Scientists seek causes of woody encroachment on Texas savanna

By on August 17, 2012


South Texas is a brutal landscape. For weeks on end each year, it’s possible to have high temperatures hovering above 100 degrees Fahrenheit, and lows that don’t dip under 90. The state, which has both large cities and vast rural areas, is extremely water-stressed.

“It’s really torturous,” says Darrin Moore, a research associate with the Department of Ecosystem Science and Management at Texas A&M University.

Moore should know – he and his team have been studying the local weather patterns for years, the latest in a long line of science that has come out of the area. Their most recent project is a remote weather station at La Copita Research Area.

La Copita was donated to Texas A&M in 1981 to “address the multi-faceted problems of grazing land and natural resources management in South Texas,” according to the university’s website dedicated to the area. Meaning “little cup” in Spanish, the 2,726-acre tract of land is located about 15 miles south and west of Alice, Texas.

Local accounts hold that the area was primarily used for cattle grazing, and a small cattle and goat herd are still kept for research purposes. In conjunction with another experimental range in College Station, La Copita functions as a place to monitor environmental changes and test out theories of land management.

The landscape of La Copita is considered a subtropical savanna – an ecosystem characterized by grass, with some trees openly spaced so the canopy is not closed. Savanna environments generally experience rainfall variation from year to year (usually concentrated in one season) and wildfires in the dry season. The amount of wood shrubs or trees can vary in a savanna, but lately, woody encroachment, as the shift away from a preponderance of grassland is known, has become a concern. Not just for La Copita, but for many savannas.

“The topic of woody encroachment into grassy areas is one of concern, “ Moore says.

The precise causes of woody encroachment in a savanna are unknown. The ecosystem is incredibly fragile – probably as fragile as a desert, says Moore, since it’s such a narrow band across the state.

Scientists are studying the problem from all angles – grazing, climate, season, precipitations, chemicals such as herbicides, fire, etc.

What is known is that clusters of woody plants, namely Mesquite, are becoming more frequent, edging out the grasses and altering the landscape to be more hospitable to their arboreal brethren.

“Is it the chicken or the egg – did the shrubs move in because the grass could no longer compete?…[We’re] trying to find what is the key source for woody encroachment into the grasslands,” says Moore.

Take precipitation, or the lack thereof, as a place to start. The proliferation of mesquite clusters could be caused by more frequent droughts. The lack of water may be giving the tree a competitive advantage over the grass. Or the ecosystem is changing in response to the water stress and the resulting mechanism favors the growth of more trees. The trees could also be a passive beneficiary of the situation, perhaps it’s the grasses that are being pummeled and can no longer outcompete the trees.

The effects of controlled burns and forest fires are similarly hard to pin down. According to the Encyclopedia of Earth, low-intensity fires keep the growth of woody species in check. But as cattle grazing has become widespread, the herds have reduced the fuel load (grasses), making fires less frequent, and providing an opening for woody encroachment. Wildlife grazing, in contrast, tends to keep such shrubbery in check.

The phenomenon of woody encroachment, which seems to be a new development, may actually reflect natural cyclical changes in vegetation. Early descriptions of the area do not mention trees, though the exact makeup of the environment is unclear.
Absent a good historical record, it is difficult to comprehend the extent and novelty of the changes currently being observed.

A salient question in the middle of all this is: Do these changes matter? Is woody encroachment on savanna grasslands actually a problem? As with most issues, says Moore, it depends on who you ask.

Ranchers need grass to graze cattle, so tree encroachment represents a negative for them. But for browsing deer and birders, the changes are welcome.

“From a scientific point of view, from a botanist point of view, we’re just interested in the change, and why that’s occurred,” Moore says.

Land management is a tricky issue in Texas, which, unlike other states, doesn’t have vast tracts of land controlled by the Bureau of Land Management (BLM). Ranchers have full control over their spreads, which may cover hundreds of square miles. Such decentralized control has not been a problem, says Moore, until a prolonged drought started 3-4 years ago.

The expansiveness of the ranches and the unpredictability of climate change can exacerbate unintentional mismanagement by individual owners. The cumulative effects of those decisions — digging new wells, cutting down trees — can add up.

Juniper, for instance, is often removed from the banks of rivers in order to raise aquifer levels and leave more water for plants and animals. While a rancher may remove those trees, and then see a water level increase in the aquifer, Moore says there’s no proof that cutting down the tree actually helped. Often, a similar native species will move into the niche left by the juniper, keeping an environmental status quo.

“You take out a Tamarisk and you get a Cottonwood,” says Moore.

Getting buy-in for altered land-management practices is an ongoing campaign. In some areas, demonstration fields are used to test new procedures and prove their effectiveness as a way to spread knowledge through the local population.

Large-scale environmental shifts may also have an effect on climate change – which, like everything else, is unclear. Different types of plants alter the microclimate of the soil in which they live. This process has implications for the amount of nitrogen in the soil (Mesquite is a nitrogen-fixer — taking the element from the air and storing it in its roots). Dry environments, Moore says, tend to be low in nitrogen.

Another key question is whether the vegetation shift will have an effect on underground carbon sequestration, and if so, what will it be. Scientists are still trying to determine the full amount of carbon stored underground in soil all over the world. This unknown variable plays a major role in global warming models because carbon dioxide acts as a multiplier for the effects of warming, speeding up the cycle. Understanding the soil microclimate and how it is changing is a key goal of Moore and his team’s research.

While the team begins to assess the incoming data, ongoing experimentation at the range in College Station may provide some insight. Researchers set up different plots of trees, each planted in a particular matrix. They then used heaters to raise the temperature of the mini-ecosystems, and also manipulated the rain cycle. The goal was to observe how the trees grew, and how different species grew together. The initial findings, Moore explains, showed that monoculture did not do as well under heat and water stress.

Monitoring will continue, as will the search for answers, and the larger trends behind the change in the savanna landscape.

“We’re just trying to determine the connectedness of everything and where that connectedness lies,” says Moore.

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