Tropical and subtropical grasslands

Grasslands can be defined as ecosystems dominated by grasses (Poaceae family) or graminoids (grasslike plants, usually monocots, for example, from the sedge family), with few or no trees. There are two principal ways that can be used to classify the vegetation of a defined area: in terms of floristic composition or in terms of vegetation physiognomy. The floristic classification is based on the presence of a set of species (that is, composition), which will be used to define this area as a floristic unit. The physiognomic classification, in contrast, takes into account parameters of the entire species set at any given point in time, such as height, estimated cover, and importance of different growth forms (that is, vegetation structure). The definition of grasslands presented above contains both floristic (large predominance of one botanical family, Poaceae) and physiognomic (comparatively low vegetation height due to dominance of herbaceous species, high grass/tree ratio, and growth form, considering the grassy habitat as predominant) elements.

However, it is widely accepted that the worldwide distribution of ecosystems, including grasslands, is mainly related to climate conditions, and this can be used for a classification of grass-dominated ecosystems. David J. Gibson, for example, uses the Köppen climate classification system to define different grassland ecosystems around the globe.

Grasslands occur in tropical wet, dry, and desert climates, as well as in subtropical, temperate, and alpine climates. These different climates encompass a large variation in factors such as soil type, mean temperature, rainfall, and moisture. Considering that different species have different adaptations and strategies to endure different environmental conditions, this climatic variation is reflected in differences of composition and structure among different grassland ecosystems.

The frequency and abundance of tree species is an important characteristic for grassland classification—especially in tropical and subtropical grasslands. This is chiefly due to the conceptual overlapping between two ecosystem types in which grasses dominate: grasslands and savannas. Following the classical definition by Heinrich Walter, savanna ecosystems are tropical formations with the presence of grasses and woody plants, but do not include open grasslands without a woody component. Pragmatically, savannas can be considered as (tropical) grassland ecosystems with more than 10 percent tree cover, usually with tussock grasses and shrubs shaping the lower layer of the vegetation. This tree cover may be relatively uniform and dense (as in some parts of the Brazilian Cerrado) or clumped, shaping a parklike landscape (as in some parts of Africa). Grasslands, on the other hand, present no or less than 10 percent of tree or shrub cover. In many regions around the world, savannas and grasslands form mosaiclike patterns in the landscape, and the limits between both formations are neither clear nor stable. In this article we will focus on grassland ecosystems. However, the grassland component of savanna ecosystems will be briefly considered.

Natural Grasslands Worldwide: A Brief Evolutionary History

Grasslands are estimated to cover 20 to 40 percent of the world's land area, depending on the definition of grassland. During past geological eras, under different climates and land mass distribution, grassland-covered areas were even larger. Although the first grasses grew under forest cover or near forest borders, current evidence indicates that expansion of grasslands and diversification of grasses took place under conditions of increased aridity. The emergence of considerable shrub or tree layers in grasslands (as we see in tropical savannas today) occurred much later, when shifts in temperature and moisture allowed the establishment of such growth forms.

Fossil records provide evidence that large grazing animals and fire were important factors in the evolutionary history of grasslands. Altogether, climatic and, in some cases, edaphic conditions, grazing, and fire are constraining forces that have shaped grassland ecosystems as we see them today. In consequence, many species in the extant grassland flora show adaptations to avoid or resist drought and disturbances such as grazing and fire.

Extant grasslands may be natural or secondary, that is, the consequence of habitat conversion by human populations. Most grassland ecosystems present in highland montane climates are natural and largely determined by climate. Temperate grasslands in central and western Europe are considered secondary, being mostly linked to historical forest cleaning and subsequent mowing, burning, and grazing. Eurasian steppes and South American and African grasslands are considered natural (or climatogenic). In places where present climatic conditions are suitable for forest establishment, open grasslands gradually gave place to savannas (for example, Africa and central Brazil), shrublands or forests (southern Brazil and Africa). Some grasslands in tropical regions are clearly secondary, for example, cultivated pastures used for cattle grazing in tropical rainforest regions, originating from destruction of natural forests. This type of grassland will not be discussed in this article.

