Desert Laboratory Research Articles

Relationship between local-scale topography and vegetation on the invasive C4 perennial bunchgrass buffelgrass (Pennisetum ciliare) size and reproduction

AbstractBuffelgrass [Pennisetum ciliare (L.) Link] is an invasive C4 perennial bunchgrass that is a threat to biodiversity in aridlands in the Americas and Australia. Topography influences P. ciliare occurrence at large spatial scales, but further investigation into the relationship between local-scale topography and P. ciliare growth and reproduction would be beneficial. Further, density-dependent effects on P. ciliare growth and reproduction have been demonstrated in greenhouse experiments, but the extent to which density dependence influences P. ciliare in natural populations warrants further investigation. Here we present a study on the relationships between local-scale topography (aspect and slope gradient) and vegetation characteristics (shrub cover, P. ciliare cover, and P. ciliare density) and their interactions on individual P. ciliare plant size and reproduction. We measured slope gradient, aspect, shrub cover, P. ciliare cover, P. ciliare density, and the total number of live culms and reproductive culms of 10 P. ciliare plants in 33 4 by 4 m plots located in 11 transects at the Desert Laboratory at Tumamoc Hill, Tucson, AZ, USA. We modeled the relationships at the local scale of (1) P. ciliare cover and density with aspect and slope gradient and (2) P. ciliare size and reproduction with abiotic (slope gradient and aspect) and biotic (P. ciliare cover and density and native shrub and cacti cover) characteristics. Aspect and slope gradient were poor predictors of P. ciliare cover and density in already invaded sites at the scale of our plots. However, aspect had a significant relationship with P. ciliare plant size and reproduction. Pennisetum ciliare plants on south-facing aspects were larger and produced more reproductive culms than plants on other aspects. Further, we found no relationship between P. ciliare density and P. ciliare plant size and reproduction. Shrub cover was positively correlated with P. ciliare reproduction. South-facing aspects are likely most vulnerable to fast spread and infilling by new P. ciliare introductions.

Complete Genome Sequences of Two Temperate Bacillus subtilis Phages Isolated at Tumamoc Hill Desert Laboratory.

ABSTRACTBacteriophages are important in structuring bacterial communities, including desert soils dominated by Bacillus species. Here, we describe two genetically similar temperate phages isolated on a Bacillus subtilis strain from soil in Tucson, Arizona. Their double-stranded DNA (dsDNA) genomes contain 98 and 102 genes, with a set of 4 genes being found in only one phage.

Local temporal trajectories explain population‐level responses to climate change in saguaro (Carnegiea gigantea)

AbstractPopulation demography is typically assumed to be strongly influenced by climatic factors, particularly with succulent plants and cacti. The saguaro cactus (Carnegiea gigantea) is a long‐lived columnar cactus of the Sonoran Desert that experiences episodic recruitment and mortality. Previous studies have attributed long‐term changes in saguaro populations to climatic factors, including increased germination and establishment during wet periods and mortality and reduced establishment during droughts and extreme freezes. We used a 48‐yr data set of marked individuals at the Desert Laboratory in Tucson, Arizona, to test the hypothesis that local, temporal population trajectories are mediated by topographic heterogeneity that interacts with fluctuating climatic conditions. We tested the influence of local slope and aspect vs. climatic variability on a population of saguaro using >5800 marked individuals that have been measured since 1964. We examined the relationship between demography and climatic variables (drought, precipitation, and extreme temperatures) and found significant differences in growth and survival among aspects and among census periods. Saguaro population growth was higher during wet and cool periods (e.g., 1964–1970), and changes in age structures suggest that topographic differences interact with climatic fluctuations to produce unexpected demographic patterns including large recruitment events during periods of relatively unfavorable climate conditions. Our results highlight the importance of long‐term data to detect demographic responses to climate that could not be predicted from short‐term studies of plant physiology and population demography.

