Sediment transportation and deposition by rain-triggered lahars at Merapi Volcano,Central Java,Indonesia

A lahar is a general term for a rapidly flowing mixture of rock debris and water (other than normal streamflow) from a volcano and refers to the moving flow. Located in the populated area of Central Java, the stratovolcano Merapi (2965 m) is prone to lahar generation, due to three main factors: (1) millions of cubic meters of pyroclastic deposits are the product of frequent pyroclastic flows, which have occurred on 2- to 4-year intervals; (2) rainfall intensity is high (often 40 mm in 2 h on average) during the rainy season from November to April; and (3) drainage pattern is very dense. Following the 22 November 1994 eruption of Merapi, 31 rain-triggered lahar events were recorded in the Boyong River between December 1994 and May 1996.On Merapi's slopes, instantaneous sediment concentration at any given time of the lahars varies widely over time and space. Lahars are transient sediment-water flows whose properties are unsteady, so that the sediment load fluctuates during the flow. The boundary between the flow types (debris flow, with sediment concentration >60% volume, or hyperconcentrated flow, with sediment concentration ranges from 20% to 60% volume) may fluctuate within the flow itself. Grain-size distribution, physical composition of sediments, shear stress, yield stress, and water temperature play each a role on this boundary. Natural self-damming and rapid breakout are partly responsible for the sediment variations of the flows.Debris-flow phases at Merapi typically last a few minutes to 10 min, and are often restricted to the lahar front. Debris-flow surges are sometimes preceded and always followed by longer hyperconcentrated flow phases. As a result, mean sediment concentration of the lahars is low, commonly from 20% to 50% volume. Besides, transient normal streamflow phases (sediment concentration <20% volume) can occur between two debris-flow surges.Low sediment load and frequent transient flows in the Merapi channels may result from at least three factors: (1) several breaks-in-slope along the channel increase the deposition rate of sediment, and hinder the bulking capacity of the lahars; (2) source material is mainly coarse debris of “Merapi-type” block-and-ash flows. Consequently, the remobilization of coarse debris is more difficult and the clast deposition is accelerated; (3) variations of rainfall intensity over time and space, common in tropical monsoon rainfall, also influence the sediment load variations of the lahars.Sedimentologic analyses of the lahar deposits in the Boyong River at Merapi encompass clast-supported and matrix-supported debris-flow deposits, hyperconcentrated flow deposits, and streamflow deposits. The stratigraphic succession of massive and stratified beds observed immediately after any given lahar event in the Boyong River indicates that the sediment concentration varies widely over time and space during a single lahar event. Sedimentation rate varies from 3 to 4.5 cm/min during relatively long-lived surges to as much as 20 cm/min during short-lived surges. These results indicate that the sediment load fluctuates during lahar flow, further demonstrating that lahars are transient sediment-water flows with unsteady flow properties.     

The role of neotectonics and glaciation on terrace formation along the Nysa K odzka River in the Sudeten Mountains (southwestern Poland)

The Nysa K odzka river drainage basin in the Sudeten Mts., SW Poland, preserves a complex late Cainozoic succession that includes eight fluvial series or terraces and deposits from two glacial episodes as well as local volcanic rocks, slope deposits and loess. Fluvial sedimentation took place during the Late Pliocene and from the early Middle Pleistocene (Cromerian), with a long erosion phase (gap) during the Early Pleistocene. Fluvial series are dated to the Late Pliocene, Cromerian, Holsteinian, late Saalian/Eemian, Weichselian, and the Holocene. Glacial deposits represent the early Elsterian and early Saalian stages. Almost all these stratigraphic units have been observed in all geomorphic zones of the river: the mountainous K odzko Basin, the Bardo Mts. (Bardo gorge) and in the mountain foreland. The main phase of tectonic uplift and strong erosion was during the Early Pleistocene. Minor uplift is documented also during the post-early Saalian and probably the post-Elsterian. The post-early Saalian and post-Elstrian uplift phases are probably due to glacio-isostatic rebound. The Quaternary terrace sequence was formed due to base-level changes, epigenetic erosion after glaciations and neotectonic movements. The Cromerian fluvial deposits/terraces do not indicate tectonic influence at all. All other Quaternary terraces indicate clear divergence, and the post-early Saalian terraces also show fault scarps. The fluvial pattern remained stable, once formed during the Pliocene, with only minor changes along the uplifted block along the Bardo gorge, inferring an antecedent origin for the Bardo gorge. Only during the post-glacial times, have epigenetic incisions slightly modified the valley.     