Grasses: The Determinants of Grassland's Physiognomy

The grass family does not only give the name to the vegetation type but also dominates most grassland systems. Current estimations place the number of Poaceae species around 12,000, distributed in 700 to 800 genera in all continents and environments except Antarctica. The importance of this family to humankind is unquestionable, since all important cereal crops and sources of forage are grasses. Moreover, almost half of the Earth's surface is covered by landscapes (natural or human-driven) chiefly characterized by grasses.

Grass species are known to thrive under a wide amplitude of ecological conditions; grasses can be found from wet tropical climates to deserts. Many grasses can resist disturbance events such as complete burning or removal by grazing, or manage to resprout immediately. These abilities are consequences of evolutionary adaptations within the family, such as reduction in many vegetative and reproductive structures, growth forms adapted to disturbance (buds protected by leaf sheaths), or high importance of vegetative propagation in many grass species. Further, different photosynthetic pathways allow for good performance under different climatic conditions.

Reduction and simplification in aboveground structures directly influence effectiveness of biomass allocation: Grasses spend less energy and grow faster than trees, for example. The trade-off for this reduction seems to be clear: Grasses are more fragile than trees, because they lack features such as secondary thickening in cell walls. However, grasses compensate for this with astonishing resistance to drought, resprouting ability after disturbances or unfavorable seasons, and ability to reach faraway sites by wind dispersal.

A considerable share of the resistance and resilience of most grasses can be attributed to their growth form. Meristematic tissues of grass shoots (“tillers”) in most grasses are located near, at, or below the surface of the soil, providing protection to disturbances. Each tiller comprises photosynthetic leaves and (potentially) reproductive structures of the species. Most grass species can produce many tillers in a single growing season. This feature leads to the formation of dense tufts and multiple flowering culms in some species. Growing tillers may shape various patterns depending on the species, but there are two generally recognized growing forms: caespitose growth (erect tussocks) and rhizomatous growth (prostrate). The production of new tillers, which may be nearly unlimited in perennial species, allows the species to expand its cover in the landscape, “moving” to new sites with potentially better resources without the need of a full seed-dependent cycle of colonization and establishment.

Grasses can be divided according to their photosynthetic pathways: C₃ (when carbon dioxide breaks up into a three-carbon compound) species are cool season grasses, usually with short life cycles, and C₄ (when carbon dioxide breaks up into a four-carbon compound) species are warm season grasses, usually with long life cycles. The two pathways are named after the first stable product of photosynthesis, consisting of three carbons in C₃ grasses and four in C₄ grasses. The C₄ pathway is optimized considering carbon assimilation and enhances the plant's water use efficiency, which is advantageous in hot and dry environmental conditions. As a consequence, there are ecological differences between C₃ and C₄ grasses: The first have more efficient physiological processes under temperate conditions, whereas the latter are more efficient under tropical conditions, in which temperatures are higher throughout the year.

The Role of Disturbance in Grassland Ecosystems

Disturbance can be defined as the partial or total removal of biomass from an individual plant. On the other hand, on the level of the plant community, grazing or fire can be considered normal or, in case of climate favorable to forest development, even necessary factors for the existence of this vegetation type. The plant species present in a community subject to recurrent disturbances show a number of adaptations to these processes, such as high regeneration ability. Fire may cause local removal of individuals and even species, but prevents shrub encroachment and keeps at bay the process of forest expansion over grasslands that takes place in ecosystems under moister conditions. Grazing may negatively affect grassland diversity when management is not adequate, and overgrazing may lead to decreased soil cover, erosion, and species removal. However, well-managed cattle-raising may be one of the most ecologically sustainable economic activities in grassland ecosystems, since native grass and legume species may be maintained and used as the primary source of forage.

Differences between Tropical and Temperate Grasslands

In comparison with their tropical and subtropical counterparts, temperate grasslands suffer colder winters and milder summers, larger amplitudes in temperatures around the year, a shorter period of vegetation growth, and much more frequent frost events. Annual rainfall is usually lower in temperate grasslands than in tropical and subtropical grasslands. However, many (but not all) tropical and subtropical grasslands show great seasonality in rainfall: a pronounced dry season, when mean rainfall may be less than 2.36 inches (60 millimeters), followed by a similarly pronounced wet season. Soils in temperate grasslands are usually deep and fertile, but plant growth is nutrient-limited because much of the soil nitrogen is inaccessible to the roots.

