Leaf-level physiological responses of Tamarix ramosissima to increasing salinity

Over the past century, the invasive halophytic shrub Tamarix ramosissima Ledeb. has increased in abundance and distribution in riparian ecosystems of western North America. These increases coincide with anthropogenic modification of river systems, which decrease the rate of periodic overbank flooding, leading to an increase in soil salinity. Increased soil salinity negatively impacts the physiology of native riparian tree species, but the impact of increased soil salinity on T. ramosissima physiology is incompletely known. To measure the impacts of soil salinity on T. ramosissima, we measured leaf-level responses across a broad range of surface-soil salinity concentrations at two sites in western Kansas. Photosynthesis at 2000 μmol m⁻² s⁻¹ (A₂₀₀₀), stomatal conductance to water (g_s), intercellular CO₂ concentration (C_i), and leaf δ¹³C showed little change over surface-soil salinities from 0.5 to 17.65 mmhos/cm. The small variation in leaf-level physiological responses suggests robust functioning of T. ramosissima across a broad range of surface-soil salinities. Leaf-level physiology and δ¹³C responses were assessed by canopy position, but responses were not significantly different. These results are among the first to show broad acclimation and robust physiological functioning for many leaf-level processes measured on mature trees grown across a wide surface-soil salinity gradient in the field.     

Vertical distribution of fine roots of Tamarix ramosissima in an arid region of southern Nevada

To increase our understanding of soil water and nitrogen use strategies of invasive Tamarix ramosissima during dry seasons, the vertical distributions of fine roots and their associations with soil properties were examined in the Virgin River floodplain, southern Nevada, United States. We measured morphological traits of fine roots, such as fine-root mass density, fine-root length density, specific root length and specific root area at 10 cm increments to a depth of 2 m. Soil properties were analyzed at 20 cm increments. More than 60% of fine root length and biomass was concentrated at depths between 20 and 60 cm. Soil nitrogen (N) concentration had strong and positive relationships with fine-root mass and length densities, suggesting that the fine-root distribution may be influenced by soil N availability. A weak but positive relationship was observed between soil moisture and fine-root mass density. Soil salinity had no relationship with root morphological traits. These findings suggest that T. ramosissima fine roots may contribute to N uptake from the upper soil layers during dry seasons. This might be an important advantage over native riparian tree species in arid riparian areas of the southwestern United States.     

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.     

Short-term and diurnal patterns of salt secretion by Tamarix ramosissima and their relationships with climatic factors

Tamarisk (Tamarix spp.) is an invasive shrub or tree species in the United States. Glands within Tamarix leaves secrete salt that may increase surface soil salinity and consequently result in an unviable ecosystem for native riparian tree species. To increase our understanding of the effects of invasive Tamarix on the ecosystem and its native riparian vegetation, we investigated salt secretion of a stand of Tamarix ramosissima in the Las Vegas Wash, Nevada. Short-term and diurnal salt secretion rates were monitored in the stand from September to October 2009. Soil samples were collected beneath the canopy so that water and salinity conditions could be determined. We used weather data from a Las Vegas database in conjunction with soil temperature data collected for this study. The major cation secreted by salt glands was sodium. Diurnal variation in the salt secretion rate had a maximum during the day, suggesting that salt secretion is dependent on salt transportation to leaves. Variations in the salt secretion rates were mostly influenced by solar radiation, suggesting that salt secretion in T. ramosissima would be enhanced by light in natural conditions.     

Capacity for water conservation in invasive (Gerbillus nigeriae) and declining rodents (Taterillus pygargus and Taterillus gracilis) that exhibit climate-induced distribution changes in Senegal

