Desert Grassland Restoration: Creosote Bush

CREOSOTE BUSH (Larrea tridentata) is generally misunderstood as an invader plant.  In fact creosote is a symptom –  but not the cause –  of dying desert grasslands. Creosote will predominate as grasslands decline but eventually it also dies.

1. Creosote bush flowers and fruits

Creosote bush is one of the most common and important plants of the warm deserts (Sonoran, Chihuahuan and Mojave Deserts) of North America, but cold winters exclude it from the Great Basin Desert. Four closely related species also are found in South America (Southern Peru. Bolivia and Argentina), which is the region where creosote bush originated and from which it spread to North America at some time during the Pleistocene (from 1 million to 10,000 years ago). Today, it is often the dominant plant over miles of dry, low-lying, flat terrain where rainfall averages as little as 8-10 cm per year (images 2, 3). It looks remarkably similar wherever it occurs. But this belies the fact that it actually exists as three distinct populations. In South America and the Chihuahuan Desert of North America the plants are diploid (with two sets of chromosomes in each nucleus), whereas in the Sonoran Desert they are tetraploid (4 sets of chromosomes) and in the Mojave Desert they are hexaploid (6 sets).

2. Creosote bush in the Chihuahuan Desert, Texas

3. Creosote bush in Sonoran Desert, Arizona

4. A single creosote bush, in the shade of which (right-hand side) is a clump of the grey, spreading, mat-forming cactus termed clavellina (Opuntia schottii) The cylindrical stems of this plant – a cholla similar to the large, erect chollas) grow to only about 10 cm high. Many cacti in deserts grow in the shade of shrubs, to minimise exposure to intense sunlight.

Drought tolerance

Creosote bush is the most drought-tolerant perennial plant of North America. It can live for at least 2 years with no water at all, by shedding its leaves and even shedding branches. But usually it is an evergreen plant, retaining its thick, green, resin-coated leaves (image 5) or allowing them to dry down and turn golden-brown in periods of seasonal drought (image 6). In this condition the leaves can still synthesise some sugars – enough to keep the plants alive. The extreme drought-tolerance of the leaves is due to several factors: the leaves are small with a low surface area for water loss (image 7), the leaf cuticle is very thick and waxy, and the high stomatal resistance. Coupled with all this, the tissues of the leaves can have very high water potentials, caused by high levels of chemicals, so they can withstand drought conditions.

5 (left) and 6 (right) show creosote bush leaves in conditions of good soil moisture and drought. These images also show young and old galls caused by the creosote gall midge (see plant galls)

Creosote bush is also remarkable for several other reasons. The individual plants live an extremely long time – often 100-200 years. And they produce clonal colonies (all plants having the same genetic make-up). This happens over a period of many centuries, as new shoots are continually produced from the outer edge of the root crown, and the older stems die in the centre of the bush. The result is that these plants gradually move outwards in a circle and then become separate plants, all derived from the original one. In contrast to this, the seeds which are produced quite abundantly by creosote bushes (image 1) germinate poorly and the seedlings show very low survival in natural environments.

In almost any area dominated by creosote bush the plants are seen to be spaced with a remarkable degree of uniformity (image 2). This has given rise to speculation that the roots secrete alleopathic chemicals – those that might inhibit the growth of nearby plants. Although there is some evidence of this, it is more likely that the plant spacing reflects the availability of water. Creosote bushes have extensive fibrous root systems, typically extending up to 4 metres from the plants. Most of these roots are shallow, to intercept any water that falls on the soil surface, but in deep sandy soils the plants can also produce tap roots to obtain water from deeper in the soil profile.

7..The tiny leaves, each with two leaflets, resemble the cloved hoof of a deer

Natural products, chemistry, and herbivores

Creosote bush gains it name from the resinous odour of the leaves. In fact, these plants are natural chemical factories – they produce a wide range of compounds that protect them from damage by insects and pathogenic fungi and that also prevent them from being eaten by herbivores. For example, an anti-oxidant compound termed nordihydroguiaretic acid (NGDA) is present on the leaf surfaces and can account for 5-10% of the total weight of the leaves. This and other resinous phenolic compounds can form complexes with proteins and prevent digestion of the leaf tissues in an animal’s gut. Jackrabbits are the only mammals known to eat the leaves of this plant – and then only in times when little else is available. There are, however, more than 60 insects associated with creosote bush, including 22 species of bee that feed only on its flowers, and a creosote katydid and creosote grasshopper that are specialised to use it as a food source.

