Jornada Trails

by David C. Tremmel

Duke University

The two major shrub species that have invaded Jornada grasslands—creosotebush and mesquite—differ markedly in form, physiological characteristics, and timing of growth. Yet both species develop "resource islands"—that is, they tend to produce a patchy pattern of soil resources, with more resources underneath the shrubs and fewer resources between shrubs. These resource islands are thought to aid the persistence of the shrubs by, among other things, buffering them against the effects of short-term droughts.

The differences between these two species, and differences in the buffering capacity of young and mature resource islands, were examined in a 3-year field study at the Jornada by Jim Reynolds, Ross Virginia, Paul Kemp, Amrita de Soyza, and Dave Tremmel, the results of which were recently published in Ecological Monographs. These researchers erected "rainout shelters" over large and small plants (with correspondingly young and mature resource islands) at three locations in the Jornada. The shelters were used to exclude either all summer rainfall (June–September) or all winter/spring rainfall (October–May) from the study plants. They examined the effects of rainfall exclusion on both plants and soil, compared to similar shrubs and soil in areas that were allowed to receive natural rainfall.

In general, more similarities than differences were found between species and between young and mature islands. Creosotebush is evergreen while mesquite is winter deciduous, but the two shrub species were relatively similar in function in the absence of drought, having maximum growth and physiological activity in the late spring. Mesquite appears to have a fixed seasonal pattern of growth, as neither drought treatment affected the timing of maximum activity of this species. Creosotebush was more flexible in its response to drought, tending to shift its time of maximum activity to early summer when subjected to winter/spring drought. Both species can compensate for seasonal drought by increasing their growth and/or physiological activity in the season following drought. There was also little evidence that the maturity of the resource island affected the growth, physiological activity, or drought response of either species. Overall, the responses of both species showed that they are well adapted to withstand season-long droughts.

One somewhat surprising finding was the rather lengthy root longevity of these species. In most ecosystems, fine roots are thought to live for less than a year, and are constantly being replaced by new roots. Root observations made by repeatedly viewing live roots in the soil using minirhizotrons indicated that the fine roots of creosotebush may live for 3–8 years, while those for mesquite live for 2–5 years.

by Dale Gillette

NOAA

Wind erosion at the Jornada is being simulated in Durham, North Carolina. In March, Fanny Minvielle arrived at NOAA’s Fluid Modeling Facility (FMF) located in Durham for a two-month visit. Fanny, a student at the University of Paris XII, is using the large FMF wind tunnel to examine the resuspension of dust from desert soils. She is working with Jornada LTER researcher Dale Gillette, who began measurements of wind erosion on the Jornada Experimental Range in 1997. This work examines the effect of sparse vegetation on aerodynamic properties of the desert surface. Gillette and his collaborators from the University of Paris, Beatrice Marticorena and Gilles Bergametti, measured vegetation roughness—the effect of vegetation on airflow and on the transfer of momentum to the soil surface—in two tarbush sites near the southeast boundary of the Jornada Experimental Range. Dale erected two 15-meter-high towers to analyze wind speed, air temperature, and roughness parameters.

Fanny is now measuring the same roughness parameters under controlled conditions in the FMF wind tunnel. In a 12- by 70-foot test section of the tunnel, Fanny has placed models of low density desert vegetation. Her results will be compared to the Jornada field study. Since the effect of vegetation porosity is poorly known, she will test roughness patterns using models that simulate the porosity of desert shrubs. The research is part of a joint CNRS (Centre National Recherche Scientifique) and National Science Foundation research grant.

by Laura F. Huenneke
   New Mexico State University
   and William H. Schlesinger
   Duke University 

Adapted from a letter to the editor that appeared in the Las Cruces Sun-Times, March 7, 1999

Within the next few weeks, the Senate may choose to ratify the United Nations Convention to Combat Desertification. A successful vote would add the United States to the list of 145 other nations that will pool their knowledge and expertise to combat land degradation throughout the world. Not without its own problems of land degradation, New Mexico has a lot to offer to the U.N. Convention.

