Ecological impact of Prosopis species invasion in Turkwel riverine forest,Kenya

The impact of Prosopis species invasion in the Turkwel riverine forest in Kenya was investigated under three contrasting: Acacia, Prosopis and Mixed species (Acacia and Prosopis) canopies. Variation amongst canopies was assessed through soil nutrients and physical properties, tree characteristics and canopy closure. Invasion impact was evaluated by comparing herbaceous species cover and diversity, and occurrence of indigenous tree seedlings. Soil characteristics under Prosopis and Mixed species canopies were similar except in pH and calcium content, and had lower silt and carbon contents than soil under Acacia canopy. Tree density was higher under Prosopis intermediate under Mixed and lower under Acacia canopies. Prosopis trees had lower diameters than Acacia tortilis trees. Diameter classes' distribution in Mixed species canopy revealed invasion of Prosopis into mature A. tortilis stands. Herbaceous species cover and diversity were negatively correlated to Prosopis tree density; thus explaining the lower herbaceous species cover and diversity under Prosopis than under Acacia and Mixed species canopies. The study suggests a gradual conversion of herbaceous rich A. tortilis woodland to herbaceous poor Prosopis species woodland or thickets, through indiscriminate Prosopis invasion.     

Spatiotemporal heterogeneity of soil fertility in the Central Monte desert (Argentina)

In arid environments, soil fertility exhibits a high degree of spatial and temporal heterogeneity, which results from high climatic variability seasonally and heterogeneous plant distribution. However, because most desert areas have been altered by human activities, heterogeneous fertility would originate from grazing or logging activities. We evaluated spatial and temporal heterogeneity of soil fertility in cattle-excluded sites under and outside woody plant cover (Prosopis flexuosa and Larrea divaricata), and in sites disturbed by tree removal during wet and dry season in Ñacuñán Biosphere Reserve (Central Monte desert of Argentina). Soil organic matter, fulvic acids, bioavailable organic matter, and nitrate were lower outside plant canopy (8.9 mg g^?1, 0.03 mg g^?1, 8.2 mg g^?1, and 4.17 mg kg^?1, respectively). Total N, humic acids, and abundance of microbial functional groups did not show differences among sites. Most parameters differed between seasons, tending to be higher in the wet season. Overall soils of Ñacuñán Reserve are characterized by: a) more homogenous spatial pattern than expected from woody plant presence; b) very heterogeneous temporal pattern; and c) after two years, tree removal does not seem to induce infertile soil formation.     

The effect of wood-boring beetles on the radial growth of Prosopis flexuosa DC. in the arid Chaco of Argentina

Traditionally, the effect of wood-boring insects has been related to mechanical damage, which in severe infestations results in breakage of branches and trunks. In contrast, few studies have evaluated the physiological effects of wood-borers on the radial growth of trees. Prosopis flexuosa is the main resource for rural inhabitants in the arid Chaco of Argentina and the cerambycid Torneutes pallidipennis is the principal cause of insect damage in these forests. The presence of annual growth rings in P. flexuosa allowed us to use dendrochronological methods to assess the effect of T. pallidipennis on radial tree growth. P. flexuosa with external infestation symptoms were sampled in Chancaní (Córdoba), central Argentina. Cores from “healthy” and “infested” branches were taken and processed following standard methods in dendrochronology. Generalized Linear Models were applied to compare the growth of healthy and affected tree-ring growth series. Our results showed a growth reduction in branches with cerambycid infestation. These data strongly suggest that the biological cycles of this heartwood-borer affect the growth and normal development of trees, in addition to “mechanical” effects previously proposed. Insects may produce weakening of the host plants and a pronounced reduction in radial tree growth in the most severe cases.     

Effects of seasonal heat on the activity rhythm,habitat use,and space use of the beira antelope in southern Djibouti

In hot, arid environments, non-burrowing mammals are at risk of overheating and dehydration, so human-induced reduction in thermal cover may constitute a threat for their survival. We studied the daytime activity rhythm, habitat use, and space use of the beira (Dorcatragus megalotis), a threatened antelope living in arid hills of the Horn of Africa, where tree cover is shrinking because of logging, and overgrazing by livestock. During the cool season (Nov.–Mar.; mean midday temperature: 28.5 °C), beira did not particularly seek shade, and alternated in the course of the day between short phases of activity and rest (median duration in Dec.–Feb.: 57.5 and 42.5 min, respectively). In contrast, during the hot season (May–Sep.; mean midday temperature: 39.1 °C), beira often foraged in a hill's shade, and midday resting phases were especially long (median duration in May–Jul.: 280 min) and spent in the shade of trees, or of rock shelters when available in the home range. Because of reduced diurnal movements when temperature was high, beira home ranges were smaller during the hot than during the cool season (mean ± SE: 0.25 ± 0.05 km² in May–Jul. vs 0.42 ± 0.10 km² in Dec.–Feb.). Whatever the season, beira mainly used areas supporting trees. The obtained results suggest that the decrease of tree cover in the areas inhabited by the beira constitutes a threat for the species survival.     

