Distribution of axis-splitting in Mojave Desert shrub species along an elevational gradient

Although highly branched from the base, all shrubs have short main axes linking canopies to root systems. Main axes become increasingly segmented into independent canopy/stem/root segments as aridity increases across continents. The resulting hydraulic modularity has been proposed as an adaption to low soil moisture that prevents runaway embolism and minimizes risk of hydraulic failure. Here we test the hypotheses that (1) at a regional scale, the importance of axis-splitting species in communities declines with increasing elevation, as a proxy for precipitation, and (2) that this decline is explained by lower occurrence of low-elevation dominant species. We evaluated all species for axis splitting and determined importance values in plots along an elevational transect in the Mojave Desert. As predicted, as elevation increased, the total importance of axis-splitting species declined from 100% at low-elevation sites to 75% at the highest elevation site. However, this decline was not due solely to the decline of the lower elevation dominant species. At the high elevation site, the influx of new species resulted in a six-fold increase in species richness and almost all of the new high elevation woody eudicotyledonous species exhibited axis splitting; non-splitting species were represented by other growth forms.     

Soil-dependent canopy die-back and plant mortality in two Mojave Desert shrubs

Recent drought has led to unprecedented levels of plant mortality across the Southwestern US. An unaddressed feature of this drought's impact is how soil characteristics and soil hydrological behavior affect desert plant canopy die-back and mortality. Here, we present a multi-year study in the Mojave Desert assessing canopy die-back and whole-plant mortality of white bursage (Ambrosia dumosa) and creosotebush (Larrea tridentata) in soils varying in surface and sub-surface horizon development, and topographic (hillslope vs. channel) positions. Canopy die-back and mortality was more widespread A. dumosa than in L. tridentata, and dead plants tended to be smaller than surviving plants, especially in channel and hillslope locations. This suggests that juveniles were particularly vulnerable where plants depended heavily on augmentation of incident precipitation by runoff. Canopy die-back was greater in young, weakly developed soils that fostered extensive plant growth, while plants growing in older, well-developed soils showed markedly lower branch and plant mortality, especially in A. dumosa. We attributed these differences in plant response in part to variation in distributions of large rocks within soil profiles, which might affect soil hydrological heterogeneity and intensity of plant competition for water.     

Horizontal and vertical zones of influence for root systems of four Mojave Desert shrubs

Horizontal and vertical zones of influence for root systems of four Mojave Desert shrubs were characterized using ³²P as a nutrient tracer. Larrea tridentata's horizontal zone of influence was sparse near the plant's stem base, with a maximum probability of accessing ³²P (P_max) of 41%. However, its horizontal zone of influence extended beyond 5 m, and the distance from the stem base at which the probability of accessing ³²P was half P_max (L₅₀3 m) was significantly greater than the other three shrubs. Ambrosia dumosa's zone of influence was dense near the plant's stem base (P_max78%), but was rare at distances >2 m (L₅₀1 m). Zones of influence for Lycium andersonii and Lycium pallidum were intermediate between those of L. tridentata and A. dumosa. For vertical zones of influence, L. tridentata was more likely to obtain ³²P from 5 m soil depths than A. dumosa, but L. pallidum was not significantly different from either A. dumosa or L. tridentata. Horizontal zones of influence did not change with treatments that altered soil water and nitrogen availability, but vertical zones of influence increased with a flood irrigation treatment that increased water availability to 5 m soil depth. These differences among species likely reflect compromises between their shoot growth strategies and their need to acquire spatially and temporally limited soil resources, especially through competitive interactions.     

