Sonoran Desert Ecosystem Research Articles

Higher phosphorus and water use efficiencies and leaf stoichiometry contribute to legume success in drylands

Abstract Legumes are essential plants in dryland ecosystems worldwide because they increase nitrogen availability, so their understanding is vital for improving knowledge and modelling in the face of climate change. This work studies the differences in resource use efficiency and their relationship with photosynthetic, photochemical, bioelemental, and stoichiometric traits of coexistent legumes and non‐legumes in a Sonoran Desert ecosystem. We found that legumes had higher photosynthetic rates, intrinsic and seasonal water use efficiency (WUE), phosphorus use efficiency (PPUE), and higher light utilisation mediated by chlorophyll content and active reaction centers, which may increase their photoprotection. Legumes can increase their WUE and PPUE with no changes in nitrogen use efficiency (PNUE). Consequently, observed trait relationships between studied traits in these legumes have significant differences with the non‐legume species in the study. Stoichiometry is helpful, in some cases, as an indicator of nutrient use efficiency and enables functional group differentiation. Our results strongly relate legumes' higher resource use efficiency with their success in dryland ecosystems. Read the free Plain Language Summary for this article on the Journal blog.

Associations between a feral equid and the Sonoran Desert ecosystem

Associations between a feral equid and the Sonoran Desert ecosystem

Effects of parity, seasonal heat stress, and colostrum collection time postpartum on colostrum quality of Holstein cattle in an arid region.

The aim of this study was to determine effects of parity (primiparous vs. multiparous), seasonal heat stress at calving (summer vs. winter), and time postpartum on some parameters associated with colostrum quality in Holstein cows reared in the Sonoran Desert ecosystem. Forty-seven cows (11 primiparous and 36 multiparous) expected to calve during summer, and 46 cows during winter (14 primiparous and 32 multiparous) were randomly selected. Management and feeding before and after parturition were similar for cows in both seasons. After parturition, colostrum from all cows was evaluated for volume, weight, temperature, density, and content of fat, protein, solids non-fat (SNF), and immunoglobulins (IGG). Data were analyzed with a model that included effects of parity status, calving season, and time postpartum, as well as all interactions. Colostrum produced in summer was warmer (P < 0.01) by almost 6°C than winter colostrum, while colostrum from multiparous was warmer (P = 0.02) by 1.2°C than that produced by primiparous cows. Colostrum volume and weight were not impacted by parity, calving season or time postpartum. Density, protein, and SNF content in colostrum were higher (P < 0.01) in multiparous vs. primiparous cows, as well as at parturition (0h postpartum) than at 12h postpartum (P < 0.01). At calving (0h), spring colostrum had higher fat content (P < 0.01) and lower (P < 0.01) IGG concentration than that collected in summer, and no difference (P > 0.05) between seasons was observed for these components at 12h postpartum. Multiparous cows produced colostrum with higher (P < 0.01) IGG concentrations than primiparous cows. In conclusion, only 0-h colostrum and that from multiparous cows was categorized as "Excellent," meanwhile the colostrum produced under summer heat stress was characterized as "Good" with reduced fat content. While the lacteal secretion collected at 12 post-partum still classified as colostrum, substantially lower contents of IGG, protein, fat, and SNF decreased its classification to "Poor" from the classification of "Excellent" at 0h postpartum.

Homeowner perceptions and responses to buffelgrass invasion risk in the Tucson, Arizona Wildland-Urban Interface