Too much of a good thing? A landscape-of-fear analysis for collared peccaries (Pecari tajacu) reveals hikers act as a greater deterrent than thorny or bitter food

To study how wildlife perceive recreating humans, we studied the habitat selection of a human commensalist, the collared peccary (Pecari tajacu (Linnaeus, 1758)). We measured peccary activity patterns in an area of high human activity (Tumamoc Hill Desert Laboratory in Tucson, Arizona, USA) using a landscape-of-fear analysis. We examined whether the perception of risk from human activity interacted with the chemical (tannin) and mechanical (thorns) antipredator mechanisms of local plant species. The peccaries avoided food stations near a hiking trail. The population foraged less near houses, i.e., moderate human activity, than in the perceived safety of a small wadi. Plant defence treatments impacted the harvesting of food only in the safe zone, suggesting that risk trumps food selectivity. The strong effect of the hiking trail on habitat selection in this disturbance-loving species is an indicator of a much larger impact on sensitive species in conservation areas.

The Ecological Society of America: Founders, Founding Stories, Foundations

The Ecological Society of America: Founders, Founding Stories, Foundations

One hundred and six years of population and community dynamics of Sonoran Desert Laboratory perennials

AbstractThis data set constitutes all information associated with the Spalding‐Shreve permanent vegetation plots from 1906 through 2012, which is the longest‐running plant monitoring program in the world. The program consists of detailed maps of all Sonoran Desert perennial plants in 30 permanent plots located on Tumamoc Hill, near Tucson, Arizona, USA. Most of these plots are 10 m × 10 m quadrats that were established by Volney Spalding and Forrest Shreve between 1906 and 1928. Analyses derived from these data have been pivotal in testing early theories on plant community succession, plant life history traits, plant longevity, and population dynamics. One of the major contributions of this data set is the species‐specific demographic traits that derived from estimating individual plant trajectories for more than 106 years. Further use of these data might shed light on spatially explicit population and community dynamics, as well as long‐term changes attributable to global change.Data presented here consist of digital versions of original maps created between 1906 and 1984 and digital data from recent censuses between 1993 and 2012. Attributes associated with these maps include location and coverage of all shrubs, and in some cases, plant height. In addition, we present plot‐specific summaries of plant cover and density for each census year and all other information collected, including seedling counts, grass coverage, and annual species enumerations. We reference the repeat photography of these plots, which began in 1906; these images are stored at the Desert Laboratory Collection of Repeat Photography in Tucson. Initial data collection consisted of grid‐mapping the plots manually on graph paper; starting in 1993, Total Stations (which allow a direct digitalization, and more accurate mapping) were used to survey root crowns and canopies.

Global Land Surface Emissivity Retrieved From Satellite Ultraspectral IR Measurements

Ultraspectral resolution infrared (IR) radiances obtained from nadir observations provide information about the atmosphere, surface, aerosols, and clouds. Surface spectral emissivity (SSE) and surface skin temperature from current and future operational satellites can and will reveal critical information about the Earth's ecosystem and land-surface-type properties, which might be utilized as a means of long-term monitoring of the Earth's environment and global climate change. In this study, fast radiative transfer models applied to the atmosphere under all weather conditions are used for atmospheric profile and surface or cloud parameter retrieval from ultraspectral and/or hyperspectral spaceborne IR soundings. An inversion scheme, dealing with cloudy as well as cloud-free radiances observed with ultraspectral IR sounders, has been developed to simultaneously retrieve atmospheric thermodynamic and surface or cloud microphysical parameters. This inversion scheme has been applied to the Infrared Atmospheric Sounding Interferometer (IASI). Rapidly produced SSE is initially evaluated through quality control checks on the retrievals of other impacted surface and atmospheric parameters. Initial validation of retrieved emissivity spectra is conducted with Namib and Kalahari desert laboratory measurements. Seasonal products of global land SSE and surface skin temperature retrieved with IASI are presented to demonstrate seasonal variation of SSE.

