Woody-plant encroachment: Precipitation, herbivory, and grass-competition interact to affect shrub recruitment.

Woody-plant encroachment is a global phenomenon that has been affecting the southwestern United States since the late 1800s. Drought, overgrazing, herbivory, and competition between grasses and shrub seedlings have been hypothesized as the main drivers of shrub establishment. However, there is limited knowledge about the interactions among these drivers. Using a rainfall manipulation system and various herbivore exclosures, we tested hypotheses about how precipitation (PPT), competition between grasses and shrub seedlings, and predation affect the germination and first-year survival of mesquite (Prosopis glandulosa), a shrub that has encroached in Southern Great Plains and Chihuahuan Desert grasslands. We found that mesquite germination and survival (1) increased with increasing PPT, then saturated at about the mean growing season PPT level, (2) that competition between grasses and shrub seedlings had no effect on either germination or survival, and (3) that herbivory by small mammals decreased seedling establishment and survival, while ant granivory showed no effect. In addition to its direct positive effect on survival, PPT had an indirect negative effect via increasing small mammal activity. Current models predict a decrease in PPT in the southwestern United States with increased frequency of extreme events. The non-linear nature of PPT effects on Mesquite recruitment suggests asymmetric responses, wherein drought has a relatively greater negative effect than the positive effect of wet years. Indirect effects of PPT, through its effects on small mammal abundance, highlight the importance of accounting for interactions between biotic and abiotic drivers of shrub encroachment. This study provides quantitative basis for developing tools that can inform effective shrub management strategies in grasslands and savannas.

The detection of a non-anemophilous plant species using airborne eDNA.

Genetic analysis of airborne plant material has historically focused (generally implicitly rather than as a stated goal) on pollen from anemophilous (wind-pollinated) species, such as in multiple studies examining the relationship of allergens to human health. Inspired by the recent influx of literature applying environmental DNA (eDNA) approaches to targeted-species and whole-ecosystem study, we conducted a proof-of-concept experiment to determine whether airborne samples reliably detect genetic material from non-anemophilous species that may not be releasing large plumes of pollen. We collected airborne eDNA using Big Spring Number Eight dust traps and quantified the amount of eDNA present for a flowering wind-pollinated genus (Bouteloua) and insect-pollinated honey mesquite (Prosopis glandulosa) that was not flowering at the time of the study. We were able to detect airborne eDNA from both species. Since honey mesquite is insect-pollinated and was not flowering during the time of this study, our results confirm that airborne eDNA consists of and can detect species through more than just pollen. Additionally, we were able to detect temporal patterns reflecting Bouteloua reproductive ecology and suggest that airborne honey mesquite eDNA responded to weather conditions during our study. These findings suggest a need for more study of the ecology of airborne eDNA to uncover its potential for single-species and whole-community research and management in terrestrial ecosystems.

Early exposure to UV radiation overshadowed by precipitation and litter quality as drivers of decomposition in the northern Chihuahuan Desert.

Dryland ecosystems cover nearly 45% of the Earth's land area and account for large proportions of terrestrial net primary production and carbon pools. However, predicting rates of plant litter decomposition in these vast ecosystems has proven challenging due to their distinctly dry and often hot climate regimes, and potentially unique physical drivers of decomposition. In this study, we elucidated the role of photopriming, i.e. exposure of standing dead leaf litter to solar radiation prior to litter drop that would chemically change litter and enhance biotic decay of fallen litter. We exposed litter substrates to three different UV radiation treatments simulating three-months of UV radiation exposure in southern New Mexico: no light, UVA+UVB+Visible, and UVA+Visible. There were three litter types: mesquite leaflets (Prosopis glandulosa, litter with high nitrogen (N) concentration), filter paper (pure cellulose), and basswood (Tilia spp, high lignin concentration). We deployed the photoprimed litter in the field within a large scale precipitation manipulation experiment: ∼50% precipitation reduction, ∼150% precipitation addition, and ambient control. Our results revealed the importance of litter substrate, particularly N content, for overall decomposition in drylands, as neither filter paper nor basswood exhibited measurable mass loss over the course of the year-long study, while high N-containing mesquite litter exhibited potential mass loss. We saw no effect of photopriming on subsequent microbial decay. We did observe a precipitation effect on mesquite where the rate of decay was more rapid in ambient and precipitation addition treatments than in the drought treatment. Overall, we found that precipitation and N played a critical role in litter mass loss. In contrast, photopriming had no detected effects on mass loss over the course of our year-long study. These results underpin the importance of biotic-driven decomposition, even in the presence of photopriming, for understanding litter decomposition and biogeochemical cycles in drylands.

