Ambrosia Dumosa Research Articles

Soil resource heterogeneity in the Mojave Desert

Heterogeneity of soil resources was investigated in the Mojave Desert to better understand spatial variability of soil attributes in a Larrea tridentata–Ambrosia dumosa community. Six soil samples were collected at each of eight microsites, with this sampling scheme replicated for three sites that were 0·7 km apart on a 10-km long bajada. The eight microsites were: five different shrub types (individual shrubs of L. tridentata, individual shrubs of Lycium pallidum , individual shrubs of A. dumosa, mixtures of shrub species with small mammal burrows, and mixtures of shrubs without small mammal burrows), a grass microsite (around individual plants of the tussock grass Pleuraphis rigida), and two microsites without perennial vegetation (one within small washes and the other within the interspace between perennial plants). Fifteen soil parameters were assessed, and all were found to differ among microsites. Nutrients were assessed as the pools extractable by vegetation. Mineral nutrients except for Ca, organic matter, and litter were higher in soils collected from shrub microsites compared to the others. Shrub microsites with small mammal burrows had significantly more N and P than those without burrows. Non-vegetated microsites in washes and plant interspaces contained very low N and P levels. Mg, Na, pH and cation exchange capacity were highest in soils under Lycium shrubs, while Ca levels were highest in wash soils. Arbuscular mycorrhizal spore densities were generally low (0–0·2 spores g −1 soil) except for Lycium soils, which contained 1·8 spores g −1 soil. Wash and interspace soils lacked spores but had mycorrhizal inoculum potential values similar to the other microsites, indicating that mycorrhizal propagules were present. Thus, the presence of perennial shrubs strongly influenced microsite soil characteristics and resulted in higher nutrient levels. Small mammal burrows further enhanced the mineral nutrient content of soils. In addition, all soil characteristics except for spore density differed among the three sites across the bajada, and interactions between microsite and site were common. Clearly, soil resources for plants are spatially heterogeneous in this desert ecosystem and differ greatly between adjacent micro- and macrosites in this L. tridentata–A. dumosa community.

Positive and negative plant interactions contribute to a north-south-patterned association between two desert shrub species.

Abiotic factors are often thought to be the predominant forces shaping desert plant communities. But both positive and negative interactions between plants are frequently observed in deserts, and it is an open question whether they can strongly affect the spatial structure of a desert community. The goal of this study was to answer this question for a plant community in the North American Mojave Desert. Two semi-shrub species, Ambrosia dumosa and Acamptopappus sphaerocephalus, were the focus of this study. At the study site, seedlings emerged predominantly on the northern side of shrubs, indicating positive effects of canopy shading on emergence, but survival of Ambrosia seedlings was much higher in open areas than at the edge of conspecific shrubs. Negative intraspecific interactions also affected Ambrosia shrubs, which did not increase in size over a 4-year period unless the nearest conspecific neighbor had been removed. These negative intraspecific interactions among different life stages of Ambrosia appear to contribute to spatial segregation observed among shrubs of this species. In contrast, Acamptopappus shrubs and their seedlings were aggregated with Ambrosia shrubs, and occurred more often on the northern side of Ambrosia than expected by chance. Removal of Ambrosia neighbors positively affected growth of Acamptopappus, but only when the neighbor was removed on the northern side. For Acamptopappus, an Ambrosia neighbor on the southern side may have some positive effects, which appear to neutralize the negative effects found for northern neighbors. These positive effects were likely at least partly due to shading. Removal of Ambrosia neighbors negatively affected predawn xylem pressure potentials of Acamptopappus, but this effect was only found during one growing season and was briefly reversed during the next. In summary, negative intraspecific interactions appear to cause spatial segregation of Ambrosia shrubs, while a combination of positive and negative interactions apparently contribute to the directional association between Ambrosia and Acamptopappus. Thus plant interactions in this desert appear to shape community structure in at least two dimensions by influencing the distances and in which directions to their neighbors plants can grow and survive.