In this study, we aimed to analyze homeowners' level of awareness and perceived risk about buffelgrass invasion in the Tucson, Arizona Wildland-Urban Interface (WUI), as well as the factors influencing their participation in buffelgrass control and fire risk mitigation efforts. Data for the study were generated through the administration of an online survey among 117 members of Home Owner Associations (HOAs) in the Tucson WUI. The results showed that the overwhelming majority of respondents were aware of buffelgrass, but their knowledge about buffelgrass control mechanisms appeared to be limited. Respondents also more frequently expressed concern about the risks posed by buffelgrass invasion to general targets, such as the Sonoran Desert ecosystem, native plants and wildlife than risks to their private property and neighborhoods. The results also showed that the level of involvement in HOAs, and leadership in HOAs had significant positive effects on homeowners' participation in buffelgrass control efforts. Homeowners' duration of residence also had a significant negative effect on participation in buffelgrass control efforts, suggesting that newcomers may be more involved than long-term residents. Similarly, the number of months respondents spent in Tucson per year had a negative effect on the number of hours spent on buffelgrass control efforts. Respondents’ perceived risk about buffelgrass invasion also had a positive effect on the hours spent on buffelgrass control as well as their level of involvement in fire risk mitigation efforts. These results highlight the importance of local institutions and community heterogeneity in social responses to threats in WUI communities. Policies aimed at building the resilience of WUI communities need to account for their complexity as coupled social-ecological systems.

Estimating social-ecological resilience: fire management futures in the Sonoran Desert.

Resilience quantifies the ability of a system to remain in or return to its current state following disturbance. Due to inconsistent terminology and usage of resilience frameworks, quantitative resilience studies are challenging, and resilience is often treated as an abstract concept rather than a measurable system characteristic. We used a novel, spatially explicit stakeholder engagement process to quantify social-ecological resilience to fire, in light of modeled social-ecological fire risk, across the non-fire-adapted Sonoran Desert Ecosystem in Arizona, USA. Depending on its severity and the characteristics of the ecosystem, fire as a disturbance has the potential to drive ecological state change. As a result, fire regime change is of increasing concern as global change and management legacies alter the distribution and flammability of fuels. Because management and use decisions impact resources and ecological processes, social and ecological factors must be evaluated together to predict resilience to fire. We found highest fire risk in the central and eastern portions of the study area, where flammable fuels occur with greater density and frequency and managers reported fewer management resources than in other locations. We found lowest fire resilience in the southeastern portion of the study area, where combined ecological and social factors, including abundant fuels, few management resources, and little evidence of past institutional adaptability, indicated that sites were least likely to retain their current characteristics and permit achievement of current management objectives. Analyzing ecological and social characteristics together permits regional managers to predict the effects of changing fire regimes across large, multi-jurisdictional landscapes and to consider where to direct resources. This study brought social and ecological factors together into a common spatial framework to produce vulnerability maps; our methods may inform researchers and managers in other systems facing novel disturbance and spatially variable resilience.

Potential changes in the distribution of Carnegiea gigantea under future scenarios.

Over the last decades several studies have identified that the directional changes in climate induced by anthropogenic emissions of greenhouse gases are affecting the ecology of desert ecosystems. In the Southwest United States, the impacts of climate change to plant abundance and distribution have already been reported, including in the Sonoran Desert ecosystem, home of the iconic Saguaro (Carnegiea gigantea). Hence, there is an urgent need to assess the potential impacts of climate change on the saguaro. The goals of this study are to provide a map of actual habitat suitability (1), describe the relationships between abiotic predictors and the saguaro distribution at regional extents (2), and describe the potential effect of climate change on the spatial distribution of the saguaro (3). Species Distribution Modeling (SDM) was used to investigate the relationships between abiotic variables and the Saguaro distribution. SDMs were calibrated using presence records, 2,000 randomly-generated pseudo absences, and ten abiotic variables. Of these, annual precipitation and max temperature of the warmest month was found to have the greatest relative influence on saguaro distribution. SDMs indicated that 6.9% and 8.1% of the current suitable habitat is predicted to be lost by 2050 and 2070, respectively. Therefore, predicted changes in climate may result in a substantial contraction of the suitable habitat for saguaro over the next century. By identifying the drivers of saguaro distribution and assessing potential changes in habitat suitability due to climate change, this study will help practitioners to design more comprehensive strategies to conserve the saguaro in the face of climate change.

