Southeastern Arizona Research Articles

Multiyear riparian evapotranspiration and groundwater use for a semiarid watershed

Riparian evapotranspiration (ET) is a major component of the surface and subsurface water balance for many semiarid watersheds. Measurement or model-based estimates of ET are often made on a local scale, but spatially distributed estimates are needed to determine ET over catchments. In this paper, we document the ET that was quantified over 3 years using eddy covariance for three riparian ecosystems along the Upper San Pedro River of southeastern Arizona, USA, and we use a water balance equation to determine annual groundwater use. Riparian evapotranspiration and groundwater use for the watershed were then determined by using a calibrated, empirical model that uses 16-day, 250–1000 m remote-sensing products for the years of 2001–2005. The inputs for the model were derived entirely from the NASA MODIS sensor and consisted of the Enhanced Vegetation Index and land surface temperature. The scaling model was validated using subsets of the entire dataset (omitting different sites or years) and its capable performance for well-watered sites (MAD=0.32 mm day −1, R 2=0.93) gave us confidence in using it to determine ET over the watershed. Three years of eddy covariance data for the riparian sites reveal that ET and groundwater use increased as woody plant density increased. Groundwater use was less variable at the woodland site, which had the greatest density of phreatophytes. Annual riparian groundwater use within the watershed was nearly constant over the study period despite an on-going drought. For the San Pedro alone, the amounts determined in this paper are within the range of most recently reported values that were derived using an entirely different approach. However, because of our larger estimates for groundwater use for the main tributary of the San Pedro, the watershed totals were higher. The approach presented here can provide riparian ET and groundwater use amounts that reflect real natural variability in phreatophyte withdrawals and improve the accuracy of a watershed's water budget.

α and β Diversity of a Tachinid Parasitoid Community Over Space and Time

Many species of Tachinidae are important parasitoids of herbivorous insects in natural and managed systems; yet, little is known about tachinid diversity and how this diversity is distributed across space and time. Here, pan trap sampling was used to analyze the richness, microhabitat speciÞcity, and seasonal diversity of a tachinid parasitoid community in an oakÐ mesquite savanna of Southeastern Arizona. Twenty-four traps were set out monthly during the growing season in three different microhabitats (open grassland, woodland understory, and woodland canopy). In total, 79 tachinid species were sampled with an estimated total diversity of 122 species. Most individual traps sampled few species; yet, variation in species composition ( diversity) among sampling dates and microhabitats was high, accounting for 40 Ð70% of the total diversity. SigniÞcant intraspeciÞc aggregation was not observed across traps or microhabitats, but it was observed across dates, suggesting that the activity of tachinid species may be associated with host phenology and seasonal periods of precipitation. The diversity associated with microhabitat and sampling date was signiÞcantly greater than expected. Tachinid species diversity was highest in the canopy traps, whereas the open-exposed traps exhibited high abundances of relatively few species, and understory traps sampled few individuals of few species. Most species tended to be sampled where their hosts would be expected to be found, although males were also frequently sampled in microhabitats associated with mate Þnding. These patterns of diversity and abundance may aid in understanding parasitoidÐ host associations and variation in rates of parasitism by tachinid sies.

Evidence of Resident Jaguars (Panthera onca) in the Southwestern United States and the Implications for Conservation

Jaguars (Panthera onca) remain virtually unstudied in the desert environments at the northern extent of their range. Historic sightings from the United States indicate a declining population of resident jaguars from the late 1800s into the 1940s, after which only occasional jaguars were reported until the present. After 2 sightings of jaguars in 1996, we established a camera monitoring program in southeastern Arizona. From March 2001 to July 2007, we maintained 9–44 trail cameras and conducted opportunistic track surveys. We documented 2 adult males and a possible 3rd unidentified jaguar with 69 photographs and 28 sets of tracks. One jaguar, originally photographed in 1996, was resighted 64 times during 2004–2007. This ≥ 13-year-old male used habitats from the Sonoran lowland desert at 877 m above sea level to pine–oak woodlands at 1,577 m, and covered 1,359 km2 in 2 mountain complexes. Despite speculation that recent sightings of jaguars in the United States represented dispersing transients on sporadic forays from Mexico, we documented jaguars in Arizona frequently, continuously, and year-round, and videotaped several scent-marking behaviors, indicating the residency of adult jaguars within Arizona. We outline the importance of maintaining cross-border connectivity for long-term survival of the wide-ranging and thinly distributed binational population of jaguars. We recommend further research and we stress the fragmentation consequences of the proposed United States–Mexico border fence to the northernmost jaguar population, and particularly to jaguars in the United States.