Effect of plant species on shrub fertile island at an oasis–desert ecotone in the South Junggar Basin, China

Shrub fertile islands are a common feature in arid ecosystems. To examine the effect of plant species on the spatial patterns of soil chemical and physical properties surrounding individual shrubs, two deciduous shrub species with different morphologies (Tamarix spp. and Haloxylon ammodendron Bge.) were studied at an oasis–desert ecotone in South Junggar Basin. Soil samples were collected under the shrub crown (canopy), at the vertically projected limit of shrub crown margin (periphery), and in the space between shrub crowns (interspace) at two depths, 0–10 and 10–20 cm, to analyze their physical and chemical properties. The results show that the fertile islands of Tamarix spp. are enriched with more soil nutrients (significantly higher, P<0.05; soil organic matter (SOM); total nitrogen (TN) and available nitrogen (AN); to a deeper depth (>20 cm) and in a larger area (beyond the canopied area) compared to that of H. ammodendron (significantly higher, P<0.05, soil nutrients detected only for AN; <20 cm in depth; smaller than the canopied area). Soil texture patterns surrounding the shrubs of the two species are even more different, with more coarse particles under the Tamarix spp. canopies compared to the interspace between shrubs but fewer under the H. ammodendron canopies compared to the interspaces. These variations are attributed to the difference in morphology of the two studied species: the Y-shaped crowns of H. ammodendron are less capable of capturing and maintaining litter under them than the hemispheroidal crowns of Tamarix spp., which leads to the less well developed fertile islands surrounding H. ammodendron shrubs.     

Plant water relations of thornscrub shrub species, north-eastern Mexico

As an approach to understand how diurnal and seasonal plant water potentials (Ψ) are related to soil water-content and evaporative demand components, the responses of six thornscrub shrubs species (Havardia pallens, Acacia rigidula, Eysenhardtia texana, Diospyros texana, Randia rhagocarpa, and Bernardia myricaefolia) of the north-eastern region of Mexico to drought stress were investigated during the course of 1 year. All study species showed the typical diurnal pattern of variation in Ψ. That is, Ψ decreased gradually from predawn (Ψ_pd) maximal values to reach minima at midday (Ψ_md) and began to recover in the late afternoon. On a diurnal basis and with adequate soil water-content (>0.20 kg kg⁻¹), diurnal Ψ values differed among shrub species and were negatively and significantly (p<0.001) correlated with air temperature (r=−0.741 to −0.883) and vapor pressure deficit (r=−0.750 to −0.817); in contrast, a positive and significant (p<0.001) relationship was found to exist with relative humidity (r=0.758–0.842). On a seasonal basis, during the wettest period (soil water-content>0.20 kg kg⁻¹), higher Ψ_pd (−0.10 MPa) and Ψ_md (−1.16 MPa) values were observed in R. rhagocarpa, whereas lower figures (−0.26 and −2.73 MPa, respectively) were detected in A. rigidula. On the other hand, during the driest period (soil water-content<0.15 kg kg⁻¹), Ψ_pd and Ψ_md values were below −7.3 MPa; i.e. when shrubs species faced severe water deficit. Soil water-content at different soil layers, monthly mean relative humidity and monthly precipitation were significantly correlated with both Ψ_pd (r=0.538–0.953; p<0.01) and Ψ_md (r=0.431–0.906; p<0.05). Average soil water-content in the 0–50 cm soil depth profile explained between 70% and 87% of the variation in Ψ_pd. Results have shown that when gravimetric soil water-content values were above 0.15 kg kg⁻¹, Ψ_pd values were high and constant; below this threshold value, Ψ declined gradually. Among all shrub species, A. rigidula appeared to be the most drought tolerant of the six species since during dry periods it tends to sustain significantly higher Ψ_pd in relation to B. myricaefolia. The remaining species showed an intermediate pattern. It is concluded that the ability of shrub species to cope with drought stress depends on the pattern of water uptake and the extent to control water loss through the transpirational flux.     

高光谱指数法用于确定多枝柽柳(Tamarix ramosissima)蒸腾速率

蒸腾速率(Tr)是植物生理生态学研究中表征蒸腾耗水的常用指标,研究植物的蒸腾耗水有助于了解当地生态系统稳定性和水资源的可持续利用,但在遥感应用尤其在干旱区遥感应用中很少被使用。本文以古尔班通古特沙漠南缘的主要建群种多枝柽柳(Tamarix ramosissima)作为研究对象,应用高光谱指数法对其Tr日变化过程进行研究,寻找和确定最佳的Tr光谱指数。选择的6个光谱指数判定系数R2介于0.06~0.73,其中简单比值(SR)光谱指数有最高的判定系数(R2=0.73)、较低的均方根误差(RMSE=0.24)和较为简单的形式,光谱范围处于近红外波段(1 645~1 655nm)/(1 775~1 785nm)。SR作为Tr最佳光谱指数,对植被水分关系变化敏感,能够较好地记录和监测Tr日变化过程,有益于揭示光谱指数物理和生理机制。     

