Temperate grasslands

Several characteristics of the temperate grassland biome have contributed to its historic and current importance in shaping people's livelihoods and customs, as well as local, regional, and now global economies. In particular, the location of temperate grasslands at mid-latitude regions ranging from 30 degrees to 60 degrees north and south of the equator is conducive to human settlement. Large swaths of these grasslands cover parts of North America, South America, Eurasia, and southern Africa, while less extensive grasslands occur in Australia and New Zealand. An agreeable climate, highly fertile soils, and the availability of forage for livestock provided incentive for a diversity of people from these regions to settle in the temperate grassland zone. The many names used to describe temperate grasslands, including prairie in North America, steppe in Eurasia, pampas in South America, and veld in South Africa, reflect how different cultures identify portions of this expansive biome. Given the history of human settlement in temperate grasslands, one of the main threats to their integrity is land-use change. Prudent stewardship is required so that the healthy functioning of this biome, which is highly productive and of great economic importance, can be maintained over the long term.

Temperate grasslands are largely defined by their lack of woody vegetation. They are typically dominated by perennial grasses and sedges (graminoids) that are well adapted to semi-arid to subhumid climates with marked extremes in temperature (both among seasons and over the course of a day) and seasonal precipitation. These grasslands also contain a variety of nongraiminoid herbaceous plants called forbs, which, although less common than the dominant graminoids, are the main contributors to plant diversity in these systems. Forbs often flower in synchrony following monsoonal moisture events, giving rise to a colorful palette of blooms that cover vast expanses of land. In some cases small shrubs are present, for example, where the local topography and soil texture favor them. More generally, plant community composition varies in relationship to many factors other than climate including edaphic (soil) conditions, fire regimes, and grazing by wild and domesticated ungulates (hoofed mammals).

Temperate grasslands are broadly considered to be water-limited in the sense that woody vegetation is typically not supported. However, there is substantial variation in the amount and timing of precipitation among regions, which gives rise to distinct differences in grass canopy height and species composition. For example, in North America, as one moves from the subhumid central lowlands in the east to the semi-arid western extent of the Great Plains, precipitation becomes more limiting. This gradient in precipitation is mirrored by a transition from tall-grass prairie in the east, to short-grass prairie in the west, with midgrass prairie occupying the area in between. In contrast, the gradient of precipitation availability in the Eurasian steppe runs from north to south, with grasses and forbs becoming shorter, less abundant, and less diverse as one moves south and precipitation becomes more limiting.

Photosynthetic Grass Pathways

An important distinction among grasses is whether they have a C₃ (when carbon dioxide breaks up into a three-carbon compound) or C₄ (when carbon dioxide breaks up into a four-carbon compound) photosynthetic pathway. The designation as a C₃ or C₄ grass indicates whether the first product of carbon fixation during photosynthesis is a three-carbon or four-carbon molecule. Several differences in the C₃ and C₄ photosynthetic pathways result in plants of each type achieving optimal photosynthetic rates under different climatic conditions. As a result, the distribution of C₃ and C₄ grasses varies strongly in relationship to water availability and temperature among regions. The C₃ grasses, often referred to as “cool season” grasses, dominate in areas with relatively cool temperatures and where precipitation is high during the winter months. These grasses germinate in early spring and achieve maximum growth when temperatures are still relatively low (approximately 20 degrees C or 68 degrees F).

In contrast, the C₄, or “warm season” grasses, dominate in areas where both temperature and average annual rainfall are higher than where C₃ grasses dominate, and where the majority of precipitation occurs during the summer. Germination of C₄ grasses is delayed until the summer months, and maximum growth occurs at quite high temperatures (approximately 35 degrees C or 95 degrees F). The distribution of C₃ and C₄ plants in relation to their climatic requirements results in gradients in their relative abundance at large spatial scales. For example, temperate grasslands in the southeastern United States are dominated by C₄ grasses, while C₃ grasses become increasingly dominant to the north and west. Differences in the chemical composition of C₃ and C₄ grasses (for example, the ratio of carbon to nitrogen in their tissues) affect ecosystem-level properties such as rates of litter decomposition and nutrient cycling.

Grasses also exhibit a number of morphological (structural) characteristics that adapt them to the precipitation and temperature extremes to which they are exposed, as well as to fire and grazing by herbivores. In particular, grasses have several characteristics that help them cope with stressful conditions, including vegetative (asexual) reproduction; buds (new growth) that occur at or below the soil surface; cell division (new growth) that occurs at the base of leaf blades rather than at the tip; and fibrous root systems that are concentrated in the shallow layers of the soil profile. Vegetative reproduction occurs when individual plants send out specialized structures such as stolons or rhizomes in order to establish a new individual. Eventually the new individual breaks off from the mother plant, forming an independent, genetically identical individual (or clone). This type of reproduction is beneficial to plants growing in harsh environments because it requires less time and energy than sexual reproduction, which depends on pollination and the formation of seeds. Once grasses have established, new buds are often produced at or below the soil surface, which protects this young and delicate tissue from fire and grazing herbivores. Additionally, cell division typically occurs at the base of leaf blades, rather than at the tip, as is common in other types of plants. This allows for quick tissue regeneration, even if the majority of a leaf is lost to herbivory. Grasses also have fibrous root systems that extend horizontally from the plant through the shallow layers of the soil profile. The fibrous roots, which are finely branched and have a high surface area, allow grasses to take maximum advantage of periodic monsoonal rains that are short in duration and infiltrate only the shallow soil layers.