Large area mapping of southwestern forest crown cover, canopy height, and biomass using the NASA Multiangle Imaging Spectro-Radiometer

A rapid canopy reflectance model inversion experiment was performed using multi-angle reflectance data from the NASA Multi-angle Imaging Spectro-Radiometer (MISR) on the Earth Observing System Terra satellite, with the goal of obtaining measures of forest fractional crown cover, mean canopy height, and aboveground woody biomass for large parts of south-eastern Arizona and southern New Mexico (>200,000 km2). MISR red band bidirectional reflectance estimates in nine views mapped to a 250 m grid were used to adjust the Simple Geometric-optical Model (SGM). The soil-understory background signal was partly decoupled a priori by developing regression relationships with the nadir camera blue, green, and near-infrared reflectance data and the isotropic, geometric, and volume scattering kernel weights of the LiSparse–RossThin kernel-driven bidirectional reflectance distribution function (BRDF) model adjusted against MISR red band data. The SGM's mean crown radius and crown shape parameters were adjusted using the Praxis optimization algorithm, allowing retrieval of fractional crown cover and mean canopy height, and estimation of aboveground woody biomass by linear rescaling of the dot product of cover and height. Retrieved distributions of crown cover, mean canopy height, and aboveground woody biomass for forested areas showed good matches with maps from the United States Department of Agriculture (USDA) Forest Service, with R2 values of 0.78, 0.69, and 0.81, and absolute mean errors of 0.10, 2.2 m, and 4.5 tons acre-1 (10.1 Mg ha-1), respectively, after filtering for high root mean square error (RMSE) on model fitting, the effects of topographic shading, and the removal of a small number of outliers. This is the first use of data from the MISR instrument to produce maps of crown cover, canopy height, and woody biomass over a large area by seeking to exploit the structural effects of canopies reflected in the observed anisotropy patterns in these explicitly multiangle data.

AN EMPIRICAL EVALUATION OF THE INSURANCE HYPOTHESIS IN DIVERSITY–STABILITY MODELS

An important stabilizing mechanism in most diversity stability models is the insurance hypothesis, which involves correlation/covariance relationships among species. These models require that species do not fluctuate synchronously over time: that is, the correlation between pairs of species does not equal 1.0. However, the strength of this stabilizing mechanism increases as correlations decline away from 1.0, especially as they become more negative and also as the summed covariance across all species pairs becomes more negative. We evaluated the importance of the insurance hypothesis as a stabilizing mechanism by examining a variety of terrestrial assemblages using long-term data from the Global Population Dynamics Database, the Breeding Bird Survey, and a long-term site in southeastern Arizona, USA. We identified co-occurring assemblages of species and calculated the Spearman rank correlations of all pairs of species and the summed covariance of the entire assemblage. We found that, in most assemblages, positive correlations were two to three times more common than negative and that the magnitude of the positive correlations tended to be stronger than the negative correlations. For all but three assemblages, the summed covariance was positive. Data from larger spatial scales tended to exhibit more positive correlations, but even at the smallest spatial scales, positive correlations outnumbered negative. We suggest that species often covary positively because coexisting species respond similarly to fluctuations in their resource base driven by climatic fluctuations. As such, our review suggests that the insurance hypothesis may not be a strong mechanism stabilizing fluctuations in natural terrestrial communities.

Comparison of Field Performance of Multiple Soil Moisture Sensors in a Semi‐Arid Rangeland1

Abstract: Automated electronic soil moisture sensors, such as time domain reflectometry (TDR) and capacitance probes are being used extensively to monitor and measure soil moisture in a variety of scientific and land management applications. These sensors are often used for a wide range of soil moisture applications such as drought forage prediction or validation of large‐scale remote sensing instruments. The convergence of three different research projects facilitated the evaluation and comparison of three commercially available electronic soil moisture probes under field application conditions. The sensors are all installed in shallow soil profiles in a well instrumented small semi‐arid shrub covered subwatershed in Southeastern Arizona. The sensors use either a TDR or a capacitance technique; both of which indirectly measure the soil dielectric constant to determine the soil moisture content. Sensors are evaluated over a range of conditions during three seasons comparing responses to natural wetting and drying sequences and using water balance and infiltration simulation models. Each of the sensors responded to the majority of precipitation events; however, they varied greatly in response time and magnitude from each other. Measured profile soil moisture storage compared better to water balance estimates when soil moisture in deeper layers was accounted for in the calculations. No distinct or consistent trend was detected when comparing the responses from the sensors or the infiltration model to individual precipitation events. The results underscore the need to understand how the sensors respond under field application and recognize the limitations of soil moisture sensors and the factors that can affect their accuracy in predicting soil moisture in situ.

