Northern Sonoran Desert Research Articles

Obsidian Sources and Source Provenance of Archaeological Obsidian in Northern Sonora: The Known and Unknown

A recent obsidian provenance study in three sites in the Valle de Altar region of northern Sonora has greatly expanded our understanding of the prehistoric relationship between the southern and northern Sonoran Desert. Based on obsidian source provenance, archaeological sites in the Valle de Altar indicate that contact with the region as far north as west-central and northern Arizona, the northern portion of the Papaquería, the Sierra Pinacate, the Sierra Madre Occidental, and the Basin and Range of Chihuahua was potentially frequent. However, the vast majority of artifacts produced from obsidian were procured apparently locally from an, as yet, unlocated source called here “SON Valle de Altar Unknown.” The purpose here is to discuss the relevant regional geology, the source provenance of the obsidian artifact assemblage, and relationships throughout the greater Sonoran Desert. This project also adds significant data to the Southwest Archaeological Obsidian Project (http://swxrflab.net/).

Seasonal Rainfall, Shrub Cover and Soil Properties Drive Production of Winter Annuals in the Northern Sonoran Desert

Seasonal Rainfall, Shrub Cover and Soil Properties Drive Production of Winter Annuals in the Northern Sonoran Desert

Hiding in plain sight: Cryptic enemies are found on cochineal (Hemiptera: Dactylopiidae), a scale insect of economic and cultural significance.

Cochineal is the common name for cactus‐feeding scale insects in the Dactylopiidae. These ruby‐red insects include the domesticated dye insect Dactylopius coccus. Dactylopius coccus and congeners have been introduced around the world, some accidentally, to become pests of prickly pear cactus species (Opuntia), and some intentionally, for dye production or biological control of pest Opuntia. In the northern Sonoran Desert (Tucson, AZ, USA), we studied the enemy complex of D. opuntiae and D. confusus on Opuntia and characterized two cryptic enemies, a coccinellid beetle predator and a parasitoid wasp. (1) Hyperaspis sp. The coccinellid predator Hyperaspis trifurcata shares a niche with a similar, typically all‐black beetle. Morphological data, crossing tests, and phylogenetic results showed the black beetle to be a distinct, undescribed species in the genus Hyperaspis, with a rare spotted phenotype that is similar in appearance to H. trifurcata. Crossing tests among black and spotted forms showed the spotted morph is inherited as a single‐locus dominant allele. (2) Formicencyrtus thoreauini. Rearing of this ant‐like parasitoid wasp (Encyrtidae) in pure culture of D. opuntiae showed it to be a semi‐gregarious primary parasitoid of cochineal. To our knowledge, this is the first confirmed instance of a cochineal parasitoid. Observations of development show early instar larvae keep their posterior end within the egg chorion, attached to an aeroscopic plate with a connection to the cochineal body wall. Late instar larvae are eventually surrounded by a membrane, likely of larval origin. Wasps then pupate in a dry air‐filled chamber within the desiccated scale remains before chewing out as an adult. Both Hyperaspis sp. and F. thoreauini may have restricted distributions. Hyperaspis sp. does not appear to be a member of the cochineal community in Mexico or Texas, and scant records suggest F. thoreauini may also be restricted to the Southwestern USA.

Relationships among climate, stem growth, and biomass δ13C in the giant saguaro cactus (Carnegiea gigantea)