Ecological Responses of Two Mojave Desert Shrubs to Soil Horizon Development and Soil Water Dynamics

In the arid southwestern United States, subtle differences in soil horizon development affect seasonal soil hydrology and consequently influence plant performance and community structure. We measured canopy development, population structure, and seasonal ecophysiology (predawn water potential, ψpd, and midday net photosynthetic assimilation, Anet) of two co-dominant warm-desert shrubs, the evergreen Larrea tridentata and drought-deciduous Ambrosia dumosa, in five Mojave Desert soils varying in surface and sub-surface soil development, and we used process-based soil hydrology modeling output to determine longer-term soil water dynamics underlying soil/plant responses. We hypothesized that ecophysiological performance would covary with plant development, which would reflect soil hydrological characteristics. Among three sites on alluvial fan deposits of different geological ages (Young Alluvial, <4000 yr BP; Intermediate Alluvial, ∼12 000 yr BP; Old Alluvial, ∼40 000 yr BP), total canopy volume of Larrea (cubic meters per 100 m2 ground area) was highest on the Young Alluvial site, in close agreement with soil modeling results showing that these coarse-textured, weakly developed soils permit deeper water infiltration. In older, stronger developed soils, infiltration and persistence of soil water was sharply reduced, which was reflected by lower individual Larrea plant volumes. However, during peak spring conditions, ψpd and Anet were highest in Larrea at the Intermediate Alluvial site (−4.2 ± 0.32 MPa and 3.2 ± 0.91 μmol·m−2·s−1), where soils had substantial surface and subsurface horizons, and at the Pavement site, where soils had strong surface layers but little subsurface development. Concurrent plant performance at the Young Alluvial site was unexpectedly low (−4.8 ± 0.49 MPa and 1.7 ± 0.56 μmol·m−2·s−1, respectively). During summer drought ψpd and Anet remained high in Intermediate Alluvial plants, but were extremely low in Pavement site Larrea (−8.17 MPa and −0.04 μmol·m−2·s−1, respectively), due to curtailed infiltration of summer precipitation. These findings suggest that Larrea growing in older soils experience greater mortality and reduced growth but are not subject to strong intra-specific competition resulting from the persistence of large individuals apparent in younger, coarser textured soils. In contrast to Larrea, density of Ambrosia increased with soil horizon development, but smaller plant sizes resulted in similar canopy volume per area, and identical ψpd and Anet across all soils where it occurred, suggesting greater plasticity to the transmittal of precipitation. These findings show that a strong geomorphology and soils context is essential for understanding the variation in plant responses and vegetation structure in desert environments.