Rainfall Intensity-Dependent Infiltration Rates on Rangeland Rainfall Simulator Plots

Most implementations of infiltration equations with rainfall-runoff models use a hydraulic conductivity parameter that is constant for a given rainfall event. However, plot data from rainfall simulator experiments and natural rainfall events have shown that infiltration rates can increase with increasing rainfall rate instead of decreasing with time or infiltrated depth, as predicted by infiltration models. This has been hypothesized to be a function off the spatial variability of the infiltration capacity across the area. In this article, an exponential model relating steady-state infiltration rate with rainfall intensity and the average areal infiltration rate when the area under consideration is contributing to runoff is evaluated using data from variable-intensity rainfall simulator experiments. The experiments were conducted on five rangeland vegetation-soil associations at the Walnut Gulch Experimental Watershed in southeastern Arizona. The results from 19 rainfall simulation runs show that the increase in infiltration rate with increasing rainfall intensity can be significant and that the exponential model represents the relationship between steady-state infiltration and rainfall intensity. The exponential model coupled with a kinematic wave model also represents the hydrographs better than the Green-Ampt Mein-Larsen infiltration model coupled with the same routing model. The time to the start of runoff is influenced more by rainfall intensity than by initial soil moisture conditions, particularly when the initial rainfall intensity was high. The rapid time to steady-state runoff at the beginning of the simulation run of the observed runoff hydrographs suggests that the infiltration rates become constant more quickly than infiltration theory would suggest.

Livestock Trampling and Lilaeopsis schaffneriana Var. Recurva (Brassicaceae)

Lilaeopsis schaffneriana var. recurva (Hill) Affolter, is a small aquatic to semi-aquatic plant restricted to mid-elevation cienega and riparian margins in southeastern Arizona and northwestern Sonora (Hendrickson and Minckley 1984). It was listed as Endangered in 1997 under the Endangered Species Act of 1973 (as amended) because of ‘‘degradation and destruction of habitat resulting from livestock overgrazing, water diversions, dredging, and groundwater pumping’’ (USFWS 1997). Trampling and overgrazing by livestock are commonly cited as threats to this species, however, we have found no documented cases or quantification of livestock effects. Herein, we document and quantify the effect of trampling by trespass livestock on Lilaeopsis schaffneriana var. recurva in Leslie Canyon National Wildlife Refuge, Cochise County, Arizona. During 2003, we established a pilot monitoring program for Lilaeopsis schaffneriana var. recurva, which occupied 12 distinct patches in Leslie Canyon National Wildlife Refuge. The protocol required taking multiple, cross-patch measurements of each patch and plotting the measurements on simplified drawings of the patches. The diagrams were then taken back to the lab, scaled polygons were drawn in TurboCAD (IMSI 2002), and from these polygons the area of each patch was estimated. The sum of the individual patch areas is the total area occupied. The trampling effect of trespass livestock was assessed in the same manner, where portions of patches destroyed by livestock were measured, described as polygons with area calculations, and summed across all trampled areas. All patch measurements were completed on 13 Jun 2003. Total area occupied by ten Lilaeopsis schaffneriana var. recurva patches along Leslie Creek was 27.11 m; individual patches averaged 2.71 m (SD 5 2.54). On 6 Jul 2004, a trespass bull entered Leslie Canyon National Wildlife Refuge and spent 4 d within the riparian area; it appeared that most of the individual’s time was spent away from the Lilaeopsis patches. During the time that the bull spent around the Lilaeopsis patches, 1.47 m (5.4% of the total area occupied) were lost to trampling. This event allowed us to quantify the effect of a single livestock individual on Lilaeopsis schaffneriana var. recurva, over a span of only 4 d. Even though resource use patterns by livestock could leave some patches unaffected, the damage caused by this single individual indicates that even a small number of livestock could eradicate most of the Leslie Creek Lilaeopsis population in several days to weeks. Therefore, based on the negative effect caused by this short-term event, our conclusion is that livestock should be excluded from areas occupied by Lilaeopsis schaffneriana var. recurva.

