Dry River Valleys Research Articles

A preliminary report of the excavation at Spitzkloof D rockshelter, Namaqualand, South Africa

This paper describes the first excavations of Spitzkloof D rockshelter located in the semi-arid desert of northern Namaqualand, South Africa. The site is in a dry river valley 30 km south of the Orange River, which currently acts as a lifeline for pastoralists with mixed sheep and goat herds. A surface survey of the site revealed pottery, lithics, domesticate remains, ostrich eggshell (OES) fragments and jewellery, glass beads and iron fragments. The stratigraphy is complex reflecting multiple occupations with six layers consisting of large hearths, ash deposits, and multiple pits, some with potential votive (faunal) offerings in their base. Faunal analysis reveals a broad subsistence strategy consisting of low-intensity sheep-keeping combined with the hunting of wild species found on the landscape today. The presence of Equus zebra, a locally extinct water obligate species, suggests occupation during a climatic period that was more humid than today. Radiocarbon dates from the upper layers confirm a Little Ice Age occupation between AD 1667-1936, when the region was cooler and wetter, and a peak in radiocarbon dates indicates a population pulse in the region. Glass trade beads, iron implements, OES beads, as well as fish remains and a limpet shell, potentially indicate that the people occupying Spitzkloof D were part of an extensive trade/interaction network. Future analysis will include increasing sample sizes through continued excavation, detailed lithic analysis, and further radiocarbon dating.

The South Asian monsoon maintains the disjunction of Rumex hastatus between the western Himalayas and the Hengduan Mountains, southwest China

The Himalayas and the Hengduan Mountains of southwest China are hotspots of both climatic and species diversity. Yet, the distribution patterns of semi‐arid plant communities that have arisen throughout the region's complex uplift history remain insufficiently understood. In particular, the striking disjunctions of plants associated with dry river valleys that solely occur on the eastern and western ends of the Himalayan arc, but are absent in between, lack a sound explanation. Here we aim to disentangle the more recent environmental drivers behind the disjunction of the semi‐woody shrub Rumex hastatus found in dry river valleys of the western Himalayas and the Hengduan Mountains. We used ecological niche modeling and enhanced random forest parameterizations (down‐sampling) to infer suitable and unsuitable habitats within the study area during the last glacial maximum, under present climate conditions, and under four future climate scenarios. Annual and seasonal precipitation contributed the strongest to the discrimination between occurrences and the distribution gap, with the latter showing much higher rainfall amounts. The gap was also characterized by low overall habitat suitability, whereas occurrence probabilities in dry river valleys inhabited by R. hastatus were very high. However, some suitable but isolated habitats within the gap were detected in Bhutan and the southeastern Tibetan Plateau. The identified distribution gap was consistent in all simulated scenarios from the last glacial maximum to the near future. Our results suggest that the South Asian summer monsoon system acts as a persistent ecological barrier maintaining the disjunct distribution of dry river valley‐dwelling plants in the western Himalayas and the Hengduan Mountains.

Genomic insights into the genotype-environment mismatch and conservation units of a Qinghai-Tibet Plateau endemic cypress under climate change.

Habitat loss induced by climate warming is a major threat to biodiversity, particularly to threatened species. Understanding the genetic diversity and distributional responses to climate change of threatened species is critical to facilitate their conservation and management. Cupressus gigantea, a rare conifer found in the eastern Qinghai–Tibet Plateau (QTP) at 3000–3600 m.a.s.l., is famous for its largest specimen, the King Cypress, which is >55 m tall. Here, we obtained transcriptome data from 96 samples of 10 populations covering its whole distribution and used these data to characterize genetic diversity, identify conservation units, and elucidate genomic vulnerability to future climate change. After filtering, we identified 145,336, 26,103, and 2833 single nucleotide polymorphisms in the whole, putatively neutral, and putatively adaptive datasets, respectively. Based on the whole and putatively neutral datasets, we found that populations from the Yalu Tsangpo River (YTR) and Nyang River (NR) catchments could be defined as separate management units (MUs), due to distinct genetic clusters and demographic histories. Results of gradient forest models suggest that all populations of C. gigantea may be at risk due to the high expected rate of climate change, and the NR MU had a higher risk than the YTR MU. This study deepens our understanding of the complex evolutionary history and population structure of threatened tree species in extreme environments, such as dry river valleys above 3000 m.a.s.l. in the QTP, and provides insights into their susceptibility to global climate change and potential for adaptive responses.