ECOLOGICAL RESPONSES OF TWO MOJAVE DESERT SHRUBS TO SOIL HORIZON DEVELOPMENT AND SOIL WATER DYNAMICS

In the arid southwestern United States, subtle differences in soil horizon development affect seasonal soil hydrology and consequently influence plant performance and community structure. We measured canopy development, population structure, and seasonal ecophysiology (predawn water potential, ψpd, and midday net photosynthetic assimilation, Anet) of two co-dominant warm-desert shrubs, the evergreen Larrea tridentata and drought-deciduous Ambrosia dumosa, in five Mojave Desert soils varying in surface and sub-surface soil development, and we used process-based soil hydrology modeling output to determine longer-term soil water dynamics underlying soil/plant responses. We hypothesized that ecophysiological performance would covary with plant development, which would reflect soil hydrological characteristics. Among three sites on alluvial fan deposits of different geological ages (Young Alluvial, <4000 yr BP; Intermediate Alluvial, ∼12 000 yr BP; Old Alluvial, ∼40 000 yr BP), total canopy volume of Larrea (cubic meters per 100 m2 ground area) was highest on the Young Alluvial site, in close agreement with soil modeling results showing that these coarse-textured, weakly developed soils permit deeper water infiltration. In older, stronger developed soils, infiltration and persistence of soil water was sharply reduced, which was reflected by lower individual Larrea plant volumes. However, during peak spring conditions, ψpd and Anet were highest in Larrea at the Intermediate Alluvial site (−4.2 ± 0.32 MPa and 3.2 ± 0.91 μmol·m−2·s−1), where soils had substantial surface and subsurface horizons, and at the Pavement site, where soils had strong surface layers but little subsurface development. Concurrent plant performance at the Young Alluvial site was unexpectedly low (−4.8 ± 0.49 MPa and 1.7 ± 0.56 μmol·m−2·s−1, respectively). During summer drought ψpd and Anet remained high in Intermediate Alluvial plants, but were extremely low in Pavement site Larrea (−8.17 MPa and −0.04 μmol·m−2·s−1, respectively), due to curtailed infiltration of summer precipitation. These findings suggest that Larrea growing in older soils experience greater mortality and reduced growth but are not subject to strong intra-specific competition resulting from the persistence of large individuals apparent in younger, coarser textured soils. In contrast to Larrea, density of Ambrosia increased with soil horizon development, but smaller plant sizes resulted in similar canopy volume per area, and identical ψpd and Anet across all soils where it occurred, suggesting greater plasticity to the transmittal of precipitation. These findings show that a strong geomorphology and soils context is essential for understanding the variation in plant responses and vegetation structure in desert environments.

The Effects of Neighbors on the Demography of a Dominant Desert Shrub (Ambrosia dumosa)

The Effects of Neighbors on the Demography of a Dominant Desert Shrub (Ambrosia dumosa)

THE EFFECTS OF NEIGHBORS ON THE DEMOGRAPHY OF A DOMINANT DESERT SHRUB (AMBROSIA DUMOSA)

We used patch-specific matrix models to test the influence of neighboring plants on the demography of Ambrosia dumosa, a dominant perennial shrub of the Colorado Desert in southern California. In the desert literature, the presence or absence of neighboring plants is reported to influence plant growth and survival and has long been associated with plant–plant interactions that range from extreme competition to facilitation. Here we consider the less addressed question of how neighbor-defined population subsets contribute differently to overall population dynamics. Demographic data collected from >6000 individual plants from a permanently mapped hectare over 10 yr were used to divide this A. dumosa population into subsets that were defined by the presence or absence of neighbors. By partitioning the population in this way, we documented differences in population growth, elasticity structure, and stable size structure among the distinct patch types, and evaluated the contributions of each patch type to overall population dynamics. Population growth was consistently higher for population subsets of plants that were isolated throughout their ontogeny, compared to those with close neighbors throughout their ontogeny. Further, overall population growth was proportionally more sensitive to perturbations involving isolated adults, despite the projected persistence of plants with close neighbors. In short, changes in survival of isolated adults had a greater influence on population growth than changes in survival of adults with neighbors. We used life table response experiments (LTREs) to test for spatiotemporal effects of neighbors. The LTRE was consistent with the elasticity analyses in showing that dynamics among adult members made the greatest contribution to the observed differences in population growth among the neighbor-defined population subsets during each census period. Neighbor effects among adults were greater and more important than temporal variation in drought levels in decreasing population growth relative to a pooled 10-yr reference matrix that ignored neighbor effects. Although neighbor effects contributed greatly to differences in population growth among the models projected, the LTRE revealed that, relative to the reference matrix, transitions among juvenile size classes decreased population growth for the predominantly drought free 1984–1989 census interval and increased population growth for the 1989–1994 interval, which included four winters of extreme drought. We hypothesize that higher mortality during periods of high rainfall was due to increased competition among plants, especially in favor of adults at the expense of juveniles. The drought period was also characterized by increased recruitment of new adults, probably reflecting diminished competition from adults for well-established juveniles capable of growing into reproductive condition after elimination of smaller juveniles during 1984–1989. Our habitat-specific partitioning of this population revealed dramatic differences in the demographic behavior of this population and showed that neighbor status is a structuring force in this plant community. Neighbor effects were shown to be dependent on plant size and suggest that conspicuous clumped distributions of adults reflect leftover individuals with diminished demographic influence remaining in a population that is largely driven by isolated adults.