Tracking acid mine-drainage in Southeast Arizona using GIS and sediment delivery models

This study investigates the application of models traditionally used to estimate erosion and sediment deposition to assess the potential risk of water quality impairment resulting from metal-bearing materials related to mining and mineralization. An integrated watershed analysis using Geographic Information Systems (GIS) based tools was undertaken to examine erosion and sediment transport characteristics within the watersheds. Estimates of stream deposits of sediment from mine tailings were related to the chemistry of surface water to assess the effectiveness of the methodology to assess the risk of acid mine-drainage being dispersed downstream of abandoned tailings and waste rock piles. A watershed analysis was preformed in the Patagonia Mountains in southeastern Arizona which has seen substantial mining and where recent water quality samples have reported acidic surface waters. This research demonstrates an improvement of the ability to predict streams that are likely to have severely degraded water quality as a result of past mining activities.

A New Species of Acalymma Barber, 1947 (Chrysomelidae: Galerucinae: Luperini), from Southeastern Arizona and New Mexico, U.S.A

Acalymma bertoluccii sp. nov. is described from southeastern Arizona and New Mexico, U.S.A.

Modeling climate change impacts – and uncertainty – on the hydrology of a riparian system: The San Pedro Basin (Arizona/Sonora)

An assessment of climate change impacts in the water resources of a semi-arid basin in southeastern Arizona and northern Sonora is presented using results from an ensemble of 17 global circulation models (GCMs) and four different climate change scenarios from the Intergovernmental Panel on Climate Change (IPCC). Annual GCM precipitation data for the region is spatially downscaled and used to derive spatially distributed recharge estimates in the San Pedro Basin. A three dimensional transient groundwater-surface water flow model is used to simulate the hydrology of the current century, from 2000 to 2100, under different climate scenarios and model estimates. Groundwater extraction in the basin was maintained constant and equal to current. The use of multiple climate model results provides a highest-likelihood mean estimate as well as a measure of its uncertainty and a range of less probable outcomes. Results suggest that recharge in the San Pedro basin will decrease, affecting the dynamics of the riparian area in the long term. It is shown that mean net stream gain, i.e. base flow, will decrease and the effects on the riparian area could be significant. The results of this work provide a basis for the inclusion of representative climate scenarios into the basin’s existing decision support system model.

Ecosystem Water Use Efficiency in a Semiarid Shrubland and Grassland Community

Ecosystem water use efficiency (EWUE) is defined as the net carbon uptake per amount of water lost from the ecosystem and is a useful measure of the functionality in semiarid shrub and grassland communities. C4 grasses have higher water use efficiency (WUE) than do C3 shrubs, although it has been postulated that C4 plants have lost much of their advantage due to the rising atmospheric CO2 concentrations. The hypothesis was that C4-grass-dominated ecosystems have a higher EWUE than C3-shrubdominated ecosystems under the present CO2 concentration and climatic variability. Evapotranspiration (ET) and CO2 fluxes were measured with Bowen ratio systems at a shrub and grass site for 6 years in southeastern Arizona. Two different methods were used to evaluate growing season EWUE using the ET and CO2 fluxes. The first method estimated a net daytime growing season EWUE for the grass site at 1.74 g CO2 ? mm 21 ET and 1.28 g CO2 ? mm 21 ET at the shrub site. The second method estimated maximum EWUE during part of the growing season at 7.35 g CO2 ? mm 21 ET for the grass site and 4.68 g CO2 ? mm 21 ET for the shrub site, which was considered a significant difference at P 5 0.056. Data variability of the first method precluded a statistical difference determination between sites, but the results indicated that the grass-dominated ecosystem was between 1.4 and 1.6 times more water use efficient than the shrub-dominated ecosystem. Mean annual growing season precipitation and ET were similar in the two ecosystems, but the higher EWUE of the grassland system enabled it to take up more carbon during the growing season than the shrub ecosystem. Ecosystem differences in CO2 and H2O flux have important management implications including primary productivity, C sequestration, and rangeland health.

