Prosopis Velutina Research Articles

Changes in riparian plant communities due to a canal barrier traversing ephemeral stream channels in the Sonoran Desert

The Central Arizona Project (CAP) is a large canal system that traverses hundreds of ephemeral stream channels in the Sonoran Desert. This longitudinal barrier alters flow during runoff events, causing water to pond behind the canal's wall. We asked: How has riparian vegetation of the ephemeral streams changed over the course of 35 years in response to canal construction? We compared field data (vegetation volume, woody plant stem density and size, and herbaceous cover) collected in distinct zones located upstream and downstream of the canal to unaltered controls.By ponding water and inducing sedimentation of fine particles, the canal has created areas that support dense vegetation. The wettest zone has the greatest vegetation volume and plant height, and supports densities of Prosopis velutina that are six times greater than in the control zone. Larrea tridentata and other desert shrubs are displaced to the border of the wettest zone, but have greater height and stem diameter than typically occur in the desert owing to increased frequency of soil wetting in the canal-associated anthropogenic-floodplain. This research aids in understanding the impacts of canal barriers on desert riparian vegetation, and can be used to predict future outcomes of proposed canals in desert environments.

Revegetating Disturbance in National Parks: Reestablishing Native Plants in Saguaro National Park, Sonoran Desert

Habitat in national parks is periodically disturbed for road maintenance, and few revegetation protocols of known financial cost exist for this disturbance, especially in deserts where extreme environments constrain natural revegetation. In Saguaro National Park of the Arizona Upland Subdivision of the Sonoran Desert, we monitored survival of 1587 outplanted individuals of 33 native perennial species for revegetating a 2006 re-construction project of the park's Cactus Forest Drive. Outplants were caged to deter vertebrate herbivory and provided with supplemental water in the hot, dry part of summer. Overall plant survival was high — 84% (1340 of 1587 outplants) — one year after planting. Survival was generally consistent across species, with survival >50% for 32 of 33 (96%) species. Survival of two tree species (Parkinsonia microphylla (yellow paloverde) and Prosopis velutina (velvet mesquite)), monitored for two years, declined little or not at all from the first to the second year and was 55% and 67%, respectively, at two years. The project met management goals of reestablishing a 1:3 lost: restored ratio of tree density required for habitat restoration of an endangered owl species and of reestablishing a range of native species for aesthetic and vegetation structural restoration. Budget estimates indicated a cost per plant of $54 from grow-out in a nursery through plant maintenance in the field. This cost also included supporting activities of site preparation, exotic plant control, and effectiveness monitoring. The monitoring data, combined with longer term observations, suggest that the National Park Service's revegetation strategy effectively established a range of native plant growth forms and met habitat restoration targets.

Ecology of the antelope jackrabbit (Lepus alleni)

The antelope jackrabbit (Lepus alleni) is a lagomorph indicative of Neotropic savanna and thornscrub in south-central Arizona, west-central Sonora, and western Sinaloa <1,200 m elevation. We found populations in Arizona are most abundant in tropic–subtropic areas of low relief, characterized by velvet mesquite (Prosopis velutina) with an herbaceous understory (= Sonoran savanna grassland), and receiving a mean annual rainfall of between 200 and 450 mm, >90 mm of which falls as summer precipitation. In Mexico, this hare most commonly occurred in savanna grasslands or thornscrub interrupted by open areas receiving between 200 and 450 mm of precipitation per annum. Cacti were important habitat components for the hares and mean annual temperatures were >18°C with <60 d a year having temperatures below 0°C. Although sympatric with Lepus californicus over portions of its range, the two species preferred different habitats and rarely occurred together, and L. alleni was more prone to occur in groups than was L. californicus. We calculated the distributional range of L. alleni as ca. 102,000 km2 in Arizona and Mexico.

Spatial patterns of grassland–shrubland state transitions: a 74‐year record on grazed and protected areas.