AbstractGiant saguaro (Carnegiea gigantea) is one of the longest‐lived and massive cacti species in the Americas. They occur throughout the Sonoran Desert region with a distribution spanning a five‐fold gradient in mean annual precipitation. Relationships between fitness traits, including stem growth, and spatio‐temporal climate patterns are still poorly understood in saguaro, but are assumed to be largely coupled to summer precipitation. To better understand patterns of climate sensitivity in giant saguaro, annual stem growth, carbon isotope ratios (δ13C) in spine tissues, and seasonal variation in stem volume, a proxy for stem water storage, were evaluated over a single growing season (2014) in six widely distributed populations in the northern Sonoran Desert, and over four consecutive growing seasons (2013–2016) in two populations with differences in mean annual precipitation and site moisture (Mi), defined as precipitation amount divided by mean atmospheric vapor pressure deficit. We hypothesized that saguaro growth and δ13C would be coupled to a complex suite of climate conditions that include winter precipitation and aridity. Annual stem growth of all populations was generally better correlated to Mi than precipitation alone and was best correlated with Mi measured over the hydrologic year, October 2013–September 2014 (F = 39.8, P < 0.0001). Likewise, mean δ13C increased with Mi, with the highest correlation with Mi calculated for July 2013–August 2014 (F = 38.4.0, P < 0.0001). Annual stem growth measured across all populations was well correlated to δ13C in spines produced during the current year of growth (F = 36.3, P < 0.0001). Annual variation in stem growth appeared to reflect annual variation in mean stem volume measured monthly from the summer of the previous year to the summer of the current year of growth. Results suggest that stem growth and photosynthetic physiology recorded by δ13C are coupled to a complex suite of climate conditions with a strong legacy effect from the previous summer and winter. These findings provide new insight on the effects of summer and winter drought and a warming climate on the photosynthesis, growth, and fitness of giant saguaro.

Variations In Saguaro Cactus (Carnegiea gigantea) Spine Length In Wet and Dry Portions of Their Range

I sampled spine length in the keystone Carnegiea gigantea (Engelm.) Britton & Rose in two northern Sonoran Desert populations where this species is ultimately limited by cold temperatures. I compared spine length (1) between two sites, (2) on north versus south facing sides of plants, (3) by level of shading, (4) by proximity to other potential conspecific competitors, and (5) by presence near sources of surface runoff from which all their water is obtained. Spine length increases, providing thermal protection and reducing photosynthesis, where temperature is more extreme (winter and summer) and where conditions are more xeric. The difference in spine length by direction on plant was much smaller at the more extreme site. Clumped plants that necessarily had increased competition for water but also greater shading were split; at the more extreme site, greater spine shading occurred where water competition was fiercest, while at the less extreme site, greater shading occurred in association with sunlight rather than water.

Demographic changes over >70 yr in a population of saguaro cacti (Carnegiea gigantea) in the northern Sonoran Desert

We studied changes in a population of saguaro (Carnegiea gigantea) cacti on a 259 ha plot in the northern Sonoran Desert (Saguaro National Park) that was first surveyed in 1941. With the help of citizen scientists, we mapped and measured the height of all detected individual saguaros. The total number of detected saguaros was 13,304 in 1941 and 9023 in 2012, a 31.4% decrease. The population in 1941 consisted of mostly larger, older individuals, whereas in 2012 it was dominated by smaller, younger individuals. Based on estimated years of saguaro establishment, a period of high establishment in the mid-late 1800s was followed by many decades of poor establishment. However, establishment surged from the 1970s to early 1990s but has been very low since. Comparisons with other long-term saguaro studies suggest that the dramatic demographic changes in this area have resulted from climatic factors, most notably recent long-term drought and higher temperatures, in combination with land-use changes, especially loss of nurse trees due to wood-cutting early in the twentieth century.

Decadal shifts in grass and woody plant cover are driven by prolonged drying and modified by topo-edaphic properties.

