Age and population structure of Joshua trees ( Yucca brevifolia ) in the northwestern Mojave Desert

Many desert perennials are long-lived, but there are few data on ages or population structures of desert plants. We used 2 methods to estimate ages of a population of Yucca brevifolia (Joshua tree) in southwestern Utah from a 14-year census of plant sizes. Plant height at the 1st census ranged from 0.08 m to 6.0 m, and trees grew in height at a mean rate of 3.75 cm ? yr -1 . Plants also increased slowly in basal diameter (0.142 cm ? yr -1 ) and branch length (0.024 m ? yr -1 ), but basal diameter varied greatly from year to year, with many plants decreasing in diameter between sequential censuses. Forms of a nonlinear growth equation (the Richards function) did not fit the data on growth in height, but a simple linear relationship fit the data well. Using height as a linear predictor of age, we estimated that ages of a sample of 69 Joshua trees ranged from 300 years old, with 75% of the trees between 20 and 100 years and 11% over 150 years old. These data support the assumption that Joshua trees are indeed long-lived.     

Fertile island development around perennial shrubs across a Mojave Desert chronosequence

Spatial heterogeneity has been examined only recently as a factor in studies of ecosystem processes. The effect of this factor on desert organisms is exemplified in the ""fertile island"" a concentration of essential soil nutrients under perennial shrubs. In this study we examined fertile island patterns in undisturbed desert areas and on abandoned roadways to see at what rate and magnitude these fertile patches reappeared after disturbance. Small-scale examination of patterns of soil P, organic matter, moisture, and bulk density showed that soils beneath Larrea tridentata (DC.) Cov. shrubs on the roads lacked the tight circular gradient in these variables that was characteristic of soils beneath control shrubs, even 88 years after road abandonment. The nature of the initial soil disturbance altered both spatial patterns of soil N and temporal patterns of fertile island development. Fertile island patterns for total soil N, available P, and organic matter were more circular than patterns for bulk density, texture, or pH. We suggest that patterns of soil heterogeneity may develop first for elements that may be limiting to desert shrub growth (N, P, organic matter), followed by spatial development in other less limiting soil factors (bulk density, texture, pH).     

Arbuscular mycorrhizae of Mojave Desert plants

Roots of 15 Mojave Desert plant species were assessed for arbuscular mycorrhizal (AM) colonization in spring and autumn 1999. Another 19 species were assessed in autumn only. Perennial species were AM colonized, whereas annual species contained very low levels of AM hyphae or were nonmycorrhizal. The nonnative tree Tamarix ramosissima was also nonmycorrhizal. Levels of hyphae changed over the season in 2 species, arbuscle levels increased in 1 species, and vesicles decreased in 7 species. Mycorrhizal inoculum potential (MIP) was assessed in the soils associated with spring-collected plants. All soils were found to contain MIP. MIP values were not correlated with AM root colonization.     

Selective herbivory by the desert woodrat ( Neotoma lepida ) on Joshua trees ( Yucca brevifolia )

We studied herbivory by the desert woodrat ( Neotoma lepida ) on Joshua trees ( Yucca brevifolia ) in the Mojave Desert to determine whether N. lepida fed selectively based on leaf nitrogen content. We measured leaf nitrogen content and the location and amount of herbivory by woodrats on Y. brevifolia trees in southwestern Utah, USA. Neotoma lepida removed the outer tips of leaves (1/3 to 2/3 of total leaf length) but left leaf bases, which had lower nitrogen content, uneaten. Herbivory by N. lepida also was concentrated on rosettes that were oriented south, and these had significantly higher nitrogen content than rosettes that were oriented north. Finally, N. lepida fed more on trees that had higher mean leaf nitrogen contents. Thus, N. lepida selectively foraged among leaf parts, among rosettes of leaves, and among trees of Y. brevifolia , and disproportionate foraging was correlated with nitrogen content of leaves.     

