Grassland
Grassland, area in which the vegetation is dominated by a nearly continuous cover of grasses. Grasslands occur in environments conducive to the growth of this plant cover but not to that of taller plants, particularly trees and shrubs. The factors preventing establishment of such taller, woody vegetation are varied.
Grasslands are one of the most widespread of all the major vegetation types of the world. This is so, however, only because human manipulation of the land has significantly altered the natural vegetation, creating artificial grasslands of cereal crops, pastures, and other areas that require some form of repetitious, unnatural disturbance such as cultivation, heavy grazing, burning, or mowing to persist.
The most extensive natural grasslands can be thought of as intermediates in an environmental gradient, with forests at one end and deserts at the other. Forests occupy the most favourable environments, where moisture is adequate for growth and survival of a tall, dense vegetation dominated by trees. Deserts are found where moisture is so lacking that a continuous, permanent vegetation cover cannot be maintained. Grasslands lie between these two extremes.
Like the savannas, deserts, and scrublands into which they commonly blend, grasslands arose during the period of cooling and drying of the global climate, which occurred during the Cenozoic Era (65.5 million years ago to the present). Indeed, the grass family itself (Poaceae or Gramineae) evolved only early in this era. The date of earliest appearance of grasslands varies from region to region. In several regions a succession of vegetation types can be recognized in the Cenozoic fossil record, as climate dried out progressively. For example, in central Australia during the past 50 million years tropical rainforest gave way successively to savanna, grassland, and, finally, desert. In some places expansion of grasslands to something approaching their modern extent occurred only during the extremely cold, dry intervals—called ice ages in north temperate regions—of the past two million years.
A dynamic balance commonly exists between grasslands and related vegetation types. Droughts, fires, or episodes of heavy grazing favour grassland at some times, and wet seasons and an absence of significant disturbances favour woody vegetation at others. Changes in the severity or frequency of these factors can cause a change from one vegetation type to another.
Grassland Environment
Grassland climates are varied, but all large regions of natural grassland are generally hot, at least in summer, and dry, though not to the extent that deserts are. In general, tropical grasslands receive 500 to 1,500 millimetres of rain in an average year and in every season experience temperatures of about 15 to 35 °C. The dry season may last as long as eight months. An excess of rainfall over evaporation, leading to ephemeral river flow, occurs only during the wet season. The tropical grassland climate overlaps very broadly with that of savanna. As previously stated, these vegetation types differ little from each other, a savanna being merely a grassland with scattered trees. Small changes in management and usage can convert one to the other.
Temperate grasslands are somewhat drier than tropical grasslands and also colder, at least for part of the year. Seasonal temperature variation may be slight in tropical grasslands but may vary by as much as 40 °C in temperate grassland areas. Mean annual rainfall in the North American grassland areas is 300 to 600 millimetres. Mean temperatures in January range from −18 °C in the north to 10 °C in the south, with corresponding values in July being 18 °C and 28 °C. Mean annual temperature in the most northerly areas of the North American grassland zone is below 0 °C.
Occurring as they do across a wide range of climatic and geologic conditions, grasslands are associated with many different types of soil. The grassland ecosystem itself influences soil formation, and this causes grassland soils to differ from other soils. The nature of grass litter and its pattern of decomposition commonly result in the development of a dark, organically rich upper soil layer that can reach 300 millimetres below the surface. This layer is absent from desert soils and is different from the surface layer of rotting leaf litter typical of forest soils. It is friable in structure and rich in plant nutrients. Lower soil layers are typically pale and yellowish, especially at depths close to two metres.
Because of its importance for grazing and other grassland agricultural production, grassland productivity has been extensively investigated using various methods. However, most studies have focused only on aboveground productivity, ignoring the important subterranean component, which can be much more substantial—as much as 10 times greater—even when the aboveground portion is at a seasonal maximum. Typical aboveground biomass (dry weight of organic matter in an area) values for North American grasslands are 2.5 to 6 metric tons per hectare, of which about three-quarters is in the form of dead shoot material; however, values up to and greater than 20 metric tons per hectare have been recorded in some tropical grasslands.
There is a relatively rapid turnover of plant matter in grasslands. Most plant organs have a life span of only one or a few seasons, leading to annual rates of overall turnover of about 20 to 50 percent. When consumption of plant parts by consumers is taken into account, annual productivity values are similar to biomass values. Aboveground annual productivity in a typical Canadian grassland was found to be 6.4 metric tons per hectare when all leaching and decomposition losses from dead shoots were taken into consideration. Values found in other grasslands have varied greatly but often have been significantly lower than this, partly because such losses have not been fully taken into account. Repeated harvesting (surrogate grazing) commonly yields 1.5 to 2.5 metric tons per hectare per year.