We are conducting studies to determine the most sustainable method to treat woody plants and reduce the rate of woody plant encroachment. These involve primarily:
- Use of prescribed fire
- Mechanical chaining treatment followed by fire
- Use of aerially applied herbicides
- Mesquite thicket-to-savanna conversion
The following sections discuss each of these main topics:
Use of Prescribed Fire
Fire is a fundamental process in southern prairie rangeland ecosystems. We are attempting to better understand the utility of fire as a management tool on rangelands that have been encroached by woody plants. Studies include:
- Determining effects of weather conditions on fire behavior
- Effects of repeated and seasonal fires on problem plants such as mesquite, juniper and prickly pear cactus
- Short and long term effects of seasonal fires on grasses and soil carbon.
- Development of management criteria for using rotational grazing systems to facilitate the use of prescribed fire to suppress brush
Mechanical Chaining Treatment Followed by Fire
Combining treatments by using fire as a follow-up to more expensive mechanical or herbicide treatments can extend the life of the initial treatment. The image below shows how grass production can be maintained at a more consistent long-term optimum if fire is used periodically after a mechanical chaining of juniper.
Ideally, if fire is the treatment of choice, there needs to be adequate grass fuel to carry it between brush plants. In addition, in some settings, brush is simply too thick to grow an adequate amount of grass as fine fuel for fire, regardless of length of grazing deferral. In these situations, it is preferable to chain brush first then follow this treatment with a fire treatment a few years later. There has been little actual field research done on combining brush treatments because of the time scales involved. We have done one study on following chaining of redberry juniper with fire (see Ansley et al. 2006). The chart below shows how it took several years before grass production began to increase in this juniper-dominated and badly eroded site.
We are currently involved in another study where fire was applied after chaining mesquite in a pasture heavily infested with pricklypear cactus. This study is on the R.A. Brown ranch in Throckmorton.
Use of Aerially Applied Herbicides
Our herbicide work has focused on use of aerial-applied herbicides for managing dense stands of mature, multi-stemmed mesquite. These treatments have been used either to maximize mesquite whole-plant mortality (root-kill) or to create more of a savanna physiognomy using treatments that cause partial canopy mortality (see next section below).
A recently completed study quantified the long-term effect of mesquite top-killing and root-killing herbicides (see Ansley et al. 2004). We found that the root-killing herbicide had a treatment life of over 20 years if herbaceous production was the criteria used to evaluate treatment longevity.
Use the link HERE to see how mesquite appeared at 10 and 20 years after a root-killing and a top-killing herbicide treatment. We used a life-sized artificial cow made of plywood and a 3 meter tall pole to provide scale. The cow is the same distance away from the camera in each image. The root-kill treatment is not void of mesquite because recruitment of new plants is continuous. However, these plots are much more open even at 20 years post treatment, than are the 10 year old top-kill plots.
Mesquite Thicket-to-Savanna Conversion
This section includes research we have conducted that has attempted to convert mesquite thickets to savanna using treatments that can cover a large amount of land area at a relatively low cost. Hand manipulation of thickets to savannas is a very expensive process and is usually only done on very limited land areas – often as a decorative activity in the entrance of a ranch or home property.
Our goal is to develop treatments that can essentially accomplish the same thing on large land areas. These treatments are designed to provide some woody cover for wildlife and plant diversity but reduce the competitive effect of brush in order to increase understory grass growth.
We have found that this can be accomplished in mesquite thickets through the use of low intensity fires and/or a light rate of clopyralid (“Reclaim”) herbicide. The chart below shows how low-intensity fires and low rates of clopyralid (0.25 to 0.30 lbs/ac) alone or followed by low intensity fires can be used to manipulate the growth form of mesquite. See Ansley 2008 for more details).
Low-intensity fires and clopyralid herbicide affect the growth form of mesquite by reducing foliage in lower portions of the mesquite canopy. Apical dominance is maintained, basal sprouting is suppressed. The net result is that mesquite has less foliage per tree and is less competitive with grasses, yet it does not produce a massive number of resprouts that will eventually become multi-stemmed thickets which is the result of top-killing treatments like high intensity fires or chaining. The remaining foliage also provides some cover for wildlife and the reduced canopy foliage stimulates grass growth.
Low rates of clopyralid (“Reclaim”) herbicide (0.25 to 0.38 lbs/ac) can produce a similar effect, although the reduced foliage tends to be scattered more throughout the canopy because the chemical is sprayed from above. We call this vertical brush sculpting. (NOTE – This process does not work when Reclaim is mixed with Remedy (triclopyr), because Remedy top-kills the trees that are not root-killed and stimulates basal sprouting – thus apical dominance of surviving plants is lost).