In addition, tropical and subtropical grasslands receive much more solar radiation throughout the year, which implies consequences concerning species composition. In tropical grasslands, C₄ grasses are dominant, and the contribution of C₃ grasses is usually reduced. However, the further south or north of the equator (that is, passing first to subtropical and then to temperate grasslands), the contribution of C₃ species enhances, considering both composition (more C₃ species) and structure (higher abundances of C₃ species).

With few exceptions (for example, the Indian subcontinent), natural grassland vegetation can be found—even though not always as a dominant vegetation type—in tropical and subtropical regions around the world. In the following we will briefly present the different vegetation types, beginning with those under a tropical climate.

South America

The South American llanos (Spanish for “plains”) follow the valleys of the Orinoco River in Venezuela and Colombia. The region is dominated by savannas, but patches of typical grasslands, with almost no trees, also occur. They can be divided into different local vegetation physiognomies, classified mostly according to soil water availability. The lower layer of the vegetation is dominated by arrow-grasses (Trachypogon spp.) in some places, and three-awn grasses are also common (Aristida spp.). Legume species are abundant, enhancing forage value. Grasslands and savannas are grazed by cattle and the native Hydrochoerus hydrochaeris (capybara or chigüiro), the largest living rodent in the world. The llanos are threatened by land conversion to crops and monospecific plantations of exotic trees.

The Brazilian Cerrado is a biome that covers about 772,204 square miles (2 million square kilometers) in South America. Although the biome as a whole can be considered a tropical savanna, its landscape actually consists of mosaics of tree-free grasslands, of different savanna physiognomies (from scattered to dense tree cover), and of gallery forests. The disturbance regime (fire and grazing) and, to some extent, soil properties shape the distribution between savanna and grasslands. Typical grasslands are locally classified according to the presence of woody elements: campos sujos contain some scattered shrubs and trees, whereas campos limpos are dominated by grasses (common genera are Echinolaena, Elyonurus, Paspalum, Trachypogon, and Tristachya), without woody species. The Cerrado is considered to be one of the biodiversity hot spots of the world; the biome as a whole and the grassland areas are threatened by land conversion to establish pastures with nonnative species, uncontrollable fires in tree-dense savannas, and invasion of exotic species (Brachiaria spp.)

Further to the south, the flooding grasslands of the Pantanal cover 54,054 square miles (140,000 square kilometers) of Brazilian, Bolivian, and Paraguayan territory. Large areas of grasslands and forests that grow along river courses are yearly flooded. Natural grasslands are mostly used for cattle-raising, sometimes with native grasses as the main source of forage. Among these grasses, especially Paspalum almum and P. plicatulum present high forage value. The fauna of the Pantanal is outstandingly rich (for example, 650 birds, 1,100 butterflies, 80 mammals, and more than 250 fishes). Hunting is a major threat for this ecosystem since it has historically depleted populations of large predators such as the jaguar (Panthera onca). The invasion of the exotic tree Vochysia divergens (cambará) threatens the grasslands, and is kept at bay with fire (which is also considered a threat, mostly to animals) and selective logging.

The Campos of southern Brazil are under subtropical climates, and harbor approximately 2,500 nonwoody plant species, many of which are endemic. At higher altitudes (greater than 700 miles or 1,127 kilometers), these grasslands occur in mosaics with Atlantic and Araucaria forests, and Andropogon lateralis (caninha grass) is the dominant grass over large areas. At lower altitudes, grassland vegetation is dominant, and forests are mostly restricted to water courses. In these lower altitudes many grasses with high forage values are found, such as Paspalum notatum, P. nicorae, P. pumilum, and Axonopus spp. Different classifications for local grassland physiognomies exist, mostly related to soil and climatic factors. The southernmost areas of the Campos stretch south to temperate areas and the pampa biome (or Rio de la Plata Grasslands) of southern Brazil, Uruguay, and Argentina. Extensive cattle-raising is historically the most important economic activity in the Campos, and land conversion to crops and exotic tree plantations are the most pressing threats to this ecosystem.