Tree and shrub abundance has increased in many grasslands causing changes in ecosystem carbon and nitrogen pools that are related to patterns of woody plant distribution. However, with regard to spatial patterns of shrub proliferation, little is known about how they are influenced by grazing or the extent to which they are influenced by intraspecific interactions. We addressed these questions by quantifying changes in the spatial distribution of Prosopis velutina (mesquite) shrubs over 74 years on grazed and protected grasslands. Livestock are effective agents of mesquite dispersal and mesquite plants have lateral roots extending well beyond the canopy. We therefore hypothesized that mesquite distributions would be random on grazed areas mainly due to cattle dispersion and clustered on protected areas due to decreased dispersal and interspecific interference with grasses; and that clustered or random distributions at early stages of encroachment would give way to regular distributions as stands matured and density-dependent interactions intensified. Assessments in 1932, 1948, and 2006 supported the first hypothesis, but we found no support for the second. In fact, clustering intensified with time on the protected area and the pattern remained random on the grazed site. Although shrub density increased on both areas between 1932 and 2006, we saw no progression toward a regular distribution indicative of density-dependent interactions. We propose that processes related to seed dispersal, grass–shrub seedling interactions, and hydrological constraints on shrub size interact to determine vegetation structure in grassland-to-shrubland state changes with implications for ecosystem function and management.

Evaluating a State-and-Transition Model Using a Long-Term Dataset

State-and-transition models (STMs) are used in natural resource management to describe ecological site scale response to natural and anthropogenic disturbances. STMs are primarily based for expert opinion and literature reviews, lacking analytical testing to support vegetation community dynamics, thresholds, and state changes. We developed a unique approach, combining ordination and permutation MANOVA (perMANOVA) with raw data interpretation, to examine vegetation data structure and identify thresholds for a STM. We used a long-term monitoring dataset for an ecological site on the Santa Rita Experimental Range, Arizona. Basal cover of perennial grasses and canopy cover of shrubs and cacti were measured on permanent transects beginning in 1957. Data were grouped by drivers identified by the STM including species invasion, grazing, drought, and mesquite treatment. Ordination by nonmetric multidimensional scaling described the structure of the data. PerMANOVA was used to test for differences between groups of sample units. Analyses of combined key species (Lehmann's lovegrass and mesquite [Prosopis velutina Woot.]) and nonkey species patterns demonstrated an irreversible transition and occurrence of a structural threshold due to Lehmann's lovegrass invasion, as well as a short-term reversible transition (restoration pathway) following mesquite treatment. Sensitivity analysis, in which key species were removed from the dataset, showed that the relative composition of nonkey species did not differ between states previously defined by the key species. This apparent disconnect between dynamics of key and nonkey species may be related to changes in the functional attributes that were not monitored during this time series. Our analyses suggest that, for this ecological site, transition to a Lehmann's lovegrass state occurs when basal cover of this species exceeds 1–2%, which often occurs within 6 yr of its arrival. Evaluation of the restoration pathway showed a recrossing of the threshold within 6 yr of treatment and when mesquite canopy cover exceeded 10%.

Soil moisture and soil-litter mixing effects on surface litter decomposition: A controlled environment assessment