Sonoran Desert rodent abundance response to surface temperature derived from remote sensing

Nocturnal rodents play a key role in the Sonoran Desert ecosystem as consumers, prey and reservoirs of disease-yet rodent distribution remains poorly mapped. We use surface temperature extracted from the Landsat Thematic Mapper thermal infrared band to model rodent abundance obtained from trapping data at the Organ Pipe Cactus National Monument in southwest Arizona. We produce response curves that describe the effects of surface temperature on species abundance. Daytime surface temperature derived from images collected in June across thirty rodent trapping grids ranged from 318 K (45.0 °C) to 332 K (59.2 °C). Results show peak seasonal surface temperature splits high species abundances into two groups: Merriam's kangaroo rat, Arizona pocket mouse, desert pocket mouse and southern grasshopper mouse species prefer habitat characterized by higher surface temperatures; and white-throated wood rat, cactus mouse, rock pocket mouse and Bailey's pocket mouse species prefer habitat characterized by lower temperatures. Abundances for the high and low temperature habitat groupings correlate linearly with surface temperature (r = +0.66 and −0.75, respectively, p < 0.001). Surface temperature integrates multiple landscape characteristics (e.g. topography, vegetation cover and soil properties) that define the dominant habitats of ORPI rodents. These results prove that simple models can inform complex ecological processes.

Plant Biotic Interactions in the Sonoran Desert: Conservation Challenges and Future Directions

Plant Biotic Interactions in the Sonoran DesertConservation Challenges and Future Directions Brigitte Marazzi (bio), Judith L. Bronstein (bio), Pacifica N. Sommers (bio), Blanca R. López (bio), Enriquena Bustamante Ortega (bio), Alberto Búrquez (bio), Rodrigo A. Medellín (bio), Clare Aslan (bio), and Kim Franklin (bio) Introduction Biotic interactions are vital to ecosystem functioning. Interactions among individuals lie at the core of population and community dynamics, and therefore play a central role in the existence and persistence of species. Plants form the food base of most terrestrial ecosystems and are therefore not surprisingly involved in a substantial portion of biotic interactions. Plants, animals, and microbes face great challenges to survival in the desert environment, and these interactions play a critical role in the survival of many species. The Sonoran Desert flora is well documented and certain of its iconic interactions are well understood. For example, saguaros and the bats that pollinate them and disperse their fruits have become textbook examples of mutualisms (e.g., Shreve and Wiggins 1964; Turner et al. 2005). However, what do we know about plant-animal, plant-plant, and plant-microbe interactions in the Sonoran Desert more generally? What role do such interactions play in the ecology and evolution of the Sonoran Desert ecosystem? How are these interactions affected by global changes, and how can we conserve interactions? These questions inspired a discussion session convened at the Next Generation Sonoran Desert Researchers (NGSDR) 2012 Summit. Ultimately, participants identified the following five critical needs regarding research and conservation. We need to (1) improve our knowledge of the natural history (diversity, ecology, evolution) of interactions, both as individual entities and as [End Page 457] players in the broader ecological community; (2) monitor interactions on broad spatio-temporal scales to be able to identify the consequences of climate change, especially for seasonal interactions involving migratory species; (3) identify the human activities with the greatest impacts on interactions; (4) develop criteria to compile a “priority interaction list” to improve and strengthen our ecosystem conservation efforts; and, finally, (5) use interactions to restore disrupted habitats and ecosystems. Here we provide a comprehensive yet concise overview of biotic interactions involving the flora, fauna, and microbiota of the Sonoran Desert, summarizing and expanding results of the NGSDR 2012 Summit discussion. We briefly present the broad categories of interspecific interactions involving Sonoran Desert plants, identify and describe threats known to negatively affect them as well as positive links with human activities, and present ongoing conservation needs and restoration efforts. We conclude by suggesting future research directions and recommendations required for urgent conservation and restoration efforts. Diversity of Biotic Interactions All living organisms on Earth are involved in interactions with other organisms. Interactions are “mutualistic” when both organisms benefit and “antagonistic” when only one benefits at the expense of the other. They are “facultative” when participants do not strictly depend on one another and interact with several other species; they are “obligate” when at least one species relies on the other and rarely interacts with other species, making the interaction a matter of life and death in some cases. Although obligate interactions represent only a minor fraction of the immense web of biotic interactions, their more constant association and higher specificity make them easier to study (Davidson and McKey 1993; Futuyma and Agrawal 2009). In contrast, facultative interactions are both more common and complicated, since species interact with many partners and the associations can fluctuate between mutualistic and antagonistic (e.g., Bentley 1977; Ness 2006). Here, we focus on interspecific interactions in which Sonoran Desert plants regularly engage. We group them based on the harm or benefits animals and microbes confer. This includes two forms of antagonism, herbivory and parasitism, and five forms of mutualism, pollination, seed dispersal, biotic protection, facilitation, and microbe-mediated nutrition. These are detailed below. An extensive bibliography is provided in appendix 1. [End Page 458] (i) Plant-Herbivore Interactions Plants are the most common terrestrial food resource for animals. Most plant-consuming animals are insects; among vertebrates, most are mammals (Herrera and Pellmyr 2002). Plants in desert habitats are particularly important for animal consumers, because they represent a critical source not only of nutrients...

