Lecture 5
Tropical Vegetation

Natural vegetation refers to vegetation unaffected by humans. This is not a precise term.
Spontaneous vegetation is probably a better term.

Factors responsible for vegetation

  • Climate (temperature, rainfall, insolation)

  • Soil

  • Topography (elevation, aspect or direction of slope)

  • Water supply

  • Human activity

    • Tropical Rainforest (Af Climate)

    Note: Evergreen Rainforest is not as good a term because evergreen rainforest also associated with California Washington Coast. Equatorial Rainforest would distinguish between the rainforest littoral climate.

    		 fig. 1

    Waterfall, Costa Rica

    fig. 2 fig. 3

    Greatest extent is in tropical South America.
    Here it is sometimes known as Selva (Spanish) or Silva (Portuguese).
    The forest is characterized by a continuous canopy of foliage.
    (Note: Woodland implies widely scattered, spaced trees).
    The tropical rainforest is scattered through the peninsular and insular (island) lowlands of Southeast Asia.
    This is found up to elevations of 2000 to 3000 feet.

    		 fig. 4
    fig. 5

    Rainforest

    		 fig. 6

    Leguminous trees, Belize

    This type of forest was unknown to the conquistadors.
    Olviedo: Spanish source of early New World Intrusion in the early 1500s described it thus:

    "A great and dark sea...The trees of these Indies are a thing that cannot be explained for the multitude."

    From the air the rainforest looks like a "sea of green cauliflower."

    		fig. 7

    Amazon

    Tree Form

  • Trees dominate, grow up to and exceed 250 ft tall but generally less, about 150 ft tall.
  • Trees typically have straight bole (trunk), tall slender trunk, branching only at canopy height.
  • Generally the bark is smooth and thin but sometimes spiny and thorny.
  • Leaves are soft and wide and evergreen.
  • There is always a gentle rain of leaves.
  • The floor of the forest is covered with brown decay of litter.
  • On the floor of the forest there is little growth because it is so dark.
  • It is quite easy to walk thought a rainforest; it is cathedral-like, dark, cool with tall columns or trunks.

    		•  fig. 8

    Amazon

    fig. 9

    Forests are stratified, typically 3 layers or strata.

    Tall trees that grow through the canopy are called "emergents."

    		 fig. 10

    Philodendrons, Belize

    Buttressed trees provides support to the tree because root system is typically shallow.

    fig. 11

    It has been suggested that they function in translocation.

    		 fig. 12
    fig. 13 fig. 14

    Plank Buttresses

    		 fig. 15
    fig. 16 fig. 17

    Belize

    Lianas are woody vines, useful for cordage when young.
    Some up to 1000 ft long.
    These are not parasites although some (strangler vine) may cause girdling.

    fig. 19

    Epiphytes such are nonparasitic and use trees as supports feeding from decaying organic matter. Examples include ferns, bromeliads (pineapple-like plants) and orchids.

    		 fig. 18

    Strangler vine

    The rainforest is not a jungle, which is a dense almost impenetrable growth where one needs a machete to traverse.

    Jungle occurs when the rainforest is removed and dense growth occurs.

    The jungle occurs in the edge of rainforest and particularly next to streams where light enters.

    		 fig. 22

    Belize forest

    fig. 20

    Jungle like undergrowth after the rainy season, Brazil

    		 fig. 21

    Flora and Vegetation

    In describing vegetation we must distinguish between flora, a list of plant kinds and vegetation aggregation of plants into communities.

    There is enormous richness of flora in the tropical rainforest, high biodiversity.

    The great variety of trees has important economic consequences.

    It is often difficult to gather and accumulate a single species!

    This is very different from temperate communities of forests where as few species predominate in the climax vegetation, the vegetation of the mature forest.

    		 fig. 23

    Jungle, Costa Rica

    Mangrove Forest

    This is a special kind of coastal vegetation which depends on silt, mud, and periodic inundation with sea water.
    Usually found near deltas of large rivers.
    Also found in brackish water where sea water is diluted with fresh water.
    This is best developed in the true tropics.
    Rhizophora is a common genus, usually short, 10 to 20 feet, but up to 90 feet in height.
    The leaves are evergreen, small and tough.
    The prop and aerial roots are the outstanding characteristics.
    They are similar to screw pine.
    Mangrove forest is particularly common in Malaysia

    		 fig. 24 fig. 25
    fig. 26 fig. 27

    Other common vegetation includes:

  • Coconut (Cocos) but often connotes a man-made vegetation

  • Screw pine (Pandanus), native to Old World, characterized by stilt-like aerial roots, long sword-shaped leaves.

    • fig. 28

    Effects of Elevation

    Mountainous vegetation in the tropics changes with elevation.

    Sub Mountain Forest

  • The structure of vegetation and flora alters with altitude.
  • The tall luxuriant growth of the lowlands, typically with three strata, give way at 2000 to 3000 feet (up to 6000 to 8000 feet) to a lower mountain (sub mountain) rainforest.
  • The trees are still evergreen but shorter.
  • There are two layers, an upper layer, 80 to 90 feet tall with a single layer beneath.
  • The flora becomes impoverished.
  • Temperate families of trees becomes common (as Fagaceae) with genera such as Quercus (oaks)-but different species from the temperate areas.
  • Buttressing of trees diminishes.

