Post-fire plant recovery in the Mojave and Sonoran Deserts of western North America

Increasing wildfire activity is one of the most pressing management concerns in arid lands of the American West. To examine post-fire recovery of perennial vegetation in the Mojave and Sonoran Deserts, I analyzed data systematically synthesized from the literature. Post-fire sprouting by desert perennials is generally limited but varies among species. For example, only 3–37% of Larrea tridentata sprouted compared to 64–86% of Yucca schidigera. Four of five studies measuring recovery of perennial cover reported close relationships (r² = 0.67–0.99) between time since fire (TSF) and cover. In fact, three studies measuring the longest TSF (≥37 years) found that cover had returned to within 10% cover of unburned areas within approximately 40 years. Conversely, post-fire species composition exhibited little convergence with unburned composition in five of six studies even 47 years after fire. Sphaeralcea ambigua, Gutierrezia spp., Achnatherum speciosum, Encelia spp., Hymenoclea salsola, and Baileya multiradiata had the highest burned:unburned abundance ratios, although overall post-fire community composition differed between the Mojave and Sonoran Deserts. Analyzing the literature as a whole suggested some generalities (e.g., that perennial cover reestablishes faster than composition), but more work is required for improving specific knowledge about plant recovery among fires, sites, species, and climates.     

Limitations to photosynthetic function across season in Larrea tridentata (creosotebush) growing on contrasting soil surfaces in the Sonoran Desert

Plant community structure in the southwestern United States co-varies with soil surface characteristics due to their role in controlling water availability. At the University of Arizona Desert Laboratory on Tumamoc Hill, we evaluated winter and summer season dynamics of photosynthesis in a dominant shrub species, Larrea tridentata (creosotebush), across soils with contrasting development of surface and sub-surface horizons. We measured pre-dawn water potential (Ψ_pd), stomatal conductance (g_s), leaf nitrogen content (%N), and determined stable carbon isotope discrimination (Δ). There were no differences in these parameters throughout the winter, although Δ was higher and %N was lower on the clay than sandy soil early in the dry summer period, and g_s was higher on the clay than sandy soil late in the summer season while precipitation decreased. We used path analysis to determine the relative influences of %N, g_s and Ψ_pd on photosynthetic function (Δ) during the winter and summer seasons. There was no influence during the summer, although g_s and %N had positive and negative impacts on Δ within the sandy soil type respectively during the winter. Across soil type, g_s was the main driver of %N and Δ. Our study highlights the need to include soil type for understanding how physiological performance of L. tridentata varies over seasons within arid and semi-arid ecosystems.     

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.     

Competition as a barrier to establishment of a native perennial grass (Elymus elymoides) in alien annual grass (Bromus tectorum) communities

The alien grass Bromus tectorum dominates stable annual-plant communities that have replaced native shrub-perennial grass communities over much of the semi-arid western United States. We conducted field competition experiments between B. tectorum and a native grass, Elymus elymoides, on two sites to determine the effects of B. tectorum competition on perennial grasses, and the role of B. tectorum competition in the stability of B. tectorum-dominated communities. B. tectorum competition acting on seedling-stage E. elymoides plants greatly reduced first-year relative growth rates and biomass which, in turn, reduced second-year survival, biomass, and flowering. However, B. tectorum competition acting on older E. elymoides plants had much less effect, which may help to explain why intact perennial-plant communities are resistant to B. tectorum invasion. At the drier site, direct effects of B. tectorum competition were less, but competition and drier habitat combined to produce greater E. elymoides mortality.     

The seedbed microclimate and active revegetation of disturbed lands in the Mojave Desert

One of the greatest challenges to the U.S. Department of Defense is maintaining the sustainability of military lands while maximizing their use for training activities. Restoration of disturbed lands in the Mojave Desert has proven to be a challenge, particularly revegetation from seed. This study presents soil moisture dynamics from three restoration sites at the National Training Center, Ft. Irwin, CA. The soil microclimate (matric potential, water balance, and temperature) was monitored beneath various surface treatments to better assess their effects on seed germination and establishment. Results show that under the typically hot and dry climatic conditions that prevail, the soil microclimate is a hostile environment. Irrigation had the most positive effect on the seedbed. Surface mulches including gravel, straw, emulsion, and bark had limited benefits often detrimental. Although seedlings germinated, survival after three months was negligible. Results presented here suggest that successful seeding with broadcast sprinkler irrigation in the central Mojave Desert is not possible unless the seedbed area is limited or advantageous environmental conditions prevail. Improved knowledge of the germination requirements and applied numerical modeling would result in more successful seedbed management.     