Following the southward shift of rainfall isolines in the Sahel at the end of the 1960s, Gerbillus nigeriae appeared in northern Senegal in the mid-1990s, and two resident Gerbillidae (Taterillus pygargus and Taterillus gracilis) subsequently declined. We investigated the causal role of the capacity to conserve water in such climate-related shifts in the distribution of these Gerbillidae by comparing the effects of a water-poor diet on the water-efflux rate (W_−out) of freshly trapped adults pre-acclimatized to a water-rich diet. During the 12-day period of water restriction in all species, 30-50% of individuals became hyperactive and showed greater weight loss and higher W_−out than the remaining inactive individuals. Such emergence of migratory strategists within populations could accelerate the expansion of G. nigeriae. On a water-poor diet, T. gracilis showed a lower capacity to conserve water (higher W_−out) than T. pygargus and G. nigeriae, in both inactive (W_−out = 44.5 ± 1.8 vs 29.6 ± 0.8 vs 27.4 ± 0.7 ml kg^−0.82.day⁻¹, respectively) and hyperactive individuals (W_−out = 60.4 ± 1 vs 45.4 ± 0.7 vs 44 ± 0.8 ml kg^−0.82.day⁻¹, respectively). We propose that the capacity to conserve water accounted for both expansion of G. nigeriae and decline of T. gracilis, whereas competition between T. pygargus and G. nigeriae could account for the decline of T. pygargus.     

Seed removal patterns in burned and unburned desert habitats: Implications for ecological restoration

In desert ecosystems, selective foraging by seed consumers affects distribution and composition of soil seed banks, influencing plant population dynamics. However, the roles of consumers in burned habitats where direct seeding is used during ecological restoration to replenish depleted seed banks have not been well established. We evaluated seed removal for nine seed species over 12 months in burned and unburned shrublands in the Mojave Desert, USA. Percentage of total seed removed was highest during spring (16% of offered seed) and summer (21%). Rodents removed much of the large-seeded Coleogyne ramosissima in both burned and unburned habitats, while seed removal of this species by ants was low in burned and moderate in unburned habitats. Ants removed the greatest amount of small-seeded species (Penstemon bicolor, Encelia farinosa, and Sphaeralcea ambigua) in unburned habitat, indicating that ants can exploit different seed masses. Seed removal imposes limitations on seed availability, particularly for large-seeded species, as both rodents and ants selected seeds of C. ramosissima. Successful restoration seeding projects in arid lands may require protecting seed from granivore pressure, and seed species selection and season of seeding warrant consideration to reduce seed loss.     

Xylem anatomy and hydraulic conductivity of three co-occurring desert phreatophytes

The tree species Populus euphratica, the shrub Tamarix ramosissima and the sub-shrub Alhagi sparsifolia are phreatophytes that grow along the southern fringe of the Taklamakan Desert (NW China). We hypothesised that in the shoot xylem of these species, the fraction of conduits with a large cross-sectional area and the hydraulic conductivity decrease in the sequence A. sparsifolia > T. ramosissima > P. euphratica according to the different ground water distances at the typical sites of the species’ occurrence. The theoretical hydraulic conductivity related to the total cross-sectional xylem area (k_ttot) or to the cross-sectional area of the conduits (k_tcond) was computed using a modified Hagen-Poiseuille equation. The percentage of wide conduits and k_tcond were highest in A. sparsifolia, and smallest, in P. euphratica. With regard to k_ttot, the diffuse-porous to semi-ring-porous P. euphratica occupied an intermediate position due to an increased percentage of conducting area related to the total cross-sectional xylem area. This figure as well as k_ttot were lowest in the semi-ring-porous to ring-porous T. ramosissima. The k_ttot values corresponded to the leaf area-related hydraulic conductivity calculated from measurements in the field. Altogether, our results match the typical occurrence of the species with respect to the ground water depth.     

Mechanical shaking and soil water affect the growth of Psammochloa villosa in the Mu Us Sandland

The Mu Us Sandland is basically characterized by water shortage and high wind. Thus, wind-induced mechanical perturbation (MP) and soil water availability are likely to interact to affect plant growth. Since high water availability and MP can induce responses that are in the opposite direction, we hypothesized that MP effects on perennial grasses might be mitigated by increased soil water availability in the Mu Us Sandland. We counducted an experiment in which seedlings of Psammochloa villosa were subjected to two levels of MP (non-MP vs. MP 1 min d⁻¹) and two levels of water availability (200 ml d⁻¹vs. 400 ml d⁻¹) and measured three plant traits. MP significantly decreased plant height, total biomass, and root/shoot ratio. There were significant interactions between MP and soil water availability on plant height and root/shoot ratio. These findings imply that MP alone is a stressful factor for P. villosa and MP effects on its growth can be partially mitigated by increased soil water availability, and also suggest that P. villosa may respond to MP in a way that allows plants to survive in the windy semiarid environments.     

