Leaf absorptances of Mohave and Sonoran desert plants

Summary Leaf absorptances to solar radiation in the 400–700 nm (photosynthetically useful wavelengths) are presented for a diversity of species in the Mohave and Sonoran Deserts of North America. As a life form shrubs are shown to have the widest range of absorptance, with perennial herbs and cacti exhibiting a smaller range, and very little variation in leaf absorptances among trees and annuals. The measurement of leaf absorptance at 625 nm is shown to be the same as the leaf absorptance to solar radiation over the 400–700 nm. Over a wide range of species and absorptances, the total solar leaf absorptance (400–3,000 nm) is shown to be closely related to the 400–700 nm leaf absorptance.     

Nutrient mobilization and processing in Sonoran desert riparian soils following artificial re-wetting

Research in river-floodplain systems has emphasized the importance of nutrient delivery by floodwaters, but the mechanisms by which floods make nutrients available are rarely evaluated. Using a laboratory re-wetting experiment, we evaluated the alternative hypotheses that increased nutrient concentrations in riparian groundwater during flash floods are due to (H1) elevated nutrient concentrations in surface floodwaters entering the riparian zone or (H2) re-mobilization of nutrients from riparian soils. We sampled soils from the riparian zone of a 400m reach of Sycamore Creek, AZ. Two sub-samples from each soil were re-wetted with a solution that mimicked the chemistry of floodwaters, with one sub-sample simultaneously treated with a biocide. We also measured structural characteristics of soils (texture, organic matter, moisture, and extractable nutrients) to investigate relationships between these characteristics and response to re-wetting. Riparian soils exhibited considerable variation in physical and chemical structure. Soil organic matter, moisture, and texture co-varied among samples. Re-wetting increased concentrations of nitrate and ammonium, and decreased SRP, relative to initial concentrations. Live soils were significantly lower in NO3-and SRP than biocide-treated samples. Extractable DIN pools were the best predictors of mobilization, and soil organic matter was strongly correlated with nitrate losses, probably via its relationship with microbial uptake. Nutrient mobilization and processing also varied considerably with depth, lateral position, and among plots. We estimate that 70–80% of N in riparian groundwater during flash floods is re-mobilized from riparian soils, but are unable to reject the hypothesis that flood inputs may be important sources of nutrients to riparian soils over longer time scales.     

Interspecific variation in extraction of buried seeds within an assemblage of sparrows

Most North American sparrows forage almost exclusively on herbaceous seeds during the winter months. Limited availability of surface seeds forces some birds to scratch for seeds buried beneath soil, snow, or litter. Artificial seed trays were used to test the ability of five different sparrow species to extract seeds buried at different depths in soil. The results suggest three functional groups based on relative scratching ability. Strong scratchers, which included eastern towhee and song and white-throated sparrows, met or exceeded their energetic requirements when foraging on seeds buried at all depths (down to a maximum depth of 1.50–2.25 cm). A weak scratching species, Savannah sparrow, scratched with the same frequency as the strong scratchers, but experienced negative energy budgets when forced to forage on sub-surface seeds. Finally, a non-scratching species, field sparrow, failed to extract any buried seeds. Level of scratching ability may influence foraging efficiency at low resource densities. As a result, interspecific differences in scratching ability may contribute to habitat selection. Strong scratchers may be adapted to foraging near woody vegetation where intense resource competition and abundant litter limit the availability of surface seeds. Weak scratchers, on the other hand, may be forced to feed away from areas where surface seeds are limited. Since woody vegetation serves as a primary source of protective cover in early successional habitats, a tradeoff between foraging efficiency and the risk of predation may promote the local coexistence of species that differ in relative scratching ability and adaptations to evading predators.     

Environmental regulation of the reproductive system in a flexibly breeding Sonoran Desert bird, the Rufous-winged Sparrow, Aimophila carpalis

