Functional ecology of shrub seedlings after a natural recruitment event at the Nevada Desert FACE Facility

Seedling recruitment is an important determinant of community structure in desert ecosystems. Positive photosynthetic growth and water balance responses to increasing atmospheric carbon dioxide (CO₂) concentrations ([CO₂]) are predicted to be substantial in desert plants, suggesting that recruitment could be stimulated. However, to date no studies have addressed the response of perennial plant recruitment in natural populations of desert shrubs exposed to elevated [CO₂]. In April 1997, we employed Free‐Air Carbon Dioxide Enrichment (FACE) in order to increase atmospheric [CO₂] in an undisturbed Mojave Desert ecosystem from ambient (～～ 370 µmol mol^?1) to elevated CO₂ (～～ 550 µmol mol^?1). From 1997 to 2001 we seasonally examined survival, growth, gas exchange and water potential responses of Larrea tridentata and Ambrosia dumosa seedlings that germinated in Fall, 1997. Recruitment densities were not influenced by [CO₂] in either species, although a two‐fold higher adult Ambrosia density under elevated [CO₂] resulted in two‐fold higher seedling density (0.87 vs 0.40 seedlings m^?2). Mortality was greatest for both species during the first summer (1998), despite above‐average rainfall during the previous Winter–Spring. A significant [CO₂] × time interaction revealed that early survival was greater under elevated CO₂, whereas a significant species time interaction revealed that overall survival was greater for Ambrosia (28%) than for Larrea (15%), regardless of [CO₂]. Microsite (understorey or interspace) alone had no significant influence on survival. Significant species, microsite and species × microsite effects on growth (seedling height, stem diameter and canopy size) were found, but elevated CO₂ had minimal impact on these parameters. Photosynthetic rates (A_sat) for both species were higher at elevated [CO₂] during certain seasons, but not consistently so. These results suggest that increased atmospheric [CO₂] may enhance carbon (C) assimilation and survival of aridland perennial shrubs during favourable growing conditions, but that it may not counteract the effects of prolonged drought on mortality.     

Xylem function of arid-land shrubs from California, USA: an ecological and evolutionary analysis

Xylem traits were examined among 22 arid-land shrub species, including measures of vessel dimensions and pit area. These structural measures were compared with the xylem functional traits of transport efficiency and safety from cavitation. The influence of evolution on trait relationships was examined using phylogenetic independent contrasts (PICs). A trade-off between xylem safety and efficiency was supported by a negative correlation between vessel dimensions and cavitation resistance. Pit area was correlated with cavitation resistance when cross species data were examined, but PICs suggest that these traits have evolved independently of one another. Differences in cavitation resistance that are not explained by pit area may be related to differences in pit membrane properties or the prevalence of tracheids, the latter of which may alter pit area through the addition of vessel-to-tracheid pits or through changes in xylem conduit connectivity. Some trait relationships were robust regardless of species ecology or evolutionary history. These trait relationships are likely to be the most valuable in predictive models that seek to examine anatomical and functional trait relationships among extant and fossil woods and include the relationship among hydraulic conductivity and vessel diameter, between vessel diameter and vessel length, and between hydraulic conductivity and wood density.     

Biogeographic diversification in Nolana （Solanaceae）, a ubiquitous member of the Atacama and Peruvian Deserts along the western coast of South America

The present paper reconstructs the biogeographic diversification for Nolana L.f. （Solanaceae）, a genus of 89 endemic species largely restricted to fog-dependent desert lomas formations of coastal Peru and Chile. Previous efforts have reconstructed a phylogenetic estimate for Nolana using a combination of molecular markers. Herein, we expand on those results to examine hypotheses of biogeographic origins and diversification patterns. Nolana occupies habitats within a continuous coastal desert and forms a terrestrial archipelago of discrete ＂islands＂ unique in size, topography, and species composition. Each locality contains at least one Nolana species and many contain multiple species in sympatry. The genus has a Chilean origin, with the basal clades confined to Chile with wide geographic and ecological distributions. Peru contains two strongly supported clades, suggesting two introductions with subsequent radiation. A Chilean clade of shrubby, small-flowered species appears to have had its origins from the same ancestors of the second line that radiated in Peru and northern Chile. Nolana galapagensis is endemic to the Islas Galapagos, with origins traced to Peruvian taxa with a divergence time of 0.35 mya. Rates of diversification over the past 4.02 mya in Nolana, in one of the driest habitats on Earth, suggest rapid adaptive radiation in several clades. Success in Nolana may be attributed to characters that confer a competitive advantage in unpredictable and water-dependent environments, such as succulent leaf anatomy and ecophysiology, and the reproductive mericarp unique to Nolana. The processes affecting or shaping the biota of western South America are discussed.     