Climatic variation and tortoise survival: Has a desert species met its match?

While demographic changes in short-lived species may be observed relatively quickly in response to climate changes, measuring population responses of long-lived species requires long-term studies that are not always available. We analyzed data from a population of threatened Agassiz’s desert tortoises (Gopherus agassizii) at a 2.59km2 study plot in the Sonoran Desert ecosystem of Joshua Tree National Park, California, USA from 1978 to 2012 to examine variation in apparent survival and demography in this long-lived species. Transect-based, mark-recapture surveys were conducted in 10 of those years to locate living and dead tortoises. Previous modeling suggested that this area would become unsuitable as tortoise habitat under a warming and drying climate scenario. Estimated adult population size declined greatly from 1996 to 2012. The population appeared to have high apparent survival from 1978 to 1996 but apparent survival decreased from 1997 to 2002, concurrent with persistent drought. The best model relating apparent survivorship of tortoises ⩾18cm over time was based on a three year moving average of estimated winter precipitation. The postures and positions of a majority of dead tortoises found in 2012 were consistent with death by dehydration and starvation. Some live and many dead tortoises found in 2012 showed signs of predation or scavenging by mammalian carnivores. Coyote (Canis latrans) scats and other evidence from the site confirmed their role as tortoise predators and scavengers. Predation rates may be exacerbated by drought if carnivores switch from preferred mammalian prey to tortoises during dry years. Climate modeling suggests that the region will be subjected to even longer duration droughts in the future and that the plot may become unsuitable for continued tortoise survival. Our results showing wide fluctuations in apparent survival and decreasing tortoise density over time may be early signals of that possible outcome.

Nest-Guarding by Female Agassiz's Desert Tortoise (Gopherus agassizii)at a Wind-Energy Facility Near Palm Springs, California

Abstract We observed behavior consistent with nest-guarding in Agassiz's desert tortoise (Gopherus agassizii) at two nests in a large wind-energy-generation facility near Palm Springs, California, locally known as the Mesa Wind Farm. As researchers approached the nests, female desert tortoises moved to the entrance of their burrows and positioned themselves sideways, directly over their nests. One female stretched her limbs outward and wedged herself into the burrow (her plastron directly above the nest). Guarding of nests is rarely observed in Agassiz's desert tortoise but can occur as a result of attempted predation on eggs by Gila monsters (Heloderma suspectum) or in direct response to the perceived threat posed by researchers. This is the first report of nest-guarding for G. agassizii in the Sonoran Desert ecosystem of California.