Africa

Grasslands and savannas are the dominant landscapes in large areas of Africa. Atlantic forests mostly follow the equator line and the Zaire River, in central Africa, and are surrounded by woody savannas. Further north toward the Sahel and then the Sahara Desert, and east toward the Horn of Africa, trees become rarer and more scattered in the landscape, and the woody savanna gives way to typical grassland. Transitional ecosystems such as arid grasslands, shrublands, and open-canopy woodlands also occur, and vegetation physiognomy is largely determined by water availability. African grasslands and savannas are known for their large migratory herds of wildebeest (Connochaetes taurinus), zebra (Equus burchelli), and eland (Taurotragus oryx); by the Thompson's gazelle (Gazella thomsonii), buffalo (Syncerus caffer), and topi (Damaliscus korrigum); and by their famous predators, the lion (Panthera leo) and the cheetah (Acinonyx jubatus). There are also giraffes (Giraffa camelopardalis) and African elephants (Loxodonta africana), mostly in Acacia savannas and in the Serengeti volcanic grasslands. Threats to African grasslands include overgrazing, conversion to croplands, and uncontrollable wildfires. Trophy hunting is also a problem, and has greatly depleted populations of mammals such as the white and black rhinos (Ceratotherium simum and Diceros bicornis).

The velds (“fields” in Dutch) are grasslands and savannas from the plateaus of South Africa and Zimbabwe. Locally, these ecosystems are divided into high velds (above 1,400 miles, or 2,253 kilometers in altitude, cooler, and with high rainfall) and low velds (below 700 miles or 1,127 kilometers in altitude, hot and dry). The dominant grasses are Cymbopogon plurinodis, Diheteropogon filifolius, Heteropogon contortus, Themeda triandra, Brachiaria serrata, Digitaria eriantha, and Setaria flabellata. In heavily grazed areas, this composition changes with more forbs, replacement of some dominant grasses, and tree encroaching.

Asia, Eurasia, and Australia

Most tropical and subtropical grasslands in southern and southeastern Asia are secondary, being the result of abandoned crop areas that were originally rainforests. Large areas of midaltitudes (300 to 700 miles, or 483 to 1,127 kilometers) are dominated by Imperata cylindrica, whereas Arundo madagascariensis is the dominant grass in higher areas (greater than 900 miles or 1,148 kilometers). The larger continuous grass-dominated ecosystem in the region is the Sehima nervosum–Dichanthium annulatum grasslands in India. In some tropical areas trees (for example, Pinus merkusii, Eucalyptus alba, and Casuarina junghuhniana) form a rather continuous layer, characterizing savanna-like landscapes.

In Australia, climate conditions and soil fertility define different grassland ecosystems. A mosaic of savannas and grasslands are distributed in a belt around the arid center of the continent. Typical savannas, with a continuous tree layer, are usually dominated by eucalypts (Eucalyptus spp.) or wattles (Acacia spp.). In low-fertility soils, there are tall-grass grasslands dominated by Themeda triandra, Schizachyrium fragile, and Sorghum spp. Additionally, tropical and subtropical tall-grass formations with Heteropogon contortus and H. triticeus, as well as midgrass grasslands with Aristida spp., Dichanthium sericeum, Bothriochloa decipiens, B. bladhii, and Chloris spp., can be found. Fire is a natural disturbance and is also used as a management practice to prevent forest expansion. In some areas, absence of fire for long periods leads to closed eucalyptus forests.

Natural grasslands can also be found in the tropics and subtropics as “islands” of often relict vegetation scattered in forest ecosystems. These grasslands usually occur at higher altitudes, where temperatures are lower throughout the year, and are mostly maintained in the present climate by fire and grazing. Such formations are common in the Brazilian Atlantic Forest, in hilltops of mountain ranges above 800 miles altitude (1,287 kilometers), and are locally known as campos rupestres. Similar altitude grasslands occur in mountain ranges of Africa and Asia, for example, on so-called inselbergs.

Land Use and Impacts on Conservation

Most grasslands originally occupied landscapes with a rather gentle topography (except in altitude grasslands), and in many cases were associated with deep and fertile soils (although grasslands may also occur on shallow, litholic soils). Also, it is easier to remove the original vegetation and plow the land in grasslands than in forests. The combination of these factors led to the conversion of many grassland ecosystems to crop production throughout the tropics and subtropics, which has led to species and habitat loss. In the past decades, natural grasslands in the tropics and subtropics have been increasingly converted to planted forests, often monocultures of exotic species, mainly to produce paper, resins, and firewood, again leading to pronounced biodiversity losses and landscape changes.