The effect of geographic range position on demographic variability in annual plants

Summary1. The abundant centre model predicts that species abundance will decline from the centre towards the periphery of the geographic range. Thus, we expect to find decreases from the centre towards the edge in variables related to population dynamics such as population density and reproductive output. However, evidence for this pattern is contradictory, suggesting that geographically peripheral sites may not be ecologically peripheral. Populations may thrive in pockets of suitable habitat at the edge of the range or may be locally adapted to peripheral conditions.2. This study examines how the position of a site within geographic and climatic ranges of 13 species is related to the population dynamics at one common location, The Desert Laboratory at Tumamoc Hill, Tucson, AZ, USA.3. We used data on survival, fecundity, germination fraction and population density from a 25‐year long‐term data set on winter annual plants to determine whether there was a relationship between distance to the centre of the range and population dynamics. Geographic distance was calculated by determining the distance from the Desert Laboratory to the centre of the observed range determined from locality records. Climatic distance was calculated using the niche modelling software,maxent, and subtracting the mean climatic profile for the species range from that of the Desert Laboratory.4. There was no relationship between mean population metrics and distance metrics. We found significant relationships between some geographic distance metrics and variance in fecundity, survival and per‐germinant fecundity, but not germination fraction or population density. We did not find a relationship with any metric of population dynamic variation and climatic distance.5.Synthesis. Our results indicate that geographic distance from the centre of the range of 13 annual plant species more strongly predicts their population dynamics than climatic distance. This study reinforces the importance of examining vital rates and their variation in order to properly capture the effect of position within a range on population dynamics.

Resolution of Respect: Professor Paul Schultz Martin 1928–2010

Resolution of Respect: Professor Paul Schultz Martin 1928–2010

Limitations to photosynthetic function across season in Larrea tridentata (creosotebush) growing on contrasting soil surfaces in the Sonoran Desert

Plant community structure in the southwestern United States co-varies with soil surface characteristics due to their role in controlling water availability. At the University of Arizona Desert Laboratory on Tumamoc Hill, we evaluated winter and summer season dynamics of photosynthesis in a dominant shrub species, Larrea tridentata (creosotebush), across soils with contrasting development of surface and sub-surface horizons. We measured pre-dawn water potential ( Ψ pd), stomatal conductance ( g s), leaf nitrogen content (%N), and determined stable carbon isotope discrimination ( Δ). There were no differences in these parameters throughout the winter, although Δ was higher and %N was lower on the clay than sandy soil early in the dry summer period, and g s was higher on the clay than sandy soil late in the summer season while precipitation decreased. We used path analysis to determine the relative influences of %N, g s and Ψ pd on photosynthetic function ( Δ) during the winter and summer seasons. There was no influence during the summer, although g s and %N had positive and negative impacts on Δ within the sandy soil type respectively during the winter. Across soil type, g s was the main driver of %N and Δ. Our study highlights the need to include soil type for understanding how physiological performance of L. tridentata varies over seasons within arid and semi-arid ecosystems.

To California with Jepson's “Phyto-Jogs” in 1913

The Second International Phytogeographical Excursion of 1913 was initiated by University of Chicago's Henry Cowles and represented the first international party of plant geographers to visit the United States. Among the twelve Europeans who participated were Cambridge University's Alfred Tansley, first president of the British Ecological Society (and later to coin the word “ecosystem”), and Adolf Engler, taxonomist and biogeographer of Berlin's Royal Botanical Gardens. Arriving in New York City at the end of July, the excursionists (nicknamed the “Phyto-jogs” by Willis Jepson) traveled by train to the West Coast, with stops in a variety of ecosystems. After arriving at San Francisco Bay from the Pacific Northwest in early September, the Phyto-jogs, guided by University of California's Jepson, proceeded to Yosemite National Park for field trips and a famous group photograph among Mariposa Grove's Giant Sequoias. On September 12 in Oakland, several of the European scientists presented major lectures at the California Botanical Society's first annual banquet. The following days included visits to sites of local interest, including Burbank's experimental garden, Mount Tamalpais and Muir Woods with Alice Eastwood, and the Carnegie Institution's Coastal Laboratory at Carmel. The last two days covered the California desert, including the Salton Sea, and Tucson's Carnegie Desert Laboratory within the Sonoran Desert. A year later Professor Tansley, reflecting on the phytogeographical diversity of America and the varied researches of its many energetic ecologists, wrote: “In that vast field of ecology America has secured a commanding position.”