Spatial Analysis of Grazed White Bursage in the Lake Mead National Recreational Area, Nevada, USA

Spatial Analysis of Grazed White Bursage in the Lake Mead National Recreational Area, Nevada, USA

Phosphorus acquisition by Bromus madritensis ssp. rubens from soil interspaces shared with Mojave Desert shrubs

Abstract Bromus madritensis ssp. rubens (L.) Husnot (Red Brome) is an invasive annual grass that is associated with increases in fire frequency and decreases in perennial plant diversity. The success of Red Brome in the Mojave Desert has been attributed to its competitive ability, but competition between Red Brome and native shrubs for below‐ground resources has not been investigated previously. In this study we present a modification of previous dual‐isotope methods that assesses competitive interactions and responses among plants under field conditions. We then use this method to (i) determine if direct competition for phosphorus (P) occurs between Red Brome and native shrubs, and (ii) evaluate the effectiveness of phosphate acquisition by Red Brome from soil interspaces shared with different Mojave Desert shrub species. Clipping Red Brome to remove ≈85% of its foliage on the day prior to labelling soil interspaces with P isotopes did not have the desired effect of inhibiting phosphorus uptake by Red Brome. Therefore we were unable to verify that direct competition for P occurred between Red Brome and native shrubs. Nonetheless, by sampling plant tissue from unclipped strips of Red Brome that were centred between two shrubs, we were able to evaluate the effectiveness of phosphate acquisition by Red Brome from interspaces shared with different shrubs. There were no differences in the amount of phosphorus acquired by Red Brome from soil interspaces shared with the evergreen shrub Larreatridentata (D.C.) Cov. versus soil interspaces shared with the drought deciduous shrub Lycium pallidum Miers. However, Red Brome acquired more than six times as much P from soil interspaces shared with Larrea than from soil interspaces shared with the drought deciduous shrub Ambrosia dumosa Payne. Less P uptake by Red Brome from soil interspaces shared with Ambrosia suggests greater effectiveness of Ambrosia compared with Larrea to deplete soil P.

Variation in hydraulic architecture and gas-exchange in two desert sub-shrubs, Hymenoclea salsola (T. & G.) and Ambrosia dumosa (Payne).

Adjustment of hydraulic architecture in response to environmental conditions was studied in two warm-desert sub-shrubs, Hymenoclea salsola and Ambrosia dumosa, both at the level of genetic adaptation along a climatic gradient and plastic response to immediate growth conditions. Individuals of both species originating from southern populations developed higher leaf-specific hydraulic conductance in the common greenhouse than individuals from northern populations. Hydraulic conductance was higher in plants grown at high temperature, but did not vary as a function of growth relative humidity. Hydraulic conductance was not correlated within species with individual variation in vessel diameter, cavitation vulnerability, or root:shoot ratio, but was strongly, negatively correlated with the fraction of total plant biomass allocated to leaves. For both species, stomatal conductance (g s) at high leaf-to-air vapor pressure difference (ν) was tightly correlated with variability in hydraulic conductance, as was the sensitivity of stomatal closure to increasing ν. Experimentally increasing shoot water potential by soil pressurization, under conditions where high ν had already caused stomatal closure, led to substantial stomatal reopening in both species, but recovery was significantly higher in H. salsola. Hydraulic conductance was higher in H. salsola than A. dumosa. H.salsola also differed from A. dumosa by being a representative of a highly specialised group of desert shrubs which use the twigs as a major photosynthetic organ. The southern population of H. salsola produced far fewer leaves and relied much more heavily on twig photosynthesis than the northern population. At the whole-plant level, increased reliance on twig photosynthesis was associated with higher leaf-specific hydraulic conductance, but equivalent whole-plant photosynthesis on either a dry weight (µmol CO2 g-1) or nitrogen basis (µmol CO2 g-1)). This suggests that twig photosynthesis might be one way of increasing hydraulic conductance per unit photosynthetic canopy by increasing allocation to an organ which simultaneously performs photosynthetic, support, and transport functions.

