Soil Ecology and Research Group
last update February 27, 2004
Demographics and Ecology of the Amargosa
Niterwort (Nitrophila mohavensis) and Ash Meadows Gumplant (Grendelia fraxino- pratensis) of the Carson Slough Area
Ash Meadows is located in the Amargosa desert, approximately 60 miles northwest of Las Vegas and 210 miles northeast of Los Angeles, California. Ash Meadows and the Amargosa drainage comprise an extensive lowland plain that is the remnant of a large Pleistocene playa, which is characterized by finely textured silts and clay with slow internal drainage and a shallow water table for much of the region (Cochrane, S.A. 1981). Ash Meadows Wildlife Refuge harbors more than 30 springs and seeps, with an estimated discharge of approximately 20.96 x 106 gallons per year (King, M. J. et al. 1998). The combination of an abundant source of water and saline soil conditions have fostered the speciation of more than 25 endemic species of plants and animals, thus making Ash Meadows one of North America's most endemically diverse regions. Much of the water originating from the springs and seeps has been capped or diverted to reservoirs for use in agriculture, ranching, and mining, but free-flowing surface water can still be observed several miles south of the major springs of Ash Meadows even in the early summer. The free-flowing water from the springs and seeps has created a desert wetland known as Carson Slough (Figure 1).
Figure 1. Free-flowing water just west of the Nevada/California state line on Ash Meadows gumplant’s critical habitat.
The hydrology of Lower Carson Slough near Death Valley Junction in California is dominated by winter runoff from the mountainous watershed and by a regional carbonate aquifer recharged from the Spring Mountains (King, M. J. et al. 1998). The streambed is dry much of the year but is underlain by a shallow groundwater table normally within 3 to 6 feet of the surface.
Carson Slough, once a massive marshland, was drained and mined for its peat in the past. In recent years, major water diversions have also depleted the amount of surface water flowing into the slough. Although shrunken in size, Carson Slough is still major drainage of the Amargosa Desert basin. The seasonally wet surface conditions, persistently high groundwater levels, and saline soil conditions have created and sustained a unique desert wetland ecosystem able to support a variety of endemic plant species, including the federally endangered Amargosa niterwort (Nitrophila mohavensis) and the federally threatened Ash Meadows gumplant (Grindelia fraxino-pratensis).
Over the past several decades, human activities such as mining, railroad construction, farming, livestock grazing, road construction and off-road vehicle travel have caused major changes in the land surface conditions. In addition, upstream water use and groundwater pumping from the regional aquifer have changed the original hydrologic regime of the slough and reduced the amount of water available to the plants. These changes have resulted in modifications to the wetland habitat of the Amargosa niterwort and the Ash Meadows gumplant. The prospect of a further drop in the region's water table increases the threat to the existence of these species.
This study focuses on the demographics and ecology of Amargosa niterwort and Ash Meadows gumplant known to Lower Carson Slough in California on the Bureau of Land Management (BLM) Death Valley Junction Water/Plant Study Site, T25N R6E sections 5-8 and T26N R6E Sections 29-33 (Figure 2, red outline). The goal of this study was twofold. First, to estimate population size and geographic range for both species of concern within the water/plant study site, and secondly, to describe the preferred habitat requirement and associated plant communities for both species of concern within the designated water/plant study area.
Figure 2. Amargosa niterwort critical habitat (bottom blue), proposed ACEC (red),
Ash meadows gumplant critical habitat (top blue).
Ecology of the Amargosa Niterwort critical habitat.
Amargosa niterwort (N. mohavensis), in the Chenopodiaceae family, is one of only two species in the Nitrophila genus known to occur in North America (Reveal J. L.1978). The Lower Carson Slough holds the largest known contiguous occurrence of Amargosa niterwort in North America, approximately 280,000 ramets (above ground shoots that are likely rhizomatously connected to other nearby shoots). Smaller occurrences of Amargosa niterwort are known to exist near Tecopa Hot Springs and within Ash Meadows Wildlife Refuge, with the largest occurrence on the Refuge of approximately 30,000 ramets just south of Crystal Spring reservoir.
