Ancient Creosote Bush Clones: A Trail of Multidisciplinary Discoveries
Pictured above: Dr. Erik Hemerlynck, standing within King Clone, Johnson Valley, June 2001.
Text and Photographs by: Dr. Joe McAuliffe, Director of Research
Across the hot deserts of the American Southwest, there is no shrub more widespread than the creosote bush, Larrea tridentata. Yet this common shrub is hardly ordinary. Scientific research has revealed many fascinating stories about its life history and ecological roles. One of the most amazing stories is how in certain places, individual creosote bush plants can live to be very old – thousands of years old.
During the last six years I’ve had the opportunity to conduct research on the environmental factors that promote the long-term survival of creosote bush at a site in the Mojave Desert, California, along with colleagues Dr. Erik Hamerlynck of Rutgers University and Dr. Martha Eppes from the University of North Carolina, Charlotte. Our multi-disciplinary teamwork, involving investigations of geology, soils, soil hydrology, plant physiology, and ecology proved essential for deciphering why large, very old creosote bush plants are common in some places but absent from others. The technical details of our research on this topic will soon be published in the Journal of Arid Environments1 but it is a pleasure to share some of these findings with you here in the The Sonoran Quarterly.
Our research builds on discoveries made over a quarter century ago by the botanists who originally recognized and established the existence of long-lived creosote bush plants. In the 1970s, Dr. Frank Vasek and colleagues from the University of California, Riverside, described large ring-shaped clusters of creosote bush stems that surrounded bare central areas. They proposed that each ring-like cluster represented the germination of a single seed long ago and the subsequent outward growth of the one plant2. Vasek suggested that the production of new peripheral branches at the bases of the plants, coupled with the eventual death of old branches in the plants’ centers, eventually yielded the ring-like growth forms. Some of the large creosote bush rings that Vasek examined were well over 25 feet (8 m) in diameter. Given the slow growth of desert plants like creosote bush, Vasek suggested that such plants could actually be several thousand years old.
Working in the same area studied by Vasek, Dr. Leonel Sternberg another biologist from the University of California, Riverside, tested Vasek’s hypothesis using molecular genetics tools3. If an individual creosote bush ring was indeed derived from a single seed and subsequent clonal growth of one plant, all parts of the resulting ring should be genetically identical, even if root systems of different parts of a ring were no longer connected. However, just like no two people (except identical twins) have the same genetic makeup, different creosote bush plants that originated from different sexually produced seeds would likely differ in some way genetically. Sternberg used an early form of genetic fingerprinting that examined the similarities (or differences) from plant to plant in the structure of particular enzymes. The very slight differences in structure of a particular enzyme from one individual organism to another (even in the same species) reflect real genetic differences.
Sternberg found that the multiple samples taken within individual, continuous creosote bush rings had identical genetic fingerprints. In contrast, the genetic fingerprints of separate ring-shaped clones differed from one another. Similarly, 20 widely separated young plants all had different genetic fingerprints. These results provided convincing evidence that the large ring-like growth forms of creosote bush were indeed the result of clonal growth of single plants.
With the clonal origin of the creosote bush rings established, another research question remained: how old were they? Since creosote bush plants don’t retain complete, year-to-year records of their ages like trees do, Vasek used a tool frequently used by geologists and archeologists to estimate ages of the clones4,5. He carefully excavated the dead remains of old bases of clone sections that remained buried within the interiors of the ring-shaped clones. He measured the distance from those buried woody remains to the outer, living perimeters of the clones. Using carbon-14 dating techniques, Vasek was able to determine how long ago the wood samples were alive and growing. Twenty-one different wood samples were older than 300 years, several exceeded 500 years, and one sample was dated at 730 years. By dividing the distance from the samples to the present-day clone perimeter by the age of the samples, he obtained estimates of long-term growth rates. With these data, Vasek estimated the clones have rates of diameter increase that average about 1.25 mm per year and he concluded that many large clones were thousands of years old. King Clone, the largest clone that has yet been discovered, has an average diameter of over 15 meters. If that giant clone experienced similar growth rates, Vasek suggested it could be slightly over 11,000 years old.