Vulnerability to xylem cavitation and the distribution of Sonoran Desert vegetation

We studied 15 riparian and upland Sonoran desert species to evaluate how the limitation of xylem pressure (Ψ(x)) by cavitation corresponded with plant distribution along a moisture gradient. Riparian species were obligate riparian trees (Fraxinus velutina, Populus fremontii, and Salix gooddingii), native shrubs (Baccharis spp.), and an exotic shrub (Tamarix ramosissima). Upland species were evergreen (Juniperus monosperma, Larrea tridentata), drought-deciduous (Ambrosia dumosa, Encelia farinosa, Fouquieria splendens, Cercidium microphyllum), and winter-deciduous (Acacia spp., Prosopis velutina) trees and shrubs. For each species, we measured the "vulnerability curve" of stem xylem, which shows the decrease in hydraulic conductance from cavitation as a function of Ψ(x) and the Ψ(crit) representing the pressure at complete loss of transport. We also measured minimum in situ Ψ(x)(Ψ(xmin)) during the summer drought. Species in desert upland sites were uniformly less vulnerable to cavitation and exhibited lower Ψ(xmin) than riparian species. Values of Ψ(crit) were correlated with minimum Ψ(x). Safety margins (Ψ(xmin)-Ψ(crit)) tended to increase with decreasing Ψ(xmin) and were small enough that the relatively vulnerable riparian species could not have conducted water at the Ψ(x) experienced in upland habitats (-4 to -10 MPa). Maintenance of positive safety margins in riparian and upland habitats was associated with minimal to no increase in stem cavitation during the summer drought. The absence of less vulnerable species from the riparian zone may have resulted in part from a weak but significant trade-off between decreasing vulnerability to cavitation and conducting efficiency. These data suggest that cavitation vulnerability limits plant distribution by defining maximum drought tolerance across habitats and influencing competitive ability of drought tolerant species in mesic habitats.

BIDIRECTIONAL FACILITATION AND INTERFERENCE BETWEEN SHRUBS AND ANNUALS IN THE MOJAVE DESERT

Composition and structure of plant communities can be strongly influenced by plant interactions. Interactions among plants commonly comprise positive and negative effects operating simultaneously and bidirectionally. Thus, a thorough understanding of plant interactions requires experimental separation and quantitative assessment of the bidirectional positive and negative effects that add up to the net effects of plant interactions. Using the close spatial association of annual plants with a desert shrub (Ambrosia dumosa) in a sandy area of the Mojave Desert of California as a test system, we separated and quantified negative and positive effects of annuals on shrubs and of shrubs on annuals. We achieved the separation of negative and positive effects with an experimental design that included reciprocal removals of neighbors and simulations of physical effects of neighbors using artificial structures. All experimental manipulations were conducted on space originally occupied by Ambrosia shrubs to focus on immediate effects of neighbors on water availability rather than on long-term microenvironmental effects (e.g., nutrient accumulation). We quantified positive effects by calculating the difference between performance parameters of neighbors growing with artificial structures (thatch to mimic the physical effects of the presence of annuals and artificial canopies to mimic the physical effects of shrubs) and those of neighbors growing alone (removals). We estimated negative effects by calculating the difference between plant performance on control plots (shrubs and annuals growing together) and performances of plants growing with the artificial structures. Annuals had simultaneously strong negative and weak positive effects on shrub water status, growth, and reproductive output. Annuals also had an impact on the sex expression of shrubs by inducing shifts toward a higher male proportion in inflorescences of monoecious Ambrosia. In contrast, we found strong positive and weak or no negative effects of the shrubs on survival, biomass production, and seed production of the entire annual community and of selected annual species (the abundant native Chaenactis fremontii and the two dominant introduced annual species Bromus madritensis ssp. rubens and Schismus barbatus). Overall, in net effect, the interaction between shrubs and annuals can be described as facilitation or positive net effects of shrubs on annuals, and interference or negative net effects of annuals on shrubs. However, during the growing season, the ratios between positive and negative effects shifted. Annual plants benefited from the presence of shrubs to the greatest extent early in the growing season, and initial negative effects of annuals on shrubs declined as annuals senesced later in the season. Results of this study support the view that an experimental resolution of bidirectional positive and negative effects is necessary to achieve an accurate, mechanistic understanding of species interactions.