The structure and rate of late Miocene expansion of C4 plants: Evidence from lateral variation in stable isotopes in paleosols of the Siwalik Group, northern Pakistan

This study uses stable isotope variation within individual Mio-Pliocene paleosols to investigate subkilometer-scale phytogeography of late Miocene vegetation change in southeast Asia between ca. 8.1 and 5 Ma, a time interval that coincides with dramatic global vegetation change. We examine trends through time in the distribution of low-latitude grasses (C4 plants) and forest (C3 plants) on Indo-Gangetic floodplains using carbon (δ13C) and oxygen isotopic (δ18O) values in buried soil carbonates in Siwalik Series sediments exposed in the Rohtas Anticline, north-central Pakistan. Revised, high-resolution magnetostratigraphy and a new 40Ar/39Ar date provide improved age control for the 2020 m Rohtas section. Carbon isotope results capture lateral variability of C3 versus C4 plants at five stratigraphic levels, R11 (8.0 Ma), R15 (6.74–6.78 Ma), R23 (5.78 Ma), R29 (4.8–4.9 Ma), and upper boundary tuff (UBT; 2.4 Ma), using detailed sampling of paleosols traceable laterally over hundreds of meters. Paleosols and the contained isotopic results can be assigned to three different depositional contexts within the fluvial sediments: channel fill, crevassesplay, and floodplain environments. δ13C results show that near the beginning (8.0 Ma) and after (4.0 Ma) the period of major ecological change, vegetation was homogeneously C3 or C4, respectively, regardless of paleo-landscape position. In the intervening period, there is a wide range of values overall, with C4 grasses first invading the drier portions of the system (floodplain surfaces) and C3 plants persisting in moister settings, such as topographically lower channel swales. Although abrupt on a geologic timescale, changes in abundance of C4 plants are modest (∼2% per 100,000 yr) compared to rates of vegetation turnover in response to glacial and interglacial climate changes in the Quaternary. Earlier research documented a sharply defined C3 to C4 transition in Pakistan between 8.1 and 5.0 Ma, based on vertical sampling, but this higher-resolution study reveals a more gradual transition between 8.0 and 4.5 Ma in which C3 and C4 plants occupied different subenvironments of the Siwalik alluvial plain. δ18O values as well as δ13C values of soil carbonate increase up section at Rohtas, similar to isotope trends in other paleosol records from the region. Spatially, however, there is no correlation between δ13C and δ18O values at most stratigraphic levels. This implies that the changes in soil hydrology brought about by the shift from forest to grassland (i.e., an increase in average soil evaporation) did not produce the shift through time in δ18O values. We interpret the trend toward heavier soil carbonate δ18O values as a response to changes in external climatic factors such as a net decrease in rainfall over the past 9 Ma.