Amargosa niterwort is most abundant on barren alkali mud flats with a layer of encrusted salt over the topsoil (Figure 3). From soil analysis of samples taken from the surrounding area, significant differences in soil compositions were revealed between Amargosa niterwort habitat and habitat that did not support Amargosa niterwort. Amargosa niterwort was most prolific in sandy loam (approximately 57% sand) with a pH of near 8.4, as opposed to the surrounding shadescale scrub with a soil composition of more than 90% sand and a pH of around 7.0. A soil sample taken from soil, which had an extremely thick layer of encrusted salt and prolific salt grass but devoid of any Amargosa niterwort, consisted of a loamy sand (greater than 82% sand) with a pH of 8.5. Refer to Appendix I for a complete table of the soil analysis.
Figure 3. Amargosa Niterwort growing on salt-encrusted soil.
The moist saline soil of the water-fed channels and seeps of Carson Slough is the preferred habitat of the niterwort and key to the species’ survival. Without adequate surface water, the niterwort’s habitat could be overtaken by saltgrass (Distichlis spicata var. stricta) which typically dominates the slightly drier portions of the slough that surround Amargosa niterwort habitat. Drier soil, or soil covered by a thick layer of salt, is usually devoid of Amargosa niterwort but densely covered with saltgrass. Saltgrass, another rhizomatous plant species, may be a major competitor to Amargosa niterwort since it has been noted that niterwort is greatly reduced or absent when saltgrass is abundant.
Another plant known to associate with Amargosa niterwort is Cleomella brevipes (stinkweed). It is sparsely distributed within the Amargosa niterwort critical habitat and it is unlikely that competition exists between these two species. Cleomella brevipes was found in close association with Amargosa niterwort, and proved to be a good indicator of Amargosa niterwort habitat.
The BLM sensitive species Tecopa birds-beak (Cordylanthus tecopensis) was not observed within the Water/Plant Study site, although it has been reported to occur on what is now designated as the Amargosa niterwort critical habitat (Munz and Roos 1955). Tecopa birds-beak is presumed extirpated within the study site, but further research should be conducted before a final judgment on extirpation is reported. The past several dry years may have inhibited its occurrence within the study site.
Associated plants known to occur with Amargosa niterwort are rather limited due to the harsh and highly specific environment needed to support the plant, with Amargosa niterwort commonly the only plant growing in the immediate area. The small assemblage of plants known to associate with Amargosa niterwort includes saltgrass (Distichlis spicata var. stricta) and stinkweed (Cleomella brevipes). On rare occasions, Atriplex confertifolia, Suaeda moquinii, and Nitrophila occidentalis were found to occur in very limited density around Amargosa niterwort. A complete list of species found in conjunction with the Amargosa niterwort is provided in Table 1.
Species found in conjunction with the Amargosa niterwort.
|Scientific name||Common name|
Distichlis spicata var. stricta
Amargosa niterwort appears to be a fairly resilient plant species surviving complete burial from State Line Road shoulder maintenance, fragmentation and burial from burrowing animals, and disturbance from off-road vehicles (figure 4). Plant adaptation to natural disturbance is a common phenomenon. Fragmentation from natural disturbances such as flash flooding may serve as a mechanism facilitating the colonization of suitable habitat.
Figure 4. Amargosa niterwort growing out of gravel deposited on the shoulder of State Line Road.
Ecology of the Ash Meadows gumplant critical habitat.
The Ash Meadows gumplant (G. fraxino-pratensis) is abundant around the fringe of the areas of Lower Carson Slough connected with flowing water during the region's rainy season (Figure 5). Soil texture where the plant is most abundant is a silty clay loam with a pH slightly over 7.0. Refer to appendix I for a complete soil analysis of the habitat. The gumplant can also be found, though in much smaller numbers, on the drier portions of the slough where shrub species dominate.
Table 2 provides a listing of all species associated with the Ash Meadows gumplant both in the areas around flowing water and in the upland areas outside the slough channel. Outside of the slough the flora is shadscale shrub typical of the Amargosa Desert region.