Clonal splitting in desert shrubs

Axis splitting is a widespread phenomenon in desert shrubs, and has been reported for shrubs from several plant families, both in old- and new-world deserts. It is so common in dwarf shrubs of arid environments as to be a defining characteristic of this growth form. Although anatomists described this phenomenon several decades ago, there has been only one ecological study of one species, Ambrosia dumosa. The anatomical nature of the various splitting mechanisms that have been found suggests axis splitting to be an extreme form of hydraulic segmentation. The adaptive advantage of clonal splitting in desert shrubs has yet to be determined, but it appears to be largely a risk-spreading mechanism that enables independent mortality of integrated hydraulic units (IHUs) or ramets. This should be especially advantageous in heterogeneous, water-limited environments, where soil water occurs in pockets too small to support a large shrub-genet. Clonal splitting may cause an increase in intraclonal competition among ramets, but there are also indications that at least some species possess mechanisms to reduce competition by minimizing root system overlap among ramets. Many desert shrub species that undergo clonal splitting maintain a dense clump growth form, possibly because such a growth form has positive effects on water and nutrient status of the soil and long-term effects on other soil properties.

Spatial patterns of mortality in a Colorado desert plant community

Spatial pattern and patterns of mortality were analyzed from a five year re-census of over 7000 mapped perennial shrubs within a one hectare study plot in Joshua Tree National Park. A five year interval revealed evidence for negative intraspecific interactions which was not evident after a one year evaluation of the same plants within the study area. Juvenile mortality was a positive function of conspecific density for Ambrosia dumosa, Eriogonum fasciculatum and Sphaeralcea ambigua. In addition, juvenile E. fasciculatum that died occurred closer to conspecific adults than would be expected by chance. The two censuses are similar in that uniform patterns of distribution among conspecifics are rare, particularly for non-succulent species.

Vulnerability to cavitation in populations of two desert species,Hymenoclea salsolaandAmbrosia dumosa, from different climatic regions

Seeds from different populations of two desert species, Hymenoclea salsola Torr. and A. Gray and Ambrosia dumosa (A. Gray) Payne, were collected along a climatic gradient, germinated in a greenhouse and the plants tested for their vulnerability to cavitation by the air-pressure method. Differences among populations were evident in A. dumosa, but not in H. salsola. Greenhouse treatments simulating regimes in temperature and relative humidity encountered in different desert environments did not cause appreciable changes in vulnerability to cavitation. It is suggested that a homeostatic mechanism may have helped in maintaining a constant water potential drop in the xylem with little need for an adjustment in the resistance to cavitation. Different plant organs had different vulnerabilities to cavitation, with roots being the most and woody stems the least susceptible. Young green twigs were intermediate. A simulation model confirmed that low water potentials are most likely to cause runaway cavitation in the roots, not in the other organs. It is hypothesized that green twigs are adapted to the favourable water conditions of the growing season, while woody stems are adapted to endure prolonged periods of drought stress.