Desert on the March

Desert on the March Raymond M. Turner On the morning of November 2, 1907, a group of adventurers gathered at a small house near the corner of East Second Street and Park Avenue in Tucson, Arizona, to begin a month-long trip to the little-known Pinacate region of northwestern Mexico. The seven-man team was led by zoologist and unexcelled raconteur William T. Hornaday. The group also included Daniel T. MacDougal, at whose home they had assembled. MacDougal was a botanist and director of Tucson’s Desert Botanical Laboratory. Other group members were Godfrey G. Sykes, geographer, engineer, and buildings and grounds superintendent at the Desert Laboratory; John M. Phillips, entrepreneur and Pennsylvania state game commissioner; Frank Cole and Jesse Jenkins of Tucson; and Charlie Foster, a Mexican from Sonoyta, Sonora, who had come to town for the express purpose of guiding the expedition to Sonoyta. Hornaday kept extensive notes from which he wove a classic book-length tale chronicling the group’s adventures on this historic trip (Hornaday 1908). As noted by Hornaday, the seven men and all their gear were loaded into a White-Water Touring Car drawn by a bay team and a pair of small mules, and a Runabout by Callahan drawn by a lazy horse and a beautiful young mare. Their first major objective was to reach Sonoyta, Sonora, 140 miles to the southwest, where they would be joined by Jeff Milton , employee of the U. S. Immigration Service, and his friend George Saunders from Philadelphia. Another friend of Jeff’s, Rube Daniels from Quitobaquito, Sonora, was to join them later. Jeff and Rube, both closely A westerner by birth, Ray Turner taught botany at the University of Arizona (1954–1962) before joining the U.S. Geological Survey, where for many years his office was at MacDougal’s Desert Laboratory on Tumamoc Hill. His interest in desert vegetation dynamics has resulted in studies of long-term permanent vegetation study plots as well as in his use of repeat photography for documenting landscape change. He is co-author of Sonoran Desert Plants: An Ecological Atlas (with Janice E. Bowers and Tony L. Burgess), The Changing Mile Revisited (with Robert H. Webb, Janice E. Bowers, and James Rodney Hastings), and The Ribbon of Green: Long-term Status of Riverine Riparian Vegetation in the Southwestern United States (with Robert H. Webb and Stanley A. Leake). He retired from the U.S. Geological Survey in 1989. Journal of the Southwest 49, 2 (Summer 2007) : 141–163 142 ✜ Journal of the Southwest familiar with the region, would guide them across the remaining miles to their final destination.1 The scientific bent of these men, leaning as it did toward the natural sciences and geography, provided Hornaday with abundant observations about their natural surroundings, and he incorporated these shared observations into his book. These observers’ keen insights were dramatically expanded by their photographic activities. Hundreds of photographs were taken by Phillips, MacDougal, Hornaday, and Sykes. These serve as accurate records of landscape conditions in 1907, augmenting and refining their field observations. In March 1959, a little more than half a century after the Hornaday expedition, I found myself in that intriguing country joined by Rod Hastings, Charles Lowe, Tad Nichols, and Robert Dubois (see Turner 2007). We were bent on recapturing on film the scenes that our predecessors had first recorded. Thus began an infatuation that has persisted to the present and has resulted in numerous repeat photos at some of the original camera stations. On the day of our 1959 departure, we gathered at the University of Arizona, just a few blocks from MacDougal’s former home and the point from which the 1907 expedition started. All of the gear for the five-man expedition was loaded into one travel-all and Hastings’ open, military-surplus Jeep. The road to the Pinacate region was now well paved, except for the final section of Mexico’s Highway 2 between Sonoyta and our turnoff to MacDougal Pass and points beyond. This was a rough gravel road that many travelers would abandon, using instead the smoother borrow pit running alongside the raised roadway. Even so, our travel time to our destination was...

Development of low-background vacuum extraction and graphitization systems for 14C dating of old (40–60 ka) samples

At the University of Arizona's Desert Laboratory, we recently constructed new low-background vacuum extraction and graphitization systems that are dedicated to preparing old (40–60 ka) samples for 14C dating. These systems are designed to minimize the amount of contaminant carbon, specifically atmospheric carbon, that is introduced to a sample during laboratory processing. Excluding contaminants is particularly important for 14C dating of old samples because the impact of contamination increases with sample age. In this study, we processed 20 pretreated and 4 untreated aliquots of Ceylon graphite (a naturally-occurring geological graphite) to determine the total procedural background level, and hence the practical limit, of our systems. Samples were heated under vacuum at 240 °C for 1 h to drive off water vapor and other atmospheric gases, and then combusted in ultra-high-purity (UHP) O 2 at 500 and 850 °C to monitor the removal of contaminants and to ensure complete combustion. After SO X , NO X , and halide species were removed, sample CO 2 was converted to graphite via catalytic reduction of CO. Fe and Zn powders used in the graphitization process were oxidized, “scrubbed”, and reduced with UHP O 2, He, and H 2, respectively, to remove sorbed atmospheric C species. Graphite targets were stored in UHP Ar until measurement by accelerator mass spectrometry (AMS) to avoid potential interaction with atmospheric gases. Based on the AMS results, the background level of our system is characterized by a nonlinear inverse relationship with sample mass (adjusted R 2=0.75; n=24). For a 1 mg graphite target, the total procedural blank, including chemical pretreatment, combustion, cleanup, graphitization, storage, and AMS measurement, is 0.05±0.01 pMC (2 σ), equivalent to a 14C “age” of 61.1±1.8 ka. This should not be taken as the upper limit of our system, however, because if the 14C activity of a sample is statistically indistinguishable from the appropriate mass-dependent blank value at the 95% confidence level (2 σ), then its age is considered to be “infinite”. Thus, for a 1 mg target, the practical limit of our system is actually ∼55 ka; for a 0.5 mg target, the practical limit is ∼50 ka. Although our extraction system can accommodate inorganic samples (e.g., calcite, aragonite), the above limits are only applicable to geological graphite, charcoal, and organic samples that are processed via combustion. Future work will be directed toward determining the appropriate background levels for inorganic materials.