Use of large Acacia trees by the cavity dwelling Black-tailed Tree Rat in the Southern Kalahari

Recent extensive harvesting of large, often dead Acacia trees in arid savanna of southern Africa is cause for concern about the conservation status of the arid savanna and its animal community. We mapped vegetation and nests of the Black-tailed Tree Rat Thallomys nigricauda to assess the extent to which the rats depend on particular tree species and on the existence of dead, standing trees. The study was conducted in continuous Acacia woodland on the southern and eastern edge of the Kalahari, South Africa. Trees in which there were tree rat nests were compared with trees of similar size and vigour to identify the characteristics of nest sites. Spatial analysis of tree rat distribution was conducted using Ripley's-L function. We found that T. nigricauda was able to utilize all available tree species, as long as trees were large and old enough so that cavities were existing inside the stem. The spatial distribution of nest trees did not show clumping at the investigated scale, and we therefore reject the notion of the rats forming colonies when inhabiting continuous woodlands. The selection of a particular tree as a nest site was furthermore depending on the close proximity of the major food plant, Acacia mellifera. This may limit the choice of suitable nest sites, since A. mellifera was less likely to grow within a vegetation patch containing a large trees than in patches without large trees.     

Groundwater use by native plants in response to changes in precipitation in an intermountain basin

Many arid basins in western North America are likely to experience future changes in precipitation timing and amount. Where shallow water tables occur, plant acquisition of groundwater and soil water may be influenced by growing season precipitation. We conducted a rainfall manipulation experiment to investigate responses of four common native plant species to ambient, increased, and decreased summer monsoon rainfall. We measured plant xylem pressure potentials (Ψ) and stable oxygen isotope signatures (δ¹⁸O) to assess effects of altered precipitation on plant water relations and water acquisition patterns. Reduced rainfall decreased Ψ more in the grasses Sporobolus airoides and Distichlis spicata than the more deeply rooted shrubs Sarcobatus vermiculatus and Ericameria nauseosa. E. nauseosa had little response to natural or experimental differences in available soil water. Plant xylem water δ¹⁸O indicated that S. airoides and D. spicata are almost entirely dependent on rain-recharged soil water, while E. nauseosa is almost entirely groundwater-dependent. Sarcobatus vermiculatus used groundwater during dry periods, but utilized precipitation from soil layers after large rainfall events. Persistent changes in precipitation patterns could cause shifts in plant community composition that may alter basin-scale groundwater consumption by native plants, affecting water availability for human and ecosystem uses.     

Biomass production, evapotranspiration and water use efficiency of arid rangelands in the Northern Cape, South Africa

Annual above-ground net primary production (ANPP), evapotranspiration (ET) and water use efficiency (WUE) of rangeland have the potential to provide an objective basis for establishing pricing for ecosystem services. To provide estimates of ANPP, we surveyed the biomass, estimated ET and prepared a water use efficiency for dwarf shrublands and arid savanna in the Riemvasmaak Rural Area, Northern Cape, South Africa. The annual production fraction was surveyed in 33 MODIS 1 km² pixels and the results regressed against the MODIS f_PAR product. This regression model was used to predict the standing green biomass (kg DM ha⁻¹) for 2009 (dry year). Using an approach which combines potential evapotranspiration (ET₀) and the MODIS f_PAR product, we estimated actual evapotranspiration (ET_a). These two models (greening standing biomass and ET_a) were used to calculate the annual WUE for 2009. WUE was 1.6 kg DM mm⁻¹ ha⁻¹ yr⁻¹. This value may be used to provide an estimate of ANPP in the absence of direct measurements of biomass and to provide a comparison of the water use efficiency of this rangeland with other rangeland types.     