ON THE GEOCHRONOLOGICAL METHOD VERSUS FLOW SIMULATION SOFTWARE APPLICATION FOR LAHAR RISK MAPPING: A CASE STUDY OF POPOCATÉPETL VOLCANO,MEXICO

Lahars are hazardous events that can cause serious damage to people who live close to volcanic areas; several were registered at different times in the last century, such as at Mt St Helens (USA) in 1980, Nevado del Ruiz (Colombia) in 1985 and Mt Pinatubo (Philippines) in 1990. Risk maps are currently used by decision‐makers to help them plan to mitigate the hazard‐risk of lahars. Risk maps are acquired based on a series of tenets that take into account the distribution and chronology of past lahar deposits, and basically two approaches have been used: (1) The use of Flow Simulation Software (FSS), which simulates flows along channels in a Digital Elevation Model and (2) The Geochronological Method (GM), in which the mapping is based on the evaluation of lahar magnitude and frequency. This study addresses the production of a lahar risk map using the two approaches (FSS and GM) for a study area located at Popocatépetl volcano – Central Mexico. Santiago Xalitzintla, a town located on the northern flank of Popocatépetl volcano, where volcanic activity in recent centuries has triggered numerous lahars that have endangered local inhabitants, has been used for the case study. Results from FSS did not provide satisfactory findings because they were not consistent with lahar sediment observations made during fieldwork. By contrast, the GM produced results consistent with these observations, and therefore we use them to assess the hazard and produce the risk map for the study area.     

Formation and failure of volcanic debris dams in the Chakachatna River valley associated with eruptions of the Spurr volcanic complex, Alaska

The formation of lahars and a debris avalanche during Holocene eruptions of the Spurr volcanic complex in south-central Alaska have led to the development of volcanic debris dams in the Chakachatna River valley. Debris dams composed of lahar and debris-avalanche deposits formed at least five times in the last 8000–10,000 years and most recently during eruptions of Crater Peak vent in 1953 and 1992. Water impounded by a large debris avalanche of early Holocene (?) age may have destabilized an upstream glacier-dammed lake causing a catastrophic flood on the Chakachatna River. A large alluvial fan just downstream of the debris-avalanche deposit is strewn with boulders and blocks and is probably the deposit generated by this flood. Application of a physically based dam-break model yields estimates of peak discharge (Q_p) attained during failure of the debris-avalanche dam in the range 10⁴<Q_p<10⁶ m³ s⁻¹ for plausible breach erosion rates of 10–100 m h⁻¹. Smaller, short-lived, lahar dams that formed during historical eruptions in 1953, and 1992, impounded smaller lakes in the upper Chakachatna River valley and peak flows attained during failure of these volcanic debris dams were in the range 10³<Q_p<10⁴ m³ s⁻¹ for plausible breach erosion rates.Volcanic debris dams have formed at other volcanoes in the Cook Inlet region, Aleutian arc, and Wrangell Mountains but apparently did not fail rapidly or result in large or catastrophic outflows. Steep valley topography and frequent eruptions at volcanoes in this region make for significant hazards associated with the formation and failure of volcanic debris dams.     

Ant communities and livestock grazing in the Great Basin, U.S.A.