Woody plant encroachment and overall declines in perennial vegetation in dryland regions can alter ecosystem properties and indicate land degradation, but the causes of these shifts remain controversial. Determining how changes in the abundance and distribution of grass and woody plants are influenced by conditions that regulate water availability at a regional scale provides a baseline to compare how management actions alter the composition of these vegetation types at a more local scale and can be used to predict future shifts under climate change. Using a remote-sensing-based approach, we assessed the balance between grasses and woody plants and how climate and topo-edaphic conditions affected their abundances across the northern Sonoran Desert from 1989 to 2009. Despite widespread woody plant encroachment in this region over the last 150years, we found that leguminous trees, including mesquite (Prosopis spp.), declined in cover in areas with prolonged drying conditions during the early 21st century. Creosote bush (Larrea tridentata) also had moderate decreases with prolonged drying but was buffered from changes on soils with low clay that promote infiltration and high available water capacity that allows for retention of water at depth. Perennial grasses have expanded and contracted over the last two decades in response to summer precipitation and were especially dynamic on shallow soils with high clay that have large fluctuations in water availability. Our results suggest that topo-edaphic properties can amplify or ameliorate climate-induced changes in woody plants and perennial grasses. Understanding these relationships has important implications for ecosystem function under climate change in the southwestern USA and can inform management efforts to regulate grass and woody plant abundances.

How long does the giant saguaro live? Life, death and reproduction in the desert

Little is known about range-wide mortality patterns and reproductive lifetimes of the keystone Carnegiea gigantea (saguaro, Cactaceae) over its range in the Sonoran Desert. The great geographic variability in reproductive effort has been documented, but variation (likely environmentally driven) across populations in life expectancy is relatively unknown, particularly in relation to reproduction over time across populations, the sustainability of populations and their longevity. Longevity is assessed in six populations characterized by different topographic and climatic conditions. Also, longevity data for an amalgamated dataset representing 50,000 km2 of the northern Sonoran Desert are tabulated. I observe that life expectancy is higher in slower growing populations. For example, the cutoff for the oldest 1% of the population ranges from 127 years in the fastest growing population to 193 years of age in the slowest. Past research has shown that more arid (and slow growing) populations delay reproduction and branching (branches increase reproductive output) to much older ages. However, even with significant delays in reproduction, slower-growing populations may still enjoy reproductive lifetimes that are similar to or greater than their faster-growing (and earlier dying) counterparts in more moist environments.

Phylogeography of the Arizona hairy scorpion (Hadrurus arizonensis) supports a model of biotic assembly in the Mojave Desert and adds a new Pleistocene refugium

AbstractAimAs data accumulate, a multi‐taxon biogeographical synthesis of the Mojave Desert is beginning to emerge. The initial synthesis, which we call the ‘Mojave Assembly Model’, was predominantly based on comparisons of phylogeographical patterns from vertebrate taxa. We tested the predictions of this model by examining the phylogeographical history of Hadrurus arizonensis, a large scorpion from the Mojave and Sonoran deserts.LocationMojave and Sonoran deserts, United States and Mexico.MethodsWe sequenced mitochondrial cytochrome c oxidase subunit I (COI) data from 256 samples collected throughout the range of H. arizonensis. We analysed sequence data using a network analysis, spatial analysis of molecular variance (SAMOVA), and a Mantel test. We then used a molecular clock to place the genetic patterns in a temporal framework. We tested for signals of expansion using neutrality tests, mismatch distributions and Bayesian skyline plots. We used Maxent to develop current and late‐glacial species distribution models from occurrence records and bioclimatic variables.ResultsPhylogenetic and structure analyses split the maternal genealogy basally into a southern clade along the coast of Sonora and a northern clade that includes six lineages distributed in the Mojave Desert and northern Sonoran Desert. Molecular dating suggested that the main clades diverged between the late Pliocene and early Pleistocene, whereas subsequent divergences between lineages occurred in the middle and late Pleistocene. Species distribution models predicted that the distribution of suitable climate was reduced and fragmented during the Last Glacial Maximum.Main conclusionsGenetic analyses and species distribution modelling suggest that the genetic diversity within H. arizonensis was predominantly structured by Pleistocene climate cycles. These results are generally consistent with the predictions of Pleistocene refugia for arid‐adapted taxa described in the Mojave Assembly Model, but suggest that a northern area of the Lower Colorado River Valley may have acted as an additional refugium during Pleistocene glacial cycles.

