- Ancient forests declined and grasslands became widespread.
- There are two main divisions of grasslands:
- Tropical grasslands or Savannas
- Temperate Grasslands
Savanna
- Savanna is grassland with scattered individual trees.
- Savannas cover almost half the surface of Africa and large areas of Australia, South America, and India.
- Climate is the most important factor in creating a savanna.
- Savannas are always found in warm or hot climates where the annual rainfall is from about 50.8 to 127 cm (20-50 inches) per year.
- Animals (which do not all occur in the same savanna) include giraffes, zebras, buffaloes, kangaroos, mice, moles, gophers, ground squirrels, snakes, worms, termites, beetles, lions, leopards, hyenas, and elephants.
- The soil of the savanna is porous, with rapid drainage of water. It has only a thin layer of humus (the organic portion of the soil created by partial decomposition of plant or animal matter), which provides vegetation with nutrients.
- Savannas are sometimes classified as forests. The predominant vegetation consists of grasses and forbs (small broad-leaved plants that grow with grasses). Different savannas support different grasses due to disparities in rainfall and soil conditions.
How are Grasslands Created?
- Savannas which result from climatic conditions are called climatic savannas.
- Savannas that are caused by soil conditions and that are not entirely maintained by fire are called edaphic savannas. These can occur on hills or ridges where the soil is shallow, or in valleys where clay soils become waterlogged in wet weather.
- A third type of savanna, known as derived savanna, is the result of people clearing forest land for cultivation.
- Farmers fell a tract of forest, burn the dead trees, and plant crops in the ashes for as long as the soil remains fertile.
- Then, the field is abandoned and, although forest trees may recolonize, grass takes over on the bare ground (succession), becoming luxuriant enough to burn within a year or so.
- In Africa, a heavy concentration of elephants in protected parkland have created a savanna by eating leaves and twigs and breaking off the branches, smashing the trunks and stripping the bark of trees.
- Elephants can convert a dense woodland into an open grassland in a short period of time. Annual fires then maintain the area as a savanna.
Destruction in the Savanna's
- Savanna has both a dry and a rainy season.
- Seasonal fires play a vital role in the savanna’s biodiversity.
- A series of violent thunderstorms, followed by a strong drying wind, signals the beginning of the dry season.
- Fire is prevalent around January, at the height of the dry season.
- Fires in savannas are often caused by poachers who want to clear away dead grass to make it easier to see their prey.
- The fires do not devastate the community.
- Most of the animals killed by the fires are insects with short life spans.
- A fire is a feast for some animals, such as birds that come to sites of fires to eat grasshoppers, stick insects, beetles, mice, and lizards that are killed or driven out by the fire.
- Underground holes and crevices provide a safe refuge for small creatures.
- Larger animals are usually able to run fast enough to escape the fire.
- There are also some environmental concerns regarding savannas such as poaching, overgrazing, and clearing of the land for crops.
Temperate Grasslands
- Temperate grasslands are characterized as having grasses as the dominant vegetation.
- Trees and large shrubs are absent.
- Temperatures vary more from summer to winter, and the amount of rainfall is less in temperate grasslands than in savannas.
- Temperate grasslands have hot summers and cold winters.
- Rainfall is moderate. The amount of annual rainfall influences the height of grassland vegetation, with taller grasses in wetter regions.
- As in the savanna, seasonal drought and occasional fires are very important to biodiversity. However, their effects aren’t as dramatic in temperate grasslands as they are in savannas.
- The soil of the temperate grasslands is deep and dark, with fertile upper layers. It is nutrient-rich from the growth and decay of deep, many-branched grass roots. The rotted roots hold the soil together and provide a food source for living plants.
- Each different species of grass grows best in a particular grassland environment (determined by temperature, rainfall, and soil conditions).
- The seasonal drought, occasional fires, and grazing by large mammals all prevent woody shrubs and trees from invading and becoming established. However, a few trees, such as cottonwoods, oaks, and willows grow in river valleys, and some nonwoody plants, specifically a few hundred species of flowers, grow among the grasses.
- The various species of grasses include purple needlegrass, blue grama, buffalo grass, and galleta. Flowers include asters, blazing stars, coneflowers, goldenrods, sunflowers, clovers, psoraleas, and wild indigos.
- Precipitation in the temperate grasslands usually occurs in the late spring and early summer.
- The annual average is about 50.8 to 88.9 cm (20-35 inches).
- The temperature range is very large over the course of the year. Summer temperatures can be well over 38° C (100 degrees Fahrenheit), while winter temperatures can be as low as -40° C (-40 degrees Fahrenheit).
- The animals include gazelles, zebras, rhinoceroses, wild horses, lions, wolves, prairie dogs, jack rabbits, deer, mice, coyotes, foxes, skunks, badgers, blackbirds, grouses, meadowlarks, quails, sparrows, hawks, owls, snakes, grasshoppers, leafhoppers, and spiders.
Desctruction in the Temperate Grasslands
- Few natural prairie regions remain because most have been turned into farms or grazing land. This is because they are flat, treeless, covered with grass, and have rich soil.
- Temperate grasslands can be further subdivided. (Prairies are grasslands with tall grasses while steppes are grasslands with short grasses. Prairie and steppes are somewhat similar but the information given above pertains specifically to prairies.
Steppes
- Steppes are dry areas of grassland with hot summers and cold winters.
- They receive 25.4-50.8 cm (10-20 inches) of rainfall a year.
- Steppes occur in the interiors of North America and Europe.
- Plants growing in steppes are usually greater than 1 foot tall.
- They include blue grama and buffalo grass, cacti, sagebrush, speargrass, and small relatives of the sunflower. Steppe fauna includes badgers, hawks, owls, and snakes.
- Today, people use steppes to graze livestock and to grow wheat and other crops.
Destruction of Steppes
- Overgrazing, plowing, and excess salts left behind by irrigation waters have harmed some steppes. - Strong winds blow loose soil from the ground after plowing, especially during droughts.
- This causes the dust storms of the Great Plains of the U.S.
Why is it important to teach to students?
Economically no other family of plants is of greater importance. People may not realize the importance of this biome. In this biome is a numerous variety of grasses. With these grasses come many different types of grains that we humans use for food. Cereal grains, such as rice, wheat, corn, barley, rye, and oats, are all grasses, and their importance as food producers for the growing populations of the world is unquestioned.
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