Mesquite trees displacing Southwestern grasslands

Mesquite trees displacing Southwestern grasslands
Their extremely deep-reaching roots give mesquite trees an edge over grasses. Credit: Greg Barron-Gafford

As the desert Southwest becomes hotter and drier, semi-arid grasslands are slowly being replaced by a landscape dominated by mesquite trees, such as Prosopis velutina, and other woody shrubs, a team of University of Arizona researchers has found.

In a "leaf-to-landscape" approach, the team combined physiological experiments on individual plants and across entire to quantify how well grasslands, compared to mesquite trees and , cope with heat and across seasonal precipitation periods.

"Our results show that even the smallest mesquites are better adapted for thriving under elevated temperatures and dry conditions – the projections for our future climate – suggesting that these woody plants are here to stay," said Greg Barron-Gafford, an associate research professor with the UA's Biosphere 2 who led the study, which is published in the scientific journal Global Change Biology.

Over the last century, the face of the Southwest has changed. Before heavy cattle grazing, flowing carpets of grass comprised of, for example, Muhly (Muhlenbergia porteri) or Grama grass (Bouteloua eriopoda) blanketed much of Southern Arizona's open range. Today, woody plants like mesquite trees dominate the landscape.

Native to the region, mesquites have been here for a long time, but not in today's abundance, Barron-Gafford said. "Visitors that see our experimental display at Biosphere 2 are always surprised to hear that the Sonoran Desert in this area used to look very different a century ago."

Scientists have evidence to believe woody plants began displacing grasslands as a result of overgrazing, but has since been propelled by changing climate.

"If there are too many cattle, they have the same effect as a lawn mower," Barron-Gafford said. "They're tilling the soil, and because they don't eat the prickly things, they stay away from the established mesquite trees. But they consume their pods and drop them off in little fertilizer islands. It's a perfect formula for landscape change."

Mesquite trees, the research team discovered, benefit not only from a changing landscape, but also from a climate shifting toward higher temperatures and greater variability in rainfall.

This was surprising, given that, evolutionarily speaking, grasses are better adapted to hot and dry conditions because they use a modified biochemical pathway for photosynthesis, the process by which plants take in carbon dioxide from the atmosphere and convert it into sugars.

"Plants have to open pores in their leaves to breathe in carbon dioxide, and while their pores are open, water diffuses out," Barron-Gafford explained. "Compared to grasses, mesquite trees have to keep their pores open longer for the same amount of sugars they make, meaning they lose more water in the process."

Mesquite trees displacing Southwestern grasslands
This is the view from a flux tower across a mesquite forest. Credit: Greg Barron-Gafford

But mesquites overcome their physiological disadvantage with roots that reach down to 160 feet or more; they can tap into groundwater not accessible to plants with shallow root systems.

"Mesquites waste more water, but they can access it much better," Barron-Gafford said. "Their roots are always out there and they find it, allowing them to bypass the grasses' evolutionary advantage. These deep-rooting shrubs and trees are accessing deeper water that was previously unavailable to drive plant biology in this area."

"It levels the playing field," he said. "In the pre-monsoon season in April and May, when the land is very dry and grasses are browning, the mesquites are leafing out. You could say they have their toes dangling in the groundwater pool. In an athletic analogy, it's like some sprinters are already running, while the competition is still getting their shoes on. "

"All of the benefits we associate with these shrubs, such as potentially greater carbon capture from our atmosphere, increased shade, attraction of wild life, and so forth come at the cost of greater water use," he said.

The research aims to improve the models used by climatologists to predict future climate. Current models, said Barron-Gafford, have mostly relied on plant physiology alone. "In an age that will be defined by our management of water resources, it is important to understand the role that this woody plant encroachment has on our regional ecohydrology."

"The known differences in photosynthetic biochemistry have led the scientific community to expect that grasses should be better adapted to a warming climate because they have evolved to be more efficient with less water in higher temperatures and drier conditions."

The new findings reveal that a plant's structure – such as deep roots – plays a more important role in how vegetation adapts to a changing environment than leaf physiology.

His research team used a "portable biosphere" to test how plants perform under different environmental conditions and temperatures. The shoebox-sized device can accommodate a leaf or a branch of a living plant and allows the researchers to seal it off from the environment.