Recent studies suggest the long-standing discrepancy between measured and modeled leaf litter decomposition in drylands is, in part, the result of a unique combination of abiotic drivers that include high soil surface temperature and radiant energy levels and soil-litter mixing. Temperature and radiant energy effects on litter decomposition have been widely documented. However, under field conditions in drylands where soil-litter mixing occurs and accelerates decomposition, the mechanisms involved with soil-litter mixing effects are ambiguous. Potential mechanisms may include some combination of enhanced microbial colonization of litter, physical abrasion of litter surfaces, and buffering of litter and its associated decomposers from high temperatures and low moisture conditions. Here, we tested how soil-litter mixing and soil moisture interact to influence rates of litter decomposition in a controlled environment. Foliar litter of two plant species (a grass [Eragrostis lehmanniana] and a shrub [Prosopis velutina]) was incubated for 32 weeks in a factorial combination of soil-litter mixing (none, light, and complete) and soil water content (2, 4, 12% water-filled porosity) treatments. Phospholipid fatty acids (PLFAs) were quantified one week into the experiment to evaluate initial microbial colonization. A complementary incubation experiment with simulated rainfall pulses tested the buffering effects of soil-litter mixing on decomposition.Under the laboratory conditions of our experiments, the influence of soil-litter mixing was minimal and primarily confined to changes in PLFAs during the initial stages of decomposition in the constant soil moisture experiment and the oscillating soil moisture conditions of the rainfall pulse experiment. Soil-litter mixing effects on CO2 production, total phospholipid concentrations, and bacterial to total PLFA ratios were observed within the first week, but responses were fairly weak and varied with litter type and soil moisture treatment. Across the entire 32-week incubation experiment, soil moisture had a significant positive effect on mass loss, but soil-litter mixing did not. The lack of strong soil-litter mixing effects on decomposition under the moderate and relatively constant environmental conditions of this study is in contrast to results from field studies and suggests the importance of soil-litter mixing may be magnified when the fluctuations and extremes in temperature, radiant energy and moisture regimes common dryland field settings are in play.

Transitions from grassland to savanna under drought through passive facilitation by grasses

AbstractQuestionsWoody plant encroachment into former grasslands currently represents a major physiognomic shift globally. Seedling establishment is a critical demographic bottleneck and is considered to be alleviated by increases in water availability and negatively impacted by interactions with grasses, particularly when water stress increases. However, interactions with grasses that are not actively competing for resources (‘passive interactions’ when grasses are dead) has seldom been considered. Could the transition from a live to a dead grass (litter) canopy favour recruitment of woody seedlings in a semi‐arid grassland of the American SW? How does the sign and intensity of grass–seedling interactions change across drastically different summer precipitation regimes with and without passive interactions?LocationSonoran Desert shrub savanna at the Santa Rita Experimental Range, near Tucson, AZ, US.MethodsFour cohorts of Prosopis velutina seeds were planted annually (2002–2005) under rainout shelters that intercepted all incoming precipitation on a soil with sandy loam texture. Summer precipitation was manipulated to simulate either a 50% increase or decrease in the long‐term mean, and cover was manipulated to simulate a grassland dominated by the C4 bunchgrass Heteropogon contortus or left unvegetated. Emergence and survival of P. velutina was monitored and compared across cover types, along with monitoring of soil water content and light interception.ResultsStrong active competition was observed with live grasses, under both summer drought and also under ample summer water supply. However, the pattern was reversed and strong passive facilitation of P. velutina was observed over time when grass canopies died and remained in place. This passive facilitation under dry summers was so strong that recruitment under dead grass conditions was comparable to that observed when ample water supply removed the effects of competition on unvegetated plots.ConclusionsAfter significant mortality of standing grass canopies, which typically compete for limited soil moisture resources, rates of recruitment by shrubs may increase even with significant seasonal drought. This work extends our understanding of interactions among co‐located organisms and their effects on plant community dynamics, and introduces a new hypothesis on how grass litter facilitates woody plant encroachment during seasonal droughts.