Effects of regional origin and genotype on intraspecific root communication in the desert shrub Ambrosia dumosa (Asteraceae)

Previous work has shown that the contact inhibition that occurs among roots of Ambrosia dumosa shrubs has a self/nonself recognition capability. In the current study, we investigated some of the geographic and genotypic dimensions of this recognition capability by using root observation chambers to observe the effects of encounters of individual roots on root elongation rates. We measured such effects in encounters between roots of plants from the same region and compared these to effects in encounters between roots of plants from two different regions. We also measured effects of encounters between roots of plants from the same clones and compared these to effects of encounters of roots of plants from different clones. Roots of plants from the same region (population) showed the usual “nonself” precipitous decline in elongation rates following contact, but when roots of plants from different regions contacted each other, elongation rates continued unchanged. When roots of separate plants from the same clone contacted each other, the same “nonself” precipitous decline in elongation rates as seen in encounters between roots of plants of different clones from the same region occurred. Meanwhile, in these same experiments “self” contacts between sister roots connected to the same plants resulted in no changes in elongation rates. Thus, differences between individuals from two geographically separate populations of Ambrosia dumosa may be sufficient to thwart the “nonself,” population‐level recognition of similarity apparently necessary for contact inhibition. Furthermore, the “self” recognition mechanism, which precludes contact inhibition between two roots on the same plant, appears to be physiological rather than genetic in nature.

Effects of Regional Origin and Genotype on Intraspecific Root Communication in the Desert Shrub Ambrosia dumosa (Asteraceae)

Previous work has shown that the contact inhibition that occurs among roots of Ambrosia dumosa shrubs has a self/nonself recognition capability. In the current study, we investigated some of the geographic and genotypic dimensions of this recognition capability by using root observation chambers to observe the effects of encounters of individual roots on root elongation rates. We measured such effects in encounters between roots of plants from the same region and compared these to effects in encounters between roots of plants from two different regions. We also measured effects of encounters between roots of plants from the same clones and compared these to effects of encounters of roots of plants from different clones. Roots of plants from the same region (population) showed the usual “nonself” precipitous decline in elongation rates following contact, but when roots of plants from different regions contacted each other, elongation rates continued unchanged. When roots of separate plants from the same clone contacted each other, the same “nonself” precipitous decline in elongation rates as seen in encounters between roots of plants of different clones from the same region occurred. Meanwhile, in these same experiments “self” contacts between sister roots connected to the same plants resulted in no changes in elongation rates. Thus, differences between individuals from two geographically separate populations of Ambrosia dumosa may be sufficient to thwart the “nonself,” population-level recognition of similarity apparently necessary for contact inhibition. Furthermore, the “self” recognition mechanism, which precludes contact inhibition between two roots on the same plant, appears to be physiological rather than genetic in nature.

Longevity, recruitment and mortality of desert plants in Grand Canyon, Arizona, USA

Abstract. The demography of woody desert plants along the Colorado River in Grand Canyon, Arizona, USA, was analyzed using 355 pairs of replicated photographs taken as long ago as 1872. Longevity, recruitment, and mortality were determined for 38 species characteristic of ungrazed desert scrub. Individual plants that survived 100 yr or more included Acacia greggii, Ambrosia dumosa, Atriplex canescens, A. confertifolia, Echinocactus polycephalus, Ephedra spp., Fouquieria splendens, Larrea tridentata, Lycium andersonii, Opuntia acanthocarpa, O. basilaris, O. erinacea, Pleuraphis rigida, and Yucca angustissima. This is the first evidence of long lifespan for most of these species, particularly the succulents. Most of the long‐lived species registered overall increases in population during the past century. Only four species with lifespans ≥ 100 yr had a net loss of individuals between 1889 and the present, and only two decreased between 1923 and the present. It seems likely that climatic fluctuations over the past century are largely responsible for these recruitment and mortality patterns; however, nurse plants, predation refuges and other biotic factors may also play a role.

