Woody plants, such as trees and shrubs, are taking over grasslands, 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 in an online issue of the scientific journal Proceedings of the National Academy of Sciences.
"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.
The study found that an increase in tree and shrub cover of 1% leads to a 2% loss in livestock production, and woody-plant cover in North America increases at a rate between 0.5 and 2% per year.
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 head of cattle.
Researchers also used remote sensors to calculate the production of grasses and shrubs. 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 levels of economic development.
Surprisingly, the presence of trees explained a larger fraction of livestock production in Argentina than in the United States.
"What's happening in Argentina seems to be a much narrower utilization of rangelands," Sala said. "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."
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 OK," Sala said. "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 tree cover is affecting the cattle ranchers."
Sala and his colleagues hope their study will be used to inform discussions as policymakers 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.