Nesting Habitat and Productivity of Swainson's Hawks in Southeastern Arizona

We studied Swainson's Hawks (Buteo swainsoni) in southeastern Arizona to assess the status of the local breeding population. Nest success (≥1 young fledged) was 44.4% in 1999 with an average of 1.43 ± 0.09 (SE) young produced per successful pair. Productivity was similar in 2000, with 58.2% nesting success and 1.83 ± 0.09 fledglings per successful pair. Mesquite (Prosopis velutina) and cottonwood (Populus fremontii) accounted for >50% of 167 nest trees. Nest trees were taller than surrounding trees and random trees, and overall there was more vegetative cover at nest sites than random sites. This apparent requirement for cover around nest sites could be important for management of the species in Arizona. However, any need for cover at nest sites must be balanced with the need for open areas for foraging. Density of nesting Swainson's Hawks was higher in agriculture than in grasslands and desert scrub. Breeding pairs had similar success in agricultural and nonagricultural areas, but the effect of rapid and widespread land-use change on breeding distribution and productivity continues to be a concern throughout the range of the species.Estudiamos a Buteo swainsoni en el sureste de Arizona para evaluar el estado de la población reproductiva local. El éxito de nidificación (≥1 pichón que deja el nido) fue 44.4% en 1999 con un promedio de 1.43 ± 0.09 (DE) pichones producidos por pareja exitosa. La productividad fue similar en el 2000, con 58.2% de éxito de nidificación y 1.83 ± 0.09 volantones por pareja exitosa. Prosopis velutina y Populus fremontii representaron >50% de 167 árboles nido. Los árboles nido fueron más altos que los árboles que los rodeaban y que los árboles elegidos al azar, y en general hubo mayor cobertura vegetal en los sitios de nidificación que en los sitios elegidos al azar. Este requerimiento aparente de cobertura alrededor de los sitios de nidificación puede ser importante para el manejo de la especia en Arizona. Sin embargo, cualquier necesidad de cobertura en sitios de nidificación debe ser balanceada con la necesidad de áreas abiertas para forrajeo. La densidad de individuos reproductivos de B. swainsoni fue mayor en tierras agrícolas que en pastizales y matorrales desérticos. Las parejas reproductivas tuvieron un éxito similar en áreas agrícolas y no agrícolas, pero el efecto del cambio rápido y extenso de uso del suelo en las distribuciones reproductivas y en la productividad continúa siendo un problema a lo largo de la distribución de la especie.

A tool for estimating impacts of woody encroachment in arid grasslands: Allometric equations for biomass, carbon and nitrogen content in Prosopis velutina

Regression equations were developed to estimate above ground biomass and carbon and nitrogen mass of foliage and stem size fractions from plant size dimensions (basal diameter, canopy area, height, canopy volume) for a tall shrub species (Prosopis velutina) that has increased in abundance in arid and semi-arid grasslands in the southwestern United States and northwestern Mexico. Regression equations were also developed to describe relationships among the dimensions of plant size. All equations were significant (p < 0.001); and all but two had r2 values >0.72. In addition to species-specific information, we found support for the global patterns of foliar biomass increasing to the ¾ power of stem biomass and height increasing to the ½ power of stem diameter. We provide a comprehensive report of all equations, which can support a variety of in situ (ground-based), modeling, and remote-sensing objectives related to quantifying changes in ecosystem function and carbon sequestration accompanying changes in woody plant abundance. We advocate that comprehensive reporting should become more common for arid and semi-arid woody species in order to support a broad spectrum of users while laying the foundation for the development of global generalizations similar to those available for forest trees.

Windows of opportunity for Prosopis velutina seedling establishment and encroachment in a semiarid grassland

Abstract Large increases in the density of woody plants in former grasslands have been documented globally over the past two centuries. Prosopis velutina (velvet mesquite) has recently expanded on over 38 million ha in the grasslands of the American southwest. Mesquite establishment and expansion is potentially influenced by competitive interactions with grasses, particle size distribution (texture) of soil and changes in the amount and/or seasonality of precipitation, amongst other factors. To investigate the relative importance of precipitation seasonality, grass competition, and soil texture on mesquite seedling and establishment, we experimentally manipulated seasonality of precipitation across 72 1.5 m × 1.7 m plots that we planted with two grasses from contrasting functional groups (or left unvegetated) across two highly contrasting soil textures within a semi-desert grassland in Arizona, USA, wherein we followed the demography of 8640 planted mesquite seeds between 2002 and 2006. Up to 70% of the variance in seedling emergence was explained by soil water content, and seedling mortality was more dependent on summer than on winter precipitation. Mortality of less drought tolerant grasses after drought in coarse textured soils created conditions most favorable for seedling establishment, whereas lower rates of seedling establishment were observed in finer textured soils or in plots dominated by more drought tolerant grasses. We conclude that mesquite encroachment is episodic, based on discontinuous patterns of seedling establishment, favored on coarse textured soils and associated with unique conditions of high summer water availability when precipitation is not limiting and when competition from grasses is lowered.