We investigated reproductive regulation in male Rufous-winged Sparrows, Aimophila carpalis, a Sonoran Desert passerine that breeds after irregular summer rains. Field and captive data demonstrate that increased photoperiod stimulates testicular development in March and maintains it until early September. Free-living birds caught in July and placed on captive long days (16L: 8D) maintained developed testes for up to 7 months, and free-living birds caught in September, during testicular regression, redeveloped testes when placed on captive long days, indicating that these birds were still photosensitive. Captive birds on long days maintained testicular development when exposed to temperatures mimicking those occurring during regression in free-living birds. In free-living birds, testicular development was observed during spring and summer, but unless this was associated with rainfall, breeding (indicated by juveniles) did not occur. Large increases in plasma luteinizing hormone (LH) in free-living males were correlated with heavy rainfall in July/August, when the birds bred, and in November, when they did not breed. In captive birds, plasma LH concentrations were unresponsive to photoperiodic changes, but may have responded to social cues. Plasma prolactin concentrations were directly correlated with photoperiod in free-living birds, but an effect of photoperiod on prolactin secretion was not seen in captive birds. It is concluded that male Rufous-winged Sparrows use long photoperiods to stimulate and maintain testicular development, but exposure to long photoperiods does not terminate breeding by inducing absolute photorefractoriness. The specific timing of reproductive behaviors is apparently determined by elevated plasma LH coinciding with long day stimulated gonad development.     

High chromosome numbers in Mojavean and Sonoran desert Atriplex canescens (Chenopodiaceae)

Cytological, flavonoid, and morphological data are provided for several varieties of the shrubby species Atriplex canescens (Pursh) Nutt. (x = 9) (fourwing saltbush) in the Mojavean and Sonoran deserts of southwestern United States and northern Mexico. These include var. linearis (S. Wats.) Munz (2x); var. angustifolia (Torr.) S. Wats. (2x, 4x); var. occidentalis (Torr. & Frem.) Welsh & Stutz (4x, 6x), the common variety; var. laciniata Parish (12x); and var. macilenta Jeps. (12x). Atriplex canescens var. grandidentatum Stutz & Sanderson (20x) is newly described. An autoploid origin from 12x var. laciniata is suggested for the 14x and 20x polyploids, through unreduced gametes. Founder populations of odd-ploid products arising during such a sequence of events could probably have returned to even-ploidy through genetic segregation and the rapid elimination of aneuploids. Morphological characters suggest an origin for 12x var. laciniata by interspecific hybridization of var. occidentalis with A. polycarpa (Torr.) S. Wats.     

Genetic differentiation in the Agave deserti (Agavaceae) complex of the Sonoran desert

The Agave deserti complex, comprising A. deserti, A. cerulata and A. subsimplex, represents a group of species and subspecies with a near allopatric distribution and clear differences in morphology. Genetic differentiation and taxonomic status with respect to spatial distribution of 14 populations of the complex were analyzed in an effort to understand the evolution and speciation process within the genus. Allelic frequencies, levels of genetic variation, expected heterozygosity (H(S)), proportion of polymorphic loci (P), and genetic differentiation (theta and Nei's genetic distance) were estimated using 41 putative RAPD loci. All three species show high levels of genetic variation (H(S)=0.12-0.29, P=63.4-95.1), and low genetic differentiation between populations and species (theta populations=0.14+/-0.02 (SE); G(st)=0.11+/-0.02). Accordingly, gene flow among populations was estimated as high by three different methods (N(m)=2.91-6.14). Nei's genetic distances between the three species were low compared to the values obtained from other Agavaceae, and there was no clear correlation with taxonomic divisions. In a UPGMA analysis, A. subsimplex and A. cerulata formed exclusive monospecific clusters, whereas the A. deserti populations appear in more than one cluster together with other species. The results were consistent with a pattern of genetic isolation by distance.     

The ants of the southern Sonoran desert: community structure and the role of trees

In contrast to other North American deserts, the southern Sonoran desert is dominated by trees that provide shaded microhabitats necessary for the establishment and survival of several plant species. Near the southern limit of the Sonoran desert in Sonora, Mexico, we evaluated the role that tree microhabitats may play in structuring ant communities. We recorded 39 species and 21 genera of ants from a 9.7-ha area. Total species richness was estimated to be between 47 and 49 species, a much greater species richness than that reported for other North American arid-zone habitats. Although species richness did not differ between open ground and tree-shaded microhabitats, species composition did. Opportunistic species, Camponotus species, Pheidole sciophila and P. titanis were more common near trees, whereas Pheidole sp. A and granivorous species were more active in open areas. The imperilment of trees in the Sonoran desert due to commercial cutting and the spread of buffelgrass Pennisetum ciliare may alter the existing composition of ant communities.     

