Tropical grassland: distribution, climate and biomass

Overview

Climate of tropical grassland and vegetation of tropical grassland.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Where is tropical grassland found?

   Model answer

   In belts on the poleward margins of the tropical forest, roughly between 5 and 20 degrees north and south, forming a transition towards the deserts. The largest expanses are in Africa, north and south of the Congo forest, from the Sahel through East Africa to the veld and miombo of the south. There are also substantial areas in the llanos and campo cerrado of South America, in northern Australia and in parts of India and South East Asia.

2. 2

   Describe the climate of tropical grassland.

   Model answer

   Temperatures are high all year, generally 25-32 degrees Celsius, with a modest annual range of about 5-8 degrees. The distinguishing feature is rainfall: total annual rainfall is moderate, commonly 500-1500 mm, but it is strongly concentrated into a single wet season of about four to six months, leaving a long, reliable dry season in which some months receive virtually no rain at all. Rainfall totals fall and the dry season lengthens as you move away from the equator.

3. 3

   When in the year is a tropical grassland at its hottest, and why is it not the middle of the wet season?

   Model answer

   The temperature peak comes at the end of the dry season, just before the rains break. Through the dry season skies are cloudless, so insolation reaching the ground is at a maximum and there is little moisture to be evaporated, meaning almost all the energy goes into heating the ground and the air. Once the rains arrive, cloud cover reflects and absorbs incoming radiation and much of the remaining energy is used as latent heat in evaporation, so temperatures fall slightly during the wettest months.

4. 4

   Why is the rainfall of tropical grassland unreliable as well as seasonal?

   Model answer

   Because the wet season depends on how far the ITCZ migrates in a given year, and that varies. If the ITCZ fails to reach as far poleward as usual, or arrives late or withdraws early, the wet season is shortened and totals fall well below average. The margin closest to the desert is the most vulnerable, since it lies at the very limit of the ITCZ's reach, and this variability is why drought is a recurrent hazard on the desert edge of the grassland rather than a rare event.

5. 5

   Describe the vegetation of tropical grassland.

   Model answer

   A continuous ground layer of tall tussock grasses, often 1-3 m high, with scattered, widely spaced trees and shrubs above it. The proportion of trees changes with rainfall: near the forest margin there is enough woodland cover to be described as savanna woodland, while towards the desert the trees thin to isolated individuals and then to open thorn scrub and grass alone. Growth is strikingly seasonal, with grasses green and lush in the wet season and straw-coloured, dormant and dead above ground through the dry.

6. 6

   What adaptations allow savanna trees and grasses to survive the dry season?

   Model answer

   Many trees are deciduous, shedding their leaves at the onset of the dry season to cut transpiration losses. Leaves are often small, waxy or reduced to thorns for the same reason. Root systems are extensive: some species send down very long tap roots to reach deep groundwater, others spread wide, shallow root mats to capture the first rain. Trunks may be thick and water-storing, as in the baobab, and bark is often thick and corky to resist fire. Grasses survive by dying back above ground and holding their growing points at or below the surface, so they can resprout from roots and rhizomes within days of the first rains. Seeds of annuals lie dormant through the drought.

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