TWO DECADES OF CHANGE IN DISTRIBUTION OF EXOTIC PLANTS AT THE DESERT LABORATORY, TUCSON, ARIZONA

A gridline survey for exotic plant species was undertaken in 1983 and repeated in 2005 at the Desert Laboratory, a 352-ha natural area just west of downtown Tucson, Arizona, USA. Coordinate data gathered during the surveys were used to plot distributions, determine frequencies (number of coordinate locations), and assess percent change. Fifty-two exotic species were encountered in all, 34 in the first survey, 44 in the second. The proportion of ornamental exotics doubled over time, reaching 50% in 2005. Casual, naturalized, and invasive exotics comprised 44%, 40%, and 15% of species found during the surveys. Minimum residence time increased significantly from casual to naturalized to invasive species, suggesting that the longer a species was present, the more likely it was to surmount barriers to naturalization and invasion. In both 1983 and 2005, casual, naturalized, and invasive species differed in mean frequency by an order of magnitude, with casual species having the lowest frequencies and invasive species the highest. Between surveys, frequency of 18 species decreased; most other species increased in frequency. The recent surge in ornamental exotics, combined with temporal trends in invasiveness, indicates that the proportion of invasive species in the flora will increase with time.

Late Quaternary paleohydrology of the central Atacama Desert (lat 22°–24°S), Chile

Research Article| March 01, 2002 Late Quaternary paleohydrology of the central Atacama Desert (lat 22°–24°S), Chile Jason A. Rech; Jason A. Rech 1Department of Geosciences and Desert Laboratory, University of Arizona, 1675 West Anklam Road, Tucson, Arizona 85745, USA Search for other works by this author on: GSW Google Scholar Jay Quade; Jay Quade 1Department of Geosciences and Desert Laboratory, University of Arizona, 1675 West Anklam Road, Tucson, Arizona 85745, USA Search for other works by this author on: GSW Google Scholar Julio L. Betancourt Julio L. Betancourt 2U.S. Geological Survey and Desert Laboratory, University of Arizona, 1675 West Anklam Road, Tucson, Arizona 85745, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2002) 114 (3): 334–348. https://doi.org/10.1130/0016-7606(2002)114<0334:LQPOTC>2.0.CO;2 Article history received: 10 Nov 2000 rev-recd: 13 Jun 2001 accepted: 24 Aug 2001 first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jason A. Rech, Jay Quade, Julio L. Betancourt; Late Quaternary paleohydrology of the central Atacama Desert (lat 22°–24°S), Chile. GSA Bulletin 2002;; 114 (3): 334–348. doi: https://doi.org/10.1130/0016-7606(2002)114<0334:LQPOTC>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract In northern Chile, precipitation in the High Andes (>3500 m) recharges groundwater systems that flow down the Pacific slope and feed large aquifers in the hyperarid Atacama Desert. Wetlands, which are often found along the base of the Andes, mark locations where the water table intersects the land surface. At these locations, paleo–wetland deposits, which are present as terraces between 3 and 20 m above modern wetlands, record past water-table heights along the Andean front and are used to reconstruct changes in groundwater discharge. Paleo–wetland deposits in the central Atacama Desert (lat 22°–24°S) record an episode (>15.4–9 ka) of high water tables followed by an episode (8–3 ka) of moderately high water tables. Elevated water tables result from increased groundwater discharge and ultimately from enhanced recharge in the Andes. The concordance of results from three separate hydrologic systems suggests that changes in groundwater discharge and recharge are regional and reflect climatic fluctuations. This interpretation is supported by close agreement with other paleoclimatic records in the region. The periods of greater groundwater discharge were separated by episodes (9–8 and 3–0 ka) of significant groundwater lowering and stream incision, implying greatly diminished discharge.The central Atacama and Andes (lat 22°– 24°S) receive precipitation mainly from moist air masses transported from the Amazon Basin by the South American Summer Monsoon (SASM). Increases in groundwater recharge are therefore thought to reflect an increase in the frequency and/or moisture content of SASM air masses crossing the Andes. Fluctuations in SASM precipitation have previously been linked to summer insolation in the Southern Hemisphere. The wettest period in the central Atacama (>15.4–9 ka), however, coincides with a minimum in austral-summer insolation at 10 ka, suggesting that regional summer insolation is not a dominant influence on the SASM. Instead, intensification of the SASM may be linked to extraregional forcings such as the Walker Circulation. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