The replacement of arborescent cactus species along a climatic gradient in the southern Chihuahuan Desert: competitive hierarchies and response to freezing temperatures

In the southern region of the southern Chihuahuan Desert three common species of arborescent cacti are distributed over a north-west to south-east climatic gradient; Opuntia leucotricha, O. streptacantha, and Myrtillocactus geometrizans. In general, O. leucotricha is more abundant in the colder north-west section; M. geometrizans in the warmer south-east zone, not occurring in the north-west; and O. streptacantha reaches its greatest abundance in the centre of the region. We studied the potential replacement process between the three species due to canopy interference as well as the effect of a disturbance, freezing temperature, on their survivorship. Canopy interference between adjacent individuals of M. geometrizans/O. streptacantha and O. streptacantha/O. leucotricha indicated a potential replacement sequence of O. leucotricha replaced by O. streptacantha which, in turn, is replaced by M. geometrizans. In contrast, the damage caused by an extreme low-temperature event hardly affected O. leucotricha. It did however cause severe damage to individuals of O. streptacantha in the north-west of its distribution with little or no damage to individuals in the more south-eastern populations studied. M. geometrizans had a similar pattern of damage to O. streptacantha over its range in the region but at each site where the two species occurred together, that damage was more severe. Our observations suggest that disturbance in the form of extreme freezing temperatures is the mechanism that limits the distribution of these three arborescent cacti in the southern Chihuahuan Desert and allows their co-existence regionally.     

Physiological responses and adjustment mechanisms of the dominate species of natural vegetation of Eastern Tengger Desert

Reaumuria soongorica and Salsola passerina have significant differences in ecophysiological characteristics, which change with the environmental variations. Although they live together for a long period of time, their adaptive mechanisms to environmental stresses are very different. As two extreme xerophytes, Reaumuria soongorica and Salsola passerina differ significantly from other psammophytes in ecophysiological characteristics; they can survive in lower water potential, and can even grow in piedmont areas. Low water potential may be related to the existence of osmosis-regulating substance, such as praline, which can strengthen the capacity of water absorption. Compared to other psammophytes, Reaumuria soongorica and Salsola passerina have a higher degree of photo-inhibition under the same condition, and the photo-inhibition can lead to destruction of the photosynthetic pigment, nevertheless, this photo-inhibition can be repaired under suitable conditions in the morning and evening.     

Evolution of soil properties on stabilized sands in the Tengger Desert, China

The spatial and temporal patterns of pedogenesis on stabilized dunes at Shapotou, northwestern China, were studied on the time sequences of 0, 18, 35 and 43 years. The spatial pattern of soil formation was estimated by measuring the thickness of accumulated sand fractions on the stabilized dune surface and by analyzing the characteristics and properties of soil. The results showed that the environment of soil formation and circulation of soil material were influenced in the processes of shifting-sand fixation, and the mean soil particle size changed from >0.2 to <0.08 mm in 0–20 cm soil depth with the succession from cultivated plants to natural vegetation. The capacity of available soil water increased fivefold. Deep infiltration of water in soil no longer occurred due to the increase in soil water capacity and the change of redistribution of soil water in profiles. Soil microorganisms evolved from simple to complex. Interaction of these processes obviously brought about accumulation of soil fertility, evolution of soil profiles and development of the profiles towards aripsamments. The difference of micro-topography is closely related to redistribution of material and energy in soil formation.     