Role of competition in restoring resource poor arid systems dominated by invasive grasses

Understanding the role competition intensity and importance play in directing vegetation dynamics is central to developing restoration strategies, especially in resource poor environments. We hypothesized 1) competition would be intense among invasive and native species, but 2) competition would be unimportant in explaining variation in target plant biomass and survivorship relative to other factors driving these variables. We performed a two year addition series field experiment to quantify competition intensity and importance. Densities of two invasive (cheatgrass and medusahead) and two native (Sandberg’s bluegrass and bluebunch wheatgrass) species were arranged in monocultures and mixtures of two, three and four species, producing varying total densities and species proportions. Multiple linear regression models predicting individual plant biomass and survivorship were developed. Based on biomass, competition intensity coefficients ranged from −0.38 to 0.63 with R² < 0.06. All survivorship data produced poor fitting regression models (R² < 0.05). Our results suggest neither competition intensity nor importance influenced plant dominance in resource poor environments during the two years of establishment. Land managers may be more successful at restoration of resource poor ecosystems by overcoming abiotic barriers to plant establishment rather than focusing on plant-plant interactions.     

Arbuscular mycorrhizal fungi improve reestablishment of Leymus chinensis in bare saline-alkaline soil: Implication on vegetation restoration of extremely degraded land

Salinization and alkalinization are increasing problems in the world. Some land has been degraded to bare saline-alkaline soil where vegetation restoration is difficult because high toxic ionic content and pH are harmful to the survival of introduced plants. We grew Leymus chinensis with and without arbuscular mycorrhizal fungi (Glomus mosseae and G. geosporum) in either pots filled with soil from bare saline-alkaline land, or transplanted seedlings into field plots, to determine the influence of AM fungi on the reestablishment of this dominant grass species in bare degraded land. Association with AM fungi increased the absorption of N, P, K⁺, Ca²⁺, but decreased Mg²⁺, Na⁺ and Cl⁻ uptake under saline-alkaline stress. Therefore, higher K/Na, Ca/Na, P/Na, and P/Cl ratios were found in the inoculated plants. Plants inoculated with AM fungi accumulated significantly higher biomass, root/shoot ratio and tiller number than non-inoculated plants. AM fungi also significantly increased the survival of seedlings when they were transplanted into a bare saline-alkaline land in the field. The improvement of survival, growth and asexual reproduction of inoculated plants indicated that the plant-AM fungi mutualism could improve the reestablishment of vegetation in bare saline-alkaline soil, drive the vegetation restoration to a community dominated by original species.     

Restoring habitat for riparian birds in the lower Colorado River watershed: An example from the Las Vegas Wash, Nevada

The success of riparian restoration projects in the arid southwestern U.S. is often measured in terms of vegetation characteristics such as growth, cover, and structure. Among low-elevation riverine environments within the Colorado River watershed, restoration is typically conducted to improve degraded habitats for birds of conservation concern by replacing the exotic tamarisk (Tamarix ramosissima) with native cottonwoods (Populus spp.) and willows (Salix spp.). The working assumption for many restoration practitioners is that replacing exotic plants with native plants will improve habitat quality and will, therefore, benefit birds. Based on data collected at exotic and restored (i.e., native) sites along the Las Vegas Wash, Nevada, not all birds benefit from restoration. Broad measures of community benefit, including benefits to birds of conservation concern and riparian obligate/dependent birds, were not detected. There were, however, some species-specific benefits. Some environmental variables that were associated with exotic and native sites were significant in explaining the composition of the bird community. For example, the richness of forbs and grass-like plants (a proxy of soil moisture), invertebrate mass, and percent shade (a proxy for canopy characteristics) were important. Considering our results and depending on restoration goals, tamarisk replacement projects may not inherently provide benefits to birds.     

