New research suggests that juniper, mesquite and other woody brush that have overrun grasslands on the Edwards Plateau of west-central Texas aren’t the water hogs that they were thought to be. Further, bulldozing this brush may not be wise, because it would remove plants that take in lots of carbon from the atmosphere, making them a potential ally in efforts to counter global warming.
These are the findings of Dr. Jim Heilman, a Texas AgriLife Research scientist and professor of environmental physics in the Department of Soil and Crop Sciences at Texas A&M University.
“People have this idea that trees are suction pumps, that if you have all of this landscape and big trees, much more water is used,” he said. “Not true. What drives water use is the energy supplied from the sun and the atmosphere.”
Heilman’s data show only “small increases” in the amount of water used due to brush encroachment. “It’s not because the trees are extracting large amounts of water that the grasses can’t reach, but because trees trap and absorb more sunlight than grasses,” he said.
Juniper is considered a “water thief” because of the belief that its deep roots are tapping into large amounts of water stored in the limestone bedrock.
However, Heilman’s study of a live oak-juniper woodland area found the live oak used more water than the juniper.
“Juniper roots are actually shallower than oak roots, and the internal plumbing of juniper greatly restricts the amount of water that can be taken up,” Heilman said.
“We found no evidence that deep roots were extracting significant amounts of water,” he said. “Limestone bedrock in the Edwards Plateau is not capable of storing large amounts of water in the deeper zones for roots to tap into.”
Heilman’s research project is being conducted on the Freeman Ranch near San Marcos with funding provided by the National Institute for Climatic Change Research-U.S. Department of Energy.
Data on the ranch have been collected using a network of meteorological towers, recording measurements such as carbon dioxide exchange, evaporation, and wind speed, Heilman said. Sensors are mounted on towers at three locations – one in a grassland, one in a grassland in the process of being overtaken by juniper and mesquite, and the other in a dense forest of juniper and oak. He said the forest represents the most extreme case of woody encroachment where water use should be the highest.
“The site being overtaken by juniper and mesquite is representative of sites that typically are subject to brush removal,” Heilman said.
Not disputing that brush encroachment is “a world problem,” Heilman’s research has him questioning the recommendation of brush removal in some instances. By removing woody species, it reduces the capability of capturing carbon dioxide from the atmosphere, he said.
“The juniper and other woody species are sequestering a lot of carbon,” he said. “We’ve seen up to a six-fold increase with the encroachment of juniper. So, if we remove brush, we might have a marginal savings of water, but we’re losing a heck of a lot of carbon. If we get into carbon trading markets, that carbon could be very valuable.”
Carbon sequestration is a fairly recent option that would create traded markets, attempting to reduce carbon emissions due to concerns with global warming.
Overall, Heilman said, the idea of brush removal to save water is a case of where “policy gets ahead of science.”
“It’s what we think we know that just ain’t so that gets us into trouble,” Heilman said, quoting Mark Twain. “It’s going to take more than just our study to get the definitive answer.”
Heilman is collaborating with Drs. Susan Schwinning of Texas State University; Marcy Litvak, University of New Mexico; Kevin McInnes, Texas A&M University; Georgianne Moore, Texas A&M University; and Keith Owens, Oklahoma State University.
Heilman’s work is currently in review with the Journal of Hydrology. The results were part of an invited paper presentation at the American Geophysical Union Joint Assembly in Fort Lauderdale, Flourida, in May.
I could not agree more. I am glad that people are incorporating the carbon sequestration potential into the woodland/grassland equation. I am also glad to see that more people are looking into ecosystem to water correlations. I am trying to convince the general population to consider energy as well as water and carbon. Plants act as solar collectors that gather sunlight energy and carbon dioxide to make carbon bonds. Plants store sugars/starches/wood and release oxygen and water vapor. These carbon bonds contain a lot of energy, which is why animals eat plants.
You can feel this energy every time you light a fire. If you ever get a chance to light a brush pile created by clearing brush or trees from the land, you will get a first hand appreciation of the quantity of energy contained in the wood and leaves of the plants removed from that particular patch of earth. As an ecologist, I agree that the clearing of brush/trees from certain areas will improve the health and biodiversity of the greater ecosystem, but the status-quo of bulldoze and burn should be stopped in most instances.
When you burn a brush-pile the size of a truck, the energy held within the carbon bonds is released in the form of heat. This heat is so intense that you cannot stand close to the pile without getting burned. The smoke (carbon) can be seen for miles as all this energy is carried away from the place where it was captured. Years worth of solar energy (and carbon sequestration) have just been released in a matter of hours. Like a lightning bolt hitting the earth, a tremendous amount of energy is released so quickly that it is difficult to capture any of this energy.
The first law of thermodynamics says that energy cannot be created nor destroyed (Law of Conservation of Energy), therefore the energy that you feel as heat will remain in the wood if it is not burned. When this energy is released over the next fifty years rather than the next fifty hours, it is released at a rate that organisms that depend on this “energy” can utilize. Numerous animals, plants, fungus, and bacteria can utilize this stored solar energy as the brush-pile is slowly converted into soil which stays on the land for decades to com.
The brush-piles provide habitat for animals, protect young plants from herbivores, and act as a seed source for native grasses. If large brush piles will not work with the planned future land use, you can still hold onto the stored energy by grinding the piles into mulch and spreading the pile across the landscape. Like mulch used in gardens, this mulch provides countless benefits to the land that a pile of ashes just cannot do. Mulch provides the underlying soil protection from the elements, protection from erosion, protection against drying out, increased groundwater recharge, a fresh source of carbon, and protection from compaction.
Sorry to go on for so long, but this is a passion of mine and I would like to see the Hill Country (and the rest of the planet) better utilize its natural resources for long term ecological and financial prosperity rather than the short sighted, ill-informed, put money in my pocket today attitudes that are lowering the quality of life for our future generations. If you have any additional suggestions, I would love to hear them. Thank you for your time and attention. 210-218-7915
Great to save those pretty trees, we disagree about the “global warming” aspect of it. Global warming is just another fraud that the universities are using to garner funding for their useless projects. There is not man made global warming, and carbon is only a natural element in the mixture of this planet, not a poison. Goodbye global warming fanatics forever. No thanks to you guys for ruining the economy.