Regional demographic trends from long-term studies of saguaro (Carnegiea gigantea) across the northern Sonoran Desert

Ten saguaro (Carnegiea gigantea) populations in the northern Sonoran Desert were monitored from 1959 to 2005 to discriminate how climate influences plant growth, abundance, reproductive potential, survivorship, age structure and regeneration trends. Thousands of saguaros were measured to determine site-specific growth rates and survivorship through time. Observed growth rates were used to predict the ages of saguaros and reconstruct local and regional regeneration patterns back to the late 18th century. Both growth rates and degree of branching generally tracked temperature and moisture gradients. Site-specific age–height models explained 89–97% of variance in observed ages, with a slope of nearly one. Regeneration was more consistent at sites in the western (hotter/drier) than eastern (cooler/wetter) sites, which exhibited clear multidecadal variability in regeneration rates. Averaged across the region, saguaro regeneration rates were highest from 1780 to 1860, coincident with wet conditions and high Pinus ponderosa recruitment in the highlands. Milder and wetter winters and protection from livestock grazing likely promoted late 20th century regeneration surges at some sites. Predictions of saguaro population dynamics in the 21st century likely will be confounded by the saguaro's episodic and asynchronous regeneration, continued urbanization, ongoing grass invasions and associated wildfires, and changing climate.

Variation in monsoon precipitation drives spatial and temporal patterns of Larrea tridentata growth in the Sonoran Desert

Summary 1. Broad-scale patterns of above-ground net primary production (ANPP) are closely coupled to climate features, particularly the distribution and magnitude of rainfall. In arid and semi-arid regions, however, the precipitation regime, together with local geomorphology and plant life history traits, combine to regulate soil water availability and patterns of growth, complicating simple correlations with climatic variables. 2. To better understand the drivers of plant growth in desert ecosystems, we characterized the rates and spatial heterogeneity of stem elongation by the dominant perennial shrub, creosote bush (Larrea tridentata) in the northern Sonoran Desert of Arizona (USA). Estimates of stem growth were made over a 5-year period (2006–2010) from 60 plots at 15 sites spanning c. 120 km across the Central Arizona–Phoenix (CAP) Long-term Ecological Research (LTER) area. 3. We observed both the highest and lowest rates of stem growth during summer, and these patterns were related to the amount of monsoon rainfall and local rates of water infiltration. The relationship between growth and precipitation in the summer was nonlinear, with rates increasing more than eightfold at plots receiving more than 100 mm of seasonal rainfall. Conversely, growth during the winter/spring was intermediate in magnitude, similar among years and poorly correlated with seasonal precipitation. 4. The spatial heterogeneity of stem growth also differed markedly between seasons and was greater both within and among sites during summer compared to winter/spring growing periods. At broad scales, spatial heterogeneity of shrub growth was correlated with seasonal changes in the spatial variability of rainfall across the study area. At small spatial scales, greater heterogeneity during the summer appears linked to local soil properties that influence infiltration and water availability following monsoon storms. 5. Overall, the strong, nonlinear growth response by L. tridentata to monsoon rainfall suggests that the recurrence interval of wet summer seasons is an important driver of ANPP for this long-lived shrub. More generally, our results illustrate how regional climate variability and local soil properties can interact to influence the rates and heterogeneity of desert plant growth at multiple scales.

Demographics of More than 12,000 Individuals of a Keystone Species in the Northern Sonoran Desert since the Mid‐1800s