Smelters and Mortality

Pope et al. (2007) provide results for reduced mortality during the 1967–1968 smelter strike in the U.S. Southwest. They ascribed mortality reduction to a decrease in ambient sulfate from the smelters. Although I found the thesis interesting, there is confounding that should be noted involving a) inconsistencies in state mortality relationships; b) the trace metal role, and possibly carbon exposure from the plant complexes; and c) ambiguities associated with SO4 sampling. The basis for the study by Pope et al. (2007) is a study by Trijonis and Yuan (1978), who analyzed the National Air Surveillance Network (NASN) SO4 and visibility. They attributed improved visibility across the Southwest to SO4 reduction during the strike. Ambient SO4 includes SO4 from oxidation of sulfur dioxide in air (secondary) and that emitted directly (primary). NASN data [e.g. U.S. Environmental Protection Agency (EPA) 1971, 1972, 1973] indicate a strike reduction in SO4 (0.1–3.6 μg/m3) at sites in the region (Trijonis and Yuan 1978; Table 16) and not Pope et al.’s uniform 2.5 μg/m3. However, the accompanying association with non–weather-related visibility change is problematic (e.g., Hidy 1984). If the smelter SO4 was present regionally, exposure to concentration gradients of SO4, SO2, and metals would be expected with distance from the plants (Eldred et al. 1983; Malm et al. 1990). Pope et al. (2007) did not differentiate their results by distance from the smelters, but some information in their article is relevant because mortality is associated mainly with population centers (cities). The risk estimates for New Mexico presented by Pope et al. (2007) in their Figure 6 (dominated by Albuquerque; the nearest smelter is 300 km south southwest) show high levels of mortality reduction in spite of an increase in annual average SO4 between 1966 and 1967–1968 and a negligible reduction during the strike. The mortality reduction in Nevada is largest of the four states presented by Pope et al. 2007 (Figure 6), even though Reno and Las Vegas (population centers) are upwind of the smelters and are far distant over mountain ranges from the nearest smelter at Ely. The smallest risk change is in Arizona, but reductions in Utah are similar to those of New Mexico. However, note that the population centers in Arizona (Phoenix and Tucson) and Utah (Salt Lake City) are close to smelters. The results of Pope et al. (2007) are further confounded by the fact that trace metals and carbon accompany the emissions from plant complexes (e.g., Leipold and Chow 1998; Small et al. 1981). Local exposure to smelter emissions involves primary SO4 as well as SO2 [the apparent SO4 concentrations are biased high by a variable SO2 filter adsorption artifact (e.g., Lee and Wagman 1966; Lipfert 1994)]. Emission reduction would reduce SO4, including the bias from SO2 adsorption; metals such as copper, lead, iron, cadmium, antimony, chromium, nickel, and arsenic; and possibly carbon. Distant exposure would be enriched in secondary SO4 up to a point, followed by decline from atmospheric dilution and deposition. Pope et al. (2007) mentioned the metal–SO4 linkage but did not explore it relative to the SO4 theory. The combined exposure in sulfur oxides and metals from the smelters preset in aerosols from many sources adds further complexity to interpreting their results, including the differences in Salt Lake City and the Arizona cities. Recent research suggests that a combination of primary SO4 and metals from oil combustion, as well as carbon emissions from motor vehicles, may be important factors in mortality risk (e.g., Grahame and Schlesinger 2007). The smelter inferences appear inconsistent with these findings. Pope et al. (2007) focused on the strike in the Southwest. However, they excluded the same period in Montana as a cross-check on their results. In the 1960s, a major copper production complex was located in Anaconda, Montana. NASN data from nearby Helena and Glacier National Park do not show a significant change in average SO4 concentrations during the strike years (U.S. EPA 1971, 1972, 1973). This appears inconsistent with the widespread reductions seen in the Southwest. This difference could be a valuable adjunct to their results if mortality data are available. Pope et al. (2007) provided a “natural” experiment in regional sulfur oxide (and metals) reduction. Their results should be examined further to insure that their interpretation is robust.

MOLECULAR EVIDENCE FOR GENETIC SUBDIVISIONS IN THE DESERT SHREW, NOTIOSOREX CRAWFORDI

Examination of cytochrome-b DNA sequences from specimens of Notiosorex crawfordi, the desert shrew, indicate that within this taxon there are at least 3 major subdivisions reflecting levels of variation more typical of species rather than subspecies. One of these cytochrome-b haplotypes was recently described as a previously unrecognized species; however, that paper was limited to the description of a species level name. We provide additional insight into subdivisions in N. crawfordi. One DNA haplotype is distributed in Texas, New Mexico, and eastern Arizona, a second haplotype in Arizona and Sonora, Mexico, and the third haplotype in Baja California, Mexico. Nucleotide sequences obtained from intron 7 of the nuclear gene, beta fibrinogen, support the conclusions that 2 of the haplotypes occur sympatrically in southeastern Arizona and are not hybridizing with each other.

