layers of vegetation. Montane forest usually grows on shallow, gray, podsolic soils.

G. Elfin woodland:
Found at the top of the highest peaks of Diablotins and Trois Pitons, it is often called the Cloud Forest, and is nearly always enveloped in clouds. It is a low dense thicket-like woodland whose woody plants are draped with mosses and other epiphytes. The effects of the wind on the plants result in smaller, thicker and almost rigid leaves to prevent tearing, and reduced growth due to the shearing effect of the wind. Many of the species found here are restricted to Dominica.

QUESTIONS TO LESSON 1

Using the information in Lesson 1, write a brief, but exact explanation for the following questions. See how your ideas match the answers that will be provided Friday Jan. 21.

Question 1: Why does the island of Dominica receive up to ten times more rainfall than is observed for the general eastern Caribbean region?

Question 2: How is it possible that on an island only 16 miles wide, two locations can have a difference in annual rainfall of nearly 25 ft. (350 in/yr or 29 ft. vs. 50 in/yr or 4.2 ft.) ?

Question 3: Before answering this question make 3 observations using the maps provided.

Note Dominica's geographical position among other islands in Caribbean Sea on the regional map .

Take a look at and and compare the position of the Intertropical Convergence Zone (area toward which the Trade Winds blow) in respect to Dominica for February and August.

Next, view the map showing the areas of hurricane formation and paths of some major storms.

Now, using your observations, explain: Why does the wet season in Dominica take place from May to December, and coincide with the island's hurricane season?

Question 4: What effects does the combination of wind, rain and altitude have on the formation of vegetative zones on Dominica's mountain slopes? In completing your answer use examples of several specific vegetative types to highlight these effects.
All Dominica lessons and photography © 1999 Lance Leonhardt


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