Grasslands are historically related to pastoral activities, dating back to the Neolithic. Today, large areas of natural grasslands in the tropics and subtropics still support herds of cattle, sheep, and horses. In many cases, the raising of grazing animals relies solely (or mainly) on native forage, which usually maintains the local biodiversity (both vegetal and animal) in good conservation status.

Abandonment, or nonmanagement, may have negative effects on natural grasslands. The absence of fire and grazing, the main nonclimatic factors that have influenced the origin, evolution, and maintenance of grassland ecosystems worldwide, may promote the dominance of tall, coarse, caespitose grasses, causing diversity losses in the plant community. This process may be followed by shrub encroachment, which leads to out shading of herbaceous plants. Under tropical and subtropical climates, where moisture conditions are suitable to forest establishment, this shrub encroachment may be followed by forest expansion and complete loss of the original natural grassland ecosystem.

Future of Tropical and Subtropical Grasslands

As human population grows, so does the demand for food. In many regions of the world, grasslands are the primary targets for conversion into croplands. The conversion rates exceed legal protection in grasslands more than in any other inland ecosystem: approximately 1 hectare of natural grassland is protected for every 10 hectares lost.

A less obvious but nonetheless serious threat to grassland biodiversity is the suppression of management and disturbance. In many tropical and subtropical grasslands, current climate conditions are suitable for development or expansion of shrubland or forest vegetation, and grazing and fire impede these successional processes. If they are suppressed, the grassland community will, in the long run, be converted into a community dominated by woody species, losing its original biodiversity and ecological properties. Conservation efforts that focus on natural grasslands therefore must take management into account.

Because of human-induced climatic change, both the global temperature and atmospheric carbon dioxide concentration are expected to rise at unprecedented rates. These changes most likely will directly affect tropical and subtropical grasslands, where C₃ (temperate grasses and most forbs, shrubs, and trees) and C₄ species (tropical grasses) coexist. The effects of climate change on grassland distribution and on the distribution and abundance of C₃ and C₄ grasses has become an important research topic.

As in many other ecosystems, invasion by alien species also threatens grasslands. In African and Australian grasslands and savannas, invasive nonnative species increase the amount of flammable biomass in natural grasslands, enhancing the intensity of fire events and thus having an impact in the community at the local scale (for example, local extinctions).

Also, many grass species that are first introduced outside their native range as forage may become dominant and may lead to losses in biodiversity and productivity. In southern Brazil, the African grasses Eragrostis plana and Brachiaria spp. that present no invasive behavior in their natural environment become immensely aggressive, outcompeting and excluding virtually any native grassland species.

No vegetation type is stable, and changes are not necessarily negative, both from conservationist and agricultural perspectives. However, as natural grasslands constitute the basis for livestock production and harbor important—and often neglected—parts of the world's biodiversity, however, management decisions should consider possible changes and aim at the stabilization of the important ecosystem functions and services provided by grassland vegetation.

Bibliography

Gibson, David J. Grasses and Grassland Ecology. Oxford UP, 2009.

Suttie, J. M., S. G. Reynolds, and C. Batello, editors. Grasslands of the World, Plant Production and Protection Series No. 34. Food and Agriculture Organization, 2005, www.fao.org/docrep/008/y8344e/y8344e00.htm. Accessed 30 July 2018.

"Tropical and Subtropical Grasslands, Savannas and Shrublands." World Wildlife Fund, 2018, www.worldwildlife.org/biomes/tropical-and-subtropical-grasslands-savannas-and-shrublands. Accessed 30 July 2018.

Walter, Heinrich. Ecology of Tropical and Subtropical Vegetation. Oliver and Boyd, 1972.

Williams, Wendy J. "Resting Subtropical Grasslands from Grazing in the Wet Season Boots Biocrust Hotspots to Improve Soil Health." Agronomy, 28 Dec. 2021, www.mdpi.com/2073-4395/12/1/62. Accessed 14 July 2022.