Responses of photosynthesis and water relations to rainfall in the desert shrub creosote bush (Larrea tridentata) as influenced by municipal biosolids

Responses of photosynthesis (P_n), stomatal conductance (g_s), pre-dawn leaf water potential (Ψ_lp) and leaf water content (ω_l) of creosote bush to 10 rainfall events in the Chihuahuan Desert were investigated. Infiltration of rainwater was manipulated by applying municipal biosolids. The responses of P_nand water relation parameters to rainfall (>10 mm) were mainly dependent upon drought severity: (1) following a moderate drought, P_n, g_s, Ψ_lpand ω_lrecovered to corresponding values of irrigated plants within 2 days after a 23-mm rainfall; (2) Ψ_lpand g_sresponded to a 15-mm rainfall within 2 days, following a 25-day drought, whereas responses of P_nand ω_lwere delayed for several days; (3) responses of P_n, g_s, Ψ_lpand ω_lto a 14·7-mm rainfall were all delayed for several weeks following a 110-day drought, but the delay was longer in P_n, g_sand ω_lthan in Ψ_lp. Creosote bush responded to small rainfall events (approximately 6 to 8 mm) with an increase in Ψ_lp, but without noticeable changes in g_sand P_n, suggesting a strong stomatal control of water loss even though xylem embolism was reduced. Biosolids applied at high rates (3·4 and 9 kgm⁻²) decreased the soil water by 2 to 4 mm following rainfall events, and this in turn delayed and decreased the responses of P_nand water relation parameters to rainfall.P_nand g_swere linearly related to ω_land exponentially related to Ψ_lp. With the generally coincidental responses of P_nor g_sand ω_lto rainfall, we concluded that the responses of P_nand g_sto rainfall were dependent on leaf rehydration which resulted from restored hydraulic conductance following drought.     

Soil physico-chemical properties affecting the distribution of biological soil crusts along an environmental transect at Zapotitlán drylands, Mexico

The composition of cyanobacteria, mosses, and lichens of biological soil crusts were correlated with soil characteristics and sun exposure along an environmental transect. The study was conducted in Zapotitlán drylands, a locality within the Tehuacán–Cuicatlán Valley, central Mexico, where a great variety of environments for crust development exist due to landscape fragmentation. Data were analyzed with redundancy and Sorensen analysis. Soil crusts consisted of different combinations of cyanobacteria (7 species), mosses (19 species), and lichens (8 species). The relative frequencies of these groups were positively correlated with soil apparent density and lichens were also positively correlated with soil pH. However, there were no significant correlations with sun exposure.     

The effects of extreme rainfall events on carbon release from biological soil crusts covered soil in fixed sand dunes in the Tengger Desert, northern China

In May to August of 2011, we assessed the effects of extreme rainfall (quantity and intensity) events on the carbon release from soils covered by different types of biological soil crusts (BSCs) in fixed sand dunes in the Tengger Desert, northern China. A Li-6400-09 Soil Chamber was used to measure the respiration rates of the BSCs immediately after the rainfall stopped, and continued until the respiration rates of the BSCs returned to the pre-rainfall basal rate. Our results showed that almost immediately after extreme rainfall events the respiration rates of algae crust and mixed crust were significantly inhibited, but moss crust was not significantly affected. The respiration rates of algae crust, mixed crust, and moss crust in extreme rainfall quantity and intensity events were, respectively, 0.12 and 0.41 μmolCO 2 /(m 2 ·s), 0.10 and 0.45 μmolCO 2 /(m 2 ·s), 0.83 and 1.69 μmolCO 2 /(m 2 ·s). Our study indicated that moss crust in the advanced succession stage can well adapt to extreme rainfall events in the short term.     

Comparison of the growth and biomass production of six acacia species in Riyadh, Saudi Arabia after 4 years of irrigated cultivation

The growth and biomass production of six acacia species were studied in the field for 4 years. The species used were Acacia asak, A. negrii, A. seyal, A. karroo, A. ampliceps, and A. stenophylla. The first three species are indigenous while the others are exotic. The results showed both A. ampliceps and A. asak with 100% survival while all A. negrii died. Acacia ampliceps attained the greatest height, diameter, relative growth rate and above-ground biomass while A. asak had the least. Height and diameter growth of acacia species decreased between warm and cold periods of the year.     