Biogeomorphology of nebkhas in the Mu Us dune field, north-central China: Chronological and morphological results

Nebkhas, also known as nebkha dunes or coppice dunes, are a unique biogeomorphological aeolian landform that is common in arid and semi-arid regions. They are often regarded as a signal of regional desertification and could be potential dust sources due to their relatively large content of fine sediments, but they also serve as “fertile islands” that enhance biodiversity at a micro-habitat. How the interactions between plant growth and aeolian sand deposition affect the formation and development of nebkhas has not been well documented, though. In this study, the chronology and morphological features of nebkhas in the Mu Us dune field, north-central China were studied using radionuclide (137Cs) dating and unmanned aerial vehicle (UAV) photogrammetric survey. The results show that the nebkhas are mainly distributed around lake basins and dry river valleys with relatively flat topography. Most nebkhas have heights around 0.5–1.5 m, and according to a new integrated age-height model, they mostly formed in the last fifty years. Widespread nebkha growth over that time is interpreted as a response to both desiccation of lake beds and river valleys and a pronounced decline in strong wind frequency. The regular rounded shape of the nebkhas in the study area may reflect the growth form of prostrate, extensively branched Nitraria tangutorum, the dominant formative shrub, and response to the low-energy wind environment. A nebkha's height and horizontal length increase synchronously in the early stage of the development, while the increase in horizontal length and projected area accelerates when the dunes become larger. Feedbacks involving both aeolian sand transport and shrub growth can explain a decline in growth rate with increasing height and an apparent height limit of about 4 m. This study also implies that nebkhas are important biogeomorphological features demonstrating how biotic and abiotic interactions can shape landforms and influence ecological functioning in arid and semi-arid environments.

Flora of the Savanna-like Vegetation in Hot Dry Valleys, Southwestern China with Implications to their Origin and Evolution

Savanna-like vegetation and dry thickets occur in hot dry valleys across southwestern China. Here, the flora and biogeography in hot dry valleys with savanna-like vegetation in SW China are studied. Native seed plants of 3217 species and varieties from 1038 genera in 163 families are recorded from these hot dry valleys. The biogeographical elements with a tropical distribution contribute 57.18%, but the ones with a temperate distribution contribute 36.45% of the total genera of the flora. This implies that the flora in hot dry valleys is of tropical margin nature and has been influenced by temperate elements. Floristic divergence across these hot dry valleys is obvious. The floras in the Yuanjiang and the Nujiang valleys are dominated by tropical elements (with a tropical distribution) (77.26% and 74.49 of the total genera, respectively), but the flora of the Jinshajiang valley is composed of half tropical (47.27%) and half temperate (44.96%) genera. Regarding floristic similarities, the Jinshajiang shows the highest similarity to the Yuanjiang although these river valleys are located a great distance from each other. We hypothesize the geological events in Yunnan could profoundly influence the flora and its divergence of the savanna-like vegetation in these deep, hot dry river valleys. Our results could be well explained from the geological events since the Cenozoic, such as the uplift of Himalayas, the extrusion of Indochina, the river capture of the Jinshajiang separating from the Yuanjiang, and the northward movement of the Burma Plate. Our study could be clues to explore the possible formation of the savanna-like vegetation.

Impacts of the channel/barrier effect and three-dimensional climate—A case study of rice water requirement and irrigation quota in Yunnan, China

Yunnan is a large rice-growing province in China that is situated in a low-latitude plateau area. In this province, the channel/barrier effect and three-dimensional climate play prominent roles and significantly influence the rice water requirement (ETc) and irrigation quota (IQ), while the research about the influence mechanism and results is lacking. We estimated the rice ETc and IQ of different hydrological years based on the experimental data from six areas of mid-season rice and the single crop efficient approach recommended by FAO-56. The contour maps were drawn with GIS, and the spatial distribution of the rice ETc and IQ under the influence of the channel/barrier effect and three-dimensional climate were analyzed. The results showed that high water ETc and IQ of rice were distributed in the hot, dry river valley area. The rice ETc was the smallest in northwestern Yunnan, and the IQ was the smallest in southern Yunnan. The ETc and IQ of rice were divided into several intervals by mountains and valleys, and varied largely among different channels and little within one channel. The three-dimensional climate results in the abrupt changes of ETc and IQ, The leading meteorological factors causing the abrupt changes were different, and there were also complex correlations between these meteorological factors. Taken together, temperature and rainfall were the most critical elements in the three-dimensional climate. The results of this study illustrated the regularity and inherent mechanism of the rice ETc and IQ under the influences of the channel/barrier effect and three-dimensional climate, providing certain references for the analyses and calculation of the rice ETc and IQ in similar circumstances.