Coordination of branch orientation and photosynthetic physiology in the Joshua tree ( Yucca brevifolia )

Despite the profusion of light in deserts, morphological adaptations to increase light interception are common among desert plants. We studied branch orientation and related physiological parameters in the Mojave Desert Joshua tree, Yucca brevifolia (Agavaceae). Azimuth and inclination were measured on all leaf rosettes of 44 Y. brevifolia trees. Interception of solar radiation was modeled for leaves in hypothetical rosettes facing due south and due north in December, March, and June. Carbon isotope discrimination, nitrogen content, and conductance of water vapor were measured in leaves from north- and south-facing rosettes. Rosette azimuths were nonrandom; rosettes predominantly faced southeast. North-facing rosettes were more steeply inclined than those facing south. The preponderance of south-facing rosettes reduces self-shading and increases interception of solar radiation during the winter-spring growth period. Stomatal conductance was higher for leaves in south-facing than in north-facing rosettes. Nevertheless, discrimination against 13 C was less in leaves of south-facing rosettes, indicating that average intercellular CO 2 concentration was also lower. South-facing whorls had higher leaf nitrogen content. Greater allocation of nitrogen to leaves in south-facing whorls probably results in those leaves having a greater photosynthetic capacity than their north-facing counterparts. Orientation of rosettes to increase interception of sunlight during the period most favorable for photosynthesis, coupled with allocation of nutrients to maintain a higher photosynthetic capacity in those rosettes, should significantly increase whole-plan carbon gain in Y. brevifolia .     

Predatory bird populations in the east Mojave Desert, California

We surveyed 7 species of predatory birds weekly during a 12-month period (December 1992 through November 1993) in the east Mojave Desert, California. The Common Raven ( Corvus corax ) was the most frequently observed species with an average of 6.9 sightings per 100 km. Turkey Vultures ( Cathartes aura ), Red-tailed Hawks ( Buteo jamaicensis ), Loggerhead Shrikes ( Lanius ludovicianus ), American Kestrels ( Falco sparverius ), Golden Eagles ( Aquila chrysaetos ), and Prairie Falcons ( Falco mexicanus ) were seen in decreasing order of frequency of observation through the study period. Ravens, Red-tailed Hawks, Loggerhead Shrikes, American Kestrels, and Prairie Falcons were seen throughout the year. Turkey Vultures were not present during winter months, while Golden Eagles were seen only during November and December. Turkey Vultures, Red-tailed Hawks, and ravens were most numerous on agricultural lands, while Loggerhead Shrikes were most common at urban areas. Raven numbers increased with increasing number of linear rights-of-way parallel to the survey route. Perching was the most common behavior type, although Turkey Vultures and ravens were often observed soaring, flying, or standing on the ground near highways. Transmission powerline towers and telephone poles were used as perch sites disproportionately to availability.     

Distributional changes and population status of amphibians in the eastern Mojave Desert

Several amphibian species historically inhabited sparsely distributed wetlands in the Mojave Desert of western North America, habitats that have been dramatically altered or eliminated as a result of human activities. The population status and distributional changes of amphibians were investigated over a 20,000-km 2 area in the eastern Mojave Desert in 2 ways. For upland sites (i.e., sites outside of major valleys and river floodplains), where wetland habitat is almost exclusively springs, encounter surveys were conducted at 128 sites in 1997-1999, and results were compared to historical (pre-1970) locality records. For lowland sites (i.e., sites within major valleys and river floodplains), locality records and field surveys in 1995-2004 were reviewed to detect changes in distribution over time. Amphibians were found at 79% of upland sites. By far the most common species was the red-spotted toad ( Bufo punctatus , 73% of sites), followed by the Pacific chorus frog ( Pseudacris regilla ), Woodhouse's toad ( B. woodhousii ), relict leopard frog ( Rana onca ), and the introduced American bullfrog ( R. catesbeiana ). Taxa observed or collected in the lowlands since 1990 were Woodhouse's toad, Pacific chorus frog, American bullfrog, and the introduced tiger salamander ( Ambystoma tigrinum ). Four taxa (Vegas Valley leopard frog [ Rana sp.], Arizona toad [ B. microscaphus ], Great Plains toad [ B. cognatus ], and Great Basin spadefoot [ Spea intermontana ]) had historical records but no evidence of occurrence in the study area within the past 5 decades. The amphibian fauna of the study area has changed dramatically in the past century, primarily at lowland sites where habitat loss and modification have been extreme. Striking changes are the nearly complete replacement of native leopard frogs (i.e., Vegas Valley and relict leopard frogs) with the introduced bullfrog, and the complete replacement of the Arizona toad in Las Vegas Valley with Woodhouse's toad or hybrids with predominantly Woodhouse's traits. In contrast, the distributions of 2 species characteristic of upland springs, red-spotted toad and Pacific chorus frog, appear to have changed little from their historical distributions, despite habitat modification at many sites.     