Linkages between woody plant proliferation dynamics and plant physiological traits in southwestern North America

Aims Woody plant encroachments in arid and semiarid ecosystems are widely reported but the physiological mechanisms still need to be further revealed. In the current study, we aim to determine whether differences in leaf physiological traits help explain grassland susceptibility to woody plant encroachment and whether distinctive physiological adaptations allow some shrub species to invade grasslands. Methods We compared physiological traits (photosynthesis, leaf water status, pigment compositions and leaf antioxidant capacities) of six species representing three functional groups: woody encroachers (Prosopis velutina, Larrea tridentata), woody non-encroachers (Acacia greggii, Lycium fremontii) and C4 grasses (Bouteloua curtipendula, Bouteloua barbata) which are naturally growing in a botanical garden in University of Arizona, USA. Important Findings We infer that P. velutina (encroacher) but not A. greggii or L. fremontii (non-encroachers) is encroaching in grasslands because the former species has higher water and light utilization efficiencies (instantaneous water use efficiency, instantaneous light use efficiency, and Fv/Fm). The extremely high carotenoid and total antioxidant capacity in its leaves appears to help the shrubL.tridentata (encroacher) survive high ambient oxidative damage caused by both drought and high light stresses in this grassland. The two C4 grass species,B.curtipendula and B. barbata, grow well in the arid ecosystem but may be susceptible to disturbances.

Forecasting climate change impacts to plant community composition in the Sonoran Desert region

AbstractHotter and drier conditions projected for the southwestern United States can have a large impact on the abundance and composition of long‐lived desert plant species. We used long‐term vegetation monitoring results from 39 large plots across four protected sites in the Sonoran Desert region to determine how plant species have responded to past climate variability. This cross‐site analysis identified the plant species and functional types susceptible to climate change, the magnitude of their responses, and potential climate thresholds. In the relatively mesic mesquite savanna communities, perennial grasses declined with a decrease in annual precipitation, cacti increased, and there was a reversal of the Prosopis velutina expansion experienced in the 20th century in response to increasing mean annual temperature (MAT). In the more xeric Arizona Upland communities, the dominant leguminous tree, Cercidium microphyllum, declined on hillslopes, and the shrub Fouquieria splendens decreased, especially on south‐ and west‐facing slopes in response to increasing MAT. In the most xeric shrublands, the codominant species Larrea tridentata and its hemiparasite Krameria grayi decreased with a decrease in cool season precipitation and increased aridity, respectively. This regional‐scale assessment of plant species response to recent climate variability is critical for forecasting future shifts in plant community composition, structure, and productivity.

Soil Coverage Reduces Photodegradation and Promotes the Development of Soil-Microbial Films on Dryland Leaf Litter

Litter decomposition is a central focus of ecosystem science because of its importance to biogeochemical pools and cycling, but predicting dryland decomposition dynamics is problematic. Some studies indicate photodegradation by ultraviolet (UV) radiation can be a significant driver of dryland decomposition, whereas others suggest soil–litter mixing controls decomposition. To test the influence of soil coverage on UV photodegradation of litter, we conducted a controlled environment experiment with shrub (Prosopis velutina) leaf litter experiencing two UV levels and three levels of coverage with dry sterile soil. Under these conditions, decomposition over 224 days was enhanced by UV, but increasing soil coverage strongly and linearly diminished these effects. In a complementary study, we placed P. glandulosa leaf litter in different habitats in the field and quantified litter surface coverage by soil films. After 180 days, nearly half of the surface area of litter placed under shrub canopies was covered by a tightly adhering film composed of soil particles and fungal hyphae; coverage was less in grassy zones between shrubs. We propose a conceptual model for the shifting importance of photodegradation and microbial decomposition over time, and conclude that (1) soil deposition can ameliorate the direct effects of UV photodegradation in drylands and (2) predictions of C losses based solely on UV effects will overestimate the importance of this process in the C cycle. An improved understanding of how development of the soil–litter matrix mediates the shift from abiotic (photodegradation) to biotic (microbial) drivers is necessary to predict how ongoing changes in land cover and climate will influence biogeochemistry in globally extensive drylands.

