Biome Background:
Grasslands generally are located in the center of continents. They are found in tropical, temperate, and Arctic zones. One fifth of the land's surface is grassland. There are about 4,500 species of grass, some quite large, including sugar cane and bamboo. How many of the ten grass crops can you name? Some names for grasslands around the world include pampas, campo, and llanos in South America; steppes between Hungary and Mongolia; veldt in southern Africa; and prairie in North America.
Whether characterized by tall, short, or mid-height grass, all grasslands receive inconsistent levels of rainfall, usually between 25-75 cm (10-40 inches) and all experience periods without rain. Taller grass species require more rainfall. Trees are kept out of the grasslands both by their relatively low and inconsistent rainfall and by fires.
Average monthly temperatures in grasslands resemble those in the deciduous forests, but the range of temperatures during the days can be extreme. Grasslands also experience drying winds, such as the Canadian "Chinook."
Grasslands are characterized by:
- rolling to flat terrain
- plant community dominated by grasses
- low and irregular rainfall
- high evaporation of soil moisture
- occasional severe droughts
- animal community dominated by burrowers and grazers
Grasses are able to withstand trampling, fire, low rainfall, and mowing. Their survival is due to their specially-adapted leaves. Grass grows from the base of the leaf (the node) and this growing area is always active. If the upper part of the grass is cut off, the plant is able to regrow at the nearest node. In addition, the plant spreads horizontal stems across the ground surface and each stem can send off leaves and roots. Grass is also strong because it is reinforced with fibers.
A grass floret can be taken apart and examined under a microscope.
Grassland soils typically are extraordinarily rich, with a thick layer of organic material (humus) from generations of grasses. It takes about four years to decompose grasses. This sod is very tough and writers around the world have remarked on the difficulty of plowing through it. The thick mulch helps keep water and nutrients. The evaporation of water from the soil surface creates a "capillary action," or pull, upwards toward the surface, preventing nutrients from leaching away.
Ruminants like buffalo and cows have evolved with the grasslands. Their stomachs are able to break down the whole grass plant and make protein from it, while humans can digest only the grain. Ruminants have several-chambered stomachs so that the tough grass is chewed and digested several times over. The first stomach chambers contain bacteria and protozoa to assist digestion. Similarly, ants and termites host a fungus to help them digest the grass. Many smaller animals, even a species of owl, live in burrows, and many grassland species migrate during dry seasons. Grasses don't need animals to disperse their seeds and therefore don't need large flowers to attract insects and animals. Instead, the wind disperses grass pollen and seeds.
Grazing recycles nutrients. Grasses decompose very slowly in dry climates. This is what is referred to as a "brittle environment." Unlike other biomes where microbial action speeds the recycling of nutrients back into the environment, chemical decomposition through drying is the driving force in the grasslands. For instance, the decomposition of a dead tree begins with the tiny outer twigs that are the first to dry and fall off, followed by the larger branches. This isn't true in a more moist "non-brittle" environment where a dead tree will host thousands of microbes and insects that help in decomposition. Because decomposition through drying is quite slow, grazing by animals plays a terribly important role in the recycling of nutrients. Buffalo, cattle, zebra, wildebeest, and other hoofed animals speed up the process. The grasslands depend upon these animals to chew off and digest the grasses and to turn over the soil, keeping decayed plant material from blocking sunlight to the growing section at the base of the plant and increasing water absorption. Without the native buffalo to perform these services to the grasslands of North America, cattle in effect act as artificial buffalo to keep the grasslands healthy and productive.
These days, most grasslands are being farmed or grazed, although portions of original carefully restored using only native species. Poor agricultural practices have led to serious erosion, salinization, and destruction of soil structure in grassland biomes. Pesticides, for example, have killed not only the bugs for which they were intended, but also soil microbes that form healthy soil clumps (“aggregates”) with their secretions. Banned in some parts of the world, these toxic pesticides are still used by farmers in other regions. Irrigation using salty water or running through salty soil can result in salt and mineral buildup in the upper layers of the soils. These are challenges that soil extension agents and soil scientists face every day. Most countries have taken strong steps to set up services to advise farmers and ranchers about these issues.