Desert rodents reduce seedling recruitment of Salsola paulsenii

Heteromyid rodents in the deserts of North America have been shown to harvest large quantities of seeds of both native and introduced plants from soil seed banks, but rarely has the impact of this seed removal been demonstrated experimentally. I used a series of fenced plots (some of which excluded rodents) to demonstrate that heteromyids at a western Nevada study site can measurably reduce seed banks and subsequent seedling establishment of Salsola paulsenii , an introduced invasive weed that has become a significant problem over much of the desert Southwest. The frequency of S. paulsenii seedlings in both 2004 and 2005 was significantly greater around the interior perimeters of plots that permitted access by rodents than in plots that excluded rodents. Density of S. paulsenii seedlings was significantly greater inside than outside rodent exclusion plots, but there was no such difference in seedling density inside versus outside plots that permitted rodent access. Salsola paulsenii has such a conspicuous presence in many desert environments that the effect of rodents in reducing its abundance may not be visually apparent; however, rodents may still ameliorate competitive effects of this weed on coexisting plants. Heteromyid rodents disperse seeds through caching, and they also consume them. Caching may enhance establishment of native plant seedlings, but is unlikely to benefit exotics such as S. paulsenii .     

Experimental manipulations of fertile islands and nurse plant effects in the Mojave Desert, USA

In a mixed desert shrub community we removed and added shrub canopies to examine above- and belowground influences of 3 species of shrubs on islands of soil fertility and the survival of transplanted Ambrosia dumosa seedlings. Soils sampled under shrubs in the wet season had higher pH, water content, organic matter, and both total and mineralizable nitrogen than soils in adjacent open areas, confirming a widely established pattern in arid lands. However, we also found species differences in soil parameters. Soils under Coleogyne ramosissima had highest pH, soils under A. dumosa had highest water content and nitrogen mineralization rates, and soils under Larrea tridentata had lowest water content. Soils sampled under shrubs in the dry season, 7 months after experimental shrub removal, maintained higher organic matter and total and mineralizable nitrogen content than adjacent open soils, but pH and water were altered by shrub manipulations. Species differences persisted only in soil water levels ( A. dumosa soils were driest). Over a 1-year period, transplanted A. dumosa seedlings had highest survivorship in shrub removal and open treatments and died most rapidly under control shrubs of all 3 species, suggesting that shrubs had a strong negative effect on seedling survival, even in the presence of higher organic matter, nutrients, and (initially) higher water content of fertile islands. Our results suggest that nurse plants and islands of soil fertility have the potential to facilitate growth of other species by nutrient additions, but that the net effect of nurse plants can be negative due to shading and/or root competition.     