Host specificity, nutrient and water dynamics of the mistletoe Viscum rotundifolium and its potential host species in the Kalahari of South Africa

The mechanisms underpinning local host specificity in mistletoes remain elusive. We determined the degree of host specificity in the mistletoe Viscum rotundifolium at Pniel Estates, near Kimberley, South Africa. We found that V. rotundifolium parasitises only Ehretia rigida and Ziziphus mucronata at this site. Both commonly parasitised host species were not the most abundant trees, were not the tallest trees, and did not have the highest water or nutrient content of trees in the area, although these factors are good predictors for mistletoe parasitism. Mistletoe seeds deposited on branches of E. rigida and Z. mucronata have a greater chance of attachment and subsequent survival, compared with those seeds deposited on co-occurring Acacia and other potential host species. The mistletoes had higher water potential than their host trees and by doing so they can passively maintain the flow of nutrients. In addition, the mistletoes had a N:Ca ratio >1, indicating active uptake from host phloem. Thus, this mistletoe species uses both passive and active uptake, which may be a selective advantage in a nutrient-poor environment or on a nutrient-deficient host species.     

Wood biomass functions for Acacia abyssinica trees and shrubs and implications for provision of ecosystem services in a community managed exclosure in Tigray,Ethiopia

In the Ethiopian highlands, remarkable recovery of vegetation has been achieved using exclosures, protecting vegetation against livestock browsing and firewood harvesting. But these emerging forest resources require tools for sustainable use, implying knowledge on biomass stocks and growth. In this study we developed biomass functions estimating total, stem and branch biomass from diameter at stump height (DSH) and tree height (H) for an 11-year old exclosure in Tigray, Ethiopia. In a systematic grid of 55 plots, DSH and H of all trees and shrubs were recorded. 40 Acacia abyssinica trees were selected for destructive sampling. Allometric relationships using a natural log–log model were established between aboveground biomass, DSH and H. Models with only DSH were found best with R² between 0.95 and 0.98. The functions were 10 fold cross-validated and R²_{_}cv ranged from 0.94 to 0.97, indicating good model performance. The models were found well in range with those of other seasonal forests in East Africa. Total aboveground biomass was estimated 25.4 ton ha⁻¹ with an annual production of 2.3 ton ha⁻¹, allowing sustainable wood fuel use for 4 persons ha⁻¹. The presented predictive functions help to harmonize between ecological and societal objectives and are as such a first step towards an integrated planning tool for exclosures.     

Wood productivity of Prosopis flexuosa D.C. woodlands in the central Monte: Influence of population structure and tree-growth habit

A balance between forest production and protection is hard to achieve in arid zones due to their low potential for wood production. Prosopis flexuosa woodlands are the major woody formations in the Monte desert and are currently in a degraded state due to intense use. The main degradation factors in the study area are overgrazing and firewood extraction. We developed allometric models to estimate the aerial biomass of P. flexuosa, compared annual growth rates of one- and multi-stemmed individuals through dendrochronological methods, and estimated the productivity of four structurally different woodlands in the central Monte. Total dry weight was best estimated by power equations. Annual increments in basal area and dry weight were initially larger for multi- than one-stemmed individuals. However, whereas multi-stemmed individuals rapidly decreased their growth rates after 60 years of age, one-stemmed trees maintained steady growth rates during the first 100 years. Depending on woodland density and tree size, total woodland biomass varied between 4000 and 15 000 kg ha⁻¹. Wood productivity was similar in all four woodlands studied (121.6-173.7 kg ha⁻¹ year⁻¹). Our results reveal the importance of tree growth habit to productivity, and suggest that regulated extraction of firewood and poles from old multi-stemmed individuals could optimize wood productivity and contribute to the sustainable use and conservation of these woodlands.     

Hydrochemical Characteristics and Sodification of Groundwater in the Shirin Sou, Hamedan, Western Iran