Mesquite trees displacing Southwestern grasslands
This modified photo illustrates how flux towers monitor eddies, or pockets of air, with instruments that measure the levels of carbon dioxide and water. "This allows us to estimate how much carbon the ecosystems are pulling in from the atmosphere versus how much water and carbon they are losing," said Greg Barron-Gafford. Credit: Greg Barron-Gafford

The team made its measurements shortly before, during and after the monsoon in the San Pedro Riparian National Conservation Area in Southeastern Arizona.

"With our portable biosphere we can obtain snapshots of plant activity," Barron-Gafford said. "The device allows us to set and control parameters like , light and relative humidity while we measure the carbon dioxide going in and out of the leaf."

He explained that the dry periods before and after the monsoon are different from each other because once the monsoon rains cease, plant metabolism is still running on high.

"Everything is cranking, except the remaining rain water is now deep underground, and we are measuring at a time when the shallow-rooted plants no longer have that pool to dangle their toes in. That is when the stress comes back for them."

Mesquite trees, on the other hand, were found to be capable of running their photosynthetic metabolism at half their maximum rate regardless of whether it was hot and dry.

"Grasses can only function in a very narrow temperature range," Barron-Gafford said. "While the they are drying out, it's like a hot day by the pool for the mesquite."

"A heavy monsoon downpour saturates the soil, but the surface dries off fast because it's a hot and dry environment," Barron-Gafford said. "But deeper down, the water remains in the soil longer, where the mesquite can access it, but not the grasses. That is a really important implication because our predicted precipitation patterns over the next century are going to favor deeper rooted mesquite shrubs over shallow-rooted grasses."

Because they rely much less on rainfall, mesquites should benefit from the forecasted increasing precipitation variability, offering a possible explanation for the observed encroachment of mesquite plants even in areas no longer subjected to heavy grazing.

To monitor the interactions among plants, water, the soil and the atmosphere on an ecosystem-wide scale, researchers with Biosphere 2 and The U.S. Department of Agriculture operate so-called eddy covariance towers in areas such as the San Pedro River Riparian Area, the Santa Rita Mountains and the Catalina Mountains, which can measure temperature, moisture and gases at defined heights above ground.

"With the towers, we can measure carbon, water and energy flow across an entire ecosystem," Barron-Gafford said.

In addition, his team has transplanted grasses and mesquite saplings into large, highly instrumented drums within Biosphere 2, where the researchers can put the plants through intentional periods of drought and temperature stress. Visitors to Biosphere 2 can see this science in action, right along the tour route, and learn more from members of the research team as they are doing the experiments.

"In the outside world, we actively monitor these leaf and ecosystem level responses to environmental stress, but to truly test the spectrum of responses, sometimes you need more control than nature affords you," Barron-Gafford said.

He said that encroachment of woody plants onto former exposes the area's semi-arid landscape to a higher risk of irreversible desertification.

"If a fire runs through and burns the thick grass cover that stabilizes the soil, you end up with areas consisting mostly of dirt with mesquite and creosote bushes. The big dust storms in certain parts of Arizona are visual proof how vulnerable that type of landscape is to erosion."

Filling gaps in knowledge about what climate predictions mean for and animals in the region and making them accessible to land management officials is an important goal of Barron-Gafford's research. "Our great historical knowledge is not enough given future climate change scenarios and we need biological data in addition to climate data to make accurate predictions."

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Mar 18, 2012
Research in the 1990s showed that the increase of mesquite trees in southwestern grasslands (Miller, Cevallos, etc.) was caused by the eradication of prarie dogs by cattle ranchers. Prarie dogs eat the roots of the mesquite and other shrubs keeping the roots from reaching the depths necessary to reach the water they need to thrive. Yet today, global warming is where the research dollars are, so to increase chance of being funded and published, it is necessary to find a global warming connection. Today the tail wags the dog.

Mar 18, 2012
Mesquite is a great plant. It produces edible, high protein seeds and its wood is great for smoking brisket.
"Mesquite beans provided a staple food for natives of the Sonoran and Chihuahuan deserts for centuries. This native bean was harvested green or dry"
I notice that now there are many who now pick prickly pear fruit as its juice is now a popular healthy drink.
This is truly sustainable agriculture for the SW.

"Native Plant Food Guides: The Desert Can Feed You"

Mar 18, 2012
If cattle grazing were stopped, would the grass be able to compete better? If so, how does cattle grazing differ from (say) bison grazing? Bison love prairie dog towns-better grass there. Are mesquite found in prairie dog towns? This is AZ, Sonora desert. How many prairie dog towns are left in this region? Any historical records on that?