Fine‐Scale Analysis of Mount Graham Red Squirrel Habitat Following Disturbance

ABSTRACT Habitat destruction and degradation are major factors in reducing abundance, placing populations and species in jeopardy. Monitoring changes to habitat and identifying locations of habitat for a species, after disturbance, can assist mitigation of the effects of human‐caused or ‐amplified habitat disturbance. Like many areas in the western United States, the Pinaleño Mountains of southeastern Arizona, USA, have suffered catastrophic fire and large‐scale insect outbreaks in the last decade. The federally endangered Mt. Graham red squirrel (Tamiasciurus hudsonicus grahamensis) is only found in the Pinaleño Mountains, and to assess effects of forest disturbance on habitat we modeled their potential habitat by identifying characteristics of cover surrounding their centrally defended middens. We classified high‐spatial resolution satellite imagery into ground cover classes, and we used logistic regression to determine areas used by squirrels. We also used known midden locations in conjunction with slope, elevation, and aspect to create a predictive habitat map. Squirrels selected areas of denser forest with higher seedfall for midden sites. Among active middens, those in the densest and least damaged forests were occupied in more seasons than those in more fragmented and damaged areas. The future conservation of red squirrels and the return of healthy mature forests to the Pinaleño Mountains will rely on preservation of mixed conifer zones of the mountain and active restoration of spruce‐fir forests to return them to squirrel habitat. Our ability to evaluate the spectrum of fine‐ to coarse‐scale disturbance effects (individual tree mortality to area wide boundaries of a disturbance) with high‐resolution satellite imagery shows the utility of this technique for monitoring future disturbances to habitat of imperiled species.

Water content estimation from hyperspectral images and MODIS indexes in Southeastern Arizona

Hyperspectral water retrievals from AVIRIS data, equivalent water thickness (EWT), were compared to in situ leaf water content and LAI measurements at a semiarid site in southeastern Arizona. Retrievals of EWT showed good correlation with field canopy water content measurements. Statistical analysis suggested that EWT was significantly different among seven community types, from savanna to agriculture. Four band–ratio indexes (NDVI, EVI, NDWI, and NDII) were derived from MODIS showing strong spatial agreement between maps of AVIRIS EWT and MODIS indexes, and good statistical agreement for the range of habitats at the site. Temporal patterns of these four indexes in all vegetation communities except creosote bush and agriculture showed distinct seasonal patterns that responded to the timing and amount of precipitation. Moreover, these time series captured different ecological responses among the different vegetation communities.

Influence of shifting flow paths on nitrogen concentrations during monsoon floods, San Pedro River, Arizona

Hydrologic flow paths control transport, and therefore are a major constraint on the cycling and availability of nutrients within stream ecosystems. This control is particularly evident in semiarid streams, where hydrologic connectivity between stream, riparian, and upland systems increases greatly during storms in the rainy season. We measured chloride concentrations in base flow, precipitation, soil water, and stream water to quantify the hydrologic connectivity and solute flux between soil water, groundwater, and the stream channel during six summer floods in 2001 (a wet year; 25 cm winter rain) and 2002 (a dry year; 5 cm winter rain) in the San Pedro River, southeastern Arizona. This hydrologic information was used to evaluate observed patterns in nitrate, dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) concentrations in floods. The first floods of each year showed increased stream nitrate concentration that was approximately two orders of magnitude higher than base flow concentration. DOC consistently doubled to tripled during storm events, while DON in 2001 showed no response and showed a marked increase in 2002. A chloride mixing model indicated that soil and groundwater contributions to storm water discharge were related to antecedent conditions and to flood magnitude. Soil and groundwater contributions were the highest early in the 2001 monsoon season following the wet winter, much lower early in 2002 following a dry winter, and lowest during the largest floods of the 2002 monsoon season when flows were derived primarily from precipitation and overland flow. Stream water nitrate‐N concentrations during floods were consistently 0.2–0.5 mg/L higher in 2002 than during 2001, suggesting greater over‐winter accumulation of soil nitrate during the drier year. This result is consistent with higher mean nitrate‐N concentrations in soil water of the riparian zone in 2002 (3.1 mg/L) than in 2001 (0.56 mg/L). These data highlight the importance of seasonal and interannual variability of hydrology in semiarid regions, and the role of water availability in driving patterns of soil nutrient accumulation and their transport to the stream.