Salinity and sodicity of groundwater are the principal water quality concerns in irrigated areas of arid and semi-arid regions. The hydrochemical characteristics and sodicity of groundwater in the Shirin Sou area, western Iran were investigated in this study by chemical analyses of groundwater samples from 49 wells. Chemical analysis of the groundwater showed that the mean concentration of the cations was in the order: Na⁺ > Ca²⁺ > Mg²⁺ > K⁺, while that for anions was SO₃ ²⁻ > Cl⁻ > HCO₃ ⁻ > NO₃ ⁻. The most prevalent water type is Na–SO₄ followed by water types Na–Cl and Ca–SO₄. The chemical evolution of groundwater is primarily controlled by water–rock interactions: mainly weathering of aluminosilicates, dissolution of sulfate minerals, and cation exchange reactions. Sulfate dissolution and pyrite weathering may both contribute to the SO₄ ²⁻ load of the groundwater. High Na⁺ concentrations in groundwater participate in ion-exchange processes, resulting in the displacement of base cations into solution and raised concentrations in groundwater. The principal component analysis (PCA) performed on groundwater identified three principal components controlling variability of groundwater chemistry. Electrical conductivity, Ca²⁺, Mg²⁺, Na⁺, SO₄ ²⁻, and Cl⁻ content were associated in the same component (PC1) (salinity), most likely linked to anthropogenic activities.     

Hydrological impacts of mesquite encroachment in the upper San Pedro watershed

Over the past century, mesquite trees (Prosopis spp.) have exhibited substantial increase in abundance throughout areas in the American Southwest that were once dominated by desert grassland. To assess hydrological consequences of mesquite encroachment, the Soil and Water Assessment Tool (SWAT) was applied to simulate progressive mesquite encroachments in the upper San Pedro watershed (U.S./Mexico). The simulated average annual basin evapotranspiration (ET) increases with mesquite encroachment, leading to the decrease of annual water yield and percolation by 9.8% and 9.7%, respectively. Substantial increase of ET (up to 19.19 mm) and decrease of percolation, and surface runoff (to −12.90 and −3.20 mm, respectively) were observed in the southeast, middle-west, and northern subwatersheds of the basin and the most significant decrease of surface runoff (around −35.8%) was simulated during the wet period. In addition, a non-linear hydrological response relative to mesquite encroachment was observed, i.e. hydrological processes changed markedly until a certain amount (approximately 40%) of grassland was removed, indicating that the strongest increase of ET occurred in the earliest stages of encroachment. Consequently, changes in vegetation physiognomy, such as mesquite encroachment, have broad implications for landuse management especially in regard to reliable water supplies in arid and semi-arid environments.     

Thiosulfate-related microbial communities from four arid soils in the Southwestern United States

Sulfur-deficiency has been observed in arid and semi-arid soils. To explore sulfur cycling in arid soils, four (4) arid soils collected from the Southwestern United States were tested for 12 chemical constituents and moisture content, and soil microbial communities were assayed for the ability to utilize a set of 31 organic substrates using EcoPlates™. Soils were cultured for the presence of sulfur-utilizing bacteria using thiosulfate as the sulfur source. The densities of thiosulfate-utilizing bacterial populations were determined using viable plate counts and differential media. Results showed that the soils contained high amounts of sulfate, calcium and other minerals. Active soil microbial communities utilized 24–31 organic substrates. The density of thiosulfate-utilizing bacterial populations ranged from 10⁵ colony forming units per gram (cfu g⁻¹) to greater than 10⁶ cfu g⁻¹. Regression analyses showed that there were no statistically significant correlations between the density of thiosulfate-reducing bacteria and other tested variables. However, the high density of sulfur-related bacteria and high organic substrate utilization by the microorganisms suggests that these organisms may be important in sulfur and carbon cycling in these soils.     

Range expansion and potential distribution of the invasive grass Bromus tectorum in southern South America on the base of herbarium records

We reconstructed the invasion history and modelled the potential distribution of the invasive grass Bromus tectorum in southern South America. On the base of herbarium data, we described two aspects of B. tectorum's range expansion over time: area of occupancy and extent of occurrence. A maximum entropy model was used to identify both climatic variables associated with B. tectorum's current distribution and potentially invasible areas. The area-of-occupancy curve showed a steady increase of the occupied area since the first collection in 1937, with no obvious asymptote. However, the extent-of-occurrence curve indicated that range expansion was not homogeneous through time, but faster between 1965 and 1980. Most invasible areas were arid and semiarid with markedly Mediterranean precipitation regime. Within this susceptible region, there were large areas containing only a few known records of B. tectorum. Our results indicate that B. tectorum has successfully expanded over much of southern South America. In addition, there seems to be room for further local invasion (i.e. an increase of its area of occupancy) over large susceptible areas within the invaded region. Overall, our results confirm the suitability of southern South America's arid environments to B. tectorum invasion, and stress the importance of long-distance dispersal in accelerating its expansion across the region.     