These amazing findings, since they were published in 1980, have captured the interest of many scientists and the general public. Since the early 1980s when I began my desert research, I have wondered, and have frequently been asked by other biologists, what factors govern the development of creosote bush clones and why large clones are only found in relatively few places?
I’m fortunate to have had the opportunity to meet and talk directly with Dr. Frank Vasek; in 1994 he kindly provided me with specific information on the location of his study area in Johnson Valley, approximately 13 miles east of Lucerne Valley, California. I visited the site in May of that year and spent some time examining the clones and marveling at the largest of them, including the singular King Clone. I came away from that trip determined to someday investigate the factors that contribute to the development of the unusual clones. I sensed that keys to answering some of the questions would involve detailed studies of soils, how water is absorbed and retained in different kinds of soil, and the physiological responses of creosote bush plants to varying supplies of soil water.
The opportunity to dive deeply into this multi-faceted ecological research problem came in 2000 when I teamed up with Dr. Erik Hamerlynck and Dr. Martha Eppes. Erik and I have worked together since 1995 on several different projects in the Mojave and Sonoran deserts. His knowledge and expertise in plant physiology provided a means to directly investigate aspects of plant responses to varying supplies of soil water. Martha (Missy) was conducting her doctoral research on geomorphology and soils of the desert terrain on the north side of the San Bernardino Mountains. Missy was then a graduate student at the University of New Mexico in the Department of Earth and Planetary Sciences, working under the direction of another of my long-time, esteemed colleagues, Dr. Les McFadden.
Dr. Frank Vasek suggested in his original publications in 1980 that large, very old creosote bush clones should be found “only on stable surfaces of long duration.” With that prediction, he recognized that erosion and deposition can significantly alter parts of the desert landscape over time periods of thousands of years. Vasek’s prediction made perfect sense – one would not expect plants that live for several thousand years to survive that long in parts of the landscape that are either being eroded away or buried by younger deposits. Vasek foresaw that understanding some of the geological details of landscapes was key to understanding more about the distribution of giant creosote bush clones.
The site of Vasek’s and our investigations in Johnson Valley, California, is located on an alluvial fan, a gently sloping landform consisting of alluvial gravels eroded from the nearby San Bernardino Mountains. Those materials were transported and deposited by flowing water. This alluvial fan, like most others in the southwestern deserts, is a complex mosaic of deposits of different ages. In some places the surface of the alluvial fan consists of materials deposited well over 11,000 years ago during the Pleistocene, the last “Ice Age.” Other surfaces consist of much younger alluvial deposits only a few thousand years old, and in some places, probably only a few centuries old. Missy’s geological expertise provided the tools necessary for determining differences in geological ages of different deposits.
Paradoxically, large creosote bush clones were absent from the oldest, long-stable parts of the alluvial fan, and the greatest concentrations of very large clones were actually found in places where the surface of the alluvial fan consisted of extremely young alluvial deposits –in some places only a few centuries old. These observations directly contradicted Vasek’s original prediction and suggested that factors other than geological stability per se were responsible for the development of long-lived clones. Nature had thrown us a counter-intuitive riddle, one that we spent several years studying in the field.
To decipher this riddle, we had to start digging, literally, for answers. In June, 2000 with mid-day temperatures reaching 100°, I dug several knee- to waist-deep trenches so Missy and I could examine the alluvial deposits in different places and the soils that had developed in them.
The different layers of sandy to gravelly alluvium we observed, measured, and sampled provided detailed records of geological activity. What we found offered at least a partial explanation of the paradox of why old creosote bush clones were located in places where young alluvial deposits covered the surface. In places where large clones were found, the geologically young surface deposits consisted only of thin layers of sandy alluvium approximately 6 to 20 inches deep. Often there would be two or three layers of these young deposits stacked like layers of a cake, the oldest deposited no more than a few thousand years ago. Those geologically young deposits buried a much older layer, which had characteristics indicating it constituted an extensive, former surface of the alluvial fan from the time of the last Ice Age, over 11,000 years ago, up to the time it was buried by the younger deposits several thousand years ago. We hypothesized that the large clones started out as young, small plants that originally established on the old surface, but the bases of those plants were repeatedly buried by younger deposits within the past few thousand years. The successive layers of young sediments each were thin enough that their accumulation apparently had no adverse effect on the developing creosote bush plants. To test this hypothesis, we examined the deposits directly beneath one large creosote bush clone that had recently been bisected by road-grading activity along a major road that crosses the study area. Data from that cross-section clearly indicated that the plant originally grew on the older surface before subsequent accumulations of more recent deposits.
