US desert grassland collapse is linked to changes in the Pacific Ocean

Researchers have discovered a long-standing connection between temperatures in the Pacific Ocean and the health of arid grasslands in the US Southwest – but climate change seems to have broken it.

For much of the 20th century, boom and bust cycles of desert grasses in the US Southwest were linked to long-term cycles of temperature in the Pacific Ocean. That link now appears to have been broken, possibly due to climate change. This could complicate efforts to restore grasslands that support biodiversity and grazing livestock.

The Glass mountains in the northern tip of the Chihuahuan desert
Luc Novovitch/Alamy

The Chihuahuan desert extends from the US Southwest into Mexico over more than 500,000 square kilometres. It contains some of the most biodiverse grasslands on the continent, across which perennial grasses are locked in a battle with woody shrubs for water and space. The grasses regularly spread and die back in cycles of collapse and recovery.

Brandon Bestelmeyer at the US Department of Agriculture and his colleagues analysed data from 1916 to 1979 on the amount of grassland cover and makeup of species over 86 square kilometres at a research plot in New Mexico. They then modelled how well different climatological factors could explain changes in the grassland.

Surprisingly, local records of temperature, precipitation and drought couldn’t predict changes in the area covered by grassland, says Bestelmeyer. Rather, the spread of grasses during this period was strongly correlated with a 40 to 60-year cycle of sea surface temperature known as the Pacific Decadal Oscillation (PDO).

The PDO can influence the climate of distant regions, as air cooled or warmed over the north-east and tropical Pacific Ocean affects wider patterns of atmospheric circulation. Grass cover was highest during the PDO’s warm phase, which is generally associated with wetter years in the Chihuahuan desert region. During the cool phase, grasslands collapsed, notably during a severe drought that occurred in the late 1940s. The researchers found a similar pattern occurred in several other grasslands in the region, though not all.

Bestelmeyer says one explanation for why the PDO can explain changes in grassland where local records can’t is that focusing on its influence integrates many climatological factors that can’t easily be measured locally, such as cloud cover or small differences in precipitation. Another explanation is that desert plants, out of necessity, tend to be less sensitive to short-term changes in precipitation and heat, and more sensitive to an “accumulation” of changes over the long term, says Andy Kleinhesselink at the University of California, Los Angeles. “They’re stoic plants.”

However, after 1979 the apparently strong link between grasslands and the PDO appears to have broken down. Since then, grassland cover has remained persistently lower than the researchers’ model would predict based on the influence of the PDO, which entered a warm phase between 1977 and 1998. Measurements of grassland cover since 2000 have included some of the lowest points in the past century.

This change could be due to increased evaporation and stresses from warming or the success of encroaching woody plants such as mesquite trees, which are more resilient to drought than grasses, says Bestelmeyer. A megadrought has affected the south-west region since 2000.

The grasslands have also been altered by livestock grazing, agriculture and other human activity, says Heather Throop at Arizona State University. “These are systems that have changed really dramatically over the last 150 years.”

If the breakdown of the link between the grassland and the PDO is due to hotter temperatures, Bestelmeyer says grasslands could have trouble recovering from their current low extent. Conversely, if the breakdown is due to woody plants moving in, the flip from the current cool phase of the PDO to its warm phase could present an opportunity to tear out shrubs and restore as much grassland as possible under favourable conditions.

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