Understanding What Causes Desertification

Long Term Ecological Research Program

We often have hectic daily schedules that make it feel like life is just blazing by. It isn’t until we turn our attention to the mountains or take a hike in a forest that we get a sense of timelessness. The plants holding stead fast to their home in the soil, and nature always coming back and forth in cycles that repeat every year for eons. From the point of view of our daily lives, our environment appears as constant as a watch that has stopped ticking. Only this stability is an illusion. The environment, especially here in the Chihuahuan Desert, is extremely variable, changing quite dramatically over the course of years.

Chihuahuan desert grassland

Chihuahuan Desert grasslands after 2014 monsoon season.

The Long Term Ecological Research Program (LTER) at The Jornada Experimental Range is part of a nationwide program supported by the National Science Foundation to understand how environments change over time. There is an ebb and flow between the grassland and shrubland environments that characterize the Chihuahuan Desert. They will transition from grassland to shrubland states and sometimes back to grasslands because they reach a tipping point. Much like the saying “the straw that broke the camel’s back”, environmental factors will accumulate until it is past a certain point and a sudden and drastic state change can occur. In Las Cruces and in semi-arid regions across the world, people like ranchers and other land users depend on grasslands for their livelihood, as do large numbers of wildlife. When desertification turns these grasslands to shrubland, it severely limits the amount of wildlife and people the land can sustain. In the Las Cruces region, grasslands occupy only 10% of what they did in the 1800’s. It is the purpose of LTER to understand these factors that cause state transitions. (For a better understanding of LTER’s background and the science behind state transitions click here.)

 

Current Science

One important question is about the relationship between climate, habitat, small mammals, and the type of vegetation growing. While it may seem that animals such as kangaroo mice and jackrabbits would be too small to have an impact, they are quite an important factor in state transitions. Different species of small mammals prefer different kinds of habitats, and these populations will fluctuate with vegetation and resources. Some of the important questions that are being ask include, "Does a change in water and nutrient resources result in a shift of species?" "Are these niches being filled in by other species?" "Are small mammals contributing to vegetation loss?"

Jack Rabbit Chihuahuan Desert

Blacktailed Jackrabbit possibly contributing to vegetation loss.

In between a grassland and shrubland area there is the ecotone, the boundary where two different vegetation zones meet. The ecotone may be abrupt or it may be very wide, but in it are characteristics of both states as it undergoes a transition. As grassland gives way to shrubland across the ecotone, one will notice that the patches of bare ground between plants gets larger and larger. These large bare patches are more sensitive to abiotic factors (wind and rain), which erode away nutrients and soil, while areas dense with plants are stabilized by biotic factors (leaf litter, fungi, etc.) and improve nutrients and soil. Grasslands are dense with plants, but as bare ground begins to appear in them, they can undergo a transition to shrublands, which are characterized by their large bare patches of soil. Researchers are asking, “At what point do these large scale abiotic processes overwhelm the small scale biotic process which stabilized plant and animal communities?” To understand this, researchers are modifying the size of the gaps in shrub and grasslands, thereby increasing the effect of wind and rain as it moves across the ground sweeping away nutrients and soil and observing the changes that occur as a result.

Mesquite brush encroaching on Chihuahuan Desert grassland

Mesquite shrubs encroaching on black grama grassland.

Another research project was stumbled upon by accident when it was noticed that soil being blown away in one particular area was affecting the growth of grasses downwind. Certain grasses like black grama do not handle being buried by soil very well, which could lead to a transition to shrubland. So the question arose, “At what point will wind erosion upwind affect the growth of grasses downwind?" To answer this question, a study is being implemented where 0%, 25%, 50%, 75%, and 100% of vegetation is being removed in plots and researchers are measuring its effect on grasses downwind.

Mesquite bush dominated Chihuahuan Desert landscape

Mesquite dominated habitat with only perennial bunch grasses remaining (notice the extent of bare ground).

Understanding the factors involved in state transitions allows researchers to create accurate predictive models that can be applied in arid and semi-arid regions across the world. It will allow anyone who is interested to know what the trigger points are, at what point degradation will occur, and when it can and cannot be reversed. These models are going to become even more accurate with the future addition of wireless meteorological stations, which will allow real-time rain, temperature, and humidity data to be applied across the board. In the end, people all over the world will be able to use the models developed at The Jornada's LTER Program to better manage their land.

 

Special thanks to John Anderson for his input on this article.

 

Photo Credits: John Anderson, LTER; USDA-ARS The Jornada Experimental Range; Johnny Ramirez

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