Rock-degrading endophytic bacteria in cacti

A plant–bacterium association of the cardon cactus (Pachycereus pringlei) and endophytic bacteria promotes establishment of seedlings and growth on igneous rocks without soil. These bacteria weather several rock types and minerals, unbind significant amounts of useful minerals for plants from the rocks, fix in vitro N₂, produce volatile and non-volatile organic acids, and reduce rock particle size to form mineral soil. This study revealed the presence of large populations of culturable endophytic bacteria inside the seeds extracted from wild plants, from seeds extracted from the guano of bats feeding on cactus fruit, in seedlings growing from these seeds, in the pulp of fruit, and in small, mature wild plants, and are comparable in size to populations of endophytic populations in some agricultural crops. The dominant culturable endophytes were isolates of the genera Bacillus spp., Klebsiella spp., Staphylococcus spp., and Pseudomonas spp. Based on partial sequencing of the 16s rRNA gene, the isolated strains had low similarity to known strains in these genera. However, these strains have higher molecular similarity among endophytes obtained from seeds, endophytes from roots, and some bacterial strains from the rhizoplane. Seedlings developed from seeds with endophytes contain the similar species of endophytes in their shoots, possibly derived from the seeds. This study shows the involvement of endophytic bacteria in rock weathering by cacti in a hot, subtropical desert and their possible contribution to primary colonization of barren rock. This study proposes that cacti capable of acquiring diverse populations of endophytes may give them an evolutionary advantage to gain a foothold on highly uncompromising terrain.     

Diversity of culturable actinomycetes in hyper-arid soils of the Atacama Desert,Chile

The Atacama Desert presents one of the most extreme environments on Earth and we report here the first extensive isolations of actinomycetes from soils at various locations within the Desert. The use of selective isolation procedures enabled actinomycetes to be recovered from arid, hyper-arid and even extreme hyper-arid environments in significant numbers and diversity. In some cases actinomycetes were the only culturable bacteria to be isolated under the conditions of this study. Phylogenetic analysis and some phenotypic characterisation revealed that the majority of isolates belonged to members of the genera Amycolatopsis, Lechevalieria and Streptomyces, a high proportion of which represent novel centres of taxonomic variation. The results of this study support the view that arid desert soils constitute a largely unexplored repository of novel bacteria, while the high incidence of non-ribosomal peptide synthase genes in our isolates recommend them as promising material in screening for new bioactive natural products.     

Water stress tolerance of shrubs in Mediterranean-type climate regions: Convergence of fynbos and succulent karoo communities with California shrub communities

Mediterranean-type climate regions are highly biodiverse and predicted to be particularly sensitive to climate change. Shrubs of the mediterranean-type climate region of South Africa are highly threatened, and their response to water stress has been comparatively little studied. Resistance to water stress induced xylem cavitation (P(50)) and xylem specific hydraulic conductivity (K(s)) were measured in 15 shrub species from fynbos and succulent karoo communities of South Africa. Species displayed a fivefold variation in cavitation resistance (P(50) of -1.9 to -10.3 MPa) with succulent karoo species displaying greater interspecific variability in P(50) than fynbos species. Principal components analysis (including P(50), minimum seasonal water potential, K(s), and xylem density) showed the response to water stress in fynbos species to be similar to chaparral species from the mediterranean-type climate region of California. The data suggest convergence of community and species-specific water stress "strategies" between these mediterranean-type climate regions with respect to their xylem traits. On the basis of the current study and reported plant death and dieback in these regions, woody species within the fynbos may be more susceptible to climate warming and drying than those within the succulent karoo that appear to be utilizing more diverse xylem strategies in response to water stress.     