Soil heterogeneity and the distribution of desert and steppe plant species across a desert-grassland ecotone

Bouteloua gracilis (Kunth) Lag. ex Griffiths (blue grama), Bouteloua eriopoda (Torr.) Torr. (black grama), and Larrea tridentata Coville (creosotebush) are dominant plants on the McKenzie Flats portion of the Llano de Manzano landform within Sevilleta National Wildlife Refuge in central New Mexico, part of the biome ecotone from the Colorado Shortgrass Steppe to the Chihuahuan Desert. In this study, we examine the hypothesis that soil heterogeneity, determined by variation in surface soil depth, carbonate accumulation, and fine-textured fraction, controls relative dominance of the three species. The area is flat, generally <1% slope; however, abrupt soil differences exist even within the flattest parts of the landscape that correspond to the pattern of buried channels incised in a petrocalcic horizon (caliche) formed in a 0.5–1.2 million year-old paleosol beneath the current surface soil. Multivariate analyses of soil-moisture-related variables suggest that B. gracilis, a Colorado Shortgrass Steppe indicator, dominates the buried paleochannels where Holocene surface deposits are deepest and the argillic (clay-rich) B-horizon is thickest. B. eriopoda, dominant in Chihuahuan Desert grasslands, is most abundant where the buried petrocalcic horizon lies within 40–60 cm of the surface and the argillic horizon is thinner and weakly developed. L. tridentata, an indicator of desertified Chihuahuan Desert shrubland, is dominant where the petrocalcic horizon is exposed or near the surface. This study illustrates the strong relationship between geomorphology, soil development and vegetation patterns in arid and semi-arid environments.     

The influence of plant removal on succession in Wyoming big sagebrush

Restoration treatments are based on the largely untested notion that desired recovery of plant communities following disturbance wouldn’t occur in the absence of active intervention. We identified rate of short-term (10 year) floristic changes following removal of plant functional groups in Wyoming big sagebrush plant communities in 1999-2005 and 2008. Treatments imposed on 6 × 6 m plots were: 1) removal of all plant functional groups, 2) perennial grass removal, 3) shrub removal and 4) control. Our data suggest recovery of the shrub component on shrub removal plots could take decades. Similarly, perennial grass cover and density on perennial grass removal plots was less than half that of unaltered plots 10 years after treatment. When all functional groups were removed, cover of annual forbs, annual grasses, and shrubs returned to unaltered levels within ten years or less. Perennial forbs were unaffected (p > 0.05) by treatment. The fact that natural recovery of some components occurred within a relatively short post-disturbance time interval (i.e. <10 years) suggests that intervention may not be necessary for some functional groups. Restoring shrubs in areas dominated by perennial grasses may require targeted reductions of competing perennial grasses. Conversely, shrub dominance may limit perennial grass re-establishment.     

Channel incision and patterns of cottonwood stress and mortality along the Mojave River, California

In 1995, mapping and classification of riparian vegetation along the Mojave River in southern California revealed an 8-km reach in which riparian cottonwoods (Populus fremontii Wats.) were stressed or dying. We tested a set of predictions based on the inference that cottonwood decline was an indirect result of lowered water-table levels following flood-related channel incision. Comparisons of topographic cross-sections from 1963 and 1997, indicated a net change in channel elevation between −0·71 and −3·6 m within zones of cottonwood stress and mortality. Ages of young cottonwood and willow stems adjacent to the present channel and radial stem growth of surviving cottonwoods were consistent with the inference that channel incision, associated with sustained flooding in January and February of 1993, lowered channel elevations throughout the affected reach. Well records and soil redoximorphic features indicate that channel incision caused net water-table declines 1·5 m on portions of the adjacent flood plain where cottonwood stand mortality ranged between 58 and 93%. In areas where water-table declines were estimated to be <1·0 m, stand mortality was 7–13%.     