Forthcoming risk of Prosopis juliflora global invasion triggered by climate change: implications for environmental monitoring and risk assessment.

Climate is a determinant factor in species distribution and climate change will affect the species abilities to occupy geographic regions. Prosopis juliflora is one of the most problematic invasive species and its biological invasion causes various negative effects in tropical, arid, and semi-arid regions of the world. As eradication efforts subsequent to the establishment of an alien invasive species are costly and time-consuming, assessing patterns of the introduction of an invasive species to new regions is among the most cost-effective means of monitoring and management of natural ecosystems. In this study by using the concept of species distribution modeling (SDM) and maximum entropy (MaxEnt) method, the effect of climate change on the current and future distribution of P. juliflora has been assessed at a global scale. Bioclimatic variables in current condition and 2050 regarding two global circulation models (GCM) and two climate change scenarios were considered as explanatory variables. Our results showed that annual mean temperature (BIO1), annual precipitation (BIO12), and temperature mean diurnal range (BIO2) represented more than 87% of the variations in the model, and with an AUC of 0.854 and TSS of 0.51, the model showed a good predictive performance. Our results indicate that on a global scale, suitable ranges for P. juliflora increase across all the GCM and RCP scenarios. In a global scale, Mediterranean Basin, Middle East, and North America are regions with the highest risk of range expansion in the future. Regarding the negative impacts of P. juliflora on structure and function of natural habitats in the invaded areas, findings of this study could be considered as a warning appliance for the environmental monitoring of the regions highly sensitive to the global invasion of the species. We suggest that assessing impacts of climate change on the global distribution of the invasive species could be used as an efficient tool to implement broad-scale and priority-setting monitoring programs in natural ecosystems.

Soil phosphorus does not keep pace with soil carbon and nitrogen accumulation following woody encroachment.

Soil carbon, nitrogen, and phosphorus cycles are strongly interlinked and controlled through biological processes, and the phosphorus cycle is further controlled through geochemical processes. In dryland ecosystems, woody encroachment often modifies soil carbon, nitrogen, and phosphorus stores, although it remains unknown if these three elements change proportionally in response to this vegetation change. We evaluated proportional changes and spatial patterns of soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) concentrations following woody encroachment by taking spatially explicit soil cores to a depth of 1.2 m across a subtropical savanna landscape which has undergone encroachment by Prosopis glandulosa (an N2 fixer) and other woody species during the past century in southern Texas, USA. SOC and TN were coupled with respect to increasing magnitudes and spatial patterns throughout the soil profile following woody encroachment, while TP increased slower than SOC and TN in topmost surface soils (0-5 cm) but faster in subsurface soils (15-120 cm). Spatial patterns of TP strongly resembled those of vegetation cover throughout the soil profile, but differed from those of SOC and TN, especially in subsurface soils. The encroachment of woody species dominated by N2 -fixing trees into this P-limited ecosystem resulted in the accumulation of proportionally less soil P compared to C and N in surface soils; however, proportionally more P accrued in deeper portions of the soil profile beneath woody patches where alkaline soil pH and high carbonate concentrations would favor precipitation of P as relatively insoluble calcium phosphates. This imbalanced relationship highlights that the relative importance of biotic vs. abiotic mechanisms controlling C and N vs. P accumulation following vegetation change may vary with depth. Our findings suggest that efforts to incorporate effects of land cover changes into coupled climate-biogeochemical models should attempt to represent C-N-P imbalances that may arise following vegetation change.

Co-occurring woody species have diverse hydraulic strategies and mortality rates during an extreme drought.