Environmental and physiological controls on the carbon isotope composition of CO2 respired by leaves and roots of a C3 woody legume (Prosopis velutina) and a C4 perennial grass (Sporobolus wrightii)

Accurate estimates of the δ(13) C value of CO(2) respired from roots (δ(13) C(R_root) ) and leaves (δ(13) C(R_leaf) ) are important for tracing and understanding changes in C fluxes at the ecosystem scale. Yet the mechanisms underlying temporal variation in these isotopic signals are not fully resolved. We measured δ(13) C(R_leaf) , δ(13) C(R_root) , and the δ(13) C values and concentrations of glucose and sucrose in leaves and roots in the C(4) grass Sporobolus wrightii and the C(3) tree Prosopis velutina in a savanna ecosystem in southeastern Arizona, USA. Night-time variation in δ(13) C(R_leaf) of up to 4.6 ± 0.6‰ in S. wrightii and 3.0 ± 0.6‰ in P. velutina were correlated with shifts in leaf sucrose concentration, but not with changes in δ(13) C values of these respiratory substrates. Strong positive correlations between δ(13) C(R_root) and root glucose δ(13) C values in P. velutina suggest large diel changes in δ(13) C(R_root) (were up to 3.9‰) influenced by short-term changes in δ(13) C of leaf-derived phloem C. No diel variation in δ(13) C(R_root) was observed in S. wrightii. Our findings show that short-term changes in δ(13) C(R_leaf) and δ(13) C(R_root) were both related to substrate isotope composition and concentration. Changes in substrate limitation or demand for biosynthesis may largely control short-term variation in the δ(13) C of respired CO(2) in these species.

Protection from livestock fails to deter shrub proliferation in a desert landscape with a history of heavy grazing

Desertification is often characterized by the replacement of mesophytic grasses with xerophytic shrubs. Livestock grazing is considered a key driver of shrub encroachment, although most evidence is anecdotal or confounded by other factors. Mapping of velvet mesquite (Prosopis velutina) shrubs in and out of exclosures in 1932, 1948, and 2006 in semiarid grasslands of southeastern Arizona, USA, afforded the opportunity to quantify livestock grazing effects on mesquite proliferation over 74 years in the absence of fire to test the widespread assumption that livestock grazing promotes shrub proliferation. In 1932, shrub cover, density, and aboveground biomass were compared on grazed (12%, 173 plants/ha, 4182 kg/ha) and newly protected areas (8%, 203 plants/ha, 3119 kg/ha). By 1948, cover on both areas increased to 18%; yet, density on the protected area increased 300% (to 620 plants/ha), nearly twice that of the grazed area (325 plants/ha). From 1932 to 1948, differences in recruitment of new plants and growth of existing plants were reflected in biomass, which was higher on the protected area (415 plants/ha, 8788 kg/ha) relative to the grazed area (155 plants/ha, 7085 kg/ha), although mortality was equally low ( 0.06%). In 2006, 42 years after an herbicide application reset mesquite cover to 10% on both areas, aboveground mesquite mass was comparable on both areas ( 4700 kg/ha), but cover and density on the protected area (22%, 960 plants/ha) exceeded that on the grazed area (15%, 433 plants/ha). Mesquite mass in 2006 was substantially below 1948 levels, so continued accrual is likely. That shrub recovery from herbicides on a biomass basis was much less than recovery on a cover basis suggests that remotely sensed biomass estimates should integrate land management history. Contrary to widely held assumptions, protection from livestock since 1932 not only failed to deter woody-plant proliferation, but actually promoted it relative to grazed areas. Results suggest (1) that thresholds for grassland resistance to shrub encroachment had been crossed by the 1930s, and (2) fire management rather than grazing management may be key to maintaining grassland physiognomy in this bioclimatic region.