Fire in Chihuahuan Desert grassland: Short-term effects on vegetation, small mammal populations, and faunal pedoturbation

A prescribed burn resulted in significant decreases in canopy cover of the grasses: Bouteloua eriopoda, Sporobolus flexuosus, and Aristida purpurea. One year post-burn, basal cover of B. eriopoda remained significantly lower in burned patches than in unburned areas but there were no differences in basal cover of the other perennial grasses. Only one species of the 14 summer annual species occurred in both burned and unburned plots. There were six species of spring annuals in burned patches but no spring annuals in the unburned grassland ten months post-burn. Fire killed 100% of the snakeweed shrubs (Gutierrezia sarothrae), 77% of the Ephedra torreyana shrubs, and 36% of the Yucca elata. All mesquite shrubs that were top-killed by fire, resprouted one month post-burn. Fire had no effect on abundance and species richness of rodents. There were fewer wolf spider, Geolycosa spp. burrows in burned areas than in unburned grassland. The area and volume of soil in termite galleries and sheeting were significantly larger in the unburned grassland than in the burned areas.     

Amelioration of soil fertility by woody perennials in cropping fields: evaluation of three tree species in the semi-arid zone of Nigeria

Soil was sampled from cropping fields in radial patterns from beneathAcacia albida, Parkia bigloboza(Jacq.) Benth. andEucalyptus camaldulensisDehnh. near Zaria, Nigeria. Results of analysis show a significant coarsening of soil texture and a decrease in organic matter and cation exchange capacity with increasing distance from all three tree species at a depth of 0–15 cm. Concentrations of nitrogen and some exchangeable cations decreased significantly with increasing distance from beneath acacia and parkia, and soil pH decreased with increasing distance from eucalyptus. The implication of the results for land management are discussed in relation to increasing productivity and soil use sustainability.     

Influence of shrubs on soil characteristics and their function in Sahelian agro-ecosystems in semi-arid Niger

Soil beneath shrubs form ‘fertile islands’ in fallow sites and millet fields in semi-arid Niger. To gain more information about this phenomena different shrub species in fallow sites following a gradient from 350–650 mm precipitation were examined. For each shrub two different areas were distinguished: an area under the canopy of the shrubs and an area in the nearby open land. Soil samples were taken from a depth of 0–10 cm and analysed for C_org, N_total, P_Bray, pH(H₂O), exchangeable cations, effective cation exchange capacity (ECEC) and soil texture. Significantly higher concentrations between 38–51% for C, N, P and 22% on ECEC for K⁺were found in the soil under the shrubs. The pH showed only slight but significant differences, whereas Al³⁺and H⁺rates on ECEC under the shrubs were increased by 44–55%. For Guiera senegalensis, the most common shrub of the studied area, enrichment ratios of most soil properties increased relatively more with increasing aridity. In general, enrichment ratios decreased with the age of the fallows, whereas concentrations showed no clear evolution. The chemical composition of the shrub litter seems to influence the degree of soil enrichment. The main step of fertile island formation takes place during the cultivation period by trapping wind-blown sediment. This work shows that shrubs are of vital importance for the accumulation of nutrients and maintenance of soil fertility within agro-ecosystems of Niger.     

Effect of nitrogen amendments on microbial biomass, above-ground biomass and nematode population in the Negev Desert soil

Nitrogen amendments (0, 25, 50 and 100 kg NH₄NO₃/ha) were used to study the responses of primary production, microbial biomass and nematode population in desert soil. The study was conducted in the Israeli Negev Desert, a region characterized by low and randomly distributed rainfall. Over a 1-year study period, nitrogen amendments resulted in a significant (p<0·01) increase in soil microbial biomass. Soil microbial biomass also increased concomitantly with the increase in soil organic matter. The number of free-living nematodes in the soil increased with the increase in soil moisture, ranging from 43,000 individuals per square metre at the end of the summer to 351,000 individuals per square metre during the rainy season. No significant correlation was found between the nitrogen treatments and the nematode population, whereas a significant positive correlation was found between the nitrogen amendments and the above-ground biomass (r²=0·94, p<0·03). The nitrate proportion of the total soluble nitrogen in the soil also increased with the increase in soil moisture. This study provides baseline data for nitrogen amendments on soil microbial status, as well as insights into the importance of nitrogen in fertility in arid environments.