Figure 5. Dry slough channel with Ash Meadows gumplant growing out of soil cracks and along the channel banks.
Species found in conjunction with the Ash Meadows gumplant
|Scientific name||Common name||Location|
Distichlis spicata var. stricta
|* indicates introduced species|
One confirmed special status plant species, the Alkali Mariposa lily (Calochortus striatus), was found in abundance on the study site within the Ash Meadows gumplant assemblage. Alkali Mariposa lily, listed as sensitive by the state of California and BLM, was found to be prolific along the banks of running streams and standing water within the study site and was strongly associated with Sporobolus airoides. A second possible finding of a special status plant within the study site was the spring-loving centaury (Centaurium namophilum var. namophilum). This species was suspected to grow in the area of the study site, but no vouchers had been taken for the state of California and were therefore not available for confirmation. From examination of the Centaurium sp. specimen collected from the study site, the original monograph published for C. n. namophilum (J.L. Reavel and J.C. Beatley 1973), and the Jepson manual, it was determined that the Centarium sp. collected was, in fact, Centarium exaltatum, not C. namophilum var. namophilum. Specimens collected from the study site will be placed in both the San Diego State University Herbarium and the San Diego Natural History Museum Herbarium for future reference.
Ash Meadows gumplant, like the Amargosa niterwort, shows a high affinity to moist soil. The plant is prolific around the main slough channels but less abundant on the outer fingers of the slough. The entire habitat within which the gumplant was found to occur includes soils with high moisture content during the wet season. The densest species occurrences were in areas that hold water for most or all of the growing season. Annual fluctuations in the gumplant population density are likely correlated to fluctuations in surface water. Dead Ash Meadows gumplant from previous growing seasons were found at the southern extent of its range within the study site (Figure 6), so it is likely that areas south of the main occurrence support the growth of Ash Meadows gumplant during wet years.
It is unknown how long this species’ seeds remain viable in the soil bank, but their longevity would play an important role in the re-linking of a once contiguous population. Prolonged drought or further reduction of free-flowing surface water and/or groundwater, on the other hand, is likely to result in reduction of the Ash Meadows gumplant’s range.
Figure 6. Ash Meadow gumplant showing old growth from a
previous growing season.
METHODS AND MATERIALS
Range determination within the study sites
The water/plant study area consisted of the proposed BLM Carson Slough Area of Critical Environmental Concern (ACEC). The majority of this area consists of shadescale scrub that is unsuitable habitat for the two species of concern. All plants found occurred within main slough channels or on the fringe of these main channels of the study site. Surveying of the two species, therefore, was concentrated around drainage channels and seeps within the study site. All drainage channels within the study site were visually inspected for plants by a two-person crew walking no more than 5 meters apart from each other. Once an individual/ramet was found, its position was recorded and flagging and/or pin flags were used to mark boundaries. Once rough estimates of range and occurrence were worked out for the two species, sampling to estimate the size of the occurrences began.
Amargosa niterwort sampling
Examination of herbarium specimens from the Rancho Santa Ana Botanical Garden clearly showed the rhizomatous nature of the Amargosa niterwort. Many specimens exhibited 4 to 6 stems arising from one node on a rhizome. A single individual may actually account for hundreds if not thousands of ramets counted. From visual inspection it is impossible to distinguish individual plants or even different nodal points on the same plant. The simplest and most objective counting system was therefore employed for this study. A count of one ramet consisted of a single stem arising from the soil.
Amargosa niterwort exhibits large variations in number of plants per area throughout its range. Therefore, the niterwort’s critical habitat was broken down into two independent sampling areas in order to estimate total population within the critical habitat. Independent sampling in both areas eliminated large statistical deviations and reduced the amount of sampling within the critical habitat.
The Amargosa niterwort is most abundant in seasonally muddy, salt-encrusted soil. A large drainage channel running through the alkali flats of the Carson Slough exemplifies this type of habitat and it was in this channel where the densest occurrence of Amargosa niterwort was found. Moving away from this occurrence, the population density drops off and distribution becomes patchier. Amargosa niterwort population within the channel was sampled separately and is referred to as Occurrence 1. The remaining area of the range was designated as Occurrence 2.
