Characteristics of tropical savannas

Climate

The climatic pattern is one of the defining characteristics of tropical savannas worldwide. In this activity, we’ll explore what are the climatic features of savanna regions.

Activity

Activity 2.1

Savanna climates - based on characteristics / Cape York / climate / climate patterns

Many of you studying this unit will be living in northern Australia or have visited here and would be familiar with the weather being hot and dry or hot and wet depending on the time of year. This weather pattern is shown in Figure 2 where the rainfall and temperature is graphed for Darwin and Daly Waters in the Northern Territory.

Have you thought about if these climate patterns are typical of savanna climates globally? Open the attached figure and refer to:

Compare your ideas to ours

A monsoonal climate with very distinct wet and dry seasons are typical of savanna ecosystems worldwide. The savanna environments are characterised by a rainy period with warm to hot conditions followed by a virtually rainless dry period with warm to cool conditions. Most commonly there is a single alternation of seasons (eg Weipa, Coen and Cooktown), however this can be bimodal (eg Bouake, Africa). Mean monthly temperatures vary from 20-35oC in the warm months to 10-25oC in the cool months.

Savanna climates can vary greatly with respect to rainfall and the length of the dry season. Annual rainfall varies from 500mm (eg Barra, Africa and Daly Waters, Australia) to over 1600mm. The rainfall in Darwin is at the higher end of the rainfall range for savanna climates. The length of the dry season ranges from 3-4 months (e.g. Goiania, S. America) to 6-8 months (e.g. Nyala, Africa).

Figure 2. Rainfall and temperature for Darwin and Daly Waters in the Northern Territory

Figure 2

Geology and soils

Activity

Activity 2.2

Geology and soils of savanna regions

The soils of the savanna regions of Australia, Africa, South America and India are similar in their underlying geology. These soils are generally low in nutrients. Why?

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Africa, South America, Australia and India were once united, forming the old continent of Gondwana. Many of the geological and soil features of these regions date from the time they formed one continent. The effects of the drifting as well as climatic changes during the Pleistocene created distinct geological features in each continent. Present-day landforms reflect common Gondwana characterisitics as well as the unique individual history of each continent since the Cretaceous.

See the following animated graphic for a view of how these continents may have been located 115 million years ago, and their subsequent break-up.

The soils are therefore formed from ancient landscapes and their poor nutrient status are partly the reflection of the extremely long history of weathering and nutrient leaching.

The poorest soils (oxisols and ultisols) are those derived from the oldest deposits, since these have been subjected to weathering and leaching for the longest times. These are common throughout the world’s tropical savanna regions.

Vegetation

As we have discussed, the savannas occur in tropical regions where there is transition between abundant rain and short-term drought within a one year period.

To cope with such conditions, savanna organisms have developed a wide range of morphological, physiological and behavioural adaptations. The seasonality of the climate also imposes a seasonal response in growth and senescence in the vegetation, particularly the grasses.

The following is a photographic time sequence from dry season to wet season and back to the dry season in the savannas around Darwin.

Activity

Activity 2.3

Look at the photographic time sequence from dry season (November) to wet season (March). What do you notice about the differences in the amount of plant growth that occurs during the wet season and dry season? Is this typical of savannas worldwide?

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These pictures show the massive differences in the amount of plant growth that occur during the wet and dry seasons, which is typical of savannas worldwide.

In savannas, the ground flora in particular changes dramatically throughout the year, with annual plants and aboveground stems of perennial herbaceous species dying off during the dry season (May to October).  At the onset of the wet season (November), many perennial herbaceous species sprout new leaves, and the seeds of annual species germinate and new seedlings establish. The wet season is the main period of flowering and growth of these species.

As you may expect, there are significant variations in both the quantity and quality of plant material during the wet and dry seasons.  Both quantity and quality are dependant on the total amount and seasonal distribution of rainfall, and on the availability of nutrients, particularly nitrogen and phosphorus (Frost et al. 1986).

The savanna vegetation exhibits a wide range of vegetative and floral phenological behaviour. Growth and flowering may occur in both the wet season and the dry season. The wet season is the main period for growth and flowering of the herbaceous species in the savanna woodland and forests. The dry season and build up to the wet are important periods of growth and flowering in the woody vegetation.

Activity

Activity 2.4

Figure 2 of Setterfield and Williams (1996, link below) shows the timing of major reproductive events for the most common evergreen trees (E. miniata and E. tetrodonta) in Kakadu.

Figure 9 of Williams et al. (1997, link below) shows the main pattern of leaf growth and phenology of 5 common canopy species in Kakadu NP. The common evergreen species are: E. miniata, E. tetrodonta and E. porrecta and two deciduous trees: Erythrophleum chlorostachys and Terminalia ferdinandiana. The phenology of other savanna species are also documented and this data set provides a comprehensive examination of the patterns of leaf growth through the seasons.

Like the reproductive phenology, there are clear and regular rhythms in the reproductive phenology.

Refer to the following:

List 5 major features about the timing and duration of leaf production and reproduction in Australian savanna trees. Are these distinct leaf and reproductive patterns exhibited by savanna species elsewhere in the world?

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Some major points apparent are:

  1. Reproductive phenology is strongly seasonal in the main tropical Eucalypts.
  2. Flowering and seed production occurs during an 8 month period in the dry season in the main tropical Eucalypts.
  3. The dominant Eucalyptus species are able to grow during the 6 month dry season.
  4. The late dry season, build up to the wet season, and the early wet season are all important periods for growth for the 5 species.
  5. The late wet season and transition from wet to dry season are characterised by low levels of growth.
  6. The timing of the major growth phases can differ between species and between years (compare the growth phases for Erythrophlem chlorostachys and Terminalia ferdinaniana).

Distinct phenological patterns such as these have been identified in savannas around the world. On the basis of the timing of growth and flowering, Sarmiento and Monasterio 1983) established phenological groups. They formed 15 phenological groups based on:

  • Their life-history (perennial or annual)
  • Whether the species assimilated carbon all year, or had a rest period
  • Growth period
  • Timing of flowering in relation to the wet season

Sarmiento and Monasterio (1983) emphasize that there is a “wide range of phenological strategies apparent” and that “in spite of the sharp seasonality of the vegetation, every period in the year appears to be favourable at least to the accomplishment of certain phenophases in on or another group of plant species.”

Fauna

Two common images of savannas are herbivory by large, native ungulates, particularly in Africa and the widespread grazing by domestic herds, particularly cattle.

Large herbivore diversity and abundance are much higher in Africa than in Australia, Asia or South America. More than 40 large wild herbivore species have been described in African savanna. In contrast, only 6 species of megapod marsupial have been considered as large herbivorous mammals in the Australian savannas, and only three species of ungulates are regarded as native South American savanna inhabitants. Domestic animals, particularly cattle, buffalos, sheep and goats are now the dominant, large herbivores in most savannas.

The more neglected group of savanna herbivores are the invertebrates, particularly grasshoppers, caterpillars, ants and termites. In Australia, insects assume a number of ecological roles (herbivory, seed predation) that are played by vertebrates elsewhere. We will discuss the role of mammal and insect herbivores in more detail in Topic 3: Determinants.

Activity

Activity 2.5

Fauna of Australian savannas

For a description of the fauna of northern Australia, refer to the Tropical Savannas CRC website at

Next topic - Determinants

Topics in this module

 

 

 

 

Termite Mound

 

 

 

Buffalo