By Jake Landers
Published May 2012
Where environmental gradients are abrupt, plant species tend to adjust to the part of the gradient where they can dominate, or, at least, survive. Water levels around a lake or along a river, geological deposits, animal activity, and many other situations, may produce striking patterns. Fence line contrasts between two pastures, or along a pasture and the highway right-of-way, often show two different groups of plants because of different growing environments. Plants species around a Harvester Ant mound in West Texas can change over a distance of inches forming narrow bands or rings.
“Zonation” is the term often used for these clusters of plants. Bare ground in the center of an ant den shows the effects of the removal of seeds of potential plants and destruction of plants that get started. But there is also a change in the soil environment that is caused by the ants which influences the survival of plants.
In photo No. 1 the prominent yellow flower, Greenthread, (Thelesperma filifolium), is also present in the surrounding area where it is not blooming. Apparently the ant activity of digging out living chambers and bringing the gravel to the surface has resulted in early blooming, probably because of an increase in soil temperature. A sandy or gravelly soil warms up more quickly in the spring than a clay soil. Also, more soil moisture should be available for the first band because of the bare ground.
Plants in the next zone include Greenthread that is not blooming, Fuzzy Plantain (Plantago helleri), Milkvetch, (Astragalus sp.), both annual plants, and a few weak plants of a perennial Threeawn, (Aristida sp.). Plants in the outer zone include Greenthread that is not blooming, Texas and California Filaree, (Erodium texanum and Erodium cicutarium), respectively, and others. If the ants die out or move on, the pattern will gradually disappear over the years.
The Threeawn grasses, also called Needlegrasses, are able to survive in the center of mounds better than most other plant species. One year I noticed that Arrowfeather Threeawn, (Aristida adscensionis), was established only in the bare ground of every Harvester Ant mound on the ranch that I saw while spraying Pricklypear. I suppose that a pasture census of ant mounds could have been taken from the pickup window by the appearance of the waving seedheads of Arrowfeather Threeawn. (I wonder if Horned Toads can home in on this grass to find the next meal of ants!)
Zonation may be very striking on a broader scale with changes in geological deposits and topographical gradients. Limestone along the Mississippi River in eastern Iowa is dominated by Oaks and Maples on the slopes and Eastern Redcedar on the ridges. The next time you drive in central Texas between Menard and Mason on Highway 29 you may notice a more gradual change from pastures on the Limestone derived soils dominated by Liveoak and Cedar to pastures on the Granitic soils dominated by Post Oak and Beebrush. The changeover between zones occurs gradually in the vicinity of Hext.
Becoming more familiar with the landscape while driving is a lot better for you than locking your view onto the center of the road ahead, or Heaven forbid, on talking or texting on a cell phone. And you might ought to get out occasionally and look at the zonation around an Ant den.
Reading The Landscape
In this pasture scene in Coke County, Texas, on Edwards Plateau rangeland, what is growing and what seems to have happened in the past 40 years across the fence? Click here for my answer.
Liveoak savanna rangeland (one large Liveoak in background), probably well managed in the past 50 years with no Mesquite visible, but with rapid spread of Redberry Cedar (Juniperus pinchotii) in past 25 years and closing in. Cedar might have been bulldozed some years ago (woody debris of cedar doesn’t decompose as fast as Mesquite), but the abundant berry crop will guarantee more seedlings. Pricklypear is increasing as the grazing pressure on grasses increases, but the pasture probably still retains most of the desirable grasses as shown in the ungrazed roadside. —JL