DIRECT MEASUREMENT OF THE SEED BANK AGE STRUCTURE OF A SONORAN DESERT ANNUAL PLANT

We describe a new approach to determining the age structure of seed banks of natural plant populations and apply it to a natural population of the Sonoran Desert winter annual, Pectocarya recurvata (Boraginaceae). Unlike other 14C techniques, tandem accelerator mass spectrometry (TAMS) counts the number of carbon isotope atoms, permitting high precision with small samples. Aboveground nuclear bomb tests caused atmospheric 14C levels to peak in 1963. Their subsequent gradual decline provides a signal for aging seed banks with TAMS. We constructed a calibration curve using seeds with known dates of production during 1980–1995, then used it to age 53 seeds sampled from a natural seed bank in 1993, at the Desert Laboratory in Tucson, Arizona. Seed number declined with age at an approximately exponential rate, with the oldest recovered seed having an estimated age of 5 yr (95% ci = ±2.3 yr). The seed bank age structure was judged more than adequate to buffer this population from typical fluctuations, based on an examination of 15 yr of population dynamic data. The TAMS technique has strong potential for answering a broad range of ecological and evolutionary questions requiring post-1963 age determinations and for which a several-year confidence interval is acceptable.

AN 85-YEAR STUDY OF SAGUARO (CARNEGIEA GIGANTEA) DEMOGRAPHY

The saguaro population in a 700-ha area at the Desert Laboratory on Tumamoc Hill, Tucson, Arizona, was censused in 1908, one year after livestock were excluded. In 1964, four 10-ha plots were established within the original area to determine the effect of slope aspect on plant growth and demography. The plots were recensused in 1970 and 1993. We developed a model for determining saguaro age, using the growth rates of over 3000 plants from 1964 to 1970. The model was verified with 1993 data. Changes in population size and the estimated age structures were then used to infer regeneration trends. Saguaro populations on all slope aspects nearly doubled since 1908. Yet, during the same period, relative abundances of saguaros remained higher on the south and east aspects than on the north and west aspects. Higher recruitment and survival of young plants rather than mortality of older ones are largely responsible for the differences between aspects. The estimated age structures show large, multi-decadal fluctuat...

Pollen reentrainment: contributions to the pollen rain in an arid environment

Abstract Atmospheric pollen was sampled from native vegetation at the Desert Laboratory, University of Arizona, Tucson, Arizona, USA, using a Burkard trap. The pollen counts disclosed two conditions of pollen preservation (1) pollen with the intine visible, and (2) pollen characterized either by loss of the intine or by a crumpled exine. The former was thought to be freshly discharged, the latter to be reentrained from depositional surfaces. 11% of the atmospheric pollen collected was reentrained. Winds accompanying convective summer storms enhanced pollen reentrainment. Chenopodiaceae-Amaranthus and Gramineae, morphologically smooth pollen taxa, totaled 86% of the reentrained pollen during 1981–82. Pollen from soils appeared to be a major source of reentrained pollen. The amount of reentrainment varied seasonally; reentrainment concentration was greatest during the summer, whereas, the percent of reentrained pollen was greatest during the winter. Pollen reentrainment has the potential to bias fossil poll...