Effects of fire and environmental variables on plant structure and composition in grazed salt desert shrublands of the Great Basin (USA)

Fire in shrub-dominated portions of the Great Basin, largely fueled by non-native annuals such as Bromus tectorum, has become an important structuring force altering vegetation composition and soil characteristics. The extent to which fire affects native species in drier portions of the Great Basin, termed salt desert, is poorly documented. We conducted a survey of grazed salt desert habitat in northwestern Nevada 5 years after wildfires burned 650,000 ha, with the goal of investigating community response to fire and factors correlating with post-fire recovery. We found that recruitment of a dominant shrub, Artemisia spinescens, is severely restricted following fire: it occurred in only 2 of the 24 burned sites. The co-dominant shrub, Atriplex confertifolia, occurred in most burned sites although on average its percent cover was one-third lower than adjacent unburned sites. Biotic soil crust cover was four times lower, and non-native species cover 5 times higher, in burned sites compared to unburned. Ordination analyses confirmed differences among plant communities in burned versus unburned sites, with environmental variables soil conductivity, plant litter, soil potassium (K⁺) and pH explaining 38% of the variance in community composition. However, we found no environmental predictors of recovery for native species in burned sites. Future recruitment is likely to be further limited, as fire frequency in the salt desert is expected to increase with invasion by non-native annual grasses and with global climate change.     

Comparative study of seed germination and growth of Kochia prostrata and Kochia scoparia (Chenopodiaceae) under salinity

Soils of arid regions of Central Asia contain salts of different types that may differentially affect seed germination and plant development. We studied effect of NaCl, Na₂SO₄, 2NaCl + KCl + CaCl₂ and 2Na₂SO₄+K₂SO₄+MgSO₄ on germination of Kochia prostrata and Kochia scoparia seeds under a range of concentrations from 0.5 to 5% and at two constant temperature regimes +22 °C and +6 °C. The observed salt tolerance limit of germination at constant temperature +22 °C for both species was 5-6%, while at low temperature (+6 °C) this limit was 2%. The salt tolerance of young plants (before flowering) was 3% for NaCl. Low concentrations of sulfuric and mixed salts had a stimulating effect on seed germination in K. prostrata. Despite similarity of salt-tolerance limits the studied species showed a significant difference in seed recovery ability, i.e. the ability of ungerminated, salt-soaked seeds to germinate after transfer to fresh water. K. scoparia demonstrated a full germination recovery after seed transfer to distilled water while K. prostrata showed only a partial recovery.     

Efficient fog harvesting by Stipagrostis sabulicola (Namib dune bushman grass)

Stipagrostis sabulicola, an endemic grass species of the central Namib Desert, grows on dune fields under conditions of very low annual precipitation punctuated by regular nocturnal fog events. The objective of this study is to determine to what extent S. sabulicola relies on water supply by fog harvesting. The following parameters were monitored: 1) climate, 2) stem runoff, 3) leaf water potential (LWP) and 4) soil water content (SWC). Collected fog water was 5.0 L (liter) per m⁻² leaf surface and therefore a total harvest of 4-5 L per fog event for a medium-sized mound of S. sabulicola. SWC close to a mound increased substantially during a fog event, with SWC at about 2.2% within a mound. LWP of S. sabulicola ranged between −1.7 MPa and −3.5 MPa. On days without fog, LWP was highest during the morning and decreased during the afternoon. No significant decrease of LWP occurred during days following a fog event. The increase of SWC at the plant base during a fog event indicates that fog harvest of S. sabulicola occurs mainly via stem flow with subsequent absorption by the root system and that fog catchment therefore represents a substantial water source for this species.     

Reducing topsoil salinity and raising carbon stocks through afforestation in Khorezm, Uzbekistan

Agricultural mismanagement of irrigated drylands results in severe soil degradation. Afforestation is an option for ameliorating such degraded land. We evaluated the impact afforestation has on the topsoil (0-20 cm) of salinized degraded cropland in regards to salinity, aggregate stability, and soil organic carbon (SOC) stocks in Uzbekistan, Central Asia. The effects of tree plantations established under either furrow or drip irrigation were studied four years following afforestation and two years after irrigation ceased. For comparative study we also sampled fallow land, land with 80 years of tree growth, natural forest, desert ecosystems, and paddy rice fields. Initial furrow irrigation showed to be most effective in improving soil fertility after four years of afforestation; the respective plantations of Populus euphratica and Ulmus pumila showed significant levels of reduced soil salinity and increased aggregate stability and improved SOC stocks. The comparison of the long-term afforested land with the short-term equivalent suggested a C sequestration rate of 0.09-0.15 t C ha⁻¹ year⁻¹. The SOC stocks of the long-term afforestation site exceeded those of the native forest. Hence, a rehabilitation of salt-affected cropland is feasible following the conversion into occasionally irrigated tree plantations, although it takes decades to reach steady-state conditions.     