Development of variable microsatellite loci and range-wide characterization of nuclear genetic diversity in the important dryland shrub antelope bitterbrush (Purshia tridentata)

Antelope bitterbrush (Purshia tridentata Pursh DC; Rosaceae) is an arid-land shrub that occupies an important ecological niche in various fire-dominated communities across much of the western United States. Because of its importance as a browse for large mammals and a food source for granivores, P. tridentata is frequently planted by Federal agencies in arid-land revegetation. We are currently analyzing the range-wide genetic diversity of this species as part of a larger effort to develop seed movement guidelines. In this study, we describe the development of eight novel nuclear microsatellite loci and characterize the amount and apportionment of range-wide nuclear genetic diversity. The eight microsatellite loci exhibited a high level of polymorphism (13–33 alleles per locus) and in general, observed levels of heterozygosity did not deviate form Hardy–Weinberg expectations. An initial screen of 196 individuals from 12 widely distributed populations revealed a moderate amount of genetic differentiation (F_ST = 0.09, p < 0.001). Moreover, these loci successfully produced PCR products in cross-species amplifications with two closely related Purshia species. These results demonstrate the utility of these markers and provide useful measures of neutral genetic diversity and population differentiation.     

公元九世纪初鄂尔多斯沙漠图图说——唐夏州、丰州境内沙漠

依据唐代文献主要是《元和郡县图志》(该书成于公元813年),考证鄂尔多斯高原公元九世纪初人文地物与自然地物,试图复原当时流沙分布情况,为研究沙漠化历史过程提供基础资料。     

Seroprevalence of Mycoplasma agassizii and tortoise herpesvirus in captive desert tortoises (Gopherus agassizii) from the Greater Barstow Area,Mojave Desert,California

Upper respiratory tract disease (URTD) has been implicated as a cause of decline of wild populations of desert tortoises, Gopherus agassizii, in the western Mojave Desert. One explanation for outbreaks of disease may be the release or escape of diseased captive tortoises into naïve wild populations. Because Mycoplasma agassizii and tortoise herpesvirus have surfaced as important pathogens, 179 captive tortoises were evaluated in the greater community of Barstow, San Bernardino County, California during 2000 and 2001 to determine pathogen exposure. An indirect enzyme-linked immunosorbent assay (ELISA) was performed to detect antibodies against Mycoplasma agassizii (n=179) and tortoise herpesvirus (n=109). Anti-mycoplasma antibodies were present in 82.7% of the tortoises while anti-herpesvirus antibodies were detected in 26.6%. A positive association was found between tortoises with anti-mycoplasma antibodies and severity of clinical signs of URTD (p=0.001) and with age categories, with adults being more likely to be positive (p<0.001). Neither association was found with herpesvirus exposure. No association was found between gender and pathogen exposure or between being positive for exposure to both pathogens. Findings suggest that captive tortoises can be a source of infection for free ranging desert tortoises.     

Plot- and landscape-level changes in climate and vegetation following defoliation of exotic saltcedar (Tamarix sp.) from the biocontrol agent Diorhabda carinulata along a stream in the Mojave Desert (USA)

The biocontrol agent, northern tamarisk beetle (Diorhabda carinulata), has been used to defoliate non-native saltcedar (Tamarix spp.) in USA western riparian systems since 2001. Biocontrol has the potential to impact biotic communities and climatic conditions in affected riparian areas. To determine the relationships between biocontrol establishment and effects on vegetation and climate at the plot and landscape scales, we measured temperature, relative humidity, foliage canopy, solar radiation, and used satellite imagery to assess saltcedar defoliation and evapotranspiration (ET) along the Virgin River in the Mojave Desert. Following defoliation solar radiation increased, daily humidity decreased, and maximum daily temperatures tended to increase. MODIS and Landsat satellite imagery showed defoliation was widespread, resulting in reductions in ET and vegetation indices. Because biocontrol beetles are spreading into new saltcedar habitats on arid western rivers, and the eventual equilibrium between beetles and saltcedar is unknown, it is necessary to monitor trends for ecosystem functions and higher trophic-level responses in habitats impacted by biocontrol.     

奎屯市大气VOCs浓度水平及物种的年变化

为了得到实时的大气挥发性有机物( VOCs)浓度组成和日变化特征,2013-2015年在新疆奎屯市区内高约25m的楼顶采用在线的气相色谱仪进行了为期3年的观测。结果表明,奎屯市TVOCs浓度年均值范围66.0-80.7ppbv,低碳烷烃、低碳稀烃、高碳烷烃、苯系物和高碳烯烃为主要的五类VOCs,其浓度之和占TVOCs浓度的84.1%。奎屯市冬季VOCS浓度值是年均值1.5倍、冬夏VOCS浓度相差3倍左右,与其他城市相比,大气挥发性污染物浓度水平整体是偏高。与前两年相比,2015年大气中卤代烃、苯系物、含氧（氮）VOCS的百分含量明显提升,分别上升15.6个百分点、6.5个百分点、5.9个百分点,大气VOCS有毒有害成分比例提升。由于在各类挥发性机物中,相同的条件下卤代烃与其它有机物相比,更容易发生光化学反应,因此,从卤代烃夏季含量很高的变化趋势看,奎屯市夏季发生光化学反应的能力不断增强。     