Soil Diversity in Temperate Grasslands

There is a great diversity of soils that underlie temperate grasslands. This diversity reflects variation in the many factors that shape soil properties, including climate, parent material (for example, bedrock), topography, and the plant and microbial (that is, bacterial and fungal) communities present in a given area. Temperate grassland soils do share some basic characteristics, however. Importantly, they tend to be extremely fertile, and because of this they have largely shaped patterns of human settlement following the discovery of agriculture. They are typically high in dissolved nutrients (for example, nitrogen, phosphorous, and potassium) and organic matter, making them amenable to crop production. The top soil layers are often dark and rich in color due to a well-developed layer of humus (organic matter). High evaporation rates and capillary action prevent the leaching of nutrients to deep soil layers, where they would be inaccessible by plant roots. Additionally, the dense fibrous root systems of native grasses effectively bind nutrients, which keep them from leaving the system, and they prevent soil erosion by anchoring the soil and trapping moisture. The texture of grassland soils, which is defined by the particle-size distribution, varies from clays to loams to sands (with clays being composed of the smallest particles and sands of the largest particles). Soil texture is important because it influences water infiltration and percolation rates, as well as water-storage capacity, all of which directly affect plant distributions.

All of the attributes discussed so far, including abiotic factors such as temperature, precipitation, and soil characteristics, and biotic factors such as plant community composition and plant canopy height, affect nutrient cycling dynamics in grasslands. In addition, microbes, which are the main decomposers of organic matter, are important in shaping nutrient cycles. Nutrients such as carbon and nitrogen move through different “compartments” of the ecosystem, including the atmosphere; living organisms such as plants, animals, and microbes; and the soil. They exist in different forms depending on which compartment they are in. Often they enter the system as inorganic molecules (for example, gaseous carbon dioxide [CO₂] or nitrogen gas [N₂] from the atmosphere) and are transformed into usable (organic) forms that can be assimilated by plants and animals. For example, during photosynthesis, plants provide the crucial service of “fixing” CO₂ into organic forms that can be used by herbivores. In turn, most plants rely on microbes that transform inorganic nitrogen into organic forms that can be used to make the proteins that catalyze photosynthesis. Given that a diversity of species shape nutrient cycling dynamics in natural grasslands, modifying native plant and soil communities (for example, by converting native grasslands to single-species crops) can have marked impacts on nutrient cycling regimes.

Fauna of Temperate Grasslands

The vegetation of temperate grasslands in large part determines the abundance and composition of the associated fauna. Grasses support several characteristic mammal assemblages, most notably herds of grazing ungulates as well as burrowing rodents and lagomorphs (for example, rabbits and hares). Bison (Bison bison americana) and pronghorns (Antilocapra americana) represent the dominant large ungulates of the North American Great Plains, while wild horses (Equus przewalskii and E. gmelini), asses (E. hemionus and E. kiang), and saiga antelope (Saiga tatarica) were prevalent on the Eurasian steppe. Other examples of grassland ungulates include the pampas deer (Ozotoceros bezoarticus celer) of South America and the sable antelope (Hippotragus niger) of the South African veld. Vast herds of these animals historically populated grassland ecosystems worldwide, acting as keystone species that shaped the abundance, distribution, and adaptive characteristics of the plants on which they grazed. While some of these species, such as the pronghorn, are still widespread, most of them have been hunted to near extinction or widely displaced due to habitat loss. For example, in a few short decades, European settlers of the American west hunted an estimated 70 million bison to near extinction.

Many reasons underlie this tragic slaughter including the attractiveness of the skins for commercial sale, the perception that bison competed with cattle for forage, and the desire of the U.S. government to pressure Native American tribes who relied on bison for food and clothing. In many cases, individuals of these once prolific keystone ungulates exist only in nature reserves or as managed semi-natural populations (as with the American bison and the pampas deer). Instead, domesticated ungulates such as cattle or sheep number in the millions in many temperate grasslands and have in some places virtually replaced the native herbivore assemblages. Unfortunately, this shift also has negative implications for many of the large carnivores that historically fed on native ungulates.