The rich information we were able to read in the layers of the geological history book provided us with only a partial explanation for the distribution of clones. Although we now understood that plants that eventually developed into clones must have originally established on older, now shallowly buried, surfaces, half of the paradox still remained: why were clones absent in places where the old geological deposits were never subsequently covered with thin layers of younger alluvium? The key to answering this question was information about how desert soils change over long periods of time, how these changes affect the soils’ capacities to absorb and store water, and the physiological responses of creosote bush plants to soil moisture (or lack, thereof).
The texture and chemistry of alluvial deposits change over time as soils develop. In deserts, the slow but relentless, accumulation of wind-blown dust provides a substantial quantity of fine soil particles (silts and clays) that change the soil’s texture, lessening the soil’s capacity to absorb precipitation6. We measured infiltration rates of water applied on the surface and found that the more clayey soils on old surfaces absorbed water at only one-tenth the rate of soils on young surfaces that had coarser, sandier textures.
As a consequence of the lower rates of absorption, less total water is stored in soils on older surfaces and this water is not stored very deep. Following two different precipitation events, we examined depths to which water percolated into soils on surfaces of different ages. In soils of younger surfaces, water percolated to over twice the depth as in the soil of older surfaces. With the passage of time (thousands of years), desert soils inexorably become less permeable to water, less water is stored, and the little water that is stored is not located at the depth that would provide the greatest benefit to a relatively deep-rooted, evergreen desert shrub like creosote bush. Creosote bush plants growing in less permeable soils never reach the sizes (heights or diameters) as those on younger surfaces with more permeable, hospitable soils.
The story about soils does not end there. In addition to the various alluvial deposits, there are also deposits of fine, wind-blown sand. These eolian deposits are actually the low mounds on which the ring-shaped clones are found. These mounds are called coppice dunes that form as the wind drops the load of sand it carries beneath a plant due to the canopy’s windbreak effect. In some places the coppice dunes are elevated more than 20 inches above the surrounding alluvial fan surface. We found that coppice dunes differ in height on the different surfaces. For any given size range of creosote bush plant, the coppice dunes beneath plants are much taller on the surfaces covered with young, sandy alluvium than on old surfaces where the more clayey soils are found. We hypothesized that the wind could easily dislodge and move materials from the loose, non-cohesive sandy alluvium and then deposit those materials beneath nearby plant canopies. In contrast, we proposed that the more clay-rich soils on the older surfaces yielded little in terms of wind-blown, sandy materials that could contribute to coppice dune development.
We tested this hypothesis by installing many small windbreak screens throughout the site. The screens were about 9 inches tall and 25 inches wide, positioned in areas between the widely spaced plants, and oriented perpendicularly to the prevailing wind from the southwest. After two years had passed, wind-blown sand had accumulated on the leeward sides of the screens in wedge-like accumulations (just like the drifts of snow behind snow fences, if you have ever experienced winters in northern climates). We then measured the volumes of those accumulations. The amount of sand that accumulated behind screens on the youngest surface was 20 times the volume accumulated behind the screens on the oldest surface. This showed us that an abundant, local source of recently deposited, unconsolidated sandy alluvium on the surface contributed to the development of large coppice dunes. The lack of these materials on the older surfaces limited the development of coppice dunes within the immediate area.
The importance of the coppice dunes to development of the clones became clear in our studies of plant water stress. We directly measured water stress in creosote bush plants in the field with an instrument called a Scholander pressure chamber. This device measures the tension at which water is held in the microscopically thin vessels that constitute the plant’s plumbing system. The measurements showed that among all of the sites, clones on the larger coppice dunes experienced less water stress than those on either smaller dunes or dunes that had partly eroded. The sand of coppice dunes readily absorbs precipitation. The tallest dunes provide the largest reservoirs of stored soil moisture and the most favorable rooting environments.