We studied a cohort‐based, long‐lived species to determine whether favorable periods that promote regeneration are driven by local, regional, or global‐scale factors. Limited demographic data exist for the keystone species Carnegiea gigantea over its range. We obtained a data set collected for 12,232 plants over an area of more than 11 km2 at a restricted military zone, located far from any other studied population. We developed the establishment pattern for the species over the last 200 yr and compared population growth trends with those for other sites and with global‐scale volcanism, which has been previously linked to the regeneration of the species. This population was significantly related to the two closer sites, suggesting a regional influence in regeneration and cohort establishment. Other eastern sites like ours also exhibited a regeneration peak in the 1920s. This population was not significantly related to volcanic eruptions, as has been documented elsewhere, though this may be due to subsequent mortality at the site, which would have effectively erased the record of establishment in past periods. Unlike past work, our study goes beyond a single population. We synthesized data from other known populations in order to produce, for the first time, a coherent picture of the patterns of regeneration over much of the species’ range. Large‐scale climatic (e.g., unusually rainy periods) and geologic (global‐scale volcanism) events promote the establishment of cohorts during the same time periods historically, though to differing extents in different populations: hot, western populations have particularly large cohorts in response to volcanism, while more eastern sites may have observable cohorts during known wetter periods. Thus, these different large‐scale phenomena appear to be of varying significance in a population on the basis of the population's location and associated environment. This provides insight into global climate change impacts on the species, whose keystone status will ultimately affect the success of and changes in the entire biome.

Functional differences between summer and winter season rain assessed with MODIS-derived phenology in a semi-arid region

Questions: We asked several linked questions about phenology and precipitation relationships at local, landscape, and regional spatial scales within individual seasons, between seasons, and between year temporal scales. (1) How do winter and summer phenological patterns vary in response to total seasonal rainfall? (2) How are phenological rates affected by the previous season rainfall? (3) How does phenological variability differ at landscape and regional spatial scales and at season and inter-annual temporal scales? Location: Southern Arizona, USA. Methods: We compared satellite-derived phenological variation between 38 distinct 625-km2 landscapes distributed in the northern Sonoran Desert region from 2000 to 2007. Regression analyses were used to identify relationships between landscape phenology dynamics in response to precipitation variability across multiple spatial and temporal scales. Results: While both summer and winter seasons show increases of peak greenness and peak growth with more precipitation, the timing of peak growth was advanced with more precipitation in winter, while the timing of peak greenness was advanced with more precipitation in summer. Surprisingly, summer maximum growth was negatively affected by winter precipitation. The spatial variations between summer and winter phenology were similar in magnitude and response. Larger-scale spatial and temporal variation showed strong differences in precipitation patterns; however the magnitudes of phenological spatial variability in these two seasons were similar. Conclusions: Vegetation patterns were clearly coupled to precipitation variability, with distinct responses at alternative spatial and temporal scales. Disaggregating vegetation into phenological variation, spanning value, timing, and integrated components revealed substantial complexity in precipitation-phenological relationships.

Urban heat islands and landscape heterogeneity: linking spatiotemporal variations in surface temperatures to land-cover and socioeconomic patterns

The urban heat island (UHI) phenomenon is a common environmental problem in urban landscapes which affects both climatic and ecological processes. Here we examined the diurnal and seasonal characteristics of the Surface UHI in relation to land-cover properties in the Phoenix metropolitan region, located in the northern Sonoran desert, Arizona, USA. Surface temperature patterns derived from the Advanced Spaceborne Thermal Emission and Reflection Radiometer for two day-night pairs of imagery from the summer (June) and the autumn (October) seasons were analyzed. Although the urban core was generally warmer than the rest of the area (especially at night), no consistent trends were found along the urbanization gradient. October daytime data showed that most of the urbanized area acted as a heat sink. Temperature patterns also revealed intra-urban temperature differences that were as large as, or even larger than, urban–rural differences. Regression analyses confirmed the important role of vegetation (daytime) and pavements (nighttime) in explaining spatio-temporal variation of surface temperatures. While these variables appear to be the main drivers of surface temperatures, their effects on surface temperatures are mediated considerably by humans as suggested by the high correlation between daytime temperatures and median family income. At night, however, the neighborhood socio-economic status was a much less controlling factor of surface temperatures. Finally, this study utilized geographically weighted regression which accounts for spatially varying relationships, and as such it is a more appropriate analytical framework for conducting research involving multiple spatial data layers with autocorrelated structures.