Competition Between Adult and Seedling Shrubs of Ambrosia dumosa in the Mojave Desert, Nevada

Seeds of the perennial shrub Ambrosia dumosa germinated in abundance following 11 days of rain during August 1983 at a study site in the northern Mojave Desert. Seedling establishment, growth, and reproduction were observed in natural vegetation and in an area that had been previously cleared of vegetation. For 5,527 A. dumosa seedlings, percent survival in April 1986 averaged 3% in the undisturbed vegetation and 58% in the denuded area. Seedlings occupying the cleared area had grown to sizes up to 0. 1 m 3 m by October 1984; some produced flowers and fruit in the spring of 1985. Surviving seedlings in the undisturbed vegetation were all smaller than 0.001 m 3 and did not reproduce. These pronounced differences in growth, survival, and reproduction associated with the presence or absence of adult shrubs demonstrated an intense competition that is incompatible with indications of mild competition from nearest-neighbor analyses. I therefore hypothesize that competition for water occurred, not by competition for water in two dimensions but by rapid use of a common resource, as if several people were drinking with straws from a common cup. This temporal mechanism would strongly favor adults over seedlings.     

Plant community patterns in unburned and burned blackbrush ( Coleogne ramosissima Torr.) shrublands in the Mojave Desert

The blackbrush vegetation type is dominated by Coleogyne ramossisima , which is thought to preclude the coexistence of many other plant species. Fire can remove blackbrush cover and possibly increase plant species richness and evenness. Fire also may increase the frequency and cover of alien annual grasses, thereby intensifying landscape flammability. We tested these predictions in unburned and burned (6-14 years postfire) blackbrush at 3 sites spanning the range of this vegetation type in the Mojave Desert. Species richness in unburned blackbrush was similar to published values for vegetation types in western North America, bur richness varied significantly among the 3 sites and 4 spatial scales (1, 10, 100, and 1000 m 2 ). Richness values declined in order from annual forbs, woody perennials, herbaceous perennials, annual grasses, cacti, to perennial grasses. Fire reduced Coleogyne cover, thus boosting species evenness. In contrast, species richness decreased after burning, although the results varied among spatial scales. Total cover was unaffected by fire because cover of wood perennials decreased, while cover of annual forbs, annual grasses, herbaceous perennials, and perennial grasses increased. Native species richness and cover decreased, whereas alien richness and cover increased after burning especially where the alien forb Erodium cicutarium was present. Fire had no effect on frequency and variable effects on cover of alien annual grasses. These results indicate that in blackbrush species richness can vary among sites and local spatial scales, and effects of fires can vary among plant life-forms and between natives and aliens.     

Effects of horse grazing in spring on survival, recruitment, and winter injury damage of shrubs

The use of domestic grazers to shift the growth advantage toward shrubs is a commonly applied tool on winter ranges managed primarily for big game. Results from horses grazing in spring indicated grazing also benefits shrub survival, seedling recruitment, and reduced winter injury damage on some species of shrubs.     

Microhabitat-specific controls on soil respiration and denitrification in the Mojave Desert: the role of harvester ant nests and vegetation

Seed harvesting ants ( Pogonomyrmex rugosus ) concentrate organic matter and nutrients near their nests and create biogeochemical hotspots in desert soil. We examined factors regulating denitrification and soil respiration in a Mojave Desert ecosystem to determine the role harvester ant colonies play in nitrogen loss and carbon mineralization. Organic matter and nutrient storage were significantly greater in colonies than under the dominant vegetation (i.e., Pleuraphis rigida , a bunch grass) and in bare soil, with standing stocks of inorganic nitrogen in colonies nearly 4-fold greater than in the other microhabitats. Soil respiration, measured with laboratory incubations, was below detection limits under ambient soil conditions. Respiration rate in soil from bare patches and under grass was limited by water and labile organic carbon, with secondary nitrate limitation evident only once carbon limitation was alleviated. Soil respiration in ant nest soil was limited by water and labile organic carbon only. Denitrification, measured by the acetylene block technique, was elevated in bare soil and under grass with the addition of nitrate. Ant nest soil also responded to a nitrate addition; however, denitrification rate was greatest with addition of glucose. Ordinarily desert soil has low rates of respiration and denitrification due to dry ambient conditions. However, ant colonies likely function as important sites for nitrogen loss and carbon mineralization following rain storms, especially if storms coincide with seed set and the temporally pulsed input of organic matter into colonies.     