The objectives of this study were to determine if metrics for ant species assemblages can be used as indicators of rangeland condition, and to determine the influence of vegetation and ground cover variables, factors often influenced by livestock grazing, on ant communities. The study was conducted in two areas in the Great Basin: a sagebrush-steppe in southeastern Idaho (n=30 sites), and a salt-desert shrub in western Utah (n=27 sites). Sites were selected based on known rangeland condition (i.e. good, fair, poor) associated with livestock grazing. Ant communities differed considerably between the two study areas. Collectively, more ant species occurred at the Idaho sites (30) than at the Utah sites (21), relatively few species (eight) occurred in both areas, species richness was significantly greater at the Idaho sites (mean=12·0 species) than the Utah sites (mean=6·9 species), and Formica spp. were diverse (total of 15 species) at the Idaho sites but rare (one species) at the Utah sites. In Idaho, all species collectively, generalists, and Formica spp. were significantly less abundant on sites in poor condition than that on sites in good or fair condition, whereas in Utah, seed harvesters and Pheidole spp. were significantly more abundant on sites in poor condition than that on sites in good or fair condition. In Idaho, species richness was significantly lower on sites in poor condition. In Idaho, species richness and relative abundances of several ant groups were significantly related to bare patch size and parameters for cover or species richness of several vegetation groups. In contrast to the comparisons involving sites in poor condition, no differences in ant communities in either Idaho or Utah were evident between sites in good and fair condition. Thus, the ant communities responded only to large changes in rangeland condition and to large differences in climatic/edaphic conditions between the two areas. Hence, ant community metrics appear to have limited utility as indicators of rangeland condition in the Great Basin.     

Alpine Plant Communities: A Statistical Assessment of Their Relation to Microclimatological,Pedological, Geomorphological,and Other Factors

Floristic composition and environmental factors vary widely among plant communities in the alpine belt. Thus far no study has attempted to measure all relevant site conditions in a larger number of alpine communities. Here we show (1) which environmental factors were highly correlated with the floristic composition of the 14 plant communities investigated in the Swiss Alps and (2) which plant communities have similar environmental affinities. In every plant community investigated, the main factors potentially having an impact on plant life were measured and the floristic composition was defined. We used nonmetric multidimensional scaling (NMDS) to determine linkage between plant communities and complex environmental gradients. The first axis of the NMDS corresponds to a climate gradient (temperature/wind speed), and the second axis corresponds to a soil gradient (soil suction/pH/Ca content). With the exception of the Nardus grassland and Carex curvula turf, plant communities belonging to the same phytosociological class are exposed to very similar combinations of environmental factors. Our study shows that the variation between phytosociological classes is much larger than within classes. Still, the variation of environmental factors within individual classes leads to a further differentiation of the floristic composition. Thus, our study reinforces the validity of the phytosociological classification.     

A comparison of classification systems for the conservation of sparsely wooded plains on Melville Island,Northern Australia

A floristic quadrat study of sparsely wooded plains on western Melville Island was conducted and used to define and describe floristic and structural vegetation types and identify their associated environmental correlates. The results were compared with an existing land‐unit survey of the area in order to identify the relative merits of these schemes for conservation planning. All schemes identify a range of broad structural types including Acacia shrubland, Grevillea pteridifolia low woodland, Lophostemon lactifluus low woodland, and Banksia dentata low woodland. However, some of the structural and land‐unit types are composed of two plant communities with distinctive environmental relations. Despite being structurally distinctive, some community types are floristically indistinguishable from surrounding forest dominated by Eucalyptus miniata and E. tetrodonta. The number of species captured by randomly selecting sites from each classificatory type revealed little difference between schemes. All the classificatory approaches were less effective at species capture than a simple iterative procedure. Some of the plant communities seem to be unique and require preservation initiatives in the face of current forestry operations and planned horticultural development.     

Diatom and thecamoebian signatures of Red River (Manitoba and North Dakota) floods: Data collected from the 1997 and 1999 spring freshets

Overbank deposits that aggraded during the severe and moderate Red River, Manitoba and North Dakota floods of 1997 and 1999 were examined to characterize the typical diatom and thecamoebian flood assemblages. The deposits contain diatom assemblages dominated by Nitzschia spp., Stephanodiscus spp. and Navicula spp. as well as large quantities of Hantzschia amphioxys, Luticola imbricata and L. mutica and lesser amounts of Aulacoseira italica, A. granulata, A. ambigua, Cyclotella meneghiniana, Amphora montana and Synedra ulna. Samples collected from the Red River in late summer are dominated by the plankton Cyclotella atomus, species of Stephanodiscus, Cyclostephanos sp. 1 (cf. delicatus), Cyclotella meneghiniana, Nitzschia reversa and Nitzschia acicularis. Late summer, floodplain lake plankton are dominated by Fragilaria capucina, Cocconeis placentula, Stephanodiscus parvus/minutulus and Cyclotella atomus. Red River thecamoebian flood assemblages are dominated by Cyclopyxis spp., with lesser amounts of Centropyxis aculeata and Arcella vulgaris. Late summer water samples from the Red River contain very few thecamoebian remains. The majority of the taxa present are Centropyxis aculeata, Arcella vulgaris and Cyclopyxis spp. Floodplain lakes are dominated by benthic thecamoebians, especially Difflugia oblonga and Cucurbitella tricuspis, a eutrophic indicator. The results of this baseline study suggest that diatom and thecamoebian assemblages can characterize flood deposits, within floodplain lake basins, during both very high and moderately high flood years, and that these assemblages are inherently different from those typically found in lakes along the Red River.     

