NM Range Professor Contends
Brush Occupying, Not Invading

By David Bowser

ALBUQUERQUE — Range deterioration is not a matter of shrubs invading grasslands, says Sandy Dick-Preddie. It's a matter of shrubs and weeds filling in open sites where something has happened to damage the grass that once was there.

Dr. William "Sandy" Dick-Preddie, professor emeritus at New Mexico State University, says that while there have apparently been climatic changes across the New Mexico region over thousands of years, the most extreme changes in vegetation patterns during the past century can be attributed to grazing.

"The principle factor dictating the occurrence of vegetation type is available moisture," Dick-Preddie says.

In New Mexico, the major vegetation types, in order of highest available moisture requirement, are forests, woodlands, grasslands and scrub lands.

"As you go down, those various vegetation types require less moisture in order to occur," he explains.

To understand changes in vegetation over a period of years, centuries and millennia, researchers have to be able to reconstruct what vegetation was present and when.

One of the sources for determining past vegetation is fossilized pollen. Imbedded in rock, the pollen grain is well protected for centuries.

"Pollen piles up when it falls in playas, and you can do cores just like you do in bogs back east where they started doing pollenology," Dick-Preddie says. "You can date them."

A new process has been developed that can date pollen fairly accurately.

The Tandem Acceleration Mass Spectrometer, or TAMS, has been perfected, and is more accurate than old methods using carbon dating.

Another way pollen has been dated is through the use of "packet middens."

"Pack rats use the same crevices and places in rock outcrops and so on for centuries and centuries," Dick-Preddie says.

The small rodents bring items like plant parts, needles and twigs to their nests, and they urinate on them.

"The urine seals it very much like rosin does on conifer trees, where you can find insects," Dick-Preddie says. "It keeps the air out so you get a record of what the vegetation was within about a half mile around the site where their nests are."

The evidence is so well preserved that it includes some species of conifers that no longer exist in New Mexico at all, Dick-Preddie says.

"They've been able to go back 40,000 years in some instances," he says. "They commonly go back 20,000 years."

For more recent changes, there are territorial surveys.

"As the country expanded from the east, they used the British system of section and range townships," Dick-Preddie explains.

Survey crews would go down those lines with a chain, noting changes in vegetation types.

"They went mile after mile," he says, "but they recorded how many chains along the line when there was a change in vegetation. They would say, 'leave grassland, enter forest or woodland.'"

These surveys in New Mexico took place in the 1850s.

There are also accounts by military units and explorers.

Looking back, Dick-Preddie says that about 18,000 years ago, large parts of New Mexico were covered by a mixed coniferous forest of Douglas fir, white fir and ponderosa pine.

There was no grassland or desert in what is now the State of New Mexico except for one small corner of the great basin.

About 12,000 years ago, there was a warmer and drier shift. About 8000 years ago there was a reduction in winter precipitation. From 5000 to 4000 years ago there was another drier shift. From 4000 clear up to 2500 years ago, there was a cooling trend.

"After that, there was a warming trend that continued until about 800 years ago," Dick-Preddie says. "Then the work of the pack rat midden people were able to show that the vegetation was fixed about 600 years ago."

Since then there has not been any climatic shift of the magnitude that would cause a change in a life form vegetation type.

"They've had a relatively stable environment since then concerning climatic conditions," Dick-Preddie says.

A map taken from the territorial survey records 150 years ago shows vast areas of grassland.

"The desert was quite limited, as you might expect," Dick-Preddie says. "In fact, I was a little surprised that there wasn't a showing of desert in the Pecos area at that time."

Even then there was a woodland savannah.

"This would have been an open savannah with scattered juniper," Dick-Preddie explains.

Yet since then, tremendous changes have taken place, he says.

Most of those changes have taken place in about the last 70 or 80 years.

"People who have come to New Mexico in the last 40 years tend to feel that the word of all this grassland was exaggerated and overdone," Dick-Preddie says. "They can't believe there was much grassland. It just doesn't seem possible that there was much."

In 1883, W.G. Rich wrote in a magazine article that the vast plains and extensive mountain ranges of Doña Ana County were covered with a species of gramma which grows in bunches, but it was always found sufficiently abundant to furnish stock with a most nutritious forage through all seasons of the year.

"It does not flourish on damp soil," Rich wrote. "Hence, it is not found on river bottoms. It thrives best in sand and gravel and is found in perfection on the dry sandy plain and rocky hill slopes. Cattle and sheep live and thrive upon this excellent grass without other feed.