The Intercultural Center for the Study of Deserts and Oceans (CEDO)

CEDO was developed in 1980 as an offshoot of a now defunct University of Arizona aquaculture project. The scope of activities within CEDO’s mission have grown from a primary focus on advancing and sharing knowledge about the NGC and surrounding Sonoran Desert ecosystem and promoting conservation, to a more active role in creating solutions and empowering coastal communities with the knowledge and tools to create sustainable livelihoods that exist in concert with the surrounding bicultural and natural environments. In addition to ecological and fisheries research, CEDO develops and delivers educational programs and environmental awareness contests for local schools, eco-tourism, and outreach to understand the needs and concerns of fishing communities throughout the NGC and to support them in applying the latest sound management practices and other sustainable solutions.

Small-scale and extensive hydrogeomorphic modification and water redistribution in a desert city and implications for regional nitrogen removal

There are numerous examples of small-scale hydrogeomorphic manipulations within urban ecosystems. These modifications are motivated both by a need to handle storm drainage and by a human desire for aquatic ecosystems as places for recreation and aesthetics. In the Phoenix Arizona metropolitan area, two examples of these local modifications are artificial lakes and stormwater retention basins. Although lakes are not a natural feature of Sonoran Desert ecosystems, numerous artificial lakes are evident in the region. Retention basins are a common landscaping practice for preventing damage from rare but potentially large storm events. Here we attempt to quantify the heretofore unknown number and extent of these designed aquatic ecosystems and consider their potential impact on hydrologic landscape connectivity and regional nitrogen (N) removal. For lakes, we found that official GIS layers from local and state agencies had significant misclassifications and omissions. We used two published GIS datasets and state impoundment-permit information to determine the number, areal extent, and water source for artificial lakes. We discovered that there are 908–1,390 lakes in the Phoenix area, with the number varying according to level of aggregation. There are no existing GIS data on retention basins, so we employed drywell-permit data to estimate that there may be 10,000 retention basins in the region. Basic data on N stocks in these ecosystems are discussed within the context of the regional N budget. Accurate data on the extent and distribution of these designed ecosystems will be vital for water-resources planning and stormwater management.

Sonoran Desert Ecosystem transformation by a C4 grass without the grass/fire cycle

AbstractAim Biological invasions facilitate ecosystem transformation by altering the structure and function, diversity, dominance and disturbance regimes. A classic case is the grass–fire cycle in which grass invasion increases the frequency, scale and/or intensity of wildfires and promotes the continued invasion of invasive grasses. Despite wide acceptance of the grass–fire cycle, questions linger about the relative roles that interspecific plant competition and fire play in ecosystem transformations.Location Sonoran Desert Arizona Upland of the Santa Catalina Mountains, Arizona, USA.Methods We measured species cover, density and saguaro (Carnegiea gigantea) size structure along gradients of Pennisetum ciliare invasion at 10 unburned/ungrazed P. ciliare patches. Regression models quantified differences in diversity, cover and density with respect to P. ciliare cover, and residence time and a Fisher’s exact test detected demographic changes in saguaro populations. Because P. ciliare may have initially invaded locations that were both more invasible and less diverse, we ran analyses with and without the plots in which initial infestations were located.Results Richness and diversity decreased with P. ciliare cover as did cover and density of most dominant species. Richness and diversity declined with increasing time since invasion, suggesting an ongoing transformation. The proportion of old‐to‐young Carnegiea gigantea was significantly lower in plots with dominant P. ciliare cover.Main conclusions Rich desert scrub (15–25 species per plot) was transformed into depauperate grassland (2–5 species per plot) within 20 years following P. ciliare invasion without changes to the fire regime. While the onset of a grass–fire cycle may drive ecosystem change in the later stages and larger scales of grass invasions of arid lands, competition by P. ciliare can drive small‐scale transformations earlier in the invasion. Linking competition‐induced transformation rates with spatially explicit models of spread may be necessary for predicting landscape‐level impacts on ecosystem processes in advance of a grass–fire cycle.