Characterization of Mechanisms and Processes Controlling Groundwater Recharge and its Quality in Drought-Prone Region of Central India (Buldhana,Maharashtra) Using Isotope Hydrochemical and End-Member Mixing Modeling

A thorough study on understanding of groundwater recharge sources and mechanisms was attempted by integrating the hydrogeological, geochemical and isotopic information along with groundwater dating and end-member mixing analysis (EMMA). This study was necessitated due to prolonged dryness and unavailability of freshwater in semi arid Deccan trap regions of Central India. In addition, groundwater resources are not characterized well in terms of their geochemical nature and recharge sources. The hydrogeochemical inferences suggest that aquifer I consists of recently recharged water dominated by Ca–Mg–HCO₃ facies, while groundwater in aquifer II shows water–rock interaction and ion exchange processes. Presence of agricultural contaminant, nitrate, in both aquifers infers limited hydraulic interconnection, which is supported by unconfined to semi-confined nature of aquifers. Groundwater in both aquifers is unsaturated with respect to carbonate and sulfate minerals indicating lesser water–rock interaction and shorter residence time. This inference is corroborated by tritium age of groundwater (aquifer I: 0.7–2 years old and aquifer II: 2–4.2 years old). Stable water isotopes (δ²H, δ¹⁸O) suggest that groundwater is a mixture of rainwater and evaporated water (surface water and irrigation return flow). EMMA analysis indicates three groundwater recharge sources with irrigation return flow being the dominant source compared to others (rainwater and surface waters). A conceptual model depicting groundwater chemistry, recharge and dynamics is prepared based on the inferences.

     

Modeling the optimal grazing regime of Acacia victoriae silvopasture in the Northern Negev,Israel

Silvopasture, the planting of suitable tree species in pastures, can improve the sustainability and productivity especially in dry rangelands. While recent studies provide information on the effects of different tree compositions on biological productivity, water and soil protection, additional parameters such as fodder production by the trees or the impact of silvopasture on the grazing potential themselves have rarely been addressed. We determined fodder production in Acacia victoriae woodland and savanna by measuring annual vegetation and tree biomass growth. We developed mathematical models for calculating the vegetative biomass available for grazing. In order to get accurate estimation for the grazing capacity, we differentiated between the grazing and the browsing fodder availability, and adapted the model to the most abundant grazers in the Negev, goats and sheep. Grazing capacity for sheep and goats was twice bigger in A. victoriae woodland than in the adjacent sustainably managed shrubland and four times higher than in degraded shrubland. The mathematical model presented in this paper can be applied in order to plan and realize high yielding and sustainable silvopasture in arid environments resistant to degradation and desertification while providing adequate fodder reserves during drought years.     

Assessing the distribution and impacts of Prosopis juliflora through participatory approaches

Invasive species pose global biological and economic challenges. Over the past four decades, Prosopis taxa have emerged as a major invader of the arid and semi-arid regions of the world. Prosopis juliflora, one of the highly invasive Prosopis species, is dominantly present in the Afar region of Ethiopia and continues to spread into the surrounding areas. The objective of this study was to aid the mapping, utilization, and management of the invasive P. juliflora in Afar, by employing participatory research techniques. We assessed the introduction history, impacts, uses, and control strategies of invasive P. juliflora by interviewing 108 pastoralists and agro-pastoralists. In addition, we used Participatory Mapping (PM), Geographic Information Systems (GIS), Global Positioning System (GPS), and remote sensing technologies and approaches to map sites invaded by P. juliflora. Sketch maps were produced by men, women, pastoralist and agro-pastoralist groups. Experts aggregated, scaled and reproduced the sketch maps with support from the Afar communities. We provided GPS and GIS trainings to selected community members who assisted us in collecting the GPS locations of more than 70 key features and landmarks. The sketch maps were digitized and geo-referenced by experts using the 70 GPS records as control points. Geo-referenced community maps were superimposed on pan-sharpened Landsat 8 satellite images and presented to the communities for verification. We overlaid the verified community maps on ancillary land-cover layers, and detected the land-cover classes that were most affected by P. juliflora invasion. Despite its uses as source of fire wood, charcoal, and animal fodder, the species has adverse impacts on native species and livestock resources. Afar communities mapped P. juliflora infestations, particularly those that occurred near their villages, using high and moderate density classes. The two highly invaded land-cover categories were dense grassland, and exposed sand & soils. Participants collaborated in creating the produced maps, suggesting that participatory research approaches are another tool for early detection of invasive species and guiding fine-scale management strategies.     

