Brush encroachment provides clues about soil health
After a long, hot day out on the range, one may begin to reflect on the things on the landscape that change and those that don't. Seeing a clump of brush that seems to move across the pasture on its own may have more to do with the air temperature, and one's current level of dehydration, than with pacing persimmons, strolling sumac or pedestrial plums. Still, there are changes occurring.
After spending many hours sampling at the same spot multiple times per year, one develops an awareness of changes in the vegetation and some theories as to what is causing them.
Fire, climate, herbivory and sometimes just bad luck explain most of the phenomena one might observe. Something that is harder to get a handle on is those things that don't change but should.
Brush encroachment is a common problem across the landscape these days, and several sites that I visit regularly are undergoing significant encroachment. However, there are areas immediately adjacent to locations that brush doesn't seem to impact. The brush gets taller and denser but doesn't spread into new territory. Why would brush grow well in one area and not so well in another?
I hadn't come up with an explanation until I started trying to push a soil probe into the ground to collect samples for a project. In areas with lots of brush, the probe goes right in; the soil has lots of organic matter, moisture and good structure. Several important forage grasses are associated with the brush. In areas not impacted by the brush, the soils often have a hardpan near the surface with little or no top soil, little moisture and organic matter, and poor structure. Vegetation on these areas is often dominated by Oldfield threeawn. So, the grasses most productive for livestock are found in the mostly inaccessible areas covered in brush, while the open, grazeable areas are covered in a species that livestock won't eat.
This is where the disagreement arises between livestock and managers as to how many grazeable acres are available. If the livestock graze less area than calculated by the land manager, animal performance and forage utilization may be unpredictable. If overgrazing takes place, brush encroachment may be encouraged.
So how should land managers treat this situation? If the topsoil is still present, indicated by the presence of the brush, then the productive grasses are probably still there. Deal with the brush, preferably with fire. If that's not an option, try herbicides or small ruminants, and lastly, mechanical methods. A combination of any or all of those methods could be useful as well.
The Oldfield threeawn presents a more difficult problem, particularly when solid stands are present. Oldfield threeawn is often indicative of poor soil fertility. When associated with a hardpan, it's not just a soil fertility issue; water infiltration is severely restricted and most roots can't penetrate deep enough to get to moisture.
These are not trivial issues to overcome. One thing a manager might consider is using the sites when feeding or storing hay since these areas have little production potential. This prevents damage to higher production areas and could add some organic matter to the site. Just about any practice that builds soil and increases organic matter will be helpful for these sites. Otherwise, there just won't be much there to ruminate on.