Density-dependent shading patterns by Sonoran saguaros

Woody plants produce shading patterns that can alter soil evaporation rates and provide nurse-plant effects for seedling establishment. These effects depend on the density of woody plants and associated characteristics of canopy architecture such as height, and can be particularly important in deserts. The tallest stature woody plants in some desert ecosystems are columnar cacti, which have distinctly different architecture than other woody plants for which shading effects have been systematically assessed. Focusing on columnar cacti of the northern Sonoran Desert, we used hemispherical photography to evaluate the effects of saguaro cacti ( Carnegiea gigantea) on microclimate along a gradient of increasing cactus density. Notably, incoming annual near-ground solar radiation was reduced by up to ∼10% for the highest density stand (156 cacti ha −1), with spatial variation in shading patterns peaking at nearly the highest stand density (133 cacti ha −1). The annual near-ground solar radiation reductions were more directly related to cacti density than to density of surrounding shrubs. Our results document that at high density, saguaro columnar cacti can have substantial effects on microclimate and, more generally, our results contribute to the growing library of relationships quantifying how shading patterns vary with woody plant architecture and density.

Using a diverse seed mix to establish native plants on a Sonoran Desert burn

Revegetating burns is a major challenge facing resource managers in the low- and unpredictable-precipitation deserts of the southwestern US. We monitored the effectiveness of using a diverse, 28-species seed mix for establishing native plants on a 1.5-ha (3.7-ac) burn in the northern Sonoran Desert. Our objective was to compare species performances, which we assessed by measuring species frequencies and cover on 5 sampling dates to capture variation during a 32-mo period following seeding. By 15 mo after seeding, desert senna ( Senna covesii (Gray) Irwin & Barneby [Fabaceae]) established best, with a frequency of 91% (based on 22, 10-m2 plots) and a relative cover of 19%. Four other seeded species also became established in ≥ 50% of plots by 32 mo after seeding. Several seeded species, including desert senna (which flowered only 7 wk after seeding) and purple threeawn ( Aristida purpurea Nutt. [Poaceae]), were observed with seed heads during one or more sampling periods. Although precipitation was only 67% of normal for 21 mo following seeding and 71% of species established in < 10% of plots, we consider the seeding to have met short-term management objectives because of the subset of highly successful species. Our results also illustrate the caution that should be used when evaluating seeding success: conclusions would have differed if the diversity of the seed mix had not included the successful species, and longer term monitoring was needed to detect some species in the seed mix that did not establish until 32 mo after seeding.

Regeneration cycles of the keystone species Carnegiea gigantea are linked to worldwide volcanism

AbstractQuestion:We know of no study that has linked volcanic eruptions occurring anywhere worldwide and the local population cycles of any species. The keystone saguaro cactus (Carnegiea gigantea) establishes in cohorts. We test whether there is a statistical relationship between Carnegiea gigantea establishment and volcanic eruptions.Location:Northern Sonoran Desert, Arizona, USA.Methods:We use both a region‐wide dataset made up of 30 populations, and a dataset from a marginal site. We incorporate data for over 750 individuals over an area of more than 50 000 km2. We created a 111‐year time series of population peaks and troughs and correlated this over the 111‐year record with the annual Weighted Historical Dust Veil Index (WHDVI). A t‐test compared establishment patterns with the WHDVI.Results:We found a significant relationship between volcanism and C. gigantea regeneration at both the marginal site, and in the region‐wide dataset.Conclusions:We suggest that while different populations are influenced by temporary global temperature ameliorations to different extents, our results show that populations do derive significant benefits from volcanic eruptions that promote their regeneration over large portions of their range, as is also exhibited locally at our marginal site, where populations are most susceptible to the inhospitable conditions that are witnessed at the edge of their range. This paper draws a link between the population fluctuations and regeneration of a species locally with geologic events from distant parts of the earth.

