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 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.

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 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:

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.