More than 100 years of basic and applied research at the Jornada Experimental Range near Las Cruces have helped improve the management of many rangelands. Several decades of field studies by faculty at the University of New Mexico and New Mexico State University have helped us understand the causes and consequences of desertification throughout the southwestern United States.

The causes of desertification, often associated with the invasion of grasslands by mesquite, are hotly debated. Livestock grazing, fire suppression, rising concentrations of atmospheric carbon dioxide, and climate fluctuations have all been implicated in the loss of productive grasslands in New Mexico. Desertification is often associated with a loss of soil fertility from erosion by wind and water. Developing better methods of sustaining vegetation in the semiarid lands of New Mexico will reduce soil erosion and the long-term loss of fertility that usually accompanies desertification. The U.N. Convention would pledge the help of U.S. scientists, range managers, and engineers to better understand and control the problem of land degradation worldwide.

And desertification is a worldwide problem. Nearly 20 percent of the world’s population lives at or near the borders of deserts, where land degradation is most severe. In most cases there is no alternative to cultivation and livestock production to sustain the local population. In recent years, the U.S. has sent humanitarian aid and peacekeeping forces to a variety of regions throughout the world where drought and crop failure have led to instability and famine.

Further research, spurred by U.S. participation in this treaty, can only help land managers better understand the process of land degradation in New Mexico, and at the same time contribute their expertise to help slow desertification in other nations of the world. To the extent that the U.N. Treaty can offer an ounce of prevention, it will be worth several pounds of cure to the U.S. taxpayer. D

June 24, 1999

Friends of the Jornada Symposium

Be sure to register for the annual Friends of the Jornada Symposium to be held at New Mexico State University on Thursday, June 24, 1999. A full day of informal research presentations will be followed by the traditional, and now famous, evening barbecue at the USDA Headquarters Ranch.

This is the Ninth Annual Symposium, and we expect that it will continue to provide a lively exchange of ideas and data on the Chihuahuan Desert environment. A registration fee of $10 covers the costs for refreshment breaks during the symposium as well as the evening barbecue.

For more information or to register, contact:

   Valerie La Plante
   USDA/ARS Jornada Experimental Range
   P.O. Box 30003, MSC 3JER
   New Mexico State University
   Las Cruces, N.M. 88003 

Or, register via email to: vlaplant@nmsu.

The American Birding Association has just published the second edition of The Birder’s Guide to the Texas Rio Grande Valley. This guide covers habitats from the Las Cruces region to the Gulf of Mexico and will be of interest to all those who visit the Chihuahuan Desert. Order from the ABA: 800-634-7736.

Jornada Trails is pleased to recognize Dr. Ross Virginia, Chair of the Environmental Studies Program at Dartmouth College, as its featured investigator. Ross has enjoyed a long history of work at the Jornada, beginning with his studies of nitrogen fixation by mesquite, in which he found active root nodules thirteen meters beneath the surface.

Working with Jim Reynolds, Ross has provided essential data comparing the water and nutrient-use efficiency of mesquite and creosotebush in a variety of habitats of the Jornada Basin. Recently, they co-authored an extensive paper in Ecological Monographs reporting on this work.

At home, Ross directs the interdisciplinary environmental studies program at Dartmouth, where he is the Albert Bradley Third Century Professor in the Sciences. A number of Dartmouth students have worked at the Jornada in the NSF-sponsored program, Research Experience for Undergraduates (REU) during the past several years.

Recently, Ross has also focused his attention on comparative studies between the Jornada and the McMurdo LTER in Antarctica. Both offer desert-like habitats, but with contrasting temperature. Ross has made extensive surveys of the diversity of soil microfauna in both habitats and comparative studies of the physiological tolerance of soil microbes.

Whether he is donning his goose-down parka or his desert sombrero, Ross is at home across the full range of the world’s desert ecosystems.