From 2011 to 2013, Texas experienced its worst drought in recorded history. This event provided a unique natural experiment to assess species-specific responses to extreme drought and mortality of four co-occurring woody species: Quercus fusiformis, Diospyros texana, Prosopis glandulosa, and Juniperus ashei. We examined hypothesized mechanisms that could promote these species' diverse mortality patterns using postdrought measurements on surviving trees coupled to retrospective process modelling. The species exhibited a wide range of gas exchange responses, hydraulic strategies, and mortality rates. Multiple proposed indices of mortality mechanisms were inconsistent with the observed mortality patterns across species, including measures of the degree of iso/anisohydry, photosynthesis, carbohydrate depletion, and hydraulic safety margins. Large losses of spring and summer whole-tree conductance (driven by belowground losses of conductance) and shallower rooting depths were associated with species that exhibited greater mortality. Based on this retrospective analysis, we suggest that species more vulnerable to drought were more likely to have succumbed to hydraulic failure belowground.

The pre-Columbian introduction and dispersal of Algarrobo (Prosopis, Section Algarobia) in the Atacama Desert of northern Chile.

Archaeological and palaeoecological studies throughout the Americas have documented widespread landscape and environmental transformation during the pre-Columbian era. The highly dynamic Formative (or Neolithic) period in northern Chile (ca. 3700-1550 yr BP) brought about the local establishment of agriculture, introduction of new crops (maize, quinoa, manioc, beans, etc.) along with a major population increase, new emergent villages and technological innovations. Even trees such as the Algarrobos (Prosopis section Algarobia) may have been part of this transformation. Here, we provide evidence that these species were not native to the Atacama Desert of Chile (18-27°S), appearing only in the late Holocene and most likely due to human actions. We assembled a database composed of 41 taxon specific AMS radiocarbon dates from archaeobotanical and palaeoecological records (rodent middens, leaf litter deposits), as well an extensive bibliographical review comprising archaeobotanical, paleoecological, phylogenetic and taxonomic data to evaluate the chronology of introduction and dispersal of these trees. Although Algarrobos could have appeared as early as 4200 yr BP in northernmost Chile, they only became common throughout the Atacama over a thousand years later, during and after the Formative period. Cultural and natural factors likely contributed to its spread and consolidation as a major silvicultural resource.

Sugary secretions of wasp galls: a want-to-be extrafloral nectar?

Background and Aims: The most widespread form of protective mutualisms is represented by plants bearing extrafloral nectaries (EFNs) that attract ants and other arthropods for indirect defence. Another, but less common, form of sugary secretion for indirect defence occurs in galls induced by cynipid wasps. Until now, such galls have been reported only for cynipid wasps that infest oak trees in the northern hemisphere. This study provides the first evidence of galls that exude sugary secretions in the southern hemisphere and asks whether they can be considered as analogues of plants' EFNs. Methods: The ecology and anatomy of galls and the chemical composition of the secretion were investigated in north-western Argentina, in natural populations of the host trees Prosopis chilensis and P. flexuosa . To examine whether ants protect the galls from natural enemies, ant exclusion experiments were conducted in the field. Key Results: The galls produce large amounts of sucrose-rich, nectar-like secretions. No typical nectary and sub-nectary parenchymatic tissues or secretory trichomes can be observed; instead there is a dense vascularization with phloem elements reaching the gall periphery. At least six species of ants, but also vespid wasps, Diptera and Coleoptera, consumed the gall secretions. The ant exclusion experiment showed that when ants tended galls, no differences were found in the rate of successful emergence of gall wasps or in the rate of parasitism and inquiline infestation compared with ant-excluded galls. Conclusions: The gall sugary secretion is not analogous to extrafloral nectar because no nectar-producing structure is associated with it, but is functionally equivalent to arthropod honeydew because it provides indirect defence to the plant parasite. As in other facultative mutualisms mediated by sugary secretions, the gall secretion triggers a complex multispecies interaction, in which the outcome of individual pair-wise interactions depends on the ecological context in which they take place.

Seasonal variability in physiological and anatomical traits contributes to invasion success of Prosopis juliflora in tropical dry forest.

We investigated whether there were consistent differences in the physiological and anatomical traits and phenotypic variability of an invasive (Prosopis juliflora (Sw.) DC.) and native species (Anadenanthera colubrina (Vell.) Brenan) in response to seasonality in a tropical dry forest. The water potential, organic solutes, gas exchange, enzymes of the antioxidant system, products of oxidative stress and anatomical parameters were evaluated in both species in response to seasonality. An analysis of physiological responses indicated that the invasive P. juliflora exhibited higher response in net photosynthetic rate to that of the native species between seasons. Higher values of water potential of the invasive species than those of the native species in the dry season indicate a more efficient mechanism for water regulation in the invasive species. The invasive species exhibits a thicker cuticle and trichomes, which can reduce transpiration. In combination, the increased epidermal thickness and the decreased thickness of the parenchyma in the dry season may contribute to water saving. Our data suggest a higher variability in anatomical traits in the invasive species as a response to seasonality, whereas physiological traits did not present a clear pattern of response.