Occurrence 1 (Figure 7) was delineated with blue pin flags and the area was mapped using a sub-meter GPS unit (Leica GS50). Occurrence 1 had nine macro-plots sampled throughout the area, the boundaries of which were randomly chosen by blindly throwing a weighted flag into the area and using the landing spot as one corner of each macro-plot. Macro-plots were 30 meters by 21 meters and were further divided into 210, 3-meter by
1-meter quadrats. Rebar was installed at two corners of each macro-plot and mapped with a GPS unit for permanent demarcation. During sampling, a stationary meter tape was stretched between the two rebar stakes and a movable meter tape was placed perpendicularly at one-meter intervals to delineate each quadrat. The 90-degree angle formed by the two tapes was checked for accuracy with compass sights/bearings and the right angle of a prefabricated 1m x 3m quadrat. With the aid of a random number chart, twenty-seven of the 210 quadrats within each macro-plot were randomly selected and sampled. See appendix II and III for macro-plot locations throughout Occurrence 1.
Because it was such a large area, Occurrence 2 (Figure 7) was not delineated with pin flags. Instead, its extent was mapped with a GPS unit (Leica GS50) to estimate total area. The area was broken into a 50 meter by 50meter grid on a topographical map with a number assigned to each separate grid. A random number chart was then used to objectively select a grid number. These selected grids became GPS coordinates by using the center of each grid as a GPS point. Each coordinate randomly selected by this process, became the location for the installation of a 30 meter by 21 meter rectangular macro-plot. The same size macro-plots used for Occurrence 1 were installed within Occurrence 2 and sampling within the macro-plots was conducted in the same fashion. A total of 15 macro-plots were constructed within Occurrence 2 with 39 quadrats sampled per macro-plot (higher than the 27 quadrats per macro-plot sampled within Occurrence 1). See appendix IV and V for macro-plot locations throughout Occurrence 2.
Once the locations and size of macro-plots were determined, counting of individual raments began. Each quadrat was hand counted to determine the average number of plants per quadrat, so an estimate on the number of plants could be extrapolated for the complete range of Amargosa niterwort within the study site.
Ash Meadows gumplant sampling
Random GPS coordinates were selected within the study site for the Ash Meadows gumplant (Figure 8) by breaking the area up into a 50 meter by 50 meter grid on a
topographical map. A random number chart was then used to objectively select a grid number. These selected grids became GPS coordinates by using the center of each grid as a GPS point. Each coordinate randomly selected by this process, became the location for the installation of a 32 meter by 32 meter square macro-plot. Each macro-plot was divided into 256 1-meter by 4-meter quadrats. Sampling methodology followed the same protocol as the Amargosa niterwort sampling design, though the size of the macro-plots was changed and the number of quadrats sample within in each macro-plot was increased to 40. See appendix VI and VII for locations of macro-plots throughout Ash Meadows gumplant area.
Sampling of Ash Meadows gumplant occurred only within the main distribution area in which the plant was found in an effort to eliminate large statistical deviations. Outside of this main occurrence outlying individuals were mapped and hand counted. Positions for all isolated individual plants were recorded with a sub-meter GPS unit (Leica GS50) and hand-counted estimates were taken. Refer to appendix VIII for location and description of outlying occurrences of Ash Meadows gumplant.
The nine macro-plots sampled within Occurrence 1 yielded an estimate of 243,478 +/- 69,337 raments at 95% confidence interval. The 20 macro-plots from Occurrence 2 yielded an estimate of 28,951 +/- 20,372 ramets at 95% confidence interval. The estimated total number of ramets within the Amargosa niterwort critical habitat is 272,429 +/- 89,709. The density per quadrat for occurrence 1 is17.917 with a standard deviation of 41.001, while the density per quadrat for occurrence 2 is 0.092 with a standard deviation of 0.429. The total area occupied by occurrence 1 (figure 7, orange polygon) is 4.08 hectares while the total area occupied by occurrence 2 (figure 7, black outlined polygon) is 169.37 hectares.