8. Holes produced by rodents around creosote bush

The soil mounds around creosote bushes are favourite places for the burrows of desert rodents, including kangaroo rats (image 8). When rodents abandon these burrows they become occupied by lizards and invertebrates.

Postscript

The vast tracts of land dominated by creosote bush have almost no commercial value for recreation or grazing of livestock. In recent years the creation of irrigation canals, drawing water from the major rivers of the southwestern USA and northern Mexico, have led to the ploughing up of large areas of this “natural wasteland” for commercial irrigated agriculture.

 

The belief that degraded grasslands have no value for recreation or grazing is as incorrect as it is widely-accepted. Below is an 11-minute video on a fast, cheap, effective way to restore these dying grasslands in which creosote bush is the last survivor. This restoration of grass and weeds does no harm to creosote or the many creatures and other plants that depend on it.

 

Desert Grassland Restoration Without Poisons from Christopher Gill on Vimeo

Posted by Chris Gill

  1. James McAllen Jr October 5, 2016 at 10:26 am

    I enjoyed your comments on the Creosote Bush and how it’s a symptom and not a problem. In deep south Texas we are experiencing problems with Tanglehead grass, spreading through large areas of ranch land at a very rapid rate. Native to Africa, tangle head is considered an noxious invasive species. Is this the same tanglehead grass I saw as a kid 30 years ago? My Dad would stop the car whenever he saw some on the side of the road, because it was a rare plant at the time. So why, all of a sudden, are we having such an aggressive spread of this plant across South Texas? From CKWRI website: “Many range scientists hypothesize that tanglehead has increased in abundance in South Texas because of climatic patterns favoring tanglehead, and an overall reduction in stocking rates in areas where it occurs as a result of the economic value of wildlife.” Personally speaking… I have seen first hand cattle eat tanglehead when left with no other choice of grass or plant eat. Cattle will eat it! It also provides carbon to the soil and cover for smaller species, such as quail, lizards, bugs, snakes, mice, etc… We may have more tanglehead in South Texas today because of a slow reduction of cattle numbers since 1980. We’ve created an environment that is “perfect” for tanglehead. Grazing is a solution to this problem.

    By the way, also enjoyed your postings on predators. Really great stuff! Reminds me of when I have a problem with mice in my feed room. What do I do? I take a few of my daughters outdoor house cats and let them spend the night at the barn. Problem solved!

    Reply

    1. Dear James,

      I have no experience with tanglehead. But I ask my South Texas friends, “When you pull the cattle and substitute burning, why are you surprised that a rest tolerant, fire tolerant grass proliferates?”

      There must be a grazing protocol that would deal with this.

      Regarding predators and mice, when they killed coyotes because they were eating quail, mice numbers exploded and they did way more damage to quail than coyotes. Killing predators always harms what we are seeking to protect. If we want fewer coyotes, try a few wolves: Wolves eat coyotes.

      Thanks for the comment.

      Reply

  2. Aldo Leopold discovered this many years ago when his policy was to kill wolves to create a better habitat for deer for hunters. The deer then proliferated and began to cause damage to the forest. So he revised his policy for wildlife management.

    Reply

  3. Thanks Sally.

    As recent research in the East African grasslands confirm:

    To be healthy, desert ranges need three things: (1) Large, concentrated migratory bison herds, or, cattle grazed to mimic bison’s migratory patterns; (2) a lot of predators of all sizes; and, (3) a high, diverse population of prey species. Remove any of these and the system collapses.

    Leopold was talking about the predator piece of this. He was an early proponent of sustainable (holistic) wildlife practices. For a while his ideas caught on but then Invasive Species “Biology” took hold. Now this thinking dominates the universities, conservation groups and wildlife agencies; they are financially interdependent with the agrochemical and agricultural cartels.

    Pushback is coming from consumers who want wholesome food and spreading out to farmers and ranchers. From these small producers, the sustainable (‘holistic’ or ‘regenerative’ if you like) thinking is now creeping into wildlife and fisheries practice.

    But it has a long way to go and is fiercely resisted in practice by most all those folks who so love to quote Aldo: There is big money in their bad practices.

    Chris

    Reply

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