Cambios anatómicos en raíces e hipocótilos de plántulas de Prosopis ruscifolia (Fabaceae) sometidas a estrés salino / Anatomical changes in roots and hypocotyls of Prosopis ruscifolia (Fabaceae) seedlings exposed to saline stress

ResumenProsopis ruscifolia es una especie arbórea pionera en áreas inundadas o salinas. El objetivo de este trabajo fue determinar cambios anatómicos en raíces e hipocótilos de plántulas de P. ruscifolia sometidas a estrés salino, bajo condiciones controladas. Las semillas se recolectaron en bosques nativos de la Región Chaqueña Occidental de Argentina. Las semillas se sembraron sobre toallas de papel humedecidas con soluciones salinas de 100, 200 y 300 mM de NaCl y un control humedecido con agua destilada. Se sembraron cuatro repeticiones de 50 semillas cada una, correspondientes a cada tratamiento, se ubicaron en cajas plásticas herméticas dentro de cámara de siembra a 27 ºC y con fotoperíodo de 12 horas. Doce días después de la siembra, se extrajeron plántulas para estudios anatómicos. Se estudiaron 35 plántulas correspondientes a cada tratamiento. Se midieron en raíces e hipocótilos las siguientes variables anatómicas: diámetro de la raíz principal e hipocótilo (µm), espesor de la corteza (µm), número de estratos celulares en la corteza, diámetro del cilindro central (µm), diámetro de la médula (µm), número de estratos celulares en el periciclo y diámetro tangencial de los vasos (µm). Se realizó ANOVA con diámetro de la raíz o hipocótilo como variable dependiente y espesor de la corteza, número de estratos celulares en la corteza, diámetro del cilindro central, diámetro de la médula, número de estratos celulares en el periciclo, diámetro tangencial de los vasos y concentración salina como variables independientes. El diámetro de la raíz disminuyó significativamente con el aumento de la concentración salina (P < 0.0001). El espesor de la corteza redujo su espesor a 100 mM (P < 0.0001) e incrementó el número de estratos celulares que la componen (P < 0.0002). El diámetro del cilindro central se redujo a la concentración salina de 100 mM (P < 0.0001) y el diámetro de la médula y el número de estratos celulares del periciclo (P < 0.0003) disminuyó progresivamente hasta 300 mM. El diámetro tangencial de los vasos (P < 0.0001) se redujo recién a 300 mM de NaCl. Estos cambios anatómicos podrían estar relacionados con la alteración de la expansión y división celular causada por la salinidad y comprometer la formación de raíces laterales y el almacenamiento de reservas. Los hipocótilos no mostraron cambios anatómicos significativos en respuesta al incremento en la salinidad, con excepción de la variación en la posición de estomas y un incremento en el espesor de la hipodermis. Estos cambios parecen indicar el estrés hídrico impuesto por el bajo potencial osmótico causado por las sales. Las plántulas de P. ruscifolia experimentaron cambios anatómicos en respuesta a las concentraciones salinas analizadas, en rasgos vinculados al almacenamiento de reservas, a la absorción y la conducción de agua y la formación de raíces laterales.