The 1m x 3m size quadrat worked best in eliminating as many zero counts or extremely high counts as possible, while still allowing a reasonable count of plants within each quadrat. The standard deviation for population estimate of Occurrence 1 (41.001) is fairly high, but is believed to be reflective of the clumped distribution of the plants. The rhizomatous growth pattern and highly specific habitat needed to support the Amargosa niterwort encourages this extremely clumped distribution across the slough and made more accurate sampling of the plant difficult. Increased sampling of the plant is not likely to lower the standard error significantly, although increased sampling with a more efficient quadrat size might lower the standard deviation.
On rare occasions a form of Amargosa niterwort was encountered that exhibited characteristics unlike the standard type found within the species. The plant was close in form to the Amargosa niterwort, but it also showed characteristics similar to those of the common niterwort (Nitrophila occidentalis). This intermediate form was rarely encountered on the study site, although one small grouping was located within one of the macro-plots. This grouping was counted as Amargosa niterwort for that macro-plot. The intermediate form is most likely a variation within the species that is not accounted for in current keys for the species, but hybridization may be a viable alternate explanation. Further research into the origin of these unique forms may be warranted.
Ash Meadows gumplant.
The 11 macro-plots yielded 241,514 +/- 69,660 plants within the study site. The density per quadrat is 3.12 with a standard deviation of 8.349. The total area sampled is 35.75 hectares (Figure 8).
Although sampling occurred in an area of fairly uniform distribution, large portions of the area were still devoid of plants. The highly dependent nature of the plant to water made for dense occurrence along slough channels followed by gaps between channels that were essentially devoid or sparsely populated. The gradient distribution of plant numbers in relation to the waterways of the slough may account for the high standard deviation.
DISCUSSION AND RECOMMENDATIONS
There appears to be a high dependency on surface/near surface water and saline soil conditions for both Amargosa niterwort and Ash Meadows gumplant which may restrict the range of both species. Although population numbers of both species are rather high, the relative area these two species occupy is just a few square miles. It is possible that any reduction of groundwater or diversion of surface water is likely to have adverse effects on current plant populations. Increasingly high demands for water from both Pahrump Valley and Las Vegas in Nevada and Inyo County in California might threaten the survival of these two species. Conservation of these two species is highly dependent on proper management of the local groundwater system.
The alkali salt flats, like those which Amargosa niterwort thrives on, have developed from the defused discharge of the regional aquifer (Threloff, D.L. 1998). It is unknown at this time whether Amargosa niterwort uses ground water discharge, or surface water from upstream discharge or a combination of both. Further study, using isotopic analysis, would provide the data needed to make this determination and is highly recommended. Physiological and ecological evidence demonstrate that water uptake is near the surface. Shallow placed spreading rhizomes with abundant numbers of plants in and along water channels of the slough is evidence of a high dependency on water occurring at or near the surface. The slow diffusion of groundwater may play a major role in supporting the plant during dry months of the year.
The proposed lower Carson Slough ACEC encompasses all known occurrences of Ash Meadows gumplant in California, but falls short in its southerly direction to cover the complete range of the Amargosa niterwort. A suitable size and shape for the ACEC to protect all known ranges is advised. Critical habitat should be expanded to include the complete range of both species. A recommended critical habitat for both species is provided in Figure 9 and 10, encompassing the current ranges of the plants and their preferred habitat as demonstrated by this study. However, a multi-year study us recommended to more accurately demonstrate the true range for each species.
Minor threats to both species include possible trampling by feral animals or livestock, and off road-vehicle damage to the habitat. A gated entrance and fencing along the state line might prevent further vehicle degradation of the area, and upkeep of the previously
Figure 9. Recommended Ash Meadows gumplant critical habitat (green outline).
Figure 10. Recommendeded Amargosa niterwort critical habitat (green outline).
installed fencing should deter most off-road enthusiasts from entering the critical habitat via State Line Road.
A precautionary measure to preventing a possible extirpation or extinction of the two species would be the establishment of an ongoing monitoring program of both plant populations to help determine the amount of water needed to sustain a viable population in the area.