Spread of Drought-Resistant Trees on Grasslands a Big Problem, Here’s Why

On every continent but Antarctica, fast-growing and drought-resistant trees are slowly taking over the grasslands. Millions of additional saplings sprouting every year may seem beneficial given how urgent the need to reduce atmospheric carbon is.

However, their spread across sensitive grasslands and shrublands is a significant issue because it is upsetting ecosystems and threatening human livelihoods. Wildlife declines, water supplies decrease, and soil health deteriorates as these areas turn into woodland. The likelihood of a catastrophic wildfire is also increasing.

Economic Problem

Researchers have demonstrated how the expansion of woodlands has a negative economic impact in a recent study. American ranchers frequently raise their livestock on rangelands devoid of trees. Due to the development of new trees, Western US landowners missed out on nearly five billion dollars worth of forage (or the plants that sheep or cattle consume between 1990 and 2019. Forage lost during those three decades amounted to 332 million tons or 22 times as many hay bales as there are in the world.

Rheinhardt Scholtz, an affiliate researcher from the University of Nebraska-Lincoln, said that The least protected and most endangered terrestrial ecosystem is grasslands. Scholtz also studies the biology of climatic change.

According to Scholtz, only less than 10% of grasslands are still unaltered, as the majority have been bulldozed for construction or plowed under for crops. Woody encroachment is one of the gravest dangers to the remaining grasslands. Scholtz is not involved in the study.

Wildfire Problem

Regular fires in the past kept trees from spreading onto grasslands by confining them to damp or rocky areas. However, trees grew in abundance as European settlers put out fires. In addition, they planted thousands of trees to serve as windbreaks for their homes and livestock. By stealing the lion's share of water and sunlight from native grasses and wildflowers, trees that invade grasslands outcompete them. A 2019 study published in the journal Science found that North America's grassland bird populations have decreased by more than 50% since 1970. Birds are frequently used as a leading indicator of ecosystem health.

Billion Dollar Forage Production Problem

According to University of Montana researcher Scott Morford, the amount of trees has steadily increased year after year over the past 30 years, increasing by 50% in the western half of the United States. Nearly 150,000 km2 of formerly treeless grasslands have been transformed into woodland. That implies that trees have already taken over an area the size of Iowa. Morford emphasizes that because they are close to seed sources, an additional 200,000 km2 of treeless rangelands are in immediate danger. Morford led the study on rangeland forage loss.

Morford and his team calculated the shift in herbaceous biomass concerning tree cover over time using satellite images in conjunction with meteorological data, topography, information about soils, and on-the-ground vegetation to determine the amount of lost forage production as a result of the woodland expansion.

The "yield gap" was calculated for each county in the 17 states that have shrubland or grassland habitats after researchers quantified the annual change in herbaceous plants.

The researchers then converted that lost data into dollars using pasture rental rates gathered by the US Department of Agriculture (USDA), and they discovered that the yield gap's value has steadily increased.

Forage was being lost at a rate of about $100 million per year in 2000, $200 million per year by 2010, and $300 million per year by 2019. This brings the cumulative loss for the 1990-2019 time frame to close to $5 billion, with an upward trend.

Barb Cooksley, a rancher in the Nebraskan Sand Hills, sets a personal objective to clear at least 1,000 trees annually to maintain the health of her family's prairie. The majority are tiny seedlings that have encroached on her pastures, Wired reports.

The study by Morford and his colleagues was recently published in the Journal of Applied Ecology.