Characteristics of Mount Graham Red Squirrel Nest Sites in a Mixed Conifer Forest

Abstract: The Mount Graham red squirrel (Tamiasciurus hudsonicus grahamensis) is constrained to the Pinaleño Mountains in southeastern Arizona, USA. The population's endangered status and extensive forest damage from insects and fire warrants a better understanding of habitat variables important for nest site selection. We examined characteristics of cavity (n = 91) and drey (n = 38) nests and compared these to random sites (n = 113). Dreys were found primarily in Engelmann spruce (Picea engelmannii) and corkbark fir (Abies lasiocarpa var. arizonica). Cavity nests occurred primarily in aspen (Populus tremuloides) and corkbark fir. Squirrels selected nest sites with higher canopy cover and more corkbark fir, decayed logs, and living trees. Forest management plans emphasizing thinning must consider how altering these habitat characteristics could affect availability and suitability of tree stands for nesting squirrels.

Quantifying water sources to a semiarid riparian ecosystem, San Pedro River, Arizona

The Upper San Pedro River Basin (Southeastern Arizona, United States) contains one of the few desert riparian areas in the Southwest, a system that is dependent on both shallow groundwater to support phreatic vegetation and baseflow for aquatic plants and animals. Proper management decisions for sustaining this biodiversity hotspot require understanding the hydrology of the riparian system and its interaction with the basin aquifer. To meet this need and to assess whether the techniques used would be efficient for evaluating other semiarid riparian ecosystems, we addressed the following questions. What are the contributions of different water sources (e.g., local recharge during monsoon flood events versus inflow of basin groundwater) to riparian groundwater and river baseflow? How does the spatial variability in water sources relate to gaining and losing reaches along of the river? We first characterize the possible water sources to the riparian system using a suite of geochemical tracers. Results indicate that, of the possible sources, basin groundwater recharged along the Huachuca Mountains to the west and local recharge of monsoon floodwaters are the dominant riparian water sources. Then, using their geochemical composition, we quantify these sources using a two end‐member mixing model. We find that riparian groundwater composition varies between gaining and losing reaches. Locally recharged monsoon floodwater comprises 60 to 85% of riparian groundwater in losing reaches whereas that of gaining reaches contains only 10% to 40%. Baseflow, sampled year round, also contains a significant component of monsoon floodwater ranging from 80% on the upstream end and decreasing to 55% after passing though several gaining reaches. These results highlight the significance of local recharge during monsoon flood events as a water source for desert riparian systems, a fact that should be addressed when constructing and calibrating hydrologic models used to evaluate these future water management decisions.

Seasonal variability in the concentration and flux of organic matter and inorganic nitrogen in a semiarid catchment, San Pedro River, Arizona

Streams and rivers in semiarid environments are characterized by highly variable discharge associated with seasonal precipitation patterns. To characterize how this variability in discharge affects the concentration and flux of organic matter and inorganic nitrogen, we measured discharge and collected water samples in the San Pedro River in southeastern Arizona from April 2001 through September 2002. Mean daily discharge during the study ranged from 2.5 × 103 m3 day−1 to 9.4 × 105 m3 day−1, with low discharge in late June followed shortly thereafter by the highest discharge in July or early August after the onset of monsoon storms. Dissolved Organic Matter (DOM) dominated carbon and nitrogen flux during the nonmonsoon season, while Particulate Organic Matter (POM) dominated flux during the monsoon. The concentrations of Dissolved Organic Carbon and Nitrogen (DOC, DON), and NO3‐N typically increased 2‐ to 10‐fold, while POM increased by 2 to 3 orders of magnitude during the monsoon season. Concurrent increases in concentration and discharge resulted in 96% of the annual organic carbon (DOC + POC) and 97% annual nitrogen (DIN + DON + PON) flux occurring during the monsoon season with the majority of both organic carbon (92%) and total nitrogen (83%) being transported as particulates. These data indicate that POM transported during the monsoon season may be a major source of carbon and nitrogen to the stream ecosystem, potentially fueling heterotrophic activity in sediments and the hyporheic zone after the monsoon season and associated floods have ended.