Although our work may appear to have many complex facets, the results show how processes that modify the desert landscape promote the development and persistence of large creosote bush clones. Highest concentrations of large clones are found where thin inputs of sandy alluvium have repeatedly covered the surface after the original establishment of young creosote bush plants. The fresh inputs of these materials promote greater infiltration of precipitation, thereby benefiting the plants. Unconsolidated sandy alluvium is reworked by the wind and accumulates in coppice dunes beneath the plant canopies, further benefiting the creosote bush plants and promoting long-term growth and survival. Few other places in the Mojave and Sonoran deserts provide this special combination of physical conditions that apparently foster the development of long-lived clones.
Understanding complex environmental processes like the ones we studied requires the collaborative efforts of multidisciplinary teams that bring diverse expertise to bear on the questions and problems at hand. Expertise in geology, soil science, soil hydrology, plant physiology, and plant ecology that Erik, Missy, and I brought to the table were all essential for deciphering questions about factors responsible for the development of large creosote bush clones. The importance of multidisciplinary efforts is all the more apparent considering the different kinds of expertise that Dr. Frank Vasek and Dr. Leonel Sternberg originally brought to bear in the 1970s on studies of ancient creosote bush clones.
Understanding the relationships between ecological and geological processes has direct implications for protection and conservation of areas that contain giant creosote bush clones, especially in Johnson Valley, the home of King Clone. Off-road vehicle traffic is a serious threat. The U.S. Dept. of the Interior Bureau of Land Management administers the 200 square mile Johnson Valley Off-Highway Vehicle Area, the unfenced boundary of which borders our study area. The area is close enough to the Los Angeles - San Bernardino area that it receives extremely heavy use on weekends during cooler seasons. The lack of fencing and regulatory signage leads to spillover use and occasional direct damage to creosote bush clones outside of the designated off-road vehicle use area. A small part of the alluvial fan has been fenced by the State of California in order to protect some of the clones (including King Clone) from this kind of damage. Nevertheless, this fence is sometimes ignored and vandalized, and vehicle tracks occasionally cut through the preserve. This kind of abuse chips away at the environmental integrity of this unique area – a kind of ecological “death by a thousand cuts.” The clones and their associated coppice dunes are extremely sensitive to disturbance. Even seemingly minor damage by vehicles to part of a creosote bush clone can reduce the windbreak effect of the plant, which can lead to wind erosion of the underlying coppice dune. With even partial erosion of a dune, a downward spiral can result due to increased water stress experienced by the plant, further declines or death of stems, fragmentation, and eventual demise of the clone.
The giant creosote bush clones in this area are some of the oldest plants on the planet. Those clones and the unique landscapes that have fostered their development deserve more comprehensive, official protection. The work that Erik, Missy, and I have recently completed will provide land managers with a clear picture of the ecology of these sensitive desert landscapes. This knowledge is a key to the better protection of these environments.
1McAuliffe, J.R., Hamerlynck, E.P, and M.C. Eppes. 2006. Landscape dynamics fostering the development and persistence of long-lived creosotebush (Larrea tridentata) clones in the Mojave Desert. Journal of Arid Environments, in press.
2Vasek, F.C., Johnson, H.B., Eslinger, D.H. 1975. Effects of pipeline construction on creosote bush scrub vegetation of the Mojave Desert. Madroño 23:1-13.
3Sternberg, L. 1976. Growth forms of Larrea tridentata. Madroño 23:408-417.
4Vasek, F.C. 1980. Creosote bush: Long-lived clones in the Mojave Desert. American Journal of Botany 67:246-255.
5Vasek, F.C. 1980. Ancient creosote bush rings in the Mojave Desert. Fremontia 7(4):10-13.
6McAuliffe, J.R. 1999. Desert soils. Pp. 87-104 in S.J. Phillips and P.W. Comus, eds. A Natural History of the Sonoran Desert. Univ. of California Press, Berkeley.
Editor’s note: The research articles listed above can be found in the Desert Botanical Garden’s library, located in the Nina Mason Pulliam Desert Research and Horticulture Center.