Abstract:Prosopis ruscifolia is a pioneer tree species in flooding or saline areas. The aim of this work was to assess anatomical changes in roots and hypocotyls of P. ruscifolia seedlings induced to saline stress under controlled conditions. Seeds, collected in natural forests of Western Chaco region in Argentina, were sown on paper towels moisturized with saline solutions of 100, 200 and 300 mM of NaCl, and a control group with distilled water. Four repetitions of 50 seeds per treatment were sown, located in hermetic polystyrene boxes, and included in a seeding chamber, at 27 ºC and 12 hours photoperiod. Were studied 35 seedlings from each saline concentration; these seedlings were processed 12 days after sown to obtain microscopic samples. The anatomical variables measured in roots and hypocotyls were the following: main root diameter (µm), bark thickness (µm), number of cell strata in bark, central cylinder diameter (µm), pith diameter (µm), number of cell strata in the pericycle and the tangential diameter of vessels (µm). ANOVA analysis were performed with hypocotyl and root diameters as the dependent variable, and bark thickness (µm), number of cell strata in the bark, the central cylinder diameter (µm), the pith diameter (µm), number of cell strata in the pericycle, the tangential diameter of vessels and the saline concentration as independent variables. Results showed that the root diameter decreased with increasing saline concentrations (P < 0.0001). The bark thickness decreased at 100 mM (P < 0.0001) and the number of cell strata of bark increased to 300 mM (P < 0.0002). The central cylinder diameter decreased at 100 mM saline concentration (P < 0.0001) and the number of cell strata of the pericycle and the pith diameter reduced progressively until 300 mM. The tangential diameter of vessels decreased at 300 mM. These anatomical changes suggested alterations in the expansion and cell division caused by the salinity, and could limit lateral roots formation and reserves storage. Hypocotyls did not show significant anatomical changes in response to increasing salinity, with exception of stomata position and an increase of the hypodermis thickness. These changes indicated that the water stress imposed by low osmotic potential is caused by increasing saline concentration. The seedlings of P. ruscifolia experienced anatomical changes in response to tested saline concentrations in traits related to reserve storage, the absorption and conduction of water, and lateral roots formation. Rev. Biol. Trop. 64 (3): 1007-1017. Epub 2016 September 01.

[Anatomical changes in roots and hypocotyls of Prosopis ruscifolia (Fabaceae) seedlings exposed to saline stress].

Prosopis ruscifolia is a pioneer tree species in flooding or saline areas. The aim of this work was to assess anatomical changes in roots and hypocotyls of P. ruscifolia seedlings induced to saline stress under controlled conditions. Seeds, collected in natural forests of Western Chaco region in Argentina, were sown on paper towels moisturized with saline solutions of 100, 200 and 300 mM of NaCl, and a control group with distilled water. Four repetitions of 50 seeds per treatment were sown, located in hermetic polystyrene boxes, and included in a seeding chamber, at 27 ºC and 12 hours photoperiod. Were studied 35 seedlings from each saline concentration; these seedlings were processed 12 days after sown to obtain microscopic samples. The anatomical variables measured in roots and hypocotyls were the following: main root diameter (µm), bark thickness (µm), number of cell strata in bark, central cylinder diameter (µm), pith diameter (µm), number of cell strata in the pericycle and the tangential diameter of vessels (µm). ANOVA analysis were performed with hypocotyl and root diameters as the dependent variable, and bark thickness (µm), number of cell strata in the bark, the central cylinder diameter (µm), the pith diameter (µm), number of cell strata in the pericycle, the tangential diameter of vessels and the saline concentration as independent variables. Results showed that the root diameter decreased with increasing saline concentrations (P < 0.0001). The bark thickness decreased at 100 mM (P < 0.0001) and the number of cell strata of bark increased to 300 mM (P < 0.0002). The central cylinder diameter decreased at 100 mM saline concentration (P < 0.0001) and the number of cell strata of the pericycle and the pith diameter reduced progressively until 300 mM. The tangential diameter of vessels decreased at 300 mM. These anatomical changes suggested alterations in the expansion and cell division caused by the salinity, and could limit lateral roots formation and reserves storage. Hypocotyls did not show significant anatomical changes in response to increasing salinity, with exception of stomata position and an increase of the hypodermis thickness. These changes indicated that the water stress imposed by low osmotic potential is caused by increasing saline concentration. The seedlings of P. ruscifolia experienced anatomical changes in response to tested saline concentrations in traits related to reserve storage, the absorption and conduction of water, and lateral roots formation.

The Potential of Algarrobo (Prosopis chilensis (Mol.) Stuntz) for Regeneration of Desertified Soils: Assessing Seed Germination Under Saline Conditions.