The influence of elevation, shrub species, and biological soil crust on fertile islands in the Mojave Desert, USA

We quantified soil nutrients and biological crust cover (bryophytes and lichens) under the canopies of three species of Mojave Desert shrubs and in interspaces between shrubs at three elevations to determine the effects of shrub species, soil crust, and elevation on islands of soil fertility. Means of pH, organic matter, total Kjeldahl nitrogen, nitrogen mineralization, and gravimetric soil moisture are significantly greater in soils under Ambrosia dumosa (Gray) Payne, Larrea tridentata Cov., and Coleogyne ramosissima Torr. than soils from adjacent interspace microhabitats. Although soil moisture and soil organic matter increase by a factor of 1.5 from the low elevation to the high elevation site, the ratio of shrub to interspace concentrations, or the difference in mean soil variables between shrubs and interspaces, is effectively constant and independent of elevation. Total bryophyte and lichen cover is relatively low (24.5%), however, there are 11 species of bryophytes and two species of lichens distributed across three elevations with the highest species richness and cover at the low-elevation site. Bryophyte and lichen cover is correlated with silt but is not related, consistently, to soil nutrients. Overall, the balance of processes controlling spatial aggregation of soil nutrients under shrubs is remarkably insensitive to potential differences in organic inputs among elevations, shrub species, and soil crust surfaces.     

The relationship between mean annual growth rates and the size of cities in the USSR

3 groups of USSR cities are used to study the correlation between city size and the mean annual city growth rates of: 1) oblast capitals of the Russian Soviet Federated Socialist Republics (RSFSR) with diversified machine building; 2) industrial cities of the RSFSR and the Ukraine based on heavy machine building; and 3) small and medium sized textile towns. Results show that the closeness of the relationship between growth rates and city size increases with the number of comparable attributes. The principal factors included are: the level of organization of the city, including its size; its functional and industrial sectoral structure; the degree of saturation with industry; and type of city, probably the key characteristic. The more highly developed a city is, the more important an ntegrated focus it becomes. On the other hand, the lower the level of organization of the city, the more its development will be determined by the function of place. Growth rates of small and medium sized textile towns over the last 20-36 years are roughly 1/5 of the growth rates of oblast capitals and large industrial cities with machine building. Overall, cities of the central economic region constitute a complex system that breaks down into the rhythms of systems of a given type and of different types at various levels of organization. Although cities that are homogeneous in terms of functional, industrial, sectoral, and territorial factors display a correlation between mean annual growth rates and population size, only by looking at these factors in combination, rather than counterposing them to one another, can we identify the objective regularities underlying the growth of cities.     

The regularities of long-term annual stream runoff fluctuations in Siberia and the Far East

We examine the time coincidence and spatial homogeneity of the annual stream runoff fluctuations in Siberia and the Far East. A regionalization of the study territory for the time coincidence of the stream runoff fluctuations has been carried out by analyzing the correlation matrix on the basis of the algorithm of automatic classification of objects through identification of groups with homogeneous attributes according to a specified homogeneity criterion. The territory’s spatial homogeneity was assessed by constructing and analyzing spatial correlation functions and accompanied by a checking for the possible anisotropy.     

Ornamental plants and the production of nature(s) in the Spanish real estate boom and bust: the case of Alicante

The Spanish real estate boom and bust of the 2000s caused immense economic, social, and environmental changes across the country. Massive urbanization, however, does not necessarily entail the end of nature and natural processes, instead marking a radical transformation of environments and societies. Through a political ecology approach, we analyze how ornamental species have become a fundamental part of new natures induced by urbanization in the Spanish province of Alicante. We connect the economic and ecological characteristics of ornamental plants like the palm tree to urbanization and real estate trends in Alicante. Our evidence indicates that, in Alicante, economic recession and environmental crises (manifested in the pests affecting palm trees) have mutually reinforced one other, creating a new geography of ecological desolation in many areas.     