Review: Sonoran Desert Life: Understanding, Insight and Enjoyment

Review: Sonoran Desert Life: Understanding, Insight, and Enjoyment By Gerald Rosenthal Reviewed by Muhammed Tayyeb Javed Pakistan Institute of Engineering and Applied Sciences, Pakistan Rosenthal, Gerald. Sonoran Desert Life: Understanding, Insight and Enjoyment. Scottsdale, AZ: University of Arizona Press, 2008. 306pp. ISBN 9780615186719. US$27.95, paper. The Sonoran Desert is a vast, arid region covering some 120,000 square miles, most of which is located in southwestern Arizona and southeastern California as well as Sonora and Baja California in Mexico. The book under review is a fascinating book on the flora and fauna of the American Sonoran Desert by a world-renowned biologist and biochemical entomologist. The unique aspect of this book is the role that beetles, bugs, bats and bees play in maintaining genetic diversity by dispersing pollen and assuring that eternal life thrives in the Sonoran Desert. More than 300 plant species from 70 families in the desert are provided in this compact multicolored book. It provides information on flowers, stems, involucres, tubercles, spines, leaves, calyx and fruit. The book is divided in three broad sections and further divided into eighteen chapters. The opening section includes information on the Sonoran Desert, how to use the book, and basic botanical information for non-professionals. The main section is distributed into seven chapters labeled and marked in the range of colors shown by Sonoran plants: Yuccas; White, White-Green and Green Flowers; Yellow Flowers; Red, Pink, Magenta and Purple Flowers; Orange and Apricot Flowers; Blue, Violet and Lavender Flowers; and Large Shrubs and Trees. The last section provides a comprehensive glossary of more than 225 terminologies used in the book along with information sources. The technical aspects related to the production of the photos are also provided. The photography was done using a Canon 1Ds MII (16.7 MP) camera equipped with a Canon 100 mm EF lens. Image manipulation was achieved using ULEAD Photo Explorer 8.5 and Photoshop 7.0 software. The book Sonoran Desert Life: Understanding, Insight and Enjoyment could be of interest to high school teachers and students as well as individuals interested in arid land biodiversity. The work is also useful in understanding the special and unique Sonoran desert ecosystem. Muhammad Tayyeb Javed, PhD. , Assistant Professor, Department of Chemical and Materials Engineering, Pakistan Institute of Engineering and Applied Sciences, P.O. Nilore, Islamabad, Pakistan. TEL: 92-51-2207381-3 x 3356. Electronic Green Journal, Issue 29, Winter 2009, ISSN:1076-7975

Comparative population analysis of desert ironwood ( Olneya tesota) in the Sonoran Desert

Olneya tesota (ironwood) is a keystone tree species that modifies the structure and function of Sonoran desert ecosystems. Presently, extraction of trees and branches in the region constitutes a severe risk for ironwood populations. We analyzed the size structure, two damage indicators (damaged basal area or “DBA” and percents of damaged individuals or “PDI”) for eleven ironwood populations in Sonora and Baja California, México. In four populations, we compared damage indicators obtained in 1992 and 2004. We found that in unexploited populations, small size categories (<500 cm 2 in basal area) and juveniles were better represented. In contrast in exploited populations large size individuals were severely damaged and lacked recruitment. Populations on the coast of Sonora have greater damage (up to 50% of the individuals), while those in the Baja California Peninsula exhibited less damage (smaller than 20%), with high juvenile recruitment (up to 40%). Historical comparisons of DBA and PDI exhibited non significant differences between 1992 and 2004 for three populations, and only in Bahia Kino population (Sonora) did the PDI decreased significantly. One population surveyed in 1992 (Sonoyta) was completely removed. Even that the damage seems to be constant, the damage indicators remain high, affecting the ecosystems dynamics.