A simple method for estimating basin-scale groundwater discharge by vegetation in the basin and range province of Arizona using remote sensing information and geographic information systems

Groundwater is a vital water resource in the arid to semi-arid southwestern United States. Accurate accounting of inflows to and outflows from the groundwater system is necessary to effectively manage this shared resource, including the important outflow component of groundwater discharge by vegetation. A simple method for estimating basin-scale groundwater discharge by vegetation is presented that uses remote sensing data from satellites, geographic information systems (GIS) land cover and stream location information, and a regression equation developed within the Southern Arizona study area relating the Enhanced Vegetation Index from the MODIS sensors on the Terra satellite to measured evapotranspiration. Results computed for 16-day composited satellite passes over the study area during the 2000 through 2007 time period demonstrate a sinusoidal pattern of annual groundwater discharge by vegetation with median values ranging from around 0.3 mm per day in the cooler winter months to around 1.5 mm per day during summer. Maximum estimated annual volume of groundwater discharge by vegetation was between 1.4 and 1.9 billion m³ per year with an annual average of 1.6 billion m³. A simplified accounting of the contribution of precipitation to vegetation greenness was developed whereby monthly precipitation data were subtracted from computed vegetation discharge values, resulting in estimates of minimum groundwater discharge by vegetation. Basin-scale estimates of minimum and maximum groundwater discharge by vegetation produced by this simple method are useful bounding values for groundwater budgets and groundwater flow models, and the method may be applicable to other areas with similar vegetation types.     

Hydrogeochemistry of Groundwater and Its Suitability for Drinking and Agricultural Use in Nahavand,Western Iran

Groundwater is the major source of drinking water in Nahavand city. However, the groundwater quality at the agricultural areas has been deteriorating in recent years. Ground water quality monitoring is a tool which provides important information for water management and sustainable development of the water resources in Nahavand. Hydrochemical investigations were carried out in an agricultural area in Nahavand, western Iran, to assess chemical composition of groundwater. In this study, 64 representative groundwater samples were collected from different irrigation wells and analyzed for pH, electrical conductivity, major ions, and nitrate. The results of the chemical analysis of the groundwater showed that concentrations of ions vary widely and the most prevalent water type is Ca–Mg–HCO₃, followed by other water types: Ca–HCO₃, Ca–Na–HCO₃, and Na–Cl, which is in relation with their interactions with the geological formations of the basin, dissolution of feldspars and chloride and bicarbonate minerals, and anthropogenic activities. Thirty-seven percent of the water samples showed nitrate (NO₃ ⁻) concentrations above the human affected value (13 mg L⁻¹). The phosphorous (P) concentration in groundwater was between 0.11 and 0.90 mg L⁻¹, with an average value of 0.30 mg L⁻¹, with all of the samples over 0.05 mg L⁻¹. The most dominant class C2-S1 (76.5%) was found in the studied area, indicating that sodicity is very low and salinity is medium, and that these waters are suitable for irrigation in almost all soils. Agronomic practices, such as cultivation, cropping, and irrigation water management may decrease the average NO₃ ⁻ concentration in water draining from the soil zone.     

Invasion by an exotic,perennial grass alters responses of a native woody species in an arid system

Grass growth has been encouraged in arid communities globally to improve grazing. Often exotic species are planted, subsequently spreading beyond their plantings to drastically alter cover and negatively impact native species. We evaluated the effects of the invasive, perennial grass Cenchrus ciliaris on the native woody species Cercidium microphyllum over two growing seasons: 2002 and 2003. To determine if C. ciliaris alters seasonal responses of C. microphyllum, we measured predawn water potential, photosynthesis, and branch sacrifice on C. microphyllum growing with either native cover or C. ciliaris. In 2003 a removal treatment of C. ciliaris was applied to assess responses in C. microphyllum during the summer. In both years, C. microphyllum water potentials closely tracked seasonal rainfall, suggesting dependence on shallow soil water. In 2002, we observed few negative impacts when C. microphyllum grew with C. ciliaris. However, the trees growing with C. ciliaris exhibited greater branch sacrifice, a typical response to drought, which likely complicated interpretation of the treatment effects. Removal of C. ciliaris resulted in increased water potentials and photosynthesis. This study suggests that exotic grasses, such as C. ciliaris, can reduce both water potential and photosynthetic rates in mature woody shrubs, like C. microphyllum, in aridland systems.