长江三峡阶地的成因机制

根据对长江三峡阶地堆积物进行的野外调查与室内分析发现三峡阶地的成因具有以下特点:构成阶地上部的河漫滩相堆积、中部的冲积砾石层与作为阶地基座的基岩平台是在不同时期形成的;阶地上部河漫滩相堆积是在中全新世气候温暖、长江三峡高水位条件下形成的.因此,长江三峡阶地是在构造上升的基础上由于气候及长江三峡流量及水位变化而形成的,并非一定是间歇性构造上升的标志.     

Analysis of raudhas vegetation in central Saudi Arabia

The present study analyses the vegetation of three raudhas in central Saudi Arabia (Kharj, Khuraim and Binban). Seventy-five stands were examined and the martrix of 68 species × 75 stands, based on species cover data, was classified using TWINSPAN and ordinated using DECORANA. Eleven vegetation groups were generated: one dominated byHammada elegansand five dominated byAcaciaspp. (A. iraquensis, A. elatior, A. seyal, A. tortilis-Ziziphus spina-christi-Rhazya strictaandA. abyssinica). TheAcaciaspp. communities represent the climax stage of the xerophytic vegetation in Saudi Arabia and have, in most cases, high species cover and low species diversity. Another five groups are dominated, fully or partly, by species that often inhabit disturbed areas (Prosopis juliflora, Heliotropium bacciferum-Tamarix arabica-Prosopis juliflora-Pulicaria crispa, Ziziphus spinsa-christi-Lagonychium farctum-Alhagi maurorum-Cynodon dactylon, Salsola imbricataandBassia eriophoria-Chloris virgata-Zygophyllum coccineum-Seidlitzia rosmarinus). Many of these groups have low species cover and relatively high species diversity. The effects of environmental factors and human impact on species diversity and abundance of these communities were assessed.     

Effect of environmental factors on the flora of alluvial fans in southern Sinai

The purpose of this study was to elucidate the floristic and structural characteristics of simple alluvial fan vegetation of southern Sinai and to relate them to environmental variation. The main question addressed was: how does the floristic composition of fan habitats, species richness and life-form vary in relation to environmental change? Thirty-seven alluvial fans, randomly selected in two mainwadis(WadiFeiran andWadiDahab) were analysed quantitively. Two-way Indicator Species Analysis (TWINSPAN) classification and Canonical Correspondence Analysis (CCA) were applied in successive stages of the data analysis to describe vegetational variation in relation to physical gradients. Seven main clusters of fans were defined on the basis of environmental variation. These clusters are divided into three main groups of species richness. Species richness varied along a moisture gradient. The richest fans were the driest habitats, indicating that maximum species richness did not occur at high moisture availability. The moisture gradient is a complex gradient correlated with elevation, slope, climatic aridity, soil texture and the nature of the soil surface. Fine sand, silt plus clay and calcium carbonate showed significant differences between the internal groups and subgroups of the first and second level of the TWINSPAN classification.     

