Iruni Kalupahana JadeTimes Staff
I. Kalupahana is a Jadetimes news reporter covering Travel
Definition and Distribution
Savannas are ecosystems dominated by an open tree canopy with a continuous grass understory. These landscapes cover about 20% of the Earth's land surface and are mostly found between tropical rainforests and deserts. African savannas, such as the Serengeti and Kruger National Park, are the most famous, but much area of savannas also occurs in South America, Australia, India, Southeast Asia, and Madagascar. Savannas first appeared at the beginning of the Cenozoic Era, about 66 million years ago, during conditions of generally falling atmospheric carbon dioxide. The lower levels of carbon dioxide favored the C4 grasses, which are highly efficient at photosynthesis under warm, low carbon conditions. These systems are dynamic and have continued to evolve with ongoing climate changes, land use practices, and fire regimes.
Climatic Conditions and Types of Savannas
Savannas mostly occur in warm to hot areas with average annual temperatures ranging from 20 to 30 degrees Celsius, which is 68 to 86 degrees Fahrenheit. The seasonal rainfall defines the areas, oscillating between 500 to 1,500 millimeters 20 to 60 inches per year. Wet, dry, and thornbush are the three major types of savannas, classified according to the duration of the dry season. Wet savannas are those that receive a lot of rainfall, thus they can support a wide range of flora and fauna, while dry savannas are marked by extensive dry periods and, as such, are very prone to frequent fires. Thornbush savannas are sparsely vegetated with thorny shrubs and usually form in those areas that have the most extreme dry season. Sometimes, referring to classification, it also refers to the density of trees within these areas, "tree savannas" or "grass savannas," depending on which is more dominant.
Ecological Characteristics and Human Influence
Savannas are considered ecotones or transitional areas between thick rainforests and dry deserts. They are known to be highly biodiverse and to have very intricate ecological functions. Such systems have been caused by the combined effects of climate, soil, and disturbance like bushfires. Fire natural or man induced is a very important agency in maintaining the savanna through preventing the encroachment of trees and encouraging grass. Plants on savannas have generally evolved fire resistant adaptations that include thick bark and underground storage organs. Human impacts, largely through hunting and grazing and land clearance for agriculture, have been among the major determinants of the current status of many savannas. Overgrazing by domestic livestock can lead to loss of soils and greater susceptibility to invasion by species that, once established, may alter the composition and competitive balances of ecosystems. Today, most of the conservation efforts directed towards these important landscapes are in the form of practicing sustainable land management to preserve ecological integrity.
Soil Composition and Nutrient Cycling
Savanna soils are often nutrient poor, commonly sandy or lateritic, and low in organic matter due to the high decomposition rate in warm climates. In spite of this, some elements within the savanna ecosystem fulfill an important role in nutrient cycling. Some elements within a savanna ecosystem, for example, trees and termites, play an important role in nutrient cycling. For instance, the presence of leguminous trees will fix atmospheric nitrogen into the soil, examples include Acacia spp. Termites, especially in African savannas, build mounds that enhance the nutrient and organic matter levels of the soil. In the South American savannas, such as the Cerrado, the acidic soils with high aluminum concentration inhibit plant growth, while African savannas exhibit varying soil types, from volcanic rich to highly leached sandy soils, which affect vegetation patterns. Although the savanna soil is generally quite infertile, fires periodically return nutrients to the soil, allowing for the rapid growth of grasses that keeps the savanna productive.