RCLUS, a new program for clustering associated species: a demonstration using a Mojave Desert plant community dataset

This paper presents a new clustering program named RCLUS that was developed for species (R-mode) analysis of plant community data. RCLUS identifies clusters of co-occurring species that meet a user-specified cutoff level of positive association with each other. The "strict affinity" clustering algorithm in RCLUS builds clusters of species whose pairwise associations all exceed the cutoff level, whereas the "coalition" clustering algorithm only requires that the mean pairwise association of the cluster exceeds the cutoff level. Both algorithms allow species to belong to multiple clusters, thus accommodating both generalist and specialist species. Using a 60-plot dataset of perennial plants occurring on the Beaver Dam Slope in southwestern Utah, we carried out RCLUS analyses and compared the results with 2 widely used clustering techniques: UPGMA and PAM. We found that many of the RCLUS clusters were subsets of the UPGMA and PAM clusters, although novel species combinations were also generated by RCLUS. An advantage of RCLUS over these methods is its ability to exclude species that are poorly represented in a dataset as well as species lacking strong association patterns. The RCLUS program also includes modules that assess the affinity of a given species, plot, or environmental variable to a given cluster. We found statistically significant correlations between some of the RCLUS species clusters and certain environmental variables of the study area (elevation and topographical position). We also noted differences in clustering behavior when different association coefficients were used in RCLUS and found that those incorporating joint absences (e.g., the phi coefficient) produced more clusters and more even numbers of species per cluster than those not incorporating joint absences (e.g., the Jaccard index). In addition to the species association application described in this paper, the RCLUS algorithms could be used for preliminary data stratification in sample (Q-mode) analysis. The indirect link between sample plots and RCLUS species clusters could also be exploited to yield a form of "fuzzy" classification of plots or to characterize species pools of plots.     

New variety of Yucca harrimaniae (Agavaceae) from Utah

Yucca harrimaniae var. sterilis Neese & Welsh is named and described from the Uinta Basin of Utah. The plant is strongly rhizomatous, evidently sterile, and has limber, sparingly filiferous leaves that tend to recline on the ground.         

Electrophoresis of isoenzymes of 16 western shrubs: technique development

Wildland shrubs have gained considerable attention in recent years due to increasing recognition of their values as animal feed, as wildlife habitat, and for land reclamation. Better management of the shrub resource will be possible through clearer taxonomic identification and better understanding of phylogenetic relationships. This study applied polyacrylamide gel electrophoresis and further developed this technique to address genetic relationships among 16 paired shrub species (genera: Artemisia, Chrysothamnus, Atriplex, Ceratoides, Sarcobatus, Purshia, Cowania, and Cercocarpus [Compositae, Chenopodiaceae, Rosaceae]). Cluster analysis of similarity values for total protein and 14 isoenzyme systems gave patterns of species relationships expected from classical morphological grounds with two minor exceptions. Isoenzyme analyses showed promise for solving taxonomic, phylogenetic, and population genetics problems.     

Effects of browsing by native ungulates on the shrubs in big sagebrush communities in Yellowstone National Park