Toxicity and biotransformation of ZnO nanoparticles in the desert plants Prosopis juliflora-velutina, Salsola tragus and Parkinsonia florida

Although arid and semiarid regions account for about 40% of world land, no nanotoxicity studies on desert plants have been reported. In this investigation, Parkinsonia florida (blue palo verde), Prosopis juliflora-velutina (velvet mesquite) and Salsola tragus (tumbleweed) were selected to determine the phytotoxicity of ZnO nanoparticles (NPs) at germination stage. Seeds were treated with ZnO NP concentrations ranging between 0 and 4000 mg L−1. From this, the germination rate, root elongation and Zn concentration in tissues were determined. Furthermore, X-ray Absorption Spectroscopic (XAS) studies were performed to obtain preliminary information on potential NP biotransformation within plant tissues. Results indicated that germination was not significantly affected (P < 0.05) in any of the three plant species. Also, root elongation in blue palo verde was reduced by 16%, with respect to control at 4000 mg ZnO L−1. Tumbleweed root size diminished by 14% and 16% at ZnO NP levels of 500 and 2000 mg L−1, respectively, and velvet mesquite root length was reduced to all ZnO NP concentrations used in this study. The 50% inhibitory concentration (IC50) for mesquite root elongation was between 1000 mg L−1 and 2000 mg L−1. XAS results demonstrated that ZnO NPs were biotransformed on/within the root and Zn was present as Zn(II) in the three desert plant species.

Spectroscopic verification of zinc absorption and distribution in the desert plant Prosopis juliflora-velutina (velvet mesquite) treated with ZnO nanoparticles

The impact of metal nanoparticles (NPs) on biological systems, especially plants, is still not well understood. The aim of this research was to determine the effects of zinc oxide (ZnO) NPs in velvet mesquite ( Prosopis juliflora-velutina). Mesquite seedlings were grown for 15 days in hydroponics with ZnO NPs (10 nm) at concentrations varying from 500 to 4000 mg L −1. Zinc concentrations in roots, stems and leaves were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). Plant stress was examined by the specific activity of catalase (CAT) and ascorbate peroxidase (APOX); while the biotransformation of ZnO NPs and Zn distribution in tissues was determined by X-ray absorption spectroscopy (XAS) and micro X-ray fluorescence (μXRF), respectively. ICP-OES results showed that Zn concentrations in tissues (2102 ± 87, 1135 ± 56, and 628 ± 130 mg kg −1 d wt in roots, stems, and leaves, respectively) were found at 2000 mg ZnO NPs L −1. Stress tests showed that ZnO NPs increased CAT in roots, stems, and leaves, while APOX increased only in stems and leaves. XANES spectra demonstrated that ZnO NPs were not present in mesquite tissues, while Zn was found as Zn(II), resembling the spectra of Zn(NO 3) 2. The μXRF analysis confirmed the presence of Zn in the vascular system of roots and leaves in ZnO NP treated plants.

Vegetation Effects on Soil Organic Carbon Quality in an Arid Hyperthermic Ecosystem

