Impact of salt stress on the features and activities of root system for three desert halophyte species in their seedling stage

Linkage between belowground and aboveground sections of ecological system is mainly depending on root system. But root system is the parts of plant that people less understand. The absorption function of root system is closely related to their morphology and activity. Moreover root system can interact with the environmental stress under the adverse situation, and adjust its system to take adaptation responses in morphology and physiology to strengthen its survival chance. This research is focused on three desert halophyte species of H. ammodendron (C.A.Mey.) Bge., S. physophora Pall., and S. nitraria Pall. under solution culture, to study the differences of their root system morphology and activity in the seedling stage under varying salt concentration conditions. The study results show that: A certain salt concentration can promote development of these three halophytes; but rather high salt concentration will restrain their growth, in particular inhibit the root system development. Under the same salt concentration condition, S. nitraria Pall. grows fast and accumulates the largest amount of biomass. Under relatively low salt concentration, the length of axial root and the total length of root system of these three halophyte species are all increased; and compared to the checking samples, S. physophora Pall. occupies the top place of root system growth, but the high salt concentration will restrain the increase of total root length; among them, the impact intensity on S. physophora Pall. is lighter than to H. ammodendron (C.A.Mey.) Bge. and S. nitraria Pall. is lighter; the salinity does not bring distinct influence on the average diameter of root system of these three plant species, but trends to reducing the size; under the solution culture conditions, the middle and lower parts of the axial root of H. ammodendron (C.A.Mey.) Bge. and S. physophora Pall. are rather equally distributed, but the central zone of S. nitraria Pall. root system is more significantly increased than the upper and lower zones; salt concentration does not bring significant impact on the root system spatial distribution of each species. The root activity of the three plants is increased along with the increase of the salt concentration. When the salt concentration is low, the root activity is not significantly increased; but when the salt concentration is high, the root activity is increased significantly. The experimental results show that the saline tolerance capacity of H. ammodendron (C.A.Mey.) Bge. is lower than the other two species, and the capacity of S. physophora Pall. ranks the top place.     

Cathedral Cave,Wellington Caves,New South Wales,Australia. A multiphase,non‐fluvial cave

Cathedral Cave is an outstanding example of a class of multiphase caves with largely non‐fluvial origins. It contains large cavities such as cathedrals and cupolas, characteristic of excavation by convection currents in rising waters. Smaller‐scale features such as rising half‐tubes, pseudonotches, curved juts, projecting corners, blades and bridges indicate intersection and exhumation of older cavities during the formation of younger ones. It is possible to recognize at least ten significant phases of speleogenesis by morphostratigraphy, in addition to the four generations of cave‐filling palaeokarst deposits intersected by the cave. The cave we see today results from the progressive integration of a number of previously disconnected or poorly connected solution cavities. Copyright © 2007 John Wiley & Sons, Ltd.     

The influence of riparian vegetation on near‐bank turbulence: a flume experiment

Measurements from a fixed‐bed, Froude‐scaled hydraulic model of a stream in northeastern Vermont demonstrate the importance of forested riparian vegetation effects on near‐bank turbulence during overbank flows. Sections of the prototype stream, a tributary to Sleepers River, have increased in channel width within the last 40 years in response to passive reforestation of its riparian zone. Previous research found that reaches of small streams with forested riparian zones are commonly wider than adjacent reaches with non‐forested, or grassy, vegetation; however, driving mechanisms for this morphologic difference are not fully explained. Flume experiments were performed with a 1:5 scale, simplified model of half a channel and its floodplain, mimicking the typical non‐forested channel size. Two types of riparian vegetation were placed on the constructed floodplain: non‐forested, with synthetic grass carpeting; and forested, where rigid, randomly distributed, wooden dowels were added. Three‐dimensional velocities were measured with an acoustic Doppler velocimeter at 41 locations within the channel and floodplain at near‐bed and 0·6‐depth elevations. Observations of velocity components and calculations of turbulent kinetic energy (TKE), Reynolds shear stress and boundary shear stress showed significant differences between forested and non‐forested runs. Generally, forested runs exhibited a narrow band of high turbulence between the floodplain and main channel, where TKE was roughly two times greater than TKE in non‐forested runs. Compared to non‐forested runs, the hydraulic characteristics of forested runs appear to create an environment with higher erosion potential. Given that sediment entrainment and transport can be amplified in flows with high turbulence intensity and given that mature forested stream reaches are wider than comparable non‐forested reaches, our results demonstrated a possible driving mechanism for channel widening during overbank flow events in stream reaches with recently reforested riparian zones. Copyright © 2007 John Wiley & Sons, Ltd.     

