Anatomy of a Savanna Grassland
- The savanna biome is part of the larger grassland biome, which is dominated by grasses rather than shrubs and trees
- Other than Antarctica, grasslands are found on every continent and covers over 20% of earth's surface
- There are two grassland sub-biomes:
- Temperate grasslands - vast open plains of grasses with few trees or shrubs, has hot summers and cold winters, with moderate rainfall; soil is fertile in the upper layers
- Tropical grasslands or savannas
- Savannas (tropical grasslands) are a transitional biome (a theoretical ecocline)
- There is a gradual, continuous change from tropical rainforest to tropical, dry savanna grassland as the number of dry months increase, with distance, from the humid tropical climate
- Tropical savannas cover almost half the surface of Africa, large areas of Australia, Central and South America and southern Asia
- From the edges of tropical rainforest, the savanna runs from wooded savanna to grass savanna into semi-desert scrub at the pole-ward margins
Characteristics | Temperate Grasslands | Savanna/Tropical Grasslands |
Location Example |
40° - 60° north and south of the equator The 'veldts' of South Africa, the 'pampas' of Argentina, and 'steppes' of Russia and the 'plains' of the USA |
Towards the outer reaches of the tropical zone on the continents of Africa, Asia, S America, northern Australia and southern N America Central Africa - Tanzania, Kenya |
Annual Precipitation |
250-750mm |
500-1200mm Dry season virtually no rainfall (< 100mm) Wet season rainfall averages 20 days/month and up to 400 mm a day |
Annual Temp Range | -40°C to 40°C | High temperature all round (27° – 30°C), with low relative humidity (53% annual average but up to 73% during wet season) |
Seasons |
Four seasons |
Distinct wet and dry season - Savanna dry season May to Sept and wet season Dec to March |
Growing Season |
During the summer (dependent on temperature) |
During the wet season (4-5 months) |
Soils |
Fertile soil |
Free draining, acidic, with a thin layer of organic humus Not very fertile, most nutrients near the surface |
Biodiversity |
Large numbers of plant and animal species Grasses, sunflowers Bison, antelopes, rabbits Grasses and trees |
Wide range of plant and animal species Grasses, baobab and acacia trees Zebras, elephants, giraffes Greatest diversity of hoofed animals |
Distribution of temperate and savanna/tropical grasslands
Savanna structure
- Climate is the most important factor in the creation of a savanna
- Savannas are found in hot/warm climates with. distinct. seasonal wet and dry periods
- Rainfall is concentrated into 6-8 months, with long periods of drought, allowing fires to occur, which maintains the grasslands
- Three types of savanna are found:
- Climatic savanna - natural succession - annual fires maintain the grasslands and certain trees cannot survive the long dry seasons
- Edaphic savanna - soil conditions keep these savannas small, but can be found in any warm climate - hills and ridges where soil is shallow, or in valleys with clay, waterlogged soils,
- Derived savanna - human activity - clearing forests for agriculture; creating national park where elephants destroy trees, eat leaves and strip bark off trees
- Savannas are characterized by a continuous cover of perennial grasses, often 1m to 1.8m tall at maturity
- They may or may not have an open canopy of drought-resistant, fire-resistant, or browse-resistant trees, or they may have an open shrub layer
- As distance from the equator increases there is distinction between tree or wooded savanna; park savanna; shrub savanna and grass savanna - a theoretical ecocline
Idealised Latitudinal Transition of a Tropical Savanna/Grassland
Wet Savanna | Dry Savanna | |||
Type of Savanna | Wooded | Park | Shrub | Grass |
Main Vegetation | Deciduous tree, with grass cover below | Tall grass and widely spaced deciduous trees | Short grass and thorny shrubs | Short, tufted, sparse grass |
Max. Tree Height (m) | 20 | 14 | 9 | 5 (limited no.) |
Max. Grass Height (m) | 5 | 3 | 2 | 1 |
No. of Dry Months Furthest Border Zone | 4 | 5 | 7 | 9 |
Lowest Mean Annual Rainfall Furthest Border Zone (mm) | 1200 | 900 | 600 | 500 |
Annual Evapotranspiration Excess (mm) | 100 | 300 | 600 | 1000 |
Average changes in a tropical grassland/savanna vegetation. Note that with distance from the equator, as the water table lowers, the roots of trees have to grow deeper, this is an arresting factor for tree and shrub growth
- The main changes from the Equator are:
- Decreasing height of vegetation
- Decreasing density of vegetation
- Changing vegetation type and structure
- Decreasing number of tree species
- Increasing vertical depth of root networks
- These changes are due to decreasing rainfall totals, along with changing lengths of wet and dry seasons, the greater the distance is from the Equator
- This in turn, changes the vegetation type and its structure
Seasonal variations
- The dry season typically begins with a series of violent thunderstorms, leading to strong, drying winds
- These winds help to promote the spread of fires, causing animal migration and plant dormancy against the coming drought
- Rivers and lakes dry up and the grasslands become dry and pale
- Once the rains arrive in March, the transformation is spectacular with lush vegetation and rapid growth of grasses
- Rivers and lakes recharge and trees and shrubs bloom; animals return to graze and spread seed in their fur or faeces
- It is the long, dry seasons that prevent trees from dominating the landscape, along with seasonal fires and large numbers of grazing animals keeping forests from growing
Exam Tip
You may be asked to describe and explain the distribution of savannas or climate patterns. Consider the following;
- Biome distribution
-
- Latitude
- Country and continent names
- Factors which affect the distribution: latitude, altitude, ocean currents, continentality
- Climate patterns
- Temperature: maximum and minimum
- Seasonal variations
- Rainfall totals, maximum and minimum