STUDIES ON TERRACES AND RIVER SYSTEM CHANGES OF THE UPPER YELLOW RIVER

Ｉ.ＴｈｅＴｅｒｒａｃｅｓｏｆｔｈｅＵｐｐｅｒＹｅｌｌｏｗＲｉｖｅｒＴｈｅｔｅｒｒａｃｅｓｏｆｔｈｅｕｐｐｅｒＹｅｌｌｏｗＲｉｖｅｒａｒｅｍｏｓｔｌｙｄｉｓｐｅｒｓｅｄａｌｏｎｇｔｈｅｗｉｄｅｒｃｏｕｒｓｅｓｉｎｓｅｖｅｒａｌＭｅｓｏｚｏｉｃＣｅｎｏｚｏｉｃｂａｓｉｎｓ(Ｆｉｇｕｒｅ1).1.ＴｅｒｒａｃｅｓｂｅｔｗｅｅｎＮｇｏｒｉｎｇＬａｋｅａｎｄＧｙａｒｉｎｇＬａｋｅＴｈｅｓｕｒｆａｃｅｌａｙｅｒｓｏｆｔｈｅｓｅｄｉｍｅｎｔｉｎｔｈｅｗｉｄｅａｎｄｓｈａｌｌｏｗｂａｓｉｎｓｗｈｅｒｅｔｈｅＹｅｌｌ…     

Lacustrine and palustrine carbonate petrography: an overview

Lacustrine limestones were formerly identified by their faunistic (limnea, planorbis) and floristic (Charophytes) content. For 30 years, indications of pedogenesis have been found in many lacustrine deposits, and consequently the concept of palustrine limestone was defined.Lacustrine fabrics are not that numerous: varved, laminated, homogeneous, peloidal, brecciated, gravelly, bioturbated (burrows), bioclastic, algal, and stromatolitic. Detrital beds are sometimes present and are interpreted as bottomset deposits. Palustrine fabrics result from exposure and pedogenesis of lacustrine mud. The main processes involved in this evolution are: cracking, with planar, curved, craze and skew planes, colonization by plants resulting in root traces, marmorization (redistribution of iron due to water table fluctuation), and redistribution of carbonates (needles, subspherical or cylindrical vertical nodules, carbonate coatings, early and late diagenetic crystals, Microcodium). Carbonate palustrine features can be associated with other minerals: palygorskite, gypsum, or silica. Alternation of lacustrine sedimentation and exposure/pedogenesis leads to the pseudo-microkarst facies resulting from enlargement of the complex network of root traces and horizontal cracks. The voids in the pseudo-microkarst facies are infilled with a polyphased internal sediment composed of carbonate and vadose silt and phreatic and vadose cements. Traces of exposure and pedogenesis are less in evidence in lacustrine bioclastic sands and algal-stromatolitic limestones. Finally, under certain conditions, the surficial laminar horizon and its associated perlitic crust (ooids) develops on palustrine muds and form a desert stromatolite.     

Use of Noise to Augment Training Data: A Neural Network Method of Mineral–Potential Mapping in Regions of Limited Known Deposit Examples

One of the main factors that affects the performance of MLP neural networks trained using the backpropagation algorithm in mineral-potential mapping isthe paucity of deposit relative to barren training patterns. To overcome this problem, random noise is added to the original training patterns in order to create additional synthetic deposit training data. Experiments on the effect of the number of deposits available for training in the Kalgoorlie Terrane orogenic gold province show that both the classification performance of a trained network and the quality of the resultant prospectivity map increasesignificantly with increased numbers of deposit patterns. Experiments are conducted to determine the optimum amount of noise using both uniform and normally distributed random noise. Through the addition of noise to the original deposit training data, the number of deposit training patterns is increased from approximately 50 to 1000. The percentage of correct classifications significantly improves for the independent test set as well as for deposit patterns in the test set. For example, using ±40% uniform random noise, the test-set classification performance increases from 67.9% and 68.0% to 72.8% and 77.1% (for test-set overall and test-set deposit patterns, respectively). Indices for the quality of the resultant prospectivity map, (i.e. D/A, D × (D/A), where D is the percentage of deposits and A is the percentage of the total area for the highest prospectivity map-class, and area under an ROC curve) also increase from 8.2, 105, 0.79 to 17.9, 226, 0.87, respectively. Increasing the size of the training-stop data set results in a further increase in classification performance to 73.5%, 77.4%, 14.7, 296, 0.87 for test-set overall and test-set deposit patterns, D/A, D × (D/A), and area under the ROC curve, respectively.     