Root Communication Mechanisms and Intracommunity Distributions of Two Mojave Desert Shrubs

Experimental studies using root observation chambers to observe the effects of encounters between individual roots on root elongation rates have revealed that the interactions among roots of Ambrosia dumosa and Larrea tridentata are more complex than simple competition for a limiting resource. Larrea roots inhibited elongation of either Larrea or Ambrosia roots in their vicinity, and Ambrosia roots inhibited elongation of contacted roots on other Ambrosia plants only. The purpose of the study reported here was to test the hypothesized involvement of inhibitory substances released by roots in these interroot encounters by attempting to remove such substances by adsorption to activated carbon. The presence of activity carbon caused a significant decrease in the inhibition of elongation of neighboring roots by Larrea roots, but activated carbon had no effect on the intraspecific responses of Ambrosia roots. These results support the hypotheses that the interaction mechanism of Larrea roots involves the release of a readily diffusible, generally inhibitory substance by Larrea roots into the soil, rather than a simple depletion of water or nutrients from around Larrea roots, and that the intraspecific, self—nonselfish—recognizing interaction mechanism of Ambrosia roots is mediated by contact and is fundamentally different from that of Larrea. These findings may enhance our understanding of Mojave desert community structure. The root—mediated allelopathy of Larrea may play a role in producing and maintaining the commonly occurring, regular distributions of Larrea. The complex communication mechanism of Ambrosia roots appears to constitute a detection and avoidance system that may allow this shrub to grow in clumped intraspecific distributions with little or no intraspecific competition for water. The interspecific interference between Larrea and Ambrosia in the field may be mechanistically asymmetrical due to their different root communication mechanisms.

Using plants as an exploration tool for gold

Abstract Creosote bush (Larrea tridentata), burr bush (Franseria dumosa or Ambrosia dumosa), and sagebrush (Artemesia californica) have been tested for Au biogeochemically in the arid environment of the Mojave Desert on the Standard Hill property, which is located approximately 100 km from Los Angeles, California. Results indicate that creosote bush leaves are a reasonable medium to use for sampling for Au in the arid environment. Gold anomalous areas were extended under colluvial cover to a depth approaching 20 m. An additional biogeochemical study was completed in the semiarid environment of the Zenda property, which is located approximately 50 km east of Bakersfield, California. In addition to semiaridity, the test also was able to measure the influence of high relief on the effectiveness of plants as sample media for Au. The plants tested in the Zenda Survey were the scrub oak (Quercus dumosa), digger pine (Pinus sabiniana), and the California juniper (Juniperus californica). Results enabled the discovery of an area not identified by an earlier rock-chip survey. It is concluded that plant sampling is a quick, inexpensive, and effective method to use for Au exploration in the arid and semiarid environments that exist in southern California.

Pattern and mortality in Colorado Desert plants.

We tested for intraspecific interference among Colorado Desert shrubs using an integrated analysis of spatial pattern and juvenile mortality. The data set included 7,000 woody perennials of 24 species in a mapped hectare of Joshua Tree National Monument, California. The site is dominated by Ambrosia dumosa (62.0% of the stems), with Larre tridentata a conspicuous secondary species (2.3% of the stems). Analyses of static pattern for common species showed: (1) aggregated adults and juveniles for Ambrosia dumosa, Erigonum fasciculatum, Mirabilis bigelovii, and Sphaeralcea ambigua, with more aggregation among juveniles than adults; (2) randomly distributed adults and juveniles for Krameria grayi, Opuntia rasmosissima, Simondsia chinensis, and Yucca schidigera. The summed volumes and distances between nearest conspecific neighbors were positively correlated for Ambrosia dumosa and Larrea tridentata, but not significantly correlated for eight remaining species with ≥100 individuals. Static pattern suggests only weak evidence for negative interactions in Ambrosia and Larrea, and little evidence for other species. Alternative mechanisms other than negative interaction that could give rise to these static patterns are discussed. Juvenile mortality was documented for four common species (Ambrosia dumosa, Eriogonum fasciculatum, Mirabilis bigelovii, and Sphaeralcea ambigua) that experienced substantial mortality. Analyses show: (1) the proportion of individuals that died was independent of the initial density of conspecifics; (2) distance to conspecific adults did not differ for juveniles that died versus those that survived; and (3) death was no more likely for juveniles that contacted other plants than for those that were isolated. The exception was a vine, Mirabilis bigelovii, whose juveniles survived better in contact with other plants. In sum, neither spatial pattern nor patterns of mortality showed clear evidence of negative intraspecific interference.