Movements of Juvenile Common Ravens in an Arid Landscape

ABSTRACT Movement patterns of juvenile birds are poorly understood, yet critically important ecological phenomena, especially for species with a prolonged juvenile period. We evaluated postfledging movements of juvenile common ravens (Corvus corax) in a western Mojave Desert landscape composed of a mosaic of natural and anthropogenic elements. Generally, ravens do not begin breeding until after their fourth year. We marked 2 annual cohorts of juvenile ravens and followed them from dispersal from their natal territory for up to 33 months. Movements of juvenile common ravens were similar for males and females. Conspecifics and confined livestock feeding operations represented important resources for juvenile ravens, and juveniles were rarely located in open desert. However, initial movements from the natal territory to the nearest communal point subsidy rather than the closest anthropogenic resource suggested juvenile dispersal was influenced by the combination of conspecifics and anthropogenic resources, rather than the distribution of those resources. Land managers concerned with growing raven populations should reduce access to concentrated anthropogenic resources such as landfills and dairies, which serve as important resources for juveniles. Because juvenile ravens rarely venture into open desert, reducing their numbers by lethal removal or other means is unlikely to lessen raven predation of desert tortoises (Gopherus agassizii).     

Habitat Use by Sonoran Desert Tortoises

ABSTRACT The distribution of desert tortoises (Gopherus agassizii) spans a wide range of biotic and abiotic conditions in the southwestern United States and northwestern Mexico, with physical and behavioral differences distinguishing tortoises inhabiting the Mojave Desert from those inhabiting the Sonoran Desert. Relative to tortoise populations in the Mojave Desert, populations in the Sonoran Desert have not been well-studied. To assess how habitat use of desert tortoises in the Sonoran Desert was influenced by topography, vegetation, geomorphology, and soil, we surveyed 40 randomly located 3-ha sites for presence of adult tortoises within a site-occupancy framework. We modeled both occupancy and detection probability as a function of environmental features, and compared those results with a logistic regression model that assumed detection probability was equal to 1. Results from both approaches agreed, suggesting that habitat selection of tortoises in the Sonoran Desert was influenced primarily by topographic and geomorphologic features rather than by vegetation. Specifically, tortoises were more likely to occupy sites that were steep (we detected tortoises on 29% of sites with mean slope <5° and 92% of sites with mean slope >15°) and predominantly east-facing (53% of sites with <5% of site facing E and 92% of sites with >20% facing E), and less likely to occupy north-facing slopes (100% of sites with <10% of site facing N and 14% of sites with >60% facing N). Our results contrast with patterns of habitat use in the Mojave Desert where tortoises primarily occupy valley bottoms. Habitat use of tortoises in Sonoran and Mojave Desert populations differ considerably, contributing to the mounting body of evidence suggesting that these geographically distinct populations may represent separate species.     

Resource availability, population dynamics and diet of the nectar-feeding bat Leptonycteris curasoae in Guaymas, Sonora, Mexico

Nectar-feeding bats play important roles in the pollination of columnar cacti. Migratory bats such as Leptonycteris curasoae are vulnerable to extinction due to their migratory behavior and cave-roosting habit. The conservation of migratory bats requires the identification and protection of key roosting sites and their floral resources. In this paper we describe the availability of resources, the population dynamics and the diet of L. curasoae in the Guaymas region, and its diet at three additional sites in coastal Sonora, Mexico. Our results indicate that L. curasoae is a seasonal resident in the Guaymas area, with local occupation of around 4 months. Seasonal occupation seems to be determined by the availability of flowers and fruits of columnar cacti as peak abundance and peak availability coincide in time. The analyses of fecal and fur samples revealed that columnar cacti constitute the major food source during their seasonal occupation of coastal Sonora. The frequency of pollen from Agave and Bombacaceae in fecal samples was generally low but increased in importance in lower latitudes. Our results suggest that the availability of resources provided by columnar cacti determine seasonal occupation of L. curasoae in Sonora. Given that we detected newborns in the studied colony, we suggest active cave protection for the conservation of the maternity roosting site in Guaymas, Sonora, Mexico.