Due to their multipurpose use, leguminous trees are desirable for the restoration of degraded ecosystems. Our aim was to investigate seed germination of the leguminous tree Prosopis chilensis in response to salinity, one of the major abiotic challenges of desertified soils. Germination percentages of seed from 12 wild P. chilensis populations were studied. Treatments included four aqueous NaCl concentrations (150, 300, 450, and 600 mM). In each population, the highest germination percentage was seen using distilled water (control), followed closely by 150 mM NaCl. At 300 mM NaCl or higher salt concentration, germination was progressively inhibited attaining the lowest value at 450 mM NaCl, while at 600 mM NaCl germination remained reduced but with large variation among group of samples. These results allowed us to allocate the 12 groups from where seeds were collected into three classes. First, the seeds from Huanta-Rivadavia showed the lowest percent germination for each salt condition. The second group was composed of moderately salt-tolerant seeds with 75% germination at 300 mM NaCl, followed by 50% germination at 450 mM NaCl and 30% germination at 600 mM NaCl. The third group from Maitencillo and Rapel areas was the most salt tolerant with an impressive seed germination level of 97% at 300 mM NaCl, 82 % at 450 mM NaCl, and 42 % at 600 mM NaCl. Our results demonstrate that P. chilensis seeds from these latter localities have an increased germination capability under saline stress, confirming that P. chilensis is an appropriate species to rehabilitate desertified soils.

Population-level consequences of herbivory, changing climate, and source-sink dynamics on a long-lived invasive shrub.

Long-lived plant species are highly valued environmentally, economically, and socially, but can also cause substantial harm as invaders. Realistic demographic predictions can guide management decisions, and are particularly valuable for long-lived species where population response times can be long. Long-lived species are also challenging, given population dynamics can be affected by factors as diverse as herbivory, climate, and dispersal. We developed a matrix model to evaluate the effects of herbivory by a leaf-feeding biological control agent released in Australia against a long-lived invasive shrub (mesquite, Leguminoseae: Prosopis spp.). The stage-structured, density-dependent model used an annual time step and 10 climatically diverse years of field data. Mesquite population demography is sensitive to source-sink dynamics as most seeds are consumed and redistributed spatially by livestock. In addition, individual mesquite plants, because they are long lived, experience natural climate variation that cycles over decadal scales, as well as anthropogenic climate change. The model therefore explicitly considered the effects of both net dispersal and climate variation. Herbivory strongly regulated mesquite populations through reduced growth and fertility, but additional mortality of older plants will be required to reach management goals within a reasonable time frame. Growth and survival of seeds and seedlings were correlated with daily soil moisture. As a result, population dynamics were sensitive to rainfall scenario, but population response times were typically slow (20-800 years to reach equilibrium or extinction) due to adult longevity. Equilibrium population densities were expected to remain 5% higher, and be more dynamic, if historical multi-decadal climate patterns persist, the effect being dampened by herbivory suppressing seed production irrespective of preceding rainfall. Dense infestations were unlikely to form under a drier climate, and required net dispersal under the current climate. Seed input wasn't required to form dense infestations under a wetter climate. Each factor we considered (ongoing herbivory, changing climate, and source-sink dynamics) has a strong bearing on how this invasive species should be managed, highlighting the need for considering both ecological context (in this case, source-sink dynamics) and the effect of climate variability at relevant temporal scales (daily, multi-decadal, and anthropogenic) when deriving management recommendations for long-lived species.

Mapping current and potential distribution of non-native Prosopis juliflora in the Afar region of Ethiopia.

We used correlative models with species occurrence points, Moderate Resolution Imaging Spectroradiometer (MODIS) vegetation indices, and topo-climatic predictors to map the current distribution and potential habitat of invasive Prosopis juliflora in Afar, Ethiopia. Time-series of MODIS Enhanced Vegetation Indices (EVI) and Normalized Difference Vegetation Indices (NDVI) with 250 m2 spatial resolution were selected as remote sensing predictors for mapping distributions, while WorldClim bioclimatic products and generated topographic variables from the Shuttle Radar Topography Mission product (SRTM) were used to predict potential infestations. We ran Maxent models using non-correlated variables and the 143 species- occurrence points. Maxent generated probability surfaces were converted into binary maps using the 10-percentile logistic threshold values. Performances of models were evaluated using area under the receiver-operating characteristic (ROC) curve (AUC). Our results indicate that the extent of P. juliflora invasion is approximately 3,605 km2 in the Afar region (AUC  = 0.94), while the potential habitat for future infestations is 5,024 km2 (AUC  = 0.95). Our analyses demonstrate that time-series of MODIS vegetation indices and species occurrence points can be used with Maxent modeling software to map the current distribution of P. juliflora, while topo-climatic variables are good predictors of potential habitat in Ethiopia. Our results can quantify current and future infestations, and inform management and policy decisions for containing P. juliflora. Our methods can also be replicated for managing invasive species in other East African countries.