从机载激光扫描数据中提取道路

提出了一种从机载激光扫描数据提取道路的方法,采用结合高程数据和强度数据方法来分析道路信息,实现了对道路的提取。     

Soil morphology and organic matter dynamics under cheatgrass and sagebrush-steppe plant communities

Widespread cheatgrass (Bromus tectorum L.) invasion represents a major shift in species dominance that may alter ecosystem processes across much of the western US. To investigate differences following such conversion, soil morphology and organic matter under cheatgrass-dominated and native shrub-steppe vegetation were compared by standard soil analysis procedures at seven paired sites in Idaho and Utah. Results suggest that, following conversion to cheatgrass dominance, increased porosity and labile organic inputs enhance microbial decomposition in near-surface horizons beneath cheatgrass compared to adjacent soils under native vegetation. Enhanced decomposition could result in depletion of long-term SOM, leading to impoverished sites difficult to restore to native perennial vegetation.     

Seasonal variation of crescentic dune morphology and morphometry,Strzelecki–Simpson Desert,Australia

The general absence of contemporary dune building in the Australian arid zone finds the occurrence of crescentic dune genesis and evolution at Gurra Gurra waterhole, a somewhat enigmatic event amongst prevalent inactive linear dunescapes. Crescentic dune construction is geographically restricted to areas of dominant unidirectional high windiness and minimal vegetation, parameters that are generally uncommon in arid Australia today. The influence of seasonal multidirectional winds can see dune form undergo continual transition between quasi‐equilibrium and disequilibrium. Such dynamic antithesis is depicted in seasonal mathematical comparisons of the planimetric attributes, length and width. Dimensional equilibrium is a transient feature at Gurra Gurra waterhole and not characteristic of this dunescape. Copyright © 2001 John Wiley & Sons, Ltd.     

Random controls on semi‐rhythmic spacing of pools and riffles in constriction‐dominated rivers

Average pool spacing between five and seven bankfull widths has been documented in environments throughout the world, but has limited theoretical justification in coarse‐bedded and bedrock environments. Pool formation in coarse‐bedded and bedrock channels has been attributed to bedrock and boulder constrictions. Because the spacing of these constrictions may be irregular in nature, it is difficult to reconcile pool‐formation processes with the supposedly rhythmic spacing of pools and riffles. To address these issues, a simulation model for pool and riffle formation is used to demonstrate that semi‐rhythmic spacing of pools with an approximate spacing of five to seven bankfull widths can be recreated from a random distribution of obstructions and minimum pool‐ and riffle‐length criteria. It is assumed that a pool–riffle couplet will achieve a minimum length based on dominant‐discharge conditions. Values for the minimum‐length assumption are based on field data collected in New England and California, while the theoretical basis relies on the demonstrated hydraulic response of individual pools to elongation. Results from the simulations show that the location of pools can be primarily random in character, but still assume an average spacing between four and eight bankfull widths for a variety of conditions. Field verification data generally support the model but highlight a highly skewed distribution of pool‐forming elements and pool spacing. The relation between pool spacing and bankfull widths is attributed to the common geometric response of these features to dominant‐discharge conditions. Copyright © 2001 John Wiley & Sons, Ltd.     

EXPERIMENTAL STUDY ON INCIPIENT MOTION OF SEDIMENT PARTICLES ON GENERALIZED SLOPING FLUVIAL BEDS

1 INTRODUCTIONWhen water flows over a fluvial bed, hydro-dynandc force induced by the flow is acting on thesediment particles lying on the bed. A further increase in flow velocity results in an increase in themagnitude of this fOrce; and sediment particles begin to move if a situation is eventu8lly reached whenthe hydro-dynandc force exceeds a certain critical value. This initial movement of sediment pallicles istermed inciPient motion. The erosion and sedimentation of nuvial beds can be…