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A 104 year old dataset reveals that a Pacific Ocean cycle determines grass cover at New Mexico’s Jornada Basin LTER—and then shows that link is now broken.

Credit: Jornada ARS, used with permission.

A quadrat at the Jornada Basin in 1935 (left) and again in 2016 (right), showing the stark difference in grass cover and ensuing shrub encroachment over the last half century.

In the 1950’s, grass cover at the Jornada Basin saw dramatic declines, losing 90% of coverage over 5 years. Since then, grass cover has remained persistently low in the desert landscape. The ecosystem consequences of this shift are immense: grass declines in the Southwest ultimately lead to lower biodiversity, reduced carbon storage, and diminished cattle feed. 

A new analysis of a 104-year old dataset links changes in grassland cover at the Jornada Basin LTER, including the mid century decline, to the Pacific Decadal Oscillation (PDO), a 30-40 year long cycle between warm and cool ocean conditions in the Pacific. The research, published in the journal Ecology, shows that warm conditions in the Pacific are a boon to grass cover, whereas cool conditions cause grass cover to decline.

In recent years, the link between the Pacific and the Southwest has broken, it appears. Despite the shift back to a warm PDO cycle in the 1970s, grasses failed to recover to the extent expected. The difference, thinks Dr. Erica Christensen, lead author of the paper, is that human impacts and climate change have overpowered the influence of the PDO in recent years. “There are these feedbacks that have kicked in that make it much more difficult for these grasses to come back,” she says. 

Isn’t the Pacific miles away?

Of course, the Pacific Ocean has no direct effect on a Southwest grassland. But cycles in our largest ocean have a dramatic influence on the atmosphere, effects that propagate across the globe. Climate scientists refer to these distant atmospheric influences as “teleconnections” —and these influences can magnify, mitigate, and otherwise alter local climate conditions at a faraway site. 

When the PDO flipped from a warm to cool phase in the late 1940’s, for example, the Pacific was simultaneously experiencing La Niña conditions. The co-occurrence of both cycles manifested in a multi-year severe drought at the Jornada, which prompted severe grassland decline. 

This paper shows that models of grass cover predicted from the PDO are more accurate than models built using shorter-term climate cycles and other local factors. Having a long-term dataset was key to making this link. “With the PDO, the periodicity is so long, unless you have 60 years of data, you’re not going to see a full cycle,” says Dr. Christensen. The Jornada LTER site was established in 1982, but these data were initially collected when the Jornada was established as an Agricultural Research Service site in 1912.

Credit: Jornada ARS, used with permission.

Research methods for some extremely long-term datasets at the Jornada remain consistent to ensure data continuity. Here, the same photo in the same location with a new generation of researchers was recreated in 2016, updating the 1935 snapshot.

Breaking the link

The most surprising result of this paper, however, isn’t that the PDO and grasses at the Jornada are linked—it’s that they’ve become uncoupled in recent years. When the PDO switched back to a warm cycle in the 1970’s, grasses didn’t recover as expected. “We really saw different species coming in,” explains Dr. Christensen, noting that the long-term severe drought had affected the Jornada’s core grass species so severely that they were beyond recovery once conditions were favorable.

While drought is the primary culprit for this shift in species composition, Dr. Christensen says human impacts likely played a role, too. Heavy grazing, a hallmark of the Jornada site in the early 20th century, causes land degradation and shrub encroachment, which both diminish the ability of core grassland species to recover. On top of this, climate change has reduced the amount of water available to plants, further hindering grassland recovery. The result is a strong shift towards transient perennial grasses, many of them still native, that are more drought tolerant than the historically prevalent species.

Old data is immensely useful, but it might need a little help

Making all 104 years of data compatible was the key to this study, says Dr. Christensen. Old datasets are like an original copy of an ancient book. Methods and data storage change, much in the same way that dialects and reading practices evolve. It takes work to translate the information in a way that’s useful to modern research.

For Dr. Christensen, the opportunities are worth the effort of digitizing all this data. “What drew me to the Jornada was how long-term their research is,” she reflects. “I thought it was a really unique opportunity to look at what’s been going on in this desert system for 100 years.” 

The early data in this dataset, started in 1916, mimics paint by numbers kits or topographic maps. These data were collected by an analog tool called a pantograph. Researchers traced plants in a plot with the tool, which translated their movements into a line drawing on a piece of paper. 

Quite a few locations across the Jornada had data that hadn’t yet been processed until Dr. Christensen took the initiative. The results of her labor are clear. “The long term patterns really stood out when I started plotting the data,” says Dr. Christensen.

Credit: Jornada ARS, used with permission.

Researchers at the Jornada take measurements using a pantograph, where a researcher with a stylus (center) traces the outline of a plant in a plot and the contraption translates the movements onto a piece of paper outside of the plot (bottom right).

A hint at the future?

Understanding the link between the PDO and grass cover at the Jornada could help maximize restoration efforts across the LTER site. Historical grassland across the Southwest has seen dramatic change, such as conversion to barren land or shrubland. “Once the grasses disappear, you get wind erosion, runoff erosion, the soil degrades quickly,” says Dr. Christensen. “That makes it really hard for anything to establish in the bare, scorched earth ground.”

Grassland restoration is of particular interest at the Jornada Experimental Range, which hosts both an LTER site and a USDA funded Long Term Agroecosystem Research (LTAR) site. The LTAR is set up to explore highly sustainable yet productive livestock management practices. Healthy grasslands underpin successful cattle production, and the site continually researches how to restore these native grasslands to their former extent to try and combat desertification. “People are doing a lot of work trying to get these native species to germinate and do well again,” says Dr. Chrisensen.

The finding that grass cover is linked to the PDO could aid recovery efforts. If grassland recovery hinges on a warm PDO stage in the Pacific, restoration efforts might be best saved for those specific periods. For now, though, the PDO is locked into a cooler cycle. 

Will grasses recover when the PDO once again swings warm? “Anything’s possible,” says Dr. Christensen. Native grasses may recover, aided by targeted restoration. Or, they may not, such as after the severe mid-century drought. Alternatively, recent research hints that a new grass community could emerge, better able to withstand human impacts. Almost certain, though, is that the Jornada LTER will be there to capture the change—and they’ll add that new data to this dataset.

By Gabriel De La Rosa