A Sense of Place: The Life and Work of Forrest Shreve by Janice Emily Bowers

382 Western American Literature and compelling thesis that contemporary critiques of American culture can be characterized by fears of, what I would term roughly, collapse, moral failure, subjection and impotence. Such fears unduly influence “the language and symbols in which policies are considered.” They also often contradict the con­ tinuing importance of the many forces that have shaped us, including the Turner thesis of the western frontier and the Puritan sense of mission. The fears Wilkinson delineates help account for the negativity of most post-war interpretations of American character. Wilkinson himself is not too hard on American culture and character. A fair critic of American excesses and extremes, he also shows the balance and judgment of a mature historian in his attack upon the stridency of recent studies in which “social-character theories become psychiatric smears.” He believes that some writers overstate their cases because ©f a failure to incor­ porate such factors as class and region. Nevertheless, he challenges strains in American character that condone selfishness at home while arousing a danger­ ous moralistic attitude abroad. Throughout this book Wilkinson balances contending values and forces in American culture such as individuality and community. One of the most illuminating portions of the book places such conflicts within specific historic and contemporary situations, which gives great credibility to Wilkinson’s theories about the nature of American culture. As history and social structure intersect in historical case studies—Northeastern revivalism, Overland companies, the Progressives, department store sales­ women, for example—the book establishes an important paradigm for cul­ tural change and continuity. Accordingly, this is a solidly documented and concisely written book that should be considered indispensable for all serious students of American culture. SAM B. GIRGUS University of Oregon A Sense of Place: The Life and Work of Forrest Shreve. By Janice Emily Bowers. (Tucson: The University of Arizona Press, 1988. 195 pages, $19.95.) “Life is one thing, art is another,” said Spanish critic Ortega y Gasset. True enough for the Continent, but in the United States, and particularly the American West, writers base their art on things far more solid to the knuckles. Despite the emotional lushness of The Desert, John C. Van Dyke perspired mightily over zoological and botanical accuracy. Western novelists as different as Wallace Stegner and Larry McMurtry want, first of all, to be authentic, to get their facts straight. The fixation reflects Westerners’ acknowledged debt to their physical circumstances and the close relationship of western life and art. With this in mind, perhaps too easily we forget the unsung historians, anthropologists, and botanists whose scholarly works have served as writers’ guides and whose dis­ coveries have thus broadened the public’s concepts of the West. Reviews 383 The oversight is understandable, since scholars rarely enjoy the fanfare surrounding, say, a Louis L’Amour or an Edward Abbey. Certainly the lack of hubbub applies to Forrest Shreve. Born in 1878 to a family of Maryland Quakers, Shreve earned a Ph.D. from Johns Hopkins University, then spent much of his life in the Southwest as director of the Carnegie Institution’s Desert Laboratory. “The world knows nothing of its greatest men,” read the dour caption under his picture in a college yearbook. It was a prophetic observation. Decade after decade, perched in the Institution’smodestbuildings on a cactus-studded hill above Tucson or off on field trips across the deserts of Arizona and Mexico, the gentle botanist measured the growth rates of giant cactuses and expanded on C. Hart Merriam’s concept of life zones. The fact of the matter is that little of this lends itself to the zippy writing that sends a biography rocketing to the best-seller lists. Nonetheless, there is the quiet drama here of intellectual development as Shreve, far ahead of the common wisdom, recognizes deserts not as “wastelands” but as wonderfully complex and fragile ecosystems. His classic Vegetation of the Sonoran Desert not only defined the issues pursued by ecologists to this day, but its revelations of nature’ssubtleties lie at the heart of such far more popular writers asJoseph Wood Krutch and Ann Zwinger. PETER WILD University of Arizona Sailing to Corinth. By Irene Wanner. (Seattle: Owl Creek...