Biogeographic regions and events of isolation and diversification of the endemic biota of the tropical Andes

Understanding the spatial and temporal evolution of biota in the tropical Andes is a major challenge, given the region's topographic complexity and high beta diversity. We used a network approach to find biogeographic regions (bioregions) based on high-resolution species distribution models for 151 endemic bird taxa. Then, we used dated molecular phylogenies of 14 genera to reconstruct the area history through a sequence of allopatric speciation processes. We identified 15 biogeographical regions and found 26 events of isolation and diversification within their boundaries that are independently confirmed with disjunct distributions of sister taxa. Furthermore, these events are spatially congruent with six geographical barriers related to warm and/or dry river valleys, discontinuities in elevation, and high peaks separating fauna from different range slopes. The most important barrier is the Marañon River Valley, which limits the boundaries of four bioregions and is congruent with eight phylogenetic distribution breaks, separating the Central and Northern Andes, where the most bioregions are found. We also show that many bioregions have diffuse and overlapping structures, with contact and transition zones that challenge previous conceptions of biogeographical regions as spatially simple in structure. This study found evidence that the drivers of our identified bioregions were processes of Andean uplift and mountain dispersal facilitated by temperature oscillations of the Pleistocene. Therefore, Andean bioregions were not formed from one simple biogeographical event in a certain time frame, but from a combination of vicariance and dispersal events, which occurred in different time periods.

增加降水对干旱河谷区云南松人工林土壤呼吸的影响

PDF HTML阅读 XML下载 导出引用 引用提醒 增加降水对干旱河谷区云南松人工林土壤呼吸的影响 DOI: 10.5846/stxb201509301987 作者: 作者单位: 四川农业大学林学院(温江校区),四川阿坝师范学院,四川农业大学林学院(温江校区),四川农业大学林学院(温江校区),四川农业大学林学院(温江校区),四川农业大学林学院(温江校区) 作者简介: 通讯作者: 中图分类号: 基金项目: 国家“十二五”科技支撑项目（2010BACO1A11） Effects of precipitation increase on soil respiration of a Pinus yunnanensis plantation in a dry river valley area Author: Affiliation: College of Forestry, Sichuan agricultural university,Sichuan ABA Teachers University,College of Forestry, Sichuan agricultural university,,College of Forestry, Sichuan agricultural university,College of Forestry, Sichuan agricultural university Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:2013年5月至2014年6月，对干旱河谷区云南松（Pinus yunnanensis）人工林进行增加降水试验，试验设置对照（CK，0 mm m-2 a-1）、增水10%（A1，80 mm m-2 a-1）、增水20%（A2，160 mm m-2 a-1）和增水30%（A3，240 mm m-2 a-1）4个处理水平。采用LI-8100开路式土壤碳通量测量系统测定每月土壤呼吸速率。结果表明，4个处理云南松人工林土壤呼吸速率均呈明显的季节变化，7月最高，2月最低。与CK相比，A1年均土壤呼吸速率无显著性差异（P > 0.05），A2显著增加了12.88%（P < 0.05），而A3明显减少了17.71%（P < 0.05）。3个增水处理均提高了土壤呼吸的温度敏感性，减弱了土壤呼吸与土壤湿度的关系。与土壤温度相比，土壤湿度对土壤呼吸的影响相对较小。增水增加了湿季土壤微生物碳、氮含量，干季对微生物碳含量无影响，但明显降低了微生物氮含量。这说明，降水增加对干旱河谷区云南松人工林土壤呼吸的影响是不尽相同的，适当的增水会促进土壤呼吸，而过量的增水会抑制土壤呼吸。 Abstract:The objective was to examine the effects of global climate change inducing precipitation pattern change on soil respiration (Rs) of Pinus yunnanensis plantation in dry river valley area of China(Shimian, Yaan, Sichuan). Based on average annual precipitation, Four treatments were set, ie, CK(natural state,0 mm m-2 a-1), A1(increasing precipitation 10%, 80 mm m-2 a-1), A2(increasing precipitation 20%, 160 mm m-2 a-1) and A3(increasing precipitation 30%, 240 mm m-2 a-1). Soil respiration rate was measured using LI-8100 monthly from Jun, 2013 to May, 2014. Results indicate that soil respiration rate exhibited obvious seasonal variation in Pinus yunnanensis plantation, with the maximum in July and the minimum in February. Precipitation increase treatment significantly increased Rs in A2 (by 12.88%), significantly decreased Rs in A3 (by 17.71%), and did not significantly affect Rs in A1. Soil respiration Q10 value calculated from soil temperature was increased under precipitation increase treatments. The relationship between soil respiration and soil moisture was weakened. Compared with soil temperature, soil moisture had less effect on the soil respiration. In wet season, microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) were increased significantly by increasing precipitation. In dry season, only MBN decreased significantly under precipitation treatments. Therefore, there was a significant effect of different increasing precipitation on soil respiration in a Pinus yunnanensis plantation in the dry river valley area. Moderate increasing rainfall would promote soil respiration, while superfluous increasing rainfall would inhibit soil respiration. 参考文献 相似文献 引证文献

