The highland grasslands of the Serra Geral mountains in southern Brazil are found on ever-higher plateaus as they approach the eastern fringe, where the mountains drop off sharply into immense canyons. Grassland vegetation and shrubs predominate in this area marked by cold winters and flatlands and dotted with rocky outcrops, small patches of forest and swampy regions rich with organic matter (peat bogs). The evident monotony of the grasslands, which some refer to as a “sea of grass,” conceals a rich plant diversity, with nearly 300 species exclusive to the region, many of them little studied until recently. “The rate of endemism is 25%, much higher than what you find in the Atlantic Forest in that region,” says botanist João Iganci of the Federal University of Rio Grande do Sul (UFRGS). Although many plants there also exist in other highland regions, both tropical and temperate, endemism is what makes the southern grasslands special. “The total number of species is also high, comparable to other centers of biodiversity, considering the small size of the area.”
Iganci, an expert in the vegetation of the Subtropical Highland Grasslands, as they are known in the region, is a member of a group from UFRGS and the Federal University of Goiás (UFG) headed by geneticist Loreta Freitas, also from UFRGS, who are trying to understand the evolutionary history of the species in the region and locate areas of high priority for conservation. The researchers divided the Serra Geral into four regions (see map), in each case starting at an elevation of 900 meters above sea level, where the typical Atlantic Forest gives way to grasslands and to forests that contain araucárias, or Brazilian pines. The first step was to map the distribution of species using three genera typical of the region to serve as indicators, all having an abundance of endemic species (index of endemism): Petunia, Calibrachoa and Adesmia. The study showed that Area 1, at the top of the mountains, hosts the greatest diversity, with 13 species, followed by Area 2 to the west, with 10 species.
High indices of endemism usually occur in stable, old-growth ecosystems, since it takes a long time for new species to emerge. That appears to be what happened in the southern highland grasslands, according to climate simulations looking back to 21,000 years ago, when the planet reached its lowest temperature since the last glacial period. The findings indicate that Area 1, followed by Area 2, maintained a more stable climate, thus confirming the clue given by the biodiversity. “In the Last Glacial Maximum, the climate was colder and drier, and therefore favorable for development of grasslands, which enabled species from this environment to move into warmer, more moist areas where forests had predominated,” says Freitas. “When they migrated to regions to which they were not adapted, the grassland species diversified, giving rise to new species and lineages.” During that period, the grasslands expanded into lower elevations to the north. But with gradual warming and increasing moisture, the forests again expanded to occupy grassland regions that, in turn, became restricted to the upland regions where they are today.
The araucária forests–which share the same environment, forming mosaics with the grasslands—also played an important role. “Over the ages, there was constant competition between grasslands and forests, with the environments alternating according to climate conditions,” says Iganci. This dynamic, which still exists today, may be responsible for the separation of certain populations that eventually formed new species. “That seems to have been the case for certain petunias pollinated by bees,” Freitas says. “The bees were unable to pass through the araucária forests, thus blocking gene flow between populations.”
The researchers also observed that biodiversity is lower to the West and North, as elevation and moisture from the ocean drop off. “The findings for biodiversity refer only to the groups studied, but the species are highly representative of the region,” Freitas says. “We also observed a strong correlation between biodiversity and climate/altitude.” In addition to identifying priority areas and helping scientists understand the origin of the regional biodiversity, the study helped reveal a richness previously unknown. “Until a short time earlier, the Southern Highland Grasslands were being completely neglected in studies that take ecological, evolutionary and conservationist considerations into account,” Iganci says. The researcher, who has made several collection trips over the past 10 years, cautions about degradation of the ecosystem and identifies its principal threat as encroachment by silviculture, which involves pine and eucalyptus plantations.
Perception of grasslands
The study argues against an emphasis on forests that limits grassland conservation efforts throughout the world. A group of grassland ecosystem specialists from Brazil, the United States, France, Belgium and South Africa is trying to change that perception within and outside the scientific community by emphasizing the high degree of grassland biodiversity. They argue that the grasslands should be regarded as old-growth ecosystems whose evolutionary history, spanning millions of years, is intimately linked to fire and the presence of herbivorous animals. Many plants feature adaptations such as belowground stems, and can sprout quickly after a fire using belowground organs such as tubercles, rhizomes and bulbs, which store water and starch in a protected location.
“The diversity of plants and other groups of grassland and savannah environments in Brazil can be considered equivalent to that of forests,” says ecologist Gerhard Overbeck, an expert in grassland vegetation, also at UFRGS. “We also have to take into account the area occupied by these ecosystems. The Pampa, for example, occupies a little over 2% of Brazil’s land area, but it contains more than 2,150 plant species in grassland environments alone,” he notes. According to Overbeck, in some grassland regions of southern Brazil one can find over 50 plant species per square meter, including a large number of grass species. Many plants in grassland environments have a long life cycle, such as several in the genus Vellozia, occurring in the rupestrian grasslands of Central Brazil, that take 100 years to reach reproductive age and can live up to 500 years. The problem is that signs of old growth in grasslands are harder to spot than tree circumferences or accumulations of organic matter in forests.
Value to humans
The grasslands also provide valuable ecological services. “These ecosystems are fundamental to regulation of the water cycle. The vegetation not only retains much less rain water than forest canopies, its slender, abundant roots act like a sponge that releases the water gradually to rivers and aquifers,” says Giselda Durigan of the Forestry Institute of São Paulo State in Assis, a forestry engineer specializing in the Cerrado savannah biome. In addition, the soil holds tubercles, bulbs and rhizomes—plant adaptations that help retain water during the dry season and enable the plants to resist fire and consumption by herbivores, which are common challenges in these environments. “These structures contribute to carbon sequestration, although it is not yet quantified,” Durigan says. “The soils are complex and take a long time to form. If they are degraded, recovery is extremely difficult.”
Lack of knowledge about grassland ecology has led to flawed conservation policies, such as incentives for silviculture, with disastrous results for biodiversity and ecological services. “Trees create shade, which prevents the growth of sun-seeking herbaceous plants and reduces biodiversity,” Durigan says. “In addition, they cause 20% to 30% of rain water to evaporate before it reaches the ground.” Another example of protection gone awry is the banning of sapecadas—fires set by highland cattle ranchers for pasture management, which were prohibited in 1992 by the State Forest Code of Rio Grande do Sul. Durigan says that fires and cattle, unless they are excessive, prevent tree densification and help maintain the stability of the structure and diversity of grassland vegetation. In addition, the variety of natural grasses in these environments can make the meat healthier than that of confined animals.
The international group of experts, including Durigan and Overbeck, published a 2015 paper in which they propose the concept of “old-growth grasslands,” a term generally applied to mature forests. The authors call attention to specific characteristics of savannah grassland ecosystems that require different conservation strategies. By expanding our understanding of these environments, they also hope to promote the inclusion of grasslands on the agenda of the environmental movement, and to cast a new spotlight on these ecosystems that will help scientists discover the riches hidden in the “sea of grass.”
VELDMAN, J. W. et al. Toward an old-growth concept for grasslands, savannas, and woodlands. Frontiers in Ecology and Environment. V. 13, No. 3, p. 154-62. April 2015.
BARROS, M. J. F. et al. Environmental drivers of diversity in Subtropical Highland Grasslands. Perspectives in Plant Ecology, Evolution and Systematics. V. 17, No. 5, p. 360-8. October 2015.