Flames engulf the approximate 10-foot-tall measuring poles during a March tall-grass fire on Jim Ansley’s seedling plots.
Time Prescribed Burning
to Target Seedlings
Aiming fire at seedlings may be key to rangeland mesquite control.
Fire must be effectively managed to get the best control of mesquite seedlings, a Texas A&M AgriLife Research rangeland ecologist said.
Jim Ansley, professor in the rangeland woody plant research program, recently published study results in the Rangeland Ecology and Management journal that indicate both season and intensity are important factors in the mitigation of mesquite seedlings by fire.
“We know adult mesquite trees are fire-resistant because they resprout following a fire,” Ansley says. “We have determined that range managers might have a greater success of mesquite control with fire if their efforts are aimed at the seedlings.”
In the study, mesquite seeds were planted in mid- and tall-grass plots in the spring and then burned in the winter at 10 months or 22 months or in late summer when they were 17 months old.
Ansley says summer fires were especially effective in the mid-grass fuels. The summer fires on 17-month-old seedlings yielded a higher death rate than earlier winter fires when seedlings were 10 months old.
However, he adds, the same advantage of summer over winter fires is not seen in higher fuel load areas, such as those with tall-grass types. In that case, both seasons had a high death rate of trees after fire.
The air temperature was, as expected, higher during summer fires, and this greatly increased fireline intensity on the mid-grass plots, but only slightly increased intensity on the tall-grass plots.
Grass fuel amounts were similar for both winter and summer fires in the mid-grass plots, about 1,700 pounds (lb.) per acre, and slightly higher in summer than winter in the tall-grass plots, 6,900 lb. vs. 5,950 lb. per acre.
The approximate 10-foot-tall measuring poles show the height of the flames of a winter fire on a mid-grass fuel mesquite seedling plot.
“One of the most important conclusions was that there was a very close positive relationship between fireline intensity and seedling mortality, especially in winter fires,” Ansley explains. “So, conditions that yield high fire intensity, such as higher fuel loads or summer burning, will have the greatest effect on seedling mortality.”
Summer fire in lower-grass fuel amounts may accomplish the same level of mesquite seedling mortality without the need for lengthy grazing deferral to accumulate sufficient grass fuel for a successful winter fire, he adds.
The published paper shows a common scenario for controlling mesquite seedlings with fire. Typically, a drought year followed by a wet spring accelerates mesquite seed germination, Ansley says.
If the pasture remains ungrazed or lightly grazed, this would create the highest probability for a natural fire to occur within 5-20 months after mesquite seedling emergence.
“This is probably how mesquite was kept in check naturally before Europeans settled the region,” he says. “The introduction of cattle and fire suppression disrupted this natural cycle and mesquite populations exploded.”
Ansley says his results show a prescribed fire applied either in winter around February at about 10 months of seedling age or in late summer at about 17 months of age reduced the number of surviving mesquite seedlings by 35%-80% in mid-grass and more than 75% in tall-grass stands. Mortality in winter fires in the mid-grass fuel type was never more than 35%.
“So you need either higher fuel or a summer fire to really be effective,” recommends Ansley.
Any removal of grass by livestock grazing during this period of high grass accumulation would obviously reduce the probability of higher fireline intensities during the brief window of time when seedlings would be vulnerable to a prescribed fire, Ansley says.
Certainly, he admits, more research is needed to explore this critical window of mesquite seedling vulnerability.
To see the full results of Ansley’s study, go to http://bit.ly/1H4vl7y.
Editor’s Note: Kay Ledbetter is a communications specialist for Texas AgriLife Communications.