Summer water relations of the desert phreatophyte Prosopis glandulosa in the Sonoran Desert of southern California

Summary Two shoot populations of the rhizomatous, patchforming herb Solidago canadensis were studied throughout a developmental cycle in two abandoned pasture sites in southern Ontario. The shoot cohorts that emerged in spring dominated the two populations; subsequent recruitment was very low. Shoot mortality was highest in June and was concentrated in the smallest size classes. Both populations showed a pronounced bimodal size structure for most of the growth cycle. Relative growth rate of shoots declined as the growing season progressed and tended to be lowest in the smallest size classes. Inflorescence production depended on shoot size. The calculated relationship between log mean weight and log density of shoots was not constant during the growth cycle and the calculated maximum biomass values do not transgress the ultimate thinning line suggested from previously published data.Address for proofs and present address: Department of Geography, University of Liverpool, P.O. Box 147, Liverpool, L69 3BX. England     

Interspecific differences in differentiation and dedifferentiation patterns in the genusNicotiana

Differentiation on hormoneless media, habituation ability and crown gall induction inNicotiana tissue cultures have been used as physiological parameters of evolutionary differentiation between species. Some of them on hormone free media differentiated whole plantlets, others produced only shoots or roots or showed undifferentiated growth (habituation), some eventually died. Moreover, the same genotypes showed a differential behaviour as far as tumor formation byAgrobacterium tumefaciens was concerned. Particularly, the competence for crown gall transformation inNicotiana species seems negatively correlated with differentiation capacity and may be ascribed to differences in the plants capacity to synthesize growth regulators. The correlation between the results obtained and the phylogenetic position of the genotypes tested is finally discussed.     

Development of skin structure and cutaneous water loss in nestling desert house sparrows from Saudi Arabia

The outer layer of the epidermis, the stratum corneum (SC), contains lipids and corneocytes, which together form layers that limit cutaneous water loss (CWL). We examined the development of structure of the SC and CWL in nestling House Sparrows (Passer domesticus) from Saudi Arabia. We measured CWL of nestlings, and characterized development of their epidermis using electron microscopy. We tested two antagonistic hypotheses, that CWL decreases as nestlings age, a response to increased thickness of SC, and an opposite idea that CWL increases as nestlings age even though the number of layers of the SC remains constant. CWL of nestling House Sparrows varied with developmental stages, in a non-linear fashion, but not significantly so. CWL of nestlings averaged 7.31+/-1.5 g H(2)O/(m(2) h), whereas for adults it was 4.95 g/(m(2) h); adult CWL was 67.7% that of nestlings. We found that morphology of the SC did not change linearly with age, but seemed to vary with developmental stage. CWL decreased as the SC thickness increased and as the total thickness of the corneocytes increased. Further, we found that CWL decreased as the thickness of the extracellular space increased, number of corneocytes increased, and proportion of the SC that is extracellular space increased.     

Relative photorefractoriness, prolactin, and reproductive regression in a flexibly breeding sonoran desert passerine, the rufous-winged sparrow, Aimophila carpalis

We tested the hypothesis that adult male rufous-winged sparrows, Aimophila carpalis, exhibit relative photorefractoriness. This condition results in partial loss of sensitivity to photoperiod as a reproductive stimulus after prolonged exposure to long photoperiods and is similar to the mammalian condition called photoperiodic memory. Captive birds were exposed either to 8 h of light/16 h of dark per day (8L) or to 16L for 11 weeks and were then exposed either to 8L, 13L, 14L, or 16L. Testicular diameter, plasma luteinizing hormone (LH), and plasma prolactin (PRL) were measured to assess reproductive system activity in response to photostimulation. In free-living birds, testicular diameter, plasma LH, and PRL were compared in birds caught in September in a year when birds were breeding and in a year when birds were not breeding to further evaluate the role of PRL in the termination of seasonal breeding. Testes completely developed after transfer from 8L to 14L or to 16L and partially developed after transfer from 8L to 13L. However, after 11 weeks of 16L exposure, transfer to 14L caused partial regression and transfer to 13L caused complete regression of the testes. Plasma LH increased in all birds that were transferred from 8L to a longer photoperiod. PRL showed a weak response to longer photoperiod treatment and was elevated in birds after chronic 16L exposure in comparison to birds exposed to chronic 8L. These data indicate that male rufous-winged sparrows lose sensitivity to photoperiod after long photoperiod exposure consistent with the relative photorefractoriness and photoperiodic memory models. Lower PRL in birds that developed testes on 13L and 14L compared to birds that regressed testes on 13L and 14L are consistent with the hypothesis that PRL regulates relative photorefractoriness. However, PRL does not appear to regulate interannual differences in the timing of testicular regression.     

