Savanna Grassland Biome (AQA A Level Geography)

• 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 

Savanna productivity

• Vegetation biomass is approximately 9% of the tropical rainforest
• Net primary production reduces with latitude and length of the dry season
• Areas with grazing animals have higher productivity, as nutrient transfer is through defecation and decomposition 
• Grazing increases growth rate of grasses and animal productivity is higher than tropical rainforests

Nutrient cycling 

• Stores are more even; with less leaching due to less precipitation
• However, there are seasonal variations across the biome (dependent on rainfall and temperature)
• The dry season, along with natural and deliberate grass fires, creates a short growing season, so the biomass store is a third less than the rainforest
• However, at the start of the dry season, there is a higher transfer from biomass to litter store because:
  • Trees lose their leaves
  • Grasses die back
• The litter store is small due to fires, and decomposition is slow during the dry season
• Transfer from litter to soil is slow
• The soil store is larger than the rainforest because:
  • Vegetation demand is lower
  • Uptake is restricted to the wet season
  • Leaching and weathering is less overall, due to the dry season

Nutrient of tropical grassland/savanna

Note the rates of transfers and that the stores are more equal as compared to tropical rainforests

Savanna soil profile

• Climate is the dominant factor influencing soil formation
• The seasonal changes determines if the movement of soil nutrients is balanced, upwards or downwards
• Differing amounts of organic matter and rock type, produces variations in soil type with latitude 
• Savanna soils are up to 2m in depth and weathering is limited to the wet season
• The soil of the savanna is porous, allowing rapid water drainage
• There is a thin layer of humus which provides vegetation with nutrients
• Leaching causes the characteristic red soils due to iron oxide being left behind (ferruginous soil)
• Soil is usually acidic with a laterite layer - this layer hardens (concretion) when exposed to the air, and forms an impermeable layer, making it difficult for roots to penetrate 

Soil profile of tropical grassland/savanna 

Note that the organic (O) horizon is limited due to quick nutrient cycling

Soil moisture

• The soil moisture budget fluctuates, as there is a surplus in the wet season and a deficit in the dry season
• The water budget shows the balance between precipitation and potential evapotranspiration 
• During the rainy season (July to October) the soils are recharged until there is surplus
• There is then a short period of usage by vegetation, but from December to July moisture is lost through evaporation, leading to soil moisture deficit

Water budget for tropical grassland/savanna environment

• All vegetation has some form of xerophytic tendency
• Some plants can store water in roots, trunks, stems and leaves, or extend their roots deep into the ground to reach the water table
• Water storage in swollen parts of the plant is known as succulence
• Trees are deciduous, have small leaves, and shed their leaves during the dry season to reduce water loss
• Drought prevents dense tree growth, allowing grasses to dominate with their branching network of deep roots
• Most grasses of the savanna are pyrophytic (adapted to fire), grow in thick tufts with bare ground in between, resistant to drought by becoming dormant during the dry season, have vertical leaves, with sharp edges and/or bitter taste
• Grass fires restores nutrients to the soil during the dry season

Animal adaptations

• Savannas are home to a wide diversity of animals, from the largest land mammal (African elephant) to the most deadly snake (black mamba) to over 40 species of hoofed mammals
• Each species of grazer has its own preference for grass, which allows for the species diversity
• To maintain a balanced ecosystem, predators are numerous - lions, cheetahs, leopards and predatory birds etc. 
• Animals adapt to the shortage of water and food through various ways:
  • Migration - roaming/moving to another area
  • Hibernation until the dry season is over
• Grazing animals, like gazelles and zebras, have stripes that create an optical illusion on the open grasslands
• Predators and prey have various camouflages to hide themselves 
  • The small caracal cat has long tufts on their ear tips, which hides them in tall grasses
  • A cheetah’s coat blends with the savanna grasses, and the spots mimic shadows
• Small animals burrow underground to avoid predators and keep cool
• Nocturnal - by being nocturnal, animals avoid the harsh heat of the day
• Animals have long, strong legs or wings that allow them to move and quickly as necessary (from fires or predators)
• Hawks and buzzards use the warm thermals (air) to hover in the sky without expending energy
• All animals of the savanna have high stamina, which saves energy, and function only when it is necessary
• Animals have an increased surface area
  • Giraffes have long necks that allow them to reach up to tall trees and eat leaves. It also enables them to see predators from far away and to communicate with other giraffes over long distances
  • An elephant's large ears act as coolants, as blood flows through them. As the ears flap, more heat is released
  • Elephants also use their ears to scare other animals by spreading them wide, making them appear larger than they already are