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.     

Seed germination responses of Periploca sepium Bunge, a dominant shrub in the Loess hilly regions of China

Experiments were conducted to determine the effects of temperature, light, osmotic stress, hydration-dehydration regime and stress relief on the seed germination of Periploca sepium Bunge (Chinese silk vine Asclepiadaceae), which is a native shrub in the Loess Plateau of Northwest China. Freshly harvested seeds germinated equally well in both light and darkness. Seeds germinated under all of tested temperature regimes, and much faster when temperature was maintained at 30, 35, and 30/20, 30/25, 35/25, 35/30 °C range, with more than 90% of seeds germinating within 2 days. Moderate osmotic stress did not inhibit germination at any tested temperature. At 30 °C, seed germination was not affected significantly above −0.8 MPa. The hydration-dehydration pretreatment showed none side effects on P. sepium seeds and the seeds germinated more rapidly as soon as optimal water condition was restored. These results show the advantages of P. sepium, which is a competitive and widespread shrub in dry areas, and provide useful information for vegetation restoration in these drought-prone regions.     

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.     

Effect of cattle breeding on habitat use of Pampas deer Ozotoceros bezoarticus celer in semiarid grasslands of San Luis, Argentina

The largest population of the Argentinean Pampas deer Ozotoceros bezoarticus celer is found in the semiarid grasslands of the San Luis province. Despite relatively high deer numbers in the region, there has been concern that expansion of farming practices could displace the species. Since the 1990’s, cattle breeding has intensified especially as a result of the replacement of natural grassland with South African digit grass (Digitaria eriantha) and African lovegrass (Eragrostis curvula). In this study, we studied how deer in “El Centenario” ranch used available habitat types, especially to assess whether it preferentially used natural over exotic pastures. We employed 8 fixed line transects to record deer numbers and habitat use during the dry season, early rainy and late rainy seasons. We estimated a mean population size of 731 ± 121 individuals, a density of 1.95 ± 0.25 deer/km². Our results also showed that deer did not appear to select natural pastures over exotic ones, though D. eriantha grasslands with cattle were used less during the late rainy season. Grazed pastures with D. eriantha without cattle were used significantly more during all time periods. Our results therefore suggest that Pampas deer are not shifted by exotic pastures but we caution that it is important to manage these habitats sustainably (e.g. cattle load adjusted to grassland nutritional supply, rotational crops with parcel rest period), to ensure the conservation of the species within agricultural areas.     

Stem radial growth indicate the options of species,topography and stand management for artificial forests in the western Loess Plateau,China

An understanding of the differences in artificial forest between tree species,slope aspects,and management options in arid environments is fundamentally important for efficient management of these artificial systems;however,few studies have quantified the spatial and temporal differences in stem radial growth of trees in the arid western Loess Plateau of China.Using dendrochronology,we assessed the growth of three woody species(the native shrub Reaumuria soongorica,the exotic shrub Tamarix ramosissima and tree Platycladus orientalis)by measuring the annual stem radial increment.We also describe the long-term growth trends and responses to climatic factors on slopes with different aspects during periods with and without irrigation.We found that precipitation during the main growing season was significantly positively correlated with ring growth for all three species and both slope aspects.In addition,supplemental water(e.g.,irrigation,rainwater harvesting)greatly relieved drought stress and promoted radial growth.Our results suggest that as the main afforestation species in the Loess Plateau used for soil and water conservation,P.orientalis is more suitable than T.ramosissima under rain-fed conditions.However,a landscape that combined a tree(P.orientalis)with a shrub(R.soongorica)and grassland appears likely to represent the best means of ecological restoration in the arid western Loess Plateau.