Trees and shrubs invading critical grasslands, diminish cattle production

Aug 18, 2014
While ranchers clearly depend on grasslands to support healthy livestock, ecosystems also provide a range of other services to humans. Stakeholders such as conservationists, farmers, builders, government entities and private landowners, depend on the land for a variety of reasons and each has different values and land use needs. Credit: Osvaldo Sala

Half of the Earth's land mass is made up of rangelands, which include grasslands and savannas, yet they are being transformed at an alarming rate. Woody plants, such as trees and shrubs, are moving in and taking over, leading to a loss of critical habitat and causing a drastic change in the ability of ecosystems to produce food—specifically meat.

Researchers with Arizona State University's School of Life Sciences led an investigation that quantified this loss in both the United States and Argentina. The study's results are published in today's online issue of the scientific journal Proceedings of the National Academy of Sciences (PNAS).

"While the phenomenon of woody plant invasion has been occurring for decades, for the first time, we have quantified the losses in ecosystem services," said Osvaldo Sala, Julie A. Wrigley Chair and Foundation Professor with ASU's School of Life Sciences and School of Sustainability. "We found that an increase in tree and shrub cover of one percent leads to a two percent loss in ." And, woody-plant cover in North America increases at a rate between 0.5 and two percent per year.

In recent years, the U.S. government shelled out millions of dollars in an effort to stop the advance of trees and shrubs. The U.S. Natural Resources Conservation Service spent $127 million from 2005-2009 on herbicides and brush management, without a clear understanding of its economic benefit.

The research team used census data from the U.S. and Argentina to find out how much livestock exists within the majority of the countries' rangelands. In both countries, the team studied swaths of rangeland roughly the size of Texas—approximately 160 million acres each. These lands support roughly 40 million heads of cattle. Researchers also used remote sensors to calculate the production of grasses and shrubs. And, to account for the effects of different socioeconomic factors, researchers quantified the impact of tree cover on livestock production in two areas of the world that have similar environments, but different level of economic development.

An increase in tree and shrub cover on grasslands in the U.S. and Argentina leads to a two percent loss in livestock production. Credit: John and Karen Hollingsworth, U.S. Fish and Wildlife Service

Surprisingly, the presence of trees explained a larger fraction of livestock production in Argentina than in the US.

"What's happening in Argentina seems to be a much narrower utilization of rangelands," added Sala. "The land there is mostly privately-owned and people who have ranches are producing predominantly meat to make a profit. But in the U.S., many people who own ranches don't actually raise cattle. They are using the land for many other different purposes."

While ranchers clearly depend on grasslands to support healthy livestock, ecosystems also provide a range of other services to humans. Stakeholders such as conservationists, farmers, builders, government entities and private landowners, depend on the land for a variety of reasons and each has different values and land use needs.

Grasslands are being transformed at an alarming rate as woody plants, such as trees and shrubs, take over. This is leading to a loss of critical habitat and causing a drastic change in the ability of ecosystems to produce food. This is one example near the Santa Rita mountains in Arizona. Credit: Osvaldo Sala

Why are trees and shrubs taking over grasslands? There are several hypotheses as to why woody plant encroachment is happening. Fire reduction, grazing intensity, climate change, and increased carbon dioxide in the atmosphere are some widely held beliefs as to the cause. However, Sala's study is focused not on the cause, but rather on the cost of this change to people.

"For each piece of land, there are different people who have an interest in that land and they all have different values. And, they are all okay," said Sala. "However, in order to negotiate how to use the land and to meet the needs of these different stakeholders, we need concrete information. We now know how much increase in is affecting the cattle ranchers."

Sala and his colleagues hope that the information found in their study will be used to inform discussions as policy makers and other stakeholders negotiate changes in land use. Researchers who took part in the study include Sala and Billie Turner II with ASU, José Anadón with City University of New York, and Elena Bennett with McGill University. National Academies Keck Futures Initiative and the U.S. National Science Foundation funded the study.

Explore further: Tropical grassy ecosystems under threat

More information: Effect of woody-plant encroachment on livestock production in North and South America, PNAS, www.pnas.org/cgi/doi/10.1073/pnas.1320585111

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NOM
5 / 5 (1) Aug 18, 2014
I spent several summers when I was at university cutting scrub on marginal farmland. The main invading tree was a native (New Zealand) plant called manuka.
Nowdays manuka honey can sell for several hundred dollars a kilo, so many farmers are now leaving it to grow.
NOM
3 / 5 (2) Aug 18, 2014
Oops, I used metric, which doesn't seem to be used too much on phys.org.
It retails for about a USD per dram.
verkle
3 / 5 (2) Aug 18, 2014
Anyone heard of a chainsaw?

Seriously, a lot of the expanse in the US is due to cutting down trees 100-200 years ago. It is not that hard.
Toiea
2.5 / 5 (2) Aug 18, 2014
Aren't these shrubs a biofuel, so popular by now? Btw big mammals from Africa (elephants, rhinoceros) would wipe out these shrubs fast and we could even save their gene pool with it. The silly Chinese would buy their horns for medicine and everyone would remain happy.
alfie_null
not rated yet Aug 19, 2014
I thought all our cattle came from feedlots in Chicago.

There's diminishing amounts of 'nature' to any aspect of the beef industry. And we now see cattle being increasingly accused of contributing significantly to greenhouse gas emissions. To see them being portrayed as exemplars of 'the natural state of things' is peculiar.

I question that this perceived threat to livestock production is a particularly important issue. From everything I see, we seem to be awash in a flood of cheap meat. Probably to the detriment of our environment.
nilbud
1 / 5 (2) Aug 19, 2014
Kill off a bunch of vegetarians, they can't be allowed to meddle with our food.

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