"Thousands of tons of this valuable hay can at any time be had for the cutting and baling in the county," Rich continued.

"Good gramma hay can be cut any day of the year," he wrote. "The best season for cutting, however, is in the months of September, October, November or at a time after the summer rains but before the frost."

Scarcity of water on the plains is a drawback, but one that can easily be overcome, Rich believed. The railroad has never failed to find water on the plains where they have bored or dug for it in Doña Ana County, he said in his writings.

"Intelligent stockmen assert profits on cattle and sheep raising will average 50 percent annually on the amount invested and that the average loss will not reach two percent," Rich claimed. "No kind of stock ever required to be winter fed or sheltered."

But while records indicate broad, sweeping pastures of black gramma in the 19th century, by the end of the 20th century, black gramma has all but disappeared. Today, much of the area is covered with creosote bush.

A study of 70 years of weather data at the Jornado Experimental Range and 92 years of weather data from New Mexico State University indicates no climatic change that could account for decline of the grass. Studies conclude that the weather over that period has nothing to do with changes in vegetation.

There has been speculation that man decreased the frequency of fires and therefore the lack of fires failed to keep shrubs out.

Studies by Agricultural Research Service scientists led them to conclude that there weren't any fires based on the topography of New Mexico ranges and the bunch grass growing there that would change vegetation patterns.

Indeed, if fire had been nature's way of combating invasive shrubs, Dick-Preddie says, there would never have been any black gramma in the area. Black gramma has no tolerance for fire at all, Dick-Preddie insists. Shrubs and forbs survive and come back after fire better and faster than black gramma.

Dick-Preddie says the use of prescribed burning on Chino gramma grass in the Big Bend area of Texas indicates that to use fire to keep shrubs out of grassland there would have had to have been frequent burns.

"They found that out on the tallgrass prairies back in the middle of the United States," he says. "They found the same thing. They found that if a burr oak was five years old, a prairie fire wouldn't kill it. It would survive."

The average incidence of prairie fire in the prairie peninsula from Alberta to Texas at any one spot was once in nine years.

"It wasn't fire that was keeping grass there," Dick-Preddie says.

Plowing under the gramma grass for cropland, of course, had something to do with declining populations of gramma grass, but the change really took off about 1920, he contends.

"That change you're going to have to attribute to heavy grazing," he says. "The climate didn't change. The fires weren't significant. You really don't have much left."

No one can be blamed, however, because the people who were coming into the state assumed they could use the same stocking rate as they had on the plains.

Overstocked rangeland soon left soil exposed to erosion. On the plains, the topsoil was relatively deep. In the Chihuahuan Desert of Southern New Mexico, the topsoil layer was thin.

With a lack of information, settlers coming into the region simply followed what had been acceptable agricultural practices of the day.

The result was a quick and extreme decline in the gramma grasses. In some black gramma areas, Dick-Preddie says, there was virtually no soil profile; the surface was too rocky.

As the grass community was decimated, the range opened up for shrubs.

"When you open that site up, you have a lot more space," Dick-Preddie says. "It didn't take much for the little bit that was on the surface to be gone, so those surfaces then got occupied by creosote bush and things like that."

Some of those areas are probably gone for good, Dick-Preddie says.

"I don't think anybody can bring it back," he says, "but in other areas, where it wasn't so severe, then it can be managed."

As juniper and creosote bush and other Chihuahuan Desert shrubs advanced into the grassland, Dick-Preddie says, it is apparent that the sites had opened up.

"Something changed that habitat, because had those plants been successful against grass, the grass wouldn't have been here," Dick-Preddie says. "Its climate hasn't changed, so consequently the grass wouldn't have been here to begin with."

But Dick-Preddie says he objects to the use of the word "invasion."

"Invasion implies a pushing out," Dick-Preddie says. "Actually, these are shrubs aren't invading. They are occupying open sites."

To characterize the increase of shrubs into grasslands in this case gives the wrong impression, Dick-Preddie says, and can lead to the wrong solution.

The growth of the shrubs is a symptom, not the problem. Consequently, trying to deal with only getting rid of the shrubs is dealing only with the symptom of the problem, not the problem itself, he maintains.

"If you're going to get rid of brush, then after you get rid of the brush you must then cease doing whatever it was that caused that brush to be allowed in," Dick-Preddie says. "It's a difficult problem. What you have to do is think, 'why did that site open up so those things got in there?'"