Effects of camel grazing on the ecology of small perennial plants in the Dubai (UAE) inland desert

Camel grazing is recognized as a primary cause of ecological degradation in the UAE. A study of perennial plant species <1 m in height was conducted along a fence separating continuously camel grazed land from land in which camels had been replaced by oryx and gazelle species for 5 years (Al Maha). Vegetation regeneration in Al Maha in the absence of camels was considerable on all substrates (gravel, stable sand, and semi-stable sand) but was greatest on the gravel substratum, indicating that ecology in this habitat is most at risk. Observed regeneration was primarily through vegetative reproduction and growth of existing plants, showing that existing species can tolerate heavy grazing. Therefore, an equilibrium grazing model of continuous and reversible vegetation dynamics is most suitable for management of this ecological zone. Species richness was greater in Al Maha due to the greater number of plants, but biodiversity was unaffected. There was some evidence of localized dune stabilization within Al Maha due to increased vegetative cover. Further recovery of vegetation within Al Maha is discussed. This study highlights the need for reduced grazing pressure throughout the Dubai inland desert, and in particular on gravel substrata.     

Nest dynamics of Constrictotermes cyphergaster (Termitidae,Nasutitermitinae) and its association with the supporting vegetation in a semiarid area,northeast, Brazil

We evaluated growth, abandonment, decay and emergence of new nests of the Neotropical termite Constrictotermes cyphergaster (Termitidae) and its association with the supporting vegetation in an area of caatinga (arid thorn scrub in the state of Paraíba, northeastern Brazil). In an area of 1 ha, a total of 272 nests were observed monthly, from March 2005 to March 2007. Of these, 245 nests were small, 9 were of medium size, and 18 were large nests. During the rainy season (from June to February), nests grew 94.2 L, whereas during the dry season (March–May), their total growth was 23 L. The number of abandoned nests was positively correlated with rainfall. Decomposition of nests was greater during the rainy season. The pattern of nest distribution was associated with the distribution of the primary supporting plants. Rainfall appears to be one of the most important factors in the dynamics of growth of nests of C. cyphergaster in the caatinga.     

Ibex diggings in the Negev Desert highlands of Israel as microhabitats for annual plants. Soil salinity, location and digging depth affecting variety and density of plant species

About 400 Nubian ibex (Capra ibex nubiana) dig many hundreds of temporary night beds, approximately 80 cm × 110 cm and 15 cm deep, on slopes near the edges of the cliffs bordering the Zin valley. The beds are used according to the wind direction and the size of the ibex group, which depends on the ibex social behaviour during the year.The diggings are gradually abandoned and become important microhabitats for seed accumulation and germination, seedling development and annual plant establishment. These are influenced by soil salinity, slope direction and angle, digging depth, and the time since the diggings were abandoned. A greater variety of plant species and plant density were found in these diggings than upslope from the diggings.     

The complex nonlinear systems with fractal as well as chaotic dynamics of annual runoff processes in the three headwaters of the Tarim River

This paper attempted to identify fractal and chaotic characteristics of the annual runoff processes in headwaters of the Tarim River. Methods of fractal analyses were used to explore several aspects of the temporal changes from 1957 to 2002. The main findings are as follows: (1) The annual runoff processes of the three headwaters of the Tarim River are complex nonlinear systems with fractal as well as chaotic dynamics. (2) The correlation dimensions of attractor derived from the time series of the annual runoff for the Hotan, Yarkand and Aksu rivers are all greater than 3.0 and non-integral, implying that all three rivers are chaotic dynamical systems that are sensitive to initial conditions, and the dynamic modeling of their annual runoff process requires at least four independent variables. (3) The time series of annual runoff in each river presents a long-term correlation characteristic. The Hurst exponent for the period of 1989 to 2002 suggests that we may expect to see an increasing trend in the annual runoff of the Aksu and Yarkand rivers in the years after 2002, but a decreasing tendency for the Hotan River in the same period.     

塔里木盆地三源河年径流过程的非线形特征

This paper attempted to identify fractal and chaotic characteristics of the annual runoff processes in headwaters of the Tarim River. Methods of fractal analyses were used to explore several aspects of the temporal changes from 1957 to 2002. The main findings are as follows: (1) The annual runoff processes of the three headwaters of the Tarim River are complex nonlinear systems with fractal as well as chaotic dynamics. (2) The correlation dimensions of attractor derived from the time series of the annual runoff for the Hotan, Yarkand and Aksu rivers are all greater than 3.0 and non-integral, implying that all three rivers are chaotic dynamical systems that are sensitive to initial conditions, and the dynamic modeling of their annual runoff process requires at least four independent variables. (3) The time series of annual runoff in each river presents a long-term correlation characteristic. The Hurst exponent for the period of 1989 to 2002 suggests that we may expect to see an increasing trend in the annual runoff of the Aksu and Yarkand rivers in the years after 2002, but a decreasing tendency for the Hotan River in the same period. Foundation: Knowledge Innovation Project of CAS, No.KZCX2-XB2-03; Major Direction of Knowledge Innovation Project of CAS, No.KZCX2-YW-127; Shanghai Academic Discipline Project (Human Geography), No.B410 Author: Xu Jianhua (1965–), specialized in geographical quantitative methods and GIS.