Nanometer-scale complexity, growth, and diagenesis in desert varnish

Nanometer-scale element mapping and spectroscopy of desert varnish from the northern Sonoran Desert in southwestern Arizona reveal a dynamic disequilibrium system characterized by postdepositional mineralogical, chemical, and structural changes activated by liquid water. Lack of equilibrium is suggested by the large variety of coexisting Mn phases. Sparse secondary Ba and Sr sulfates also occur, as do carbonaceous particles. Individual Mn-oxide particles contain variable concentrations of Ba and Ce, reflecting their role as repositories of trace elements, presumably derived from atmospheric aerosols. Desert varnish is analogous to more familiar sediments in displaying authigenic and diagenetic structures, but with total sediment thicknesses <1 mm and structures at the nanometer scale. As such, it is neither a weathering rind nor patina, but a unique subaerial sediment that is in dynamic disequilibrium. Our results suggest continuing adjustment of varnish to changing environmental conditions.

Variation in age and height of onset of reproduction in the saguaro cactus (Carnegiea gigantea) in the Sonoran Desert

The objective of this study is to determine the height and age at which reproduction begins (i.e., production of flowers and fruits; the transition to adulthood) in the giant saguaro cactus (Carnegiea gigantea) in four geographically and environmentally distinct populations, and to relate observed variability to environmental differences. The onset of reproduction has been estimated at a height of 2.2 m in near optimal conditions. This value has been widely accepted and applied to populations in less optimal conditions, although variations under less ideal conditions have not been investigated. In addition, previous research has demonstrated that Carnegiea growth rates are highly variable over their range. Thus, even if 2.2 m is a consistent transition height to adulthood over their range, the age of individuals in different populations would be different. I investigate the age and height at which this transition occurs. The author sampled the heights of the shortest reproductive individuals and the tallest non-reproductive individuals to estimate the mean height of the onset of flowering in each of four locales in the northern Sonoran Desert. Using a previously published age-height-growth model, the mean age of the start of reproduction was also computed for the four sites. ANOVA and t-tests were used to compare the average transition to adulthood across sites by both age and height. Statistical results are robust and significant variations in the onset of reproduction are observed by both age and height across the four sites. Saguaro National Park and Organ Pipe Cactus National Monument individuals are transitioning to adulthood, on average, at younger ages and shorter heights than the other two locales. At the arid and marginal Kofa site, individuals that established during the regeneration peak of the late 1800s-early 1900s are only now becoming reproductive (individuals that established around 1899), while at Saguaro National Park, on average, individuals that established in the 1950s are already transitioning to adulthood. These results have implications for regeneration, particularly in marginal locales where regeneration is already limited.

HAS CLIMATIC WARMING ALTERED SPRING FLOWERING DATE OF SONORAN DESERT SHRUBS?

With global warming, flowering at many locations has shifted toward earlier dates of bloom. A steady increase in average annual temperature since the late 1890s makes it likely that flowering also has advanced in the northern Sonoran Desert of the southwestern United States and northwestern Mexico. In this study, phenological models were used to predict annual date of spring bloom in the northern Sonoran Desert from 1894 to 2004; then, herbarium specimens were assessed for objective evidence of the predicted shift in flowering time. The phenological models were derived from known flowering requirements (triggers and heat sums) of Sonoran Desert shrubs. According to the models, flowering might have advanced by 20–41 d from 1894 to 2004. Analysis of herbarium specimens collected during the 20th century supported the model predictions. Over time, there was a significant increase in the proportion of shrub specimens collected in flower in March and a significant decrease in the proportion collected in May. Thus, the flowering curve – the proportion of individuals in flower in each spring month – shifted toward the start of the calendar year between 1900 and 1999. This shift could not be explained by collection activity: collectors showed no tendency to be active earlier in the year as time went on, nor did activity toward the end of spring decline in recent decades. Earlier bloom eventually could have substantial impacts on plant and animal communities in the Sonoran Desert, especially on migratory hummingbirds and population dynamics of shrubs.