Mar 18, 2012
"As the desert Southwest becomes hotter and drier"

Go here.


Pick Precipitation & Annual.

1) The Southwest has been a lot drier in the 1950s. Any trend is miniscule.

Pick Temperature.

2) 1934 was a lot hotter, and 20 years were warmer than 2010, 23 years were warmer than 2009, and 33 years were warmer than 2011.

The trend from 2000 is -1.4F per decade.

Mar 18, 2012
Hi Everyone ~ I'm glad to see such informed commenters on this forum. I was the lead scientist on the study, and I wanted to respond to some of your points.

johnwbales- You're right, there have been many thoughts as to why we have seen a rise in the abundance of woody plants across semiarid landscapes. Scientists have found that the presence and/or absence of certain animal species can be important; This work was conducted in an area that has had limited grazing / rodent influence, so we wanted to see how might different attributes of the plant's physiology would be important b/c clearly there is more to the story than cattle or prairie dogs. Science is always evolving our thinking and understanding.

Mar 18, 2012
To ryggesogn2 - Yes~ mesquite are awesome plants; I love working with them. Being from TX originally, I am very familiar with their sue in smoking brisket. We know that the mesquites use a lot more water than the native grasses, though. Our job is to put numbers to the debate. Land managers and city planners out in the rural areas have to think about how to balance this stately tree that absorbs lots of carbon dioxide out of the atmosphere, the shade they provide, the nutrition to wildlife, and the fact that they actually feed nitrogen into the soil (all the great things) with the fact that they could drop the depth of the water table.
Roland ~ Great question. The work from this study and previous efforts from other researchers suggests that No, the grasses do not compete better in the absence of grazing. You can't see the graphs in the paper in this news article, but we showed that the mesquites were conducting much more photosynthesis than grasses, especially in dry times.

Mar 18, 2012
Response to NotParker ~ Thanks for the link to this NOAA website. You are right that there have been warmer times. When I was referring to the Southwest becoming warmer and drier, I was referring to the summer growing season. I used the link you provided and did a plot for all the years on record for July and saw was I anticipated. Lots of annual variability, but a rising trend, with the last ~15 years being above average (save 2009 which was right on avg). More than anything are the climate model projections for the coming century. Given that the modelers are doing a better job of narrowing their estimates, us biologists are trying to figure out which plants have a better ability to handle those conditions - this work suggested that the mesquites will do better, so we should expect them to stick around, regardless of grazing patterns.
I sincerely hope this answers some of your thoughts/concerns; I appreciate the conversation.

Mar 18, 2012
Thanks for your interesting research and followup, AZguy.

Mar 18, 2012
AZguy, I think July precipitation is very normal. And I think if you put faith in the climate modelers, you are in trouble. Many of the southwest temeprature stations are in cities like Phoenix which displays grotesque amounts of UHI.

On the other hand, humans (in desert regions) tend to provide more humidity for plants -- all that warm moist air coming out of A/C units.

Mar 18, 2012
NotParker, I agree that PHX is a semiarid anomaly (thankfully!) and I was afraid you were referring to all of that warm air from the talking heads. I was mainly looking at the July Temperatures. You're absolutely right - the trend and predictions for precipitation are a lot more unsure. It looks like we may get nearly the same amount of rain, but it may come in big dumps, with long inter-storm periods, rather than more daily monsoon-type rains. Those are the sorts of differences that might not be captured in that NOAA site, but are important since the grasses love those shallow-surface wetting rains, but the mesquites have the access to deeper waters. I'm not sure where you are located, but in Tucson this year, we were slated to be somewhere around the 5th driest monsoon on record, and then we got some huge October rains, and we were towards the highest rainfall seasons. The grand sum doesn't tell the whole story, but you've highlighted how we need to show that sort of information

Mar 19, 2012
Drought is cyclical. It has been much worse, and it has been better.

Take a look at the 30s and 50s.


Mar 19, 2012
"Standard compression-cycle air conditioners (both window units and whole-house models) remove moisture through condensation, as described above. Their effectiveness at moisture removal varies, however. Moisture-removal is reported several ways. On product literature, pints per hour or liters per hour is usually listed. A modest-sized, 12,000 Btu air conditioner, for example, such as the LG model in the Building Green office that's pictured here, typically removes between 3 and 4 pints of water per hour. "


Humans sweat. Showers and dishwashers and other human endeavors add moisture to a home. A/C units dump that moisture outside.

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