    • Mountain Forest (6000-8000 to 10,000 feet)

  • The trees are shorter still (10 to 30 feet), gnarled, less neat.
  • Lianas are rare.
  • There is typically one layer of trees.
  • Epiphytes are very common, particularly mosses.
  • Mountain forest is often known as a Mossy or Cloud Forest or seja de la montana in Spanish, "the eyebrow of the mountain."
  • Because of the altitude it is typically very wet.
  • Condensation is very common.
  • Cloud cover is almost continuous.
    •

    Alpine Forest

  • About 10,000 feet it gets dryer.
  • Trees are very low.
  • This is sometimes called an elfin forest and trees are shrub like.
  • Alpine grasslands are common.
    •

    Typical crops of lowland and mountain forests.

  • Lowland: Rubber, taro, manioc, maize
  • Lower mountain: Coffee, temperate latitude grains, ornamentals grown under shade.
  • Mountain: Barley, potato
  • Alpine: Pasture for dairy

    • Monsoon Forests (Am Climates)

    The monsoon climates are warm all year but there is a short dry season and a concentration of very heavy precipitation in the rainy season.
    The spontaneous vegetation reflects the differences in rainfall pattern.
    This climate is typical in Burma, Thailand, Indonesia, South China, and West Coast of India.
    In the south Malabar coast of India there is a very short dry season but the dry season extends as you go North.
    The vegetation changes reflect this.

    		 fig. 29

    The forest is stratified as the tropical forests.
    However, the top layer is deciduous in the dry winter.

    There is usually enough moisture in the soil to maintain growth throughout the year.
    In the deciduous trees the leaves fall and flower occurs in the dry season.
    The forest is less rich that the lowland tropical rainforest.
    The trees are further apart and less luxuriant.
    Epiphytes and lianas are less common.
    A common species is teak (Tectona grandis) now planted all over the world.

    		 fig. 30

    Savanna(h) Climates (Aw)

    This known as tropical wet-and-dry, or winter-dry tropical.
    The origin of the word is from Cuba.
    The name "savanna" is the Spanish spelling of an Indian name.
    This is a tropical or subtropical grassland containing scattered trees and xerophilous undergrowth.
    The savanna vegetation however is not confined to Aw climates.
    Good examples are found outside of tropical wet and dry such as Mediterranean climates of California, mountains country of the American southwest, and in temperate Australia.

    fig. 31

    Typical savanna contains spreading trees, palms, or pines.
    Cover is not continuous

    There are a number of classical vegetation types in savanna.

  • Woodland: Forest condition between rain forest and savanna.

  • Thornforest: Low forest of trees that are thorny with small leaves. These are known as xerophytic vegetation, vegetation adapted to dry climate. The Kiave forest of Hawaii is a good example. The thorns are an adaptation to protect against grazing.

  • Grassland: Continuous grass. Only trees on river or stream banks.
    •

    Much of vegetation consists of a mosaic of these types of vegetation.

    The Brazilian terms are very descriptive

    Campo cerrado: "Closed" field, Trees are touching
    Campo limpo: "Clean"or open grassy field
    Campo sujo:: "Dirty" or scattered trees.

    In Africa, trees are evergreen in moist savanna; trees are deciduous in dry savanna.

    		 fig. 32

    Cerrado, Brazil

    fig. 33 fig. 34

    Cerrado, Brazil

    		 fig. 35

    Cerrado, Brazil

    fig. 37 fig. 38

    Thorny Trees, Belize

    		 fig. 36

    Cerrado soil

    Tropical Grasslands

    However in many areas despite ample rainfalll top support forest, the vegetation is grassy.
    Climate is not the only factor determining vegetation.
    The extensive grasslands in these areas is known as Anthropogenic grasslands.
    They are due to a combination of grazing, cutting, and burning as a result of human interference.
    Burning is very common in the dry season and fire is very damaging.
    The constant smoky atmosphere in the dry season in the savanna of Brazil is known as Broma seca or dry fog.

    		 fig. 39
    fig. 40

    Cerrado Burning, Brazil

    		 fig. 41

    After Burning

    fig. 42

    Another factor giving rise to extensive grasslands are the presence of hardpan development in old eroded soils. The water accumulates as the plains are flooded. Many trees are nonadapted to "wet feet" and die out while grasses take over. Grasslands are very adapted to this condition. In some cases the trees that survive are palms (Llanos do Orinoco). This occurs because palms are fire resistant. The fires spread easily across the Llanos.

    fig. 43

    Savanna Landscape changes along rivers.
    Gallery (galleria) forest occurs along the river.

    The mosaic pattern may reflect different sequence of fire.
    Other factors include the felling of forests and development of horticulture and agriculture.

    fig. 44

    Desert Vegetation, Arizona

    		 fig. 45

    Saguaro Cactus

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