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Chapter 5 Spatial Distribution of Tropical Rainforests and Mangroves -…
Chapter 5
Spatial Distribution of
Tropical Rainforests and Mangroves
Unit 1
What is Natural Vegetation?
plant life that grows in specific parts of the world's land areas and develops without human interference
climate
of a given location
determines the type of natural vegetation
found there
temperate grassland
temperate deciduous forest
mediterranean forest
tropical rainforest
tundra
temperate coniferous forest
desert
tropical grassland
Unit 2
What is the Tropical Climate?
climate
-
average weather conditions
of a place
over a long period of time
(usually >30 years)
tropical climate
- climate experienced by areas found
within the tropics (between the Tropic of Cancer and the Tropic of Capricorn)
characterised by a
high annual rainfall
(2000ml on average)
there is
rainfall throughout the year
, no month in which rainfall is very low or absent
temperatures are high
throughout the year
tropical rainforests
and
mangroves
are found in this climate
mangroves cover about 70% of the coastlines in the tropics
Unit 3
What are Tropical Rainforests and Where are They Found?
main type of natural vegetation in the tropics
found in
Central and South America
,
West and Central Africa
,
South and Southeast Asia
most of these regions are located very close to the
Equator
Unit 4
What are the Characteristics of Tropical Rainforests?
areas with tropical climate
receive abundant sunlight and high rainfall
year-round →
very suitable for plant growth
majority of tropical rainforest plants are
evergreen
- they
do not shed all their leaves
at particular times of the year
plants
continuously grow new leaves
to replace the older ones
contains a very
large variety of plant species
due to the favourable climatic conditions
e.g. tembusu tree, tempinis tree
the
intense competition for sunlight
among the plants causes them to
grow as tall as possible
not all plants can grow to the same height → gives the tropical rainforest a
distinct vertical forest structure
, with three main layers:
Emergent
the tallest trees in the tropical rainforest are able to grow to 30m or more
the crowns of these trees form the emergent layer
Canopy
most trees grow to about 20-30m in height →
grow very close
to one another, their
crowns interlock to form a thick and near-continuous mass of branches and leave
→ canopy layer
prevents 97-98 % of the sunlight from passing through to reach the forest floor
Undergrowth
found beneath the canopy layer
very little sunlight
is able to reach →
not many plants
are able to grow here, except smaller ones and the seedlings of taller trees
amount of vegetation
growing near the ground surface is
very sparse
Unit 5
How Have Plants in Tropical Rainforests Adapted to Their Envionment?
individual plants have developed special features or
adaptations
in order
to cope with the intense competition for sunlight and high rainfall
Broad Leaves
leaves of most plants are broad, which means that they have a
large surface area
→ enables the plant to
absorb as much sunlight as possible
in order to make food for its own survival and growth
Waxy Leaves
leaves are also waxy, which gives their surfaces a
glossy
appearance → helps the plant to
reduce the amount of water vapour that it loses to the atmosphere (transpiration)
as a result of high temperatures in the tropical rainforest
Drip Tips
some plants have
leaves with small, narrow tips
that point downwards → drip tips
together with the leaves' waxy texture,
allow rainwater that falls onto them to flow off easily
→
helps the leaves to dry quickly
and thus
prevents the growth of fungi/bacteria
on them
Buttress Roots
trees that grow to great heights usually have buttress roots to
keep them upright and prevent them from toppling over
can grow up to 5m above the ground surface
trees require the support provided by the buttress roots because the rest of the roots do not extend very deep beneath the ground surface
nutrients are concentrated in the topmost layer of the soils
(released when dead leaves and branches decompose) → trees tend to
have shallow underground roots so that they can absorb nutrients quickly
Unit 7
What are the Characteristics of Mangroves?
lower diversity of plant species
compared to tropical rainforests
only a total of
60-70 mangrove tree species worldwide
not many plant species have evolved to survive in waters of high salinity
evergreen
less dense
than a tropical rainforest →
less competition for sunlight
coastal environment is difficult for plants to survive in,
mangrove plants need to obtain as much energy from the Sun
as possible →
cannot tolerate shaded conditions
mangrove plants are
relatively uniform in terms of height
some species
are
able to survive in water of higher salinity and withstand longer periods of flooding
by the tide
exhibit a
horizontal zonation
that is largely
determined by the high and low tide levels
zonation
-
distribution of plants in specific areas
according to certain parameters, each
characterised by its dominant species
Low tide level
low-lying parts
of the coast are
flooded for longer periods of time
compared to areas which are further inland
species that are found here include
Sonneratia
and
Avicennia
High tide level
species which are not as tolerant of these conditions are found closer to the high tide level where the
duration of flooding by the tide is shorter
e.g.
Rhizophora
and
Bruguiera
Unit 8
How Have the Plants in Mangroves Adapted to Their Environment?
mangrove plants have developed various adaptations to help them survive in the saline water
Salt-secreting leaves
Avicennia
enable plants to
remove salt from the saline water
that their roots have absorbed
concentrated salt solution
secreted by the leaves
evaporates
→
salt crystals
are left behind on the leaf surfaces →
removed by rain or wind
Sonneratia
deposit excess salt in older leaves
which they eventually
shed
prevent salt from building up
within the plant
Salt-excluding roots
Bruguiera
have roots which
prevent salt from entering
Aerial roots
soil is
flooded
for several hours of the day →
waterlogged
and
very poor in oxygen
,
soft
and
unstable
→ aerial roots to adapt to these soil conditions
-
grow partially above the soil surface
which enables them to
take in oxygen directly from the air
when they are exposed during low tide
roots help to
anchor the plants
to the soft soil,
preventing them from being uprooted and washed away by strong waves
Rizophora
curved prop
roots
help the plant
breathe
and provide
support
Bruguiera
knee-bend
roots,
bend upwards
to emerge above soil surface before
bending downwards
into the soil
at regular intervals
Unit 6
What are Mangroves and Where are They Found?
often found along or very close to the coast, in areas that experience the tropical climate
made up of plant species known as mangroves, which are able to
grow in water that has a higher salinity compared to fresh water
lack of competition from other types of plants which are unable to grow in such conditions
mangrove plants are
unable to withstand freezing conditions
grow best where the
average air and water temperature do not fall below 20°C
unable to survive in places very far north or south of the tropics
mangrove plants also require
calm water conditions so that their seedlings are able to take root and grow without getting washed away by strong waves
calm water conditions encourage the
accumulation of fine sediments containing nutrients, which mangrove plants require to sustain their growth
hence, mangrove forests are usually found only in
sheltered environments such as shallow river mouths or behind islands