Vegetation controls on soil organic carbon dynamics in an arid, hyperthermic ecosystem

The large land area occupied by arid lands, roughly 36% to 40% globally, underscores the importance for understanding how these ecosystems function in the global carbon cycle. Few studies have directly examined soil organic carbon (SOC) dynamics and the effect of vegetation on SOC and microbial community structure in arid ecosystems. The objective of this study was to determine the effect of vegetation type on SOC dynamics in an arid, hyperthermic Sonoran Desert ecosystem. We specifically examined the influence of Prosopis velutina (mesquite), Larrea tridentata (creosote), and a combination of Bouteloua barbata, Bouteloua aristidoides, Aristida adscensionis, and Cynodon dactylon (mixed grass) vegetation types on SOC dynamics by quantifying: (i) local scale SOC stocks; (ii) soil aggregate stability; (iii) SOC turnover; and (iv) soil microbial community composition. There was significantly greater SOC in mesquite A-horizons relative to creosote and grass sites with values of 46.7, 30.4, and 24.4 g m − 2 , respectively. Subsurface SOC content did not vary significantly between vegetation types. Aggregate stability determined using an ultrasonic dispersion technique was found to be similar among vegetation types. The only significant difference noted was greater energy required to disperse stable aggregates in mesquite relative to grass soils, 1500 and 735 J g − 1 soil, respectively. Laboratory incubations were performed to determine SOC dynamics, pool sizes, and active pool mean residence times (MRT) for each vegetation type. Incubation results indicated significant variation in the cumulative respired CO 2 under mesquite, creosote, and grasses with 151, 186, and 207 mg C g − 1 soil C respired from each respective vegetation type. The incubation data indicated that 7–11% of total SOC was highly labile across all vegetation types with modeled active pool SOC MRT averaging 17 days. Bacterial community analysis by Terminal Restriction Fragment Length Polymorphism (TRFLP) indicated significant differences in microbial community structure among vegetation types. Microbial composition was highly correlated with soil pH and electrical conductivity. Furthermore, community composition was correlated with cumulative respired CO 2, suggesting an interaction among vegetation type, soil properties and microbial community control SOC dynamics in this ecosystem. The combined results indicated significant variation in SOC dynamics within a specific ecosystem by vegetation type. Understanding local-scale vegetation controls of soil carbon cycling may improve efforts to model regional carbon dynamics in arid environments.

Patch dynamics of soil biotic feedbacks in the Sonoran Desert

Soil fertility in arid ecosystems is spatially and temporally heterogeneous, due primarily to biotic feedbacks from perennial plants. While the “island of fertility” effect is well-established, little is known about the influence of plant functional characteristics on soil nutrients, or changes in nutrient levels following mortality or microsite succession. To assess these patterns, we studied a functionally diverse Sonoran Desert ecosystem in which all microsite state transitions were determined. The woody species (Ambrosia deltoidea, Larrea tridentata, Parkinsonia microphylla) significantly enhanced soil organic matter, nitrogen and phosphorous levels (referred to generally as fertility) to varying degrees. Effect sizes differed significantly among species but were surprisingly similar. However, following mortality, fertility remained constant beneath Ambrosia and increased significantly beneath decaying Parkinsonia, contrary to expected declines. These results suggest that nutrients are recycled within a microsite, resulting in similar pool sizes beneath live plants across species, regardless of morphological or phonological differences. Microsites occupied by cacti exhibited no differences from bare soil, indicating that cacti may erase islands of fertility. However, nitrate and phosphate levels spiked following mortality of the large cactus, Carnegiea gigantea. In general, fertility effects were correlated with morphological characteristics that should be applicable across most arid systems.

Effects of Urbanization-Induced Environmental Changes on Ecosystem Functioning in the Phoenix Metropolitan Region, USA