Stress tolerance and ecophysiological ability of an invader and a native species in a seasonally dry tropical forest.

Ecophysiological traits of Prosopis juliflora (Sw.) DC. and a phylogenetically and ecologically similar native species, Anadenanthera colubrina (Vell.) Brenan, were studied to understand the invasive species' success in caatinga, a seasonally dry tropical forest ecosystem of the Brazilian Northeast. To determine if the invader exhibited a superior resource-capture or a resource-conservative strategy, we measured biophysical and biochemical parameters in both species during dry and wet months over the course of two years. The results show that P. juliflora benefits from a flexible strategy in which it frequently outperforms the native species in resource capture traits under favorable conditions (e.g., photosynthesis), while also showing better stress tolerance (e.g., antioxidant activity) and water-use efficiency in unfavorable conditions. In addition, across both seasons the invasive has the advantage over the native with higher chlorophyll/carotenoids and chlorophyll a/b ratios, percent N, and leaf protein. We conclude that Prosopis juliflora utilizes light, water and nutrients more efficiently than Anadenanthera colubrina, and suffers lower intensity oxidative stress in environments with reduced water availability and high light radiation.

Effects of copper sulfate on seedlings of Prosopis pubescens (screwbean mesquite).

Phytoextraction is an established method of removal of heavy metals from contaminated soils worldwide. Phytoextraction is most efficient if local plants are used in the contaminated site. We propose that Prosopis pubescens (Screw bean mesquite) would be a successful phytoextractor of copper in our local soils. In order to determine the feasibility of using Screw bean mesquite, we utilized inductively-coupled plasma-optical emission spectroscopy (ICP-OES) and elemental analysis to observe the uptake of copper and the effects on macro and micro nutrients within laboratory-grown seedlings. We have previously shown that P. pubescens is a hyperaccumulator of copper in soil-grown seedlings. Light and transmission electron microscopy demonstrated death of root cells and ultrastructural changes due to the presence of copper from 50 mg/L - 600 mg/L. Ultrastructural changes included plasmolysis, starch accumulation, increased vacuolation and swollen chloroplasts with disarranged thylakoid membranes in cotyledons. Inductively coupled plasma-optical emission spectroscopy analyses of macro- and micro-nutrients revealed that the presence of copper sulfate in the growth medium of Petri-dish grown Prosopis pubescens seedlings resulted in dramatic decreases of magnesium, potassium and phosphorus. At 500-600 mg/L of copper sulfate, a substantial increase of sulfur was present in roots.

Response of three semi-arid plant species to fluoride; consequences for chlorophyll florescence.

The study was done to investigate the ability of three semi-arid plant species viz. Acacia tortilis, Cassia fistula and Prosopis juliflora to adapt to fluoride (F) stress. Here we examined the changes in activities of chlorophyll a fluorescence and photosynthetic pigment concentration during early growth of these plants. One month old plants were treated with 10, 20, and 50 mg kg(-1) F in soilrite. We did not observe any major change in photosynthetic performance of these plants during early growth. This was revealed by ETR, ETRmax, PPFD-sat and deltaF/Fm'-sat values which were higher in these plants. The decrease in chl a, chl b and total chl concentrations were significant only at 5 days. For most of the parameters, C. fistula was found to be more sensitive to F stress and P. juliflora showed least damage from F. The lesser inhibition in the parameters reflected the F tolerant nature of these plants with respect to photosynthesis. This opens the possibility of potential use of these species for treatment of F contaminated soil and water.

Groundwater availability mediates the ecosystem effects of an invasion of Prosopis pallida.