中国西南山区公路沿线乡村聚落景观格局演变

山区乡村聚落演化机制是聚落地理学研究的热点,为探究山区公路对乡村聚落变化的驱动机理,以贵州省麻江县为研究对象,基于1992,2012年乡村聚落数据,运用GIS技术和景观分析软件,对山区不同地貌的公路沿线乡村聚落景观格局变化进行研究。结果表明:(1)与整个研究区(行政单元)相比,公路影响范围内乡村聚落景观快速增加,景观趋于破碎化和形状复杂化。乡村聚落景观演变速度呈现岩溶槽谷区低山丘陵区中山峡谷区的特点。(2)距公路越近的缓冲区,景观格局变化更加剧烈。在3个公路距离缓冲区,乡村聚落景观演变速度都呈岩溶槽谷区低山丘陵区中山峡谷区的特点。(3)在0~1 500 m距离缓冲区,低坡度的乡村聚落景观格局变化速度明显快于中坡度与高坡度地区。低坡度的景观格局演变速度呈现出岩溶槽谷区低山丘陵区中山峡谷区的特点,而中坡度与高坡度的景观格局演变速度呈现出中山峡谷区低山丘陵区岩溶槽谷区的特点。这说明距公路远近和坡度的变化是影响山区公路影响范围内乡村聚落景观格局演变差异的重要因素。     

Late Quaternary activity of faults and recurrence interval of earthquakes in the eastern Hokuriku region, northern central Japan, on the basis of precise cryptotephra analysis of fluvial terrace sequences

About 2000 active faults are known to exist within the land area of Japan. Most of these active faults have deformed the topographic surfaces which were formed in the late Quaternary, including fluvial terraces; and the formative ages of these terraces are estimated mainly by tephrochronology. Fluvial terraces in the eastern Hokuriku region, comprising the Toyama, Tonami, and Kanazawa Plains, northern central Japan, are widely distributed and have been deformed by reverse active faults. The formative age of terraces in this area has not been reported, as volcanic ash deposits are rarely visible within terrace deposits and the overlying loamy soil, and outcrops of fluvial terraces are quite scarce in this area. In the present study, we carried out a drilling survey on these terraces to obtain samples of the overlying loamy soil and upper part of terrace deposits. From these samples, we extracted some well-known widespread volcanic ash, from which we were able to estimate the approximate age of the terraces and the vertical slip rate of the active faults. Late Quaternary fluvial terraces in eastern Hokuriku are divided into 12 levels: Terraces 1 to 12 in descending order. Widespread tephras such as the Kikai-Tozurahara Tephra (K-Tz: 95 ka) are contained in the lowest part of the loamy soil in Terrace 4 and the Daisen-Kurayoshi Pumice (DKP: 55 ka) is present in the lowest part of the loamy soil in Terrace 6. From the ages and the vertical displacements of the fluvial terraces, the late Quaternary average vertical slip rates of active faults in eastern Hokuriku are estimated to be 0.2–0.9 mm/year (Uozu fault), 0.1–0.4 mm/year (Kurehayama fault), 0.1–0.3 mm/year (Takashozu fault), 0.1–0.4 mm/year (Hohrinji fault), and 0.5–0.8 mm/year (Morimoto-Togashi fault). We also estimated the recurrence interval of earthquakes related to active faults from displacement per event and ages of terraces and no significant difference in vertical displacement per single earthquake for different active faults, and recurrence intervals tend to be inversely proportional to vertical displacement rates. This study demonstrates that a combination of drilling of loamy soil and precise cryptotephra analysis of fluvial terraces can be used to estimate the formative age of the terraces and the average slip rate of active faults in areas where volcanic ash deposits are rare.     

福建武夷山风景名胜区及其附近地区的地貌研究

武夷山风景名胜区及其附近地区由花岗岩、火山熔岩、变质岩和砂页岩等构成的中山、低山占最大面积.最高海拔约1800m,在海拔350～1200m,有7级剥蚀夷平面,形态有山顶缓坡型、山坡肩膀型、谷底裂点型(占绝大多数).丹霞地貌的顶面坡以圆弧形为主,陡崖坡有罕见的切层洞穴群和最大的晒布岩,崖麓缓坡多由红层组成.河流纵谷常沿着NNE走向较软的中生界砂页岩发育,断层谷断续长数十公里;横谷常沿着NWW的走向节理发育.据6个TL年龄认定河流阶地的沉积年代:第一级阶地Q3 ,第二级阶地Q2或Q2为主,第三级阶地Q2.     