The effects of elk ( Cervus elaphus ), pronghorn ( Antilocapra americana ), and mule deer ( Odocoileus hemionus ) browsing on shrubs in big sagebrush ( Artemisia tridentata ) communities were monitored over a 31-year period in Yellowstone National Park. Ungulates were restricting Wyoming big sagebrush (spp. wyomingensis ) heights, size, and recruitment on the lower-elevation stratum only, while no such suppression was observed on the high-elevation stratum. Parallel increases in mountain big sagebrush (spp. vaseyana ) densities and cover occurred over the study period on both browsed and unbrowsed sites at the higher-elevation stratum, although big sagebrush, green rabbitbrush ( Chrysothamnus viscidiflorus ), and horsebrush ( Tetradymia canescens ) were slightly taller and crown sizes were slightly larger on unbrowsed than browsed sites. Wyoming big sagebrush utilization (percent leader use) was eight times higher (￣ x = 87 ± 7.2% by pronghorns, mule deer, and elk) on the low-elevation winter ranges stratum (the Boundary Line Area [BLA] portion of the winter range), while mostly mountain big sagebrush with leader use averaged only 11 ± 4.1% (nearly all by elk) on the high-elevation range stratum. In addition, annual aboveground biomass production of big sagebrush did not differ between browsed and unbrowsed study sites on the high-elevation stratum of the winter range. Population turnover was higher on browsed plots versus unbrowsed plots. No difference was observed in percent dieback of big sagebrush adult plants between browsed and unbrowsed plots at the higher stratum. Browsing did not influence the number of leaves or seedstalks per plant ( P > .05), but leaves averaged 45% longer and seedstalks 42% longer on browsed big sagebrush. Ungulate browsing, however, apparently suppressed production, germination, and survival of Wyoming big sagebrush on the low-elevation stratum. Numbers of Wyoming big sagebrush declined 43% and cover declined 29%, 1957-1990, on browsed sites on the BLA. Annual biomass production on browsed sites at the low-elevation stratum was only 6-35% that of unbrowsed sites, and big sagebrush recruitment was less on browsed sites. Percent leader use of big sagebrush did not differ between the period of ungulate reductions, 1962-1969, and the 1980s on the lower stratum (￣ x = 87% leader use), but utilization was less on higher portions of the winter range during the period of elk reductions (￣ x = 2%) than during the 1980s following cessation of elk controls (￣ x = 11%).     

Dispersal characteristics of the yucca weevil ( Scyphophorus yuccae ) in a flowering field of Yucca whipplei

Dispersal characteristics were measured for a population of yucca weevils ( Scyphophorus yuccae ) in a plot consisting of flowering and nonflowering Yucca whipplei . We compared weevil dispersal to yucca distribution, pheonolgy, and caudex temperature. We also compared weevil movement to wind patterns and time of day. Captured weevils were marked and released into both flowering and nonflowering home plants in the field. Distance traveled, weevil flight direction, and target plant characteristics were recorded. We found that yucca weevils moved only between 1600 h on the release day and 0600 h of the following day. We recorded movement from both nonflowering and old flowering (> 1/2 stalk had reached anthesis) yuccas to new flowering (> 1/2 stalk pre-anthesis) yuccas. The pattern of weevil movement did not match the pattern of flowering yuccas in the field. Yucca weevils moved a mean distance of 33 ± 8 m. Caudex temperature appeared to be important to maintain a population of weevils on a plant. Wind direction was the best predictor of weevil dispersal direction. Weevils consistently moved into the wind, suggesting that they are active fliers. Dispersal characteristics of the yucca weevil have implications for the evolution of the semelparous flowering strategy of Y. whipplei and S. yuccae life history.     

Reptile population changes with manipulation of Sonoran Desert shrub

The diversity and abundance of reptiles were studied in three vegetation types on the Santa Rita Experimental Range, Arizona. Total reptile sightings were greatest in undisturbed mesquite and mesquite with irregularly shaped clearings. No zebra-tailed lizards ( Callisaurus draconoides ) or desert spiny lizards ( Sceloporus magister ) were seen, and significantly fewer western whiptails ( Cnemidophorus tigris ) were in the mesquite-free area. Only the Sonora spotted whiptail ( Cnemidophorus sonorae ) was significantly more abundant in the mesquite-free area than in the undisturbed mesquite. In an effort to increase grass production for cattle in mesquite grasslands, it is preferable to clear irregularly shaped areas rather than to attempt total mesquite removal, if reptiles are to be considered.