Leaf-twig carbon isotope ratio differences in photosynthetic-twig desert shrubs

Summary Carbon isotope ratios were determined for bulk tissues of both leaves and current season twigs of 29 species of Mohave Desert shrubs. Leaf and twig tissues were found to differ in their carbon isotope ratios, only in those species which had photosynthetic twigs. These data suggest that the twigs of these species operate at lower intercellular CO₂ values than leaves, an interpretation which is consistent with available gas-exchange data. An effect of microhabitat was also evident between the mean isotope ratios of leaves from wash versus slope habitats.This research has been supported by NSF Grant BSR-8410943 and by DOE Grant DE-FGO2-86ER60399.     

Interspecific differences in aluminium tolerance in relation to root cation-exchange capacity

Genotypic differences in aluminium (Al) tolerance hold considerable promise in overcoming an important limitation to plant growth in acid soils. Little is known, however, about the biochemical basis of such differences. Extracellular properties, particularly low root cation-exchange capacity (CEC), have been associated with Al tolerance, since roots of low CEC adsorb less Al than do those of high CEC. A solution culture study was conducted in which 12 plant species (monocots and dicots) were grown in solution culture of low ionic strength (ca 2 mM) for 8 d at four Al concentrations (0, 16, 28 and 55 M). The species differed significantly in Al tolerance as shown by differences in root length. Root length relative to that of the same species grown in the absence of Al varied from 6 to 117% at 16 M Al, and from 6 to 75% at 28 M Al. Species tolerance of Al was not closely associated with differences in root CEC. Although in some species Al sensitivity was associated with high adsorption of Al during a 10- or 40-min exposure to Al (expressed on a fresh mass or root length basis), this was not a good predictor of Al tolerance across all species studied.     

The genetic structure of a columnar cactus with a disjunct distribution: Stenocereus gummosus in the Sonoran desert

Stenocereus gummosus is a columnar cactus endemic to the Sonoran desert that exhibits a disjunct distribution: it is widely distributed in Baja California and restricted to a small coastal area in mainland Sonora. In this paper, we examine the genetic structure and the mating system of this species in order to explore the origin of the disjunction and describe aspects of the pollination biology. Flowers are nocturnal, pollinated mainly by sphingids and self-incompatible. Polymorphism for allozymes (11 loci) was relatively high (P=75%) but moderate levels of heterozygosity were detected (H(o)=0.103 and H(e)=0.261). Sonoran populations exhibited higher levels of genetic variation than peninsular populations. H(e) declined with latitude when just peninsular and two island populations are included. Substantial levels of inbreeding within populations (f=0.60), moderate differentiation among populations (theta=0.10), and no evidence of isolation by distance were detected. The neighbor-joining phenogram showed Sonoran and island populations nested within peninsular populations. Mainland populations showed greater genetic similarity to island populations, supporting a dispersal hypothesis for the origin of the disjunction. Future studies using DNA markers are suggested in order to better understand the forces that have shaped the genetic structure of this species.     

Interspecific variation in avian blood parasites and haematology associated with urbanization in a desert habitat

Many avian species are negatively impacted by urbanization, but other species survive and prosper in urbanized areas. One factor potentially contributing to the success of some species in urban areas is the reduced presence of predators or parasite vectors in urban compared to rural areas. In addition, urban areas may provide increased food and water resources, which can enhance immune capacity to resist infection and the ability to eliminate parasites. We determined patterns of blood parasitism, body condition, and immune cell profiles in urban and rural populations of five adult male songbird species that vary in their relative abundance within urban areas. Urban birds generally exhibited less blood parasitism than rural birds. This difference was particularly evident for the urban-adaptable Abert's towhee Pipilo aberti . In contrast, no difference in haemoparasitism was seen between urban and rural populations of the curve-billed thrasher Toxostoma curvirostre , a less-urban adaptable species. In two closely related species, the curve-billed thrasher and the northern mockingbird Mimus polyglottos , urban birds had a higher leukocyte count and a higher heterophil to lymphocyte ratio, which is often associated with chronic stress or current infection, than rural birds. Urban northern mockingbirds were in better condition than rural counterparts, but no habitat-related differences in condition were detected for other species. Parasitic infection was correlated with body condition in only one species, the canyon towhee Pipilo fuscus . Parasitic infection in most species was correlated with changes in leukocyte abundance and profile. The findings suggest that interspecific differences in parasitic infection cannot be attributed entirely to differences in vector abundance or body condition. Interactions between immune function, parasite infection risk, and resource availability may contribute to determining the relative ability of certain species to adapt to cities.