Grazing effects on patchy dryland vegetation in northern Patagonia

In this study the spatial patterns and dynamics of vegetation patches along a grazing gradient in the steppe ofLarrea divaricataandStipaspp. in NE Patagonia (Argentina) are described. A general effect of grazing is the reduction of total plant cover resulting from the decrease in cover of perennial grasses (Stipa speciosa, Poa ligularis, Stipa tenuis) and some tall shrubs (Chuquiraga hystrix, Bougainvillea spinosa, Lycium chilense). Dwarf shrubs (Nassauvia fueguianaandJunellia seriphioides) increase their cover under medium and/or high grazing pressures. Plant species are spatially grouped into patches which alternate with areas of bare soil. Eleven types of vegetation patch differing in the dominant plant functional type or species, floristic richness and size were identified with different relative frequency along the grazing gradient. Based on these results, it is postulated that grazing forces the replacement of large patches dominated by tall shrubs with high species richness, byLarrea divaricatapatches or small dwarf shrub patches with low species richness and the extinction of grass patches. This results from: (1) disruption of local balances of species deletions and additions; (2) fragmentation of large patches; and (3) formation of new vegetation patches. These changes lead to differing plant spatial organization and heterogeneity along the grazing gradient which may be described by characteristic arrays of vegetation patches.     

Land degradation and vegetation distribution in Chott El Beida wetland,Algeria

The aim of this study is to explore the environmental factors that determine plant community distribution in northeast Algeria. This paper provides a quantitative analysis of the vegetation–environment relationships for a study site in the Chott El Beida wetland, a RAMSAR site in Setif, Algeria. Sixty vegetation plots were sampled and analysed using TWINSPAN and Detrended Correspondence Analysis (DCA) in order to identify the principal vegetation communities and determine the environmental gradients associated with these. 127 species belonging to 41 families and 114 genera were recorded. Six of the recorded species were endemic representing 4.7% of the total species. The richest families were Compositae, Gramineae, Cruciferae and Chenopodiaceae. Therophytes and hemicryptophytes were the most frequent life forms. The Mediterranean floristic element is dominant and is represented by 39 species. The samples were classified into four main community types. The principal DCA axes represent gradients of soil salinity, moisture and anthropogenic pressure. The use of classification in combination with ordination techniques resulted in a good discrimination between plant communities and a greater understanding of controlling environmental factors. The methodology adopted can be employed for improving baseline information on plant community ecology and distribution in often critically endangered Mediterranean wetland areas.     

Probabilistic Estimates of Number of Undiscovered Deposits and Their Total Tonnages in Permissive Tracts Using Deposit Densities

Empirical evidence indicates that processes affecting number and quantity of resources in geologic settings are very general across deposit types. Sizes of permissive tracts that geologically could contain the deposits are excellent predictors of numbers of deposits. In addition, total ore tonnage of mineral deposits of a particular type in a tract is proportional to the type’s median tonnage in a tract. Regressions using size of permissive tracts and median tonnage allow estimation of number of deposits and of total tonnage of mineralization. These powerful estimators, based on 10 different deposit types from 109 permissive worldwide control tracts, generalize across deposit types. Estimates of number of deposits and of total tonnage of mineral deposits are made by regressing permissive area, and mean (in logs) tons in deposits of the type, against number of deposits and total tonnage of deposits in the tract for the 50th percentile estimates. The regression equations (R ² = 0.91 and 0.95) can be used for all deposit types just by inserting logarithmic values of permissive area in square kilometers, and mean tons in deposits in millions of metric tons. The regression equations provide estimates at the 50th percentile, and other equations are provided for 90% confidence limits for lower estimates and 10% confidence limits for upper estimates of number of deposits and total tonnage. Equations for these percentile estimates along with expected value estimates are presented here along with comparisons with independent expert estimates. Also provided are the equations for correcting for the known well-explored deposits in a tract. These deposit-density models require internally consistent grade and tonnage models and delineations for arriving at unbiased estimates.