Early performance of Pinus radiata provenances in the earthquakeravaged dry river valley area of Sichuan, southwest China

A provenance experiment involving five native provenances and an Australian landrace of Pinus radiata (D. Don) was established over three sites in the dry river valley area of Sichuan, southwest China in 2004 in order to select the most suitable provenance for environmental planting on the dry, steep and degraded slopes to reduce soil erosion. Although with much lower soil moisture supply and mean minimum temperatures in winter compared to P. radiata provenance trials established elsewhere in the world, these sites are within the working limits of the species defined by previous climate modelling and matching. Because of the difficult site conditions and severe natural disturbances after the experiment was established, mortality was high across the three sites in comparison to provenance trials in other countries. The average mortality rate among the provenance by replicate planting units over the three sites varied from 16% to 76% four years after planting, and from 40% to 88% five years after planting. The repeated measurements of tree size over time were analysed using multilevel linear mixed models to derive growth curves for the mean, median, the 75th and the 90th percentiles of the size distribution of each provenance at each site. There were significant site effects on tree growth, but no significant interactions between site and provenance was detected. Among the six provenances, Cambria was the best performer in diameter, height and stem volume growth across all sites. The better than average and the best trees of this provenance, as represented by the 75th and 90th percentiles of the nominal stem volume distribution, were significantly larger than the Australian landrace, Ano Nuevo, and the two island provenances, Guadalupe and Cedros. Monterey was overall the second best performer behind Cambria. The Australian landrace, Guadalupe and Ano Nuevo had similar performances in general. Cedros was significantly and consistently inferior to all other native provenances and the Australian land race. Because the genetic base of the present Australian plantations was derived largely from Ano Nuevo and Monterey, the superior early growth performance of Cambria at such difficult sites brings a new promise to the search of P. radiata provenances for the vast dryland areas in New South Wales and other parts of Australia.

A sub‐basin scale dust plume source frequency inventory for southern Africa, 2005–2008

AbstractWe present a dust plume source inventory for southern Africa. In order to locate and track the local, short‐lived plume events, source and frequency data have been derived from Meteosat Second Generation (MSG) thermal infrared composite data (4 km data using 8.7, 10.8, and 12.0 µm) and Moderate Resolution Imaging Spectroradiometer (MODIS) visible composite data (0.25 km data utilizing 0.620 – 0.670 µm, 0.545 – 0.565 µm, and 0.459 – 0.479 µm). Between January 2005 and December 2008, a total of 328 distinct daytime dust plumes more than 10 km in length were detected. These plumes were attributed to 101 distinct point sources, consisting largely of ephemeral inland lakes, coastal pans as well as dry river valleys in Namibia, Botswana, and South Africa. These data also provided sub‐basin scale source observations for large basins such as Etosha and Makgadikgadi Pans.

Survey of damaging pests and preliminary assessment of forest health risks to the long term success of Pinus radiata introduction in Sichuan, southwest China

Pinus radiata was introduced to the summer rainfall environments of Sichuan Province, China in the 1990s as a part of an affore-station program for soil and water conservation in the arid and semi-arid river valley area of Aba Prefecture. Within this region a total area of 26 000 ha have been identified through climate matching as suitable and a further 63 000 ha potentially suitable for environmental plantings of P. radiata. The plantations are being established in widely separated small patches on steep and degraded slopes along the dry river valley. The newly introduced P. radiata are exposed to two kinds of forest health risks: they may be attacked by (a) indigenous pathogens and pests against which they may not possess any resistance or (b) by inadvertently introduced foreign pests or pathogens. This paper presents a survey of the potential damaging pests and a preliminary assessment of forest health risks facing the P. radiata plantations over a much longer timeframe than the initial phase of introduction and early plantation establishment. An empirical approach was adopted to evaluate forest health risks by a combination of literature review, examination of historical records of pest and disease outbreaks in the surrounding coniferous forests, field surveys and inspections, specimen collection and identification, and most importantly, expert analysis of the likelihood of attack by specific pests and pathogens and the subsequent impact of such attacks. The assessment identified some specific forest health risks to the long-term success of P. radiata introduction in this area. These risks are closely associated with the indigenous pests and pathogens of the two native pine species, P. tabulaeformis and P. armondii since these pests and pathogens are considered more likely to establish on P. radiata over time. Exotic pests and pathogens are of a quarantine concern at present. Based on the results of assessment, recommendations are provided to improve forest vigour and to reduce the forest health risks posed by indigenous as well as exotic pests and pathogens to the introduced P. radiata. Ways to increase the ability to manage the forest health risks once a particular pest infestation and disease eventuates are also recommended. Although detrimental to the survival and growth of the introduced P. radiata, the impact of identified forest health risks are not considered to be fatal to the long term success of P. radiata in this area.