Urban ecosystems are profoundly modified by human activities and thereby provide a unique ‘‘natural laboratory‘‘ to study potential ecosystem responses to anthropogenic environmental changes. Because urban environments are now affected by urban heat islands, carbon dioxide domes, and high-level nitrogen deposition, to some extent they portend the future of the global ecosystem. Urbanization in the metropolitan region of Phoenix, Arizona (USA) has resulted in pronounced changes in air temperature (Tair), atmospheric CO2 concentration, and nitrogen deposition (Ndep). In this study, we used a process-based ecosystem model to explore how the Larrea tridentata dominated Sonoran Desert ecosystem may respond to these urbanization-induced environmental changes. We found that water availability controls the magnitude and pattern of responses of the desert ecosystem to elevated CO2, air temperature, N deposition and their combinations. Urbanization effects were much stronger in wet years than normal and dry years. At the ecosystem level, aboveground net primary productivity (ANPP) and soil organic matter (SOM) both increased with increasing CO2 and Ndep individually and in combinations with changes in Tair. Soil N (Nsoil) responded positively to increased N deposition and air temperature, but negatively to elevated CO2. Correspondingly, ANPP and SOM of the Larrea ecosystem decreased along the urban‐suburban‐ wildland gradient, whereas Nsoil peaked in the suburban area. At the plant functional type (FT) level, ANPP generally responded positively to elevated CO2 and Ndep, but negatively to increased Tair .C 3 winter annuals showed a greater ANPP response to higher CO2 levels (>420 ppm) than shrubs, which could lead over the long term to changes in species composition, because competition among functional groups is strong for resources such as soil water and nutrients. Overall, the combined effects of the three environmental factors depended on rainfall variability and nonlinear interactions within and between plant functional types and environmental factors. We intend to use these simulation results as working hypotheses to guide our field experiments and observations. Experimental testing of these hypotheses through this process should improve our understanding of urban ecosystems under increasing environmental stresses.

Leaf litter decomposition in a southern Sonoran Desert ecosystem, northwestern Mexico: Effects of habitat and litter quality

Leaf litter decomposition of dominant woody perennial species in the three most common habitats of the southern Sonoran Desert was studied using the litter-bag method. Our objective was to assess the influence of litter quality on decomposition rates in three contrasting desert environments. The hypotheses were: (1) decomposition rates within the same litter type are faster in more mesic habitats, (2) decomposition rates are lower in higher lignin content or lower nutrient quality substrates, and (3) species-rich substrates enhance decomposition rates. For all litter types and habitats, a rapid loss of mass occurred during the summer rains at the start of the experiment, but total loss within the same litter type differed significantly among habitats. Decay rates were not higher in the more mesic habitat, but in the dry plains where solar irradiance and termite activity were highest. While termite activity was less important in the arroyos and absent in the hillsides habitats, proliferation of fungal mycelium in these sites was much higher than in the plains, suggesting that biotic and abiotic factors act both independently of litter richness. Lignin content seems to be an important factor controlling the loss of litter, because decay rates were inversely related to litter initial lignin content in all three habitats. Leaf litter diversity did not enhance rates of decomposition. The leaf litter mixture had k-values similar to the most recalcitrant monospecific litter in all three habitats, indicating a neutral or even antagonistic role of species-specific compounds in decomposition rates.

GARP modeling of natural and human factors affecting the potential distribution of the invasives Schismus arabicus and Brassica tournefortii in ‘El Pinacate y Gran Desierto de Altar’ Biosphere Reserve

Invasive plant encroachments threaten biodiversity and ecosystem viability worldwide. Effects of invasives on native Sonoran Desert ecosystems are, for example, a growing concern among ecologists and land managers. Potential distributions of the exotic Brassica tournefortii and Schismus arabicus were modeled for ‘El Pinacate y Gran Desierto de Altar’ Biosphere Reserve (PBR) in Sonora Mexico using genetic algorithms within a geographic information system (GIS) database. Spatial models were created using known presence points for B. tournefortii and S. arabicus, and the merged and separated influence of natural and human-related variables, respectively. Best performing models were summed to produce probability models for each species and suite of variables. Models derived from human related variables showed higher significance and accuracy values for both invasives. Combined probability models were reclassified to isolate areas of high-predicted presence, which served to identify individual variables that favor most the potential occurrence of invasives. Variables showing greatest predictive skill were isolated and recombined to produce models of merged conditions of high probability presence. Internal road networks were strongest predictors of presence, revealing the potential importance of humans as vectors of invasiveness in the PBR.