Arid lands occupy substantial global land area and thus may play an important role in the terrestrial carbon cycle. This study examined a facet of arid land carbon cycling by examining variation in plant and soil organic carbon quality and the physical partitioning of carbon into aggregate and mineral-associated fractions for soils dominated by Prosopis velutina (mesquite), Larrea tridentata (creosote), and a combination of Bouteloua barbata, Bouteloua aristidoides, Aristida adscensionis, and Cynodon dactylon (mixed grass) vegetation types in an arid hyperthermic ecosystem. We used a combination of density fractionation to quantify physical distribution of organic carbon, in addition to isotopic, thermal, and spectroscopic techniques to quantify carbon quality in each fraction. Data indicated that most of the soil carbon across all vegetation types was concentrated in free light fractions, with little role for aggregate occlusion or mineral adsorption. Differential thermal analysis and derivative thermogravimetry indicated that all vegetation types were dominated by thermally labile material (exothermic peak and mass loss at ∼350°C), with the greatest differences in carbon quality noted among the respective plant materials. Diffuse reflectance Fourier transform infrared spectroscopy also indicated that the most substantial variation in organic carbon chemical quality was among the various plant materials. In particular, the creosote plant material exhibited a distinct high-temperature exothermic peak near 515°C, the greatest specific enthalpy (20 kJ g -1 biomass), and a relatively greater proportion of aromatic components as determined by diffuse reflectance Fourier transform infrared spectroscopy. Furthermore, the data indicated substantial alteration of chemical and thermal properties as organic material progressed from plant material to the soil fractions with a convergence on organic material dominated by polysaccharides and substantial reduction in specific enthalpy in the soil fractions.

Inter- and under-canopy soil water, leaf-level and whole-plant gas exchange dynamics of a semi-arid perennial C4 grass

It is not clear if tree canopies in savanna ecosystems exert positive or negative effects on soil moisture, and how these might affect understory plant carbon balance. To address this, we quantified rooting-zone volumetric soil moisture (θ(25 cm)), plant size, leaf-level and whole-plant gas exchange of the bunchgrass, bush muhly (Muhlenbergia porteri), growing under and between mesquite (Prosopis velutina) in a southwestern US savanna. Across two contrasting monsoon seasons, bare soil θ(25 cm) was 1.0-2.5% lower in understory than in the intercanopy, and was consistently higher than in soils under grasses, where θ(25 cm) was similar between locations. Understory plants had smaller canopy areas and volumes with larger basal diameters than intercanopy plants. During an above-average monsoon, intercanopy and understory plants had similar seasonal light-saturated leaf-level photosynthesis (A(net-sat)), stomatal conductance (g(s-sat)), and whole-plant aboveground respiration (R(auto)), but with higher whole-plant photosynthesis (GEP(plant)) and transpiration (T(plant)) in intercanopy plants. During a below-average monsoon, intercanopy plants had higher diurnally integrated GEP(plant), R(auto), and T(plant). These findings showed little evidence of strong, direct positive canopy effects to soil moisture and attendant plant performance. Rather, it seems understory conditions foster competitive dominance by drought-tolerant species, and that positive and negative canopy effects on soil moisture and community and ecosystem processes depends on a suite of interacting biotic and abiotic factors.

Seasonally Pulsed Heterogeneity in Microclimate: Phenology and Cover Effects along Deciduous Grassland–Forest Continuum

Much of the terrestrial biosphere can be viewed as part of a gradient, with varying amounts of woody plant cover ranging from grassland to forest—the grassland–forest continuum. Woody plant cover directly impacts the soil microclimate through modifications of near‐ground solar radiation and soil temperature, and these interactive effects are relevant for key ecohydrological processes such as soil evaporation. Trends in how increasing woody plant cover affect soil surface microclimate have recently been evaluated for gradients of evergreen woody plants, but analogous trends for deciduous plants, where phenology should be influential, are lacking. We evaluated season‐dependent changes in soil microclimate along a deciduous grassland–forest continuum of velvet mesquite (Prosopis velutina Wooton) using repeated hemispherical photography and continuous soil temperature measurements at the 5‐cm depth. Both near‐ground solar radiation and soil temperature decreased with increasing canopy cover, even during the leafless season. The trends varied substantially among seasons, however, with differences between canopy and intercanopy patches readily evident only during the period of full leaf‐out, during which the correlation between near‐ground solar radiation and soil temperature was strongest. Our results provide a more comprehensive understanding about the interactions of canopy cover, canopy structure attributes, and plant phenology that produce seasonally pulsed heterogeneity in the soil surface microclimate. Notably, our results add a new dimension to the moisture “pulse dynamics” perspective commonly applied to dryland ecohydrology, highlighting seasonally pulsed heterogeneity in soil microclimate that could influence soil moisture dynamics in drylands.