Groundwater levels in arid environments are dropping worldwide due to human extraction, and precipitation events are predicted to become rarer and more intense in many arid areas with global climate change. These changes will likely alter both primary productivity and plant­soil nutrient cycles. To better understand the nature of such alterations, we examined effects of groundwater availability on plant­soil nitrogen (N) cycling in areas invaded by the N-fixing phreatophyte, Prosopis pallida, on the dry leeward coast of Hawai'i Island. Our aims were to quantify effects of groundwater availability to P. pallida on rates of litterfall N inputs and accretion in soils and to quantify effects of groundwater availability on N mineralization and leaching rates of inorganic N under natural rainfall conditions and simulated rain events. Stem water δ18O values indicate that P. pallida trees in lowland plots accessed shallow groundwater, while in upland plots they relied solely on rainfall. During drought periods, P. pallida at upland plots experienced water stress, evidenced by lower stem water potentials, higher water-use efficiency, and lower predawn photosynthetic performance than at lowland plots. Prosopis pallida basal area was 5.3 times greater at lowland plots, and these plots exhibited 17 times higher carbon (C), 24 times higher N, and 35 times higher phosphorus (P) additions via litterfall, indicating that productivity of this phreatophyte was decoupled from rainfall where groundwater was present. Total N mass in soils was 4.7 times greater where groundwater was accessible, supporting the case that groundwater access increased N2 fixation at a stand level. In contrast, N mineralization and leaching losses from soils, though substantially greater in lowland relative to upland areas, were strongly controlled by rainfall. Results provide clear examples of how invasive species with particular functional attributes (i.e., N-fixing phreatophytes) exploit otherwise inaccessible resources to dramatically alter the functioning of the systems they invade and how anthropogenic changes to hydrological processes can also alter ecosystem-level impacts of biological invasions. Results also illustrate a mechanism by which regional groundwater drawdown may reduce soil nutrient accretion and availability in arid regions.

Different sodium salts cause different solute accumulation in the halophyte Prosopis strombulifera.

The success of Prosopis strombulifera in growing under high NaCl concentrations involves a carefully controlled balance among different processes, including compartmentation of Cl(-) and Na(+) in leaf vacuoles, exclusion of Na(+) in roots, osmotic adjustment and low transpiration. In contrast, Na(2) SO(4) causes growth inhibition and toxicity. We propose that protection of the cytoplasm can be achieved through production of high endogenous levels of specific compatible solutes. To test our hypothesis, we examined endogenous levels of compatible solutes in roots and leaves of 29-, 40- and 48-day-old P. strombulifera plants grown in media containing various concentrations of NaCl, Na(2) SO(4) or in mixtures of both, with osmotic potentials of -1.0,-1.9 and -2.6 MPa, as correlated with changes in hydric parameters. At 24 h after the last pulse plants grown in high NaCl concentrations had higher relative water content and relatively higher osmotic potential than plants grown in Na(2) SO(4) (at 49 days). These plants also had increased synthesis of proline, pinitol and mannitol in the cytoplasm, accompanied by normal carbon metabolism. When the sulphate anion is present in the medium, the capacities for ion compartmentalisation and osmotic adjustment are reduced, resulting in water imbalance and symptoms of toxicity due to altered carbon metabolism, e.g. synthesis of sorbitol instead of mannitol, reduced sucrose production and protein content. This inhibition was partially mitigated when both anions were present together in the solution, demonstrating a detrimental effect of the sulphate ion on plant growth.

Prosopis laevigata and Mimosa biuncifera (Leguminosae), jointly influence plant diversity and soil fertility of a Mexican semiarid ecosystem.

Prosopis laevigata and Mimosa biuncifera are frequently found in arid and semiarid shrublands, but scarce information is available about their influence on plant community structure and soil fertility. We compared plant community structure, diversity and soil nutrients of three semiarid shrubland sites located in Mezquital Valley, Mexico. These sites differ in their dominant species: Site 1 (Bingu) P. laevigata, Site 2 (González) M. biuncifera, and Site 3 (Rincón) with the presence of both legumes. The results showed that the plant community with P. laevigata and M. biuncifera (Site 3) had more cover, taller plants and higher plant diversity than sites with only one legume (Site 1 and Site 2). Soil organic matter (SOM), soil organic carbon (SOC), total nitrogen (TN), phosphorus-Olsen (P) and C mineralization were higher in the soil under the canopy of both legumes than in bare soil. In contrast, soil cation concentrations were lower under the canopy of P. laevigata, but not for M. biuncifera. In addition, the density of arbuscular mycorrhizal fungi spores was higher within the soil under the canopy of M. biuncifera than in the soil under the canopy of P. laevigata. Thus, resource islands (RI) created by P. laevigata increased the amounts of SOC, TN and P when compared with the RI of M. biuncifera. This study provided evidences about the importance of species identity in order to expand the niche availability for the establishment of other plants, and highlights that P. laevigata and M. biuncifera jointly influencing plant colonization within semiarid ecosystems.