HKDSE Geography/M6/Vegetation

Luxuriant vegetation is found in tropical rainforests because of the luxuriant sunlight and rainfall. The characteristics of vegetation here is often tested in examinations and should be studied in detail.

Biomass and Biodiversity

Biomass - the total dry weight of dead organic matter in an area.

Biomass is a measure of the volume of organisms in an area. Imagine you take all the plants in a unit area, kill them, dry them, and weigh them. This is biomass.

Tropical rainforests have large biomass because of the abundant rainfall and sunlight.

Biodiversity - the variety of plant and animal species in an areas

A species is a type of animal, e.g. dog, cat, cattle, sheep. The more species we can find in a unit area, the higher the biodiversity. In tropical rainforests, the biodiversity is the largest among all the biomes. There are several reasons leading to this:

Abundant sunlight and rainfall is present.
Interspecies competition occurs. Different organisms in tropical rainforests compete for different resources. In the end, the resources may be divided, e.g. Animal A takes Shelter A and Animal B takes Shelter B. (This is not biology, so we will not go into details.)
The tropical rainforest provides a diverse range of habitats as the vegetation is stratified (see below).

Stratification of Vegetation

The five layers of a tropical rainforest. Just ignore the foreign text here, and focus on the trees and the numbers.

The tropical rainforest is divided into five strata, each with its own characteristics. (Refer to the figure on the right.)

Emergent layer: The highest layer, 40-50 m above the ground. Trees are widely spaced.
Canopy layer: The second layer, 30-40 m above the ground. Trees are continuous.
Understorey/Young tree layer: The third layer, 20-30 m above the ground. Tree seedlings of the above layers are sparse.
Shrub layer: The fourth layer, 10-20 m above the ground. Shrubs and tree seedlings of the above layers are sparse.
Undergrowth: The bottom layer, on the forest floor. Ferns, fungi, etc., are sparse.

Vegetation Characteristics

There are numerous vegetation characteristics to be memorised:

Layer	Characteristic	Explanation
General	Evergreen^[1] - no seasonal pattern in defoliation, new leaves quickly replace shed leaves; no seasonal pattern in flowering or fruiting	No seasonal pattern in insolation or rainfall
General	Low root-to-shoot ratio (1:4 to 1:5)/High shoot-to-root ratio(4:1 to 5:1) - Most of the biomass storage is in the above-ground part, little biomass in roots	Infertile soil, abundant insolation above the ground
General	Cauliflory - trunk flowers and fruits are found on the trunk and main branches	Pollination by wind is impossible, so they are made accessible to pollinators^[2] (insects who can pollinate the trunk) and animals which can eat the fruit
General	Broad leaves	Absorb more sunlight
General	Waxy leaves with drip-tips	Quickly shed rainwater when needed (high annual rainfall...). If the crown does not quickly shed rainwater, it may become too heavy because of the intercepted water, the leaves may be infected, and the water may inhibit sunlight penetration.
General	Thin bark	No need to protect the tree from the cold, facilitate transpiration
General	Smooth bark	Prevent lianas (see below) from climbing, shed water quickly
General	Shallow roots	All nutrients are concentrated on the topsoil, so shallow roots can absorb them more efficiently
Emergent	Wide, umbrella-shaped crowns^[3]	Absorb more sunlight, space is abundant
Emergent	Discontinuous, widely-spaced crowns	Absorb more sunlight
Emergent, Canopy	Straight trunks	Support the plants
Emergent, Canopy	Buttress roots - roots that extend laterally, up to 4 m high	Absorb nutrients from a larger area, support the tree's great weight
Emergent, Canopy	Tall branches, appearing only on the top third of the trunk	Absorb more sunlight
Emergent, Canopy	Lighter leaves	It is easier to absorb sunlight on the emergent and canopy layers, so they have less chlorophyll
Emergent, Canopy	Heliophytes (sun-loving plants)	More sunlight is available
Canopy	Rounded, oval-shaped crowns	Absorb more sunlight
Canopy	Continuous crowns	Absorb more sunlight
Canopy, Understorey (near rivers and streams)	Stilt roots	Support the trees
Understorey	Darker leaves	Absorb more sunlight with more chlorophyll
Understorey	Narrower crowns	Less sunlight available, so growth is hindered
Understorey	Slim trunks	Less support is needed
Understorey, shrub, undergrowth	Sparse vegetation	Insufficient sunlight for growth, relative humidity is too high and this hinders tranpiration
Shrub, undergrowth	Skiophytes (shade-loving plants) such as fungi, ferns	Less sunlight is available

↑ Some plants are deciduous and only shed leaves in autumn.
↑ It refers to the process by which plants reproduce, involving the spreading of pollen
↑ Crowns are the top part of the trees with many branches and leaves. It is the defining characteristic of the five strata.

Vegetation Types

Woody plants, including trees and woody shrubs, are found. They are heliophytes (sun-loving plants) who take advantage of the large amount of insolation.
Ferns and fungi are skiophytes which grow well in the dark.
Some plants rely on other plants (which are their hosts):
- Climbers grow on tall trees for physical support. They start at ground level, hook onto and climb along their hosts, and reach the canopy layer. They may spread to other trees to maximise the amount of sunlight received. Lianas are an example.
- Epiphytes are small shrubs or herbs that attach themselves on tree trunks or branches. They do not have roots in the soil, and have aerial roots that capture moisture from the air. They can then grow in the canopy layer and capture sunlight. They are harmless to their hosts.
- Stranglers/Strangler figs are the transitional species between lianas and epiphytes. They send their roots to the ground and surround the host trees. They grow rapidly and soon cover the crown of the hosts, blocking out their sunlight. The host will die so the strangler can inherit its place in the canopy.
- Parasites grow directly on their hosts, and absorb nutrients from them. Large amounts of litter in tropical rainforests provides a nutritious environment for parasites. Rafflesia is an example.

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[1] Some plants are deciduous and only shed leaves in autumn.

[2] It refers to the process by which plants reproduce, involving the spreading of pollen

[3] Crowns are the top part of the trees with many branches and leaves. It is the defining characteristic of the five strata.