Fossorial rodents and lagomorphs have also been important in shaping temperate grasslands, due in large part to the effect that their burrowing has on soil characteristics and plant distributions. The natural disturbances created by these ground-dwelling mammals, including the black-tailed prairie dog (Cynomys ludovicianus) of North America and marmots (Marmota sibirica) and social voles (Microtus socialis) of the Eurasian steppe, help to maintain plant and animal biodiversity in grassland systems. In lieu of other protective structures such as trees or caves, burrows provide shelter to a large variety of these relatively small, herbivorous animals. Burrow excavation and maintenance act to mix the soil layers, often times redistributing deeper soils to the surface and reducing soil compaction. Burrows also serve as habitat “microsites” that are different from the surrounding intact grassland. For example, some forb species recruit almost exclusively on disturbed soil created by burrowing, while insects and spiders often prefer the cooler, more humid habitat provided by underground burrows. In many cases, burrowing rodents live socially, in groups of related individuals that share tasks such as raising offspring and keeping watch for predators. Social living concentrates the number of individuals occupying a given area, leading to pronounced effects on habitat structure and function. For example, the preference of black-tailed prairie dogs for native grasses gives co-occurring forbs a competitive advantage, eventually leading to pronounced differences between plant community composition on and off prairie dog colonies.

Impact of Human Activities

The history and future of temperate grasslands cannot be fully considered without understanding how human activities have altered this biome over time. Because of intensive land-use change, temperate grasslands today have been mostly altered from their pristine state. One of the earliest impacts humans had on grassland ecosystems occurred in the veld of southern Africa, where people used fire as a tool in hunting game. Fires were set to corner large herbivores, thereby facilitating their capture, or to create flushes of nutritious new plant growth that would attract herds of grazers. However, because periodic fires are a natural occurrence in most temperate grasslands, the use of fire by early peoples may not have had a large negative impact (although human-caused fires likely differed in frequency and intensity from natural fires). Indeed, in wetter regions, periodic fire, along with sustained grazing, may actually be required to prevent the encroachment of woody vegetation.

A more challenging issue in terms of land-use change, at least in regard to the maintenance of native biodiversity and ecosystem health, is the conversion of millions of acres of native grassland to cropping systems or for use as intensive livestock operations. The spread and intensification of agriculture, in large part due to its mechanization over the last 200 years, is associated with marked increases in human populations as well as a more sedentary lifestyle. Because agriculture-based societies have access to a stable food supply, they have largely circumvented the need to migrate in search of the plants and animals they historically used for food or clothing. Unfortunately, the modern agricultural practices that became widespread in the early 20th century tend to be largely unsustainable, having led to the degradation of grassland habitats worldwide. For example, soil erosion due to poor farming practices and overgrazing of livestock has threatened the integrity of temperate grasslands in North America, Eurasia, and South America.

A classic example of the hardship associated with unsustainable farming practices is provided by the 1930s Dust Bowl era of the American west. After the Civil War, the government provided incentive for large numbers of settlers from Europe and the eastern United States to homestead the Great Plains. During this time, when precipitation levels happened to be much higher than usual, agriculture became widespread and intense, eventually replacing millions of acres of native grasses with cultivated crops. Many circumstances, including overplowing and severe drought, eventually led to extreme soil erosion across the American and Canadian Great Plains. The unprecedented loss of topsoil rendered the land useless for farming, forcing countless farmers to abandon their homesteads for lack of food and livelihood. The devastating ecological and social impacts that resulted are captured by pictures of denuded landscapes and massive dust clouds rolling across the plains and engulfing abandoned homesteads. Some of the dust clouds traveled far enough to darken the skies of far-away cities like Chicago and Boston.

The issue of soil erosion is not unique to the North American prairie. For example, as sedentary agriculture replaced nomadic grazing in northern China, overgrazing led to erosion and salinization (an increase in salt content) of the soil. The same issue arose in the Argentine pampas following the introduction of horses, cattle, and sheep and the advent of sedentary agriculture in areas that experienced periodic drought. Fortunately, an increased awareness of the biological properties of these systems has led to the implementation of sound policy, which has to some extent reversed the degradation associated with early intensive agriculture.

Indeed, the concept of sustainable agriculture is gaining traction today as different stakeholders acknowledge that maximizing agricultural production at the expense of ecosystem functioning and socioeconomic justice is shortsighted. However, the movement toward more sustainable practices must be balanced with the obligation to feed and clothe an ever-growing human population that has come to rely in large part on industrial-scale agriculture. Several organizations have begun to advocate a responsible transition from industrial agriculture back to smaller family-owned farms. They argue several benefits for such a transition including a reinvigoration of local farming economies; an incentive to maintain the natural resources (for example, water and healthy soil) that maintain profitable farms over the long term; a reduction in harmful pollutants due to large-scale application of chemical pesticides and fertilizers; and healthier foods derived from organic farming practices. Such organizations include the National Sustainable Agriculture Coalition in the United States and the European Initiative for Sustainable Development in Agriculture. There are also agencies that specifically advocate sustainable ranching practices, for example, by adjusting stocking rates (that is, the number of animals per unit area) to levels that are sustainable given the various ecological characteristics of the land. In fact, agricultural land is coming to be viewed as an important resource in the face of increasing urbanization worldwide. Given that much of the world's agricultural land is concentrated in the temperate grassland biome, long-term solutions for its sustainable use are needed.