Stratigraphy and palaeoenvironment of the dinosaur-bearing Upper Cretaceous Iren Dabasu Formation, Inner Mongolia, People's Republic of China

New field observations and sedimentological analyses of the dinosaur-bearing Upper Cretaceous Iren Dabasu Formation in the Iren Nor region of Inner Mongolia (People's Republic of China) have led to a better understanding of its palaeoenvironment. The fluvial deposits represent a braided river that, due to the large amount of fines, does not fit the classical model for braided rivers. The study area is divided into two parts: in the northern part, sediments of the main channel belt of the ancient braided river system are exposed along a dry river valley on the northern edge of the Iren Nor salt lake, while in the southern part, comprising all other studied exposures, different facies of the ancient floodplain are represented, including minor channels, temporary ponds, and palaeosols. The difference between the northern and southern parts is also reflected in the fossil content; only the southern exposures have yielded dinosaur remains. The ancient braided river had a broad, vegetated floodplain populated by a diverse dinosaur fauna. Four species of charophytes are described and illustrated from the Iren Nor site, together with eight species of ostracods, one of which ( Cypridea irennorensis sp. nov.) is new. Contrary to the vertebrate data, both groups of microfossils indicate a latest Cretaceous age (Campanian–Maastrichtian) for the Iren Dabasu Formation, and suggest a possible correlation with the Nemegt Formation, which would allow the age estimation to be refined to latest Campanian–Early Maastrichtian.

Introduction ofPinus radiata for afforestation: a review with reference to Aba, China

Pinus radiata D. Don, native to a California coastal environment, has been introduced to many parts of the world as an exotic species for afforestation. It is now a major plantation species in the Southern Hemisphere. In 1990, it was introduced to the heavily degraded, dry Min river valley area in Aba prefecture of Sichuan Prevince, P. R. China. Survival and growth of young trees planted st several sites appear to be reasonable. This review is to serve as an introduction to the large body of literature onP. radiata for forest scientists in Chine. It convers the following aspects:P. radiata in its native environment and in ex situ plantations, provenance and genetic variations, environmental limitations and climate niche, diseases and pests, lessons from unsuccessful introductions, and the use ofP. radiata for ecological restoration. The early growth ofP. radiata planted in the dry river valley area is briefly described. Potential problems associated with the introduction ofP. radiata in Aba and future research needs are also identified.

Windows of opportunity and the temporal structuring of foraging activity in a desert solitary bee

Summary1. Females of the desert solitary bee Anthophora pauperata collect nectar and pollen almost exclusively from Alkanna orientalis (Boraginaceae). The bee and plant are found together in the early spring, living in the bottom of steep‐sided wadis (dry river valleys) at an altitude of 1500 m in Egyptian Sinai.2. Female A. pauperata showed clear morning and afternoon peaks in foraging activity, separated by a 2–3 h midday period spent in their underground nests. This study analyses the following in order to identify the factors structuring this daily pattern: thermal aspects of the bee and its environment, temporal patterns of resource provision by the plant, and female nectar and pollen foraging behaviour.3. Although A. pauperata can generate substantial heat endothermically, morning and evening ambient temperatures well below 10 °C defined a thermal window within which foraging occurred. Maximum air temperatures were moderate (25–30 °C), and examination of the physiology and behaviour of A. pauperata suggests that the midday reduction in flight activity was not due to thermal constraints.4. Alkanna orientalis produces protandrous hermaphroditic flowers. Female A. pauperata collected pollen from male‐phase flowers and harvested nectar preferentially from female‐phase flowers. Although the nectar standing crop was relatively constant throughout the day, pollen availability peaked strongly in the early afternoon.5. Female A. pauperata visited young male‐phase flowers as soon as they opened, generating an early afternoon peak in pollen foraging activity and depleting the pollen standing crop rapidly. A morning peak in pollen foraging occurred when females gleaned remnant pollen from flowers that had opened the previous day. Pollen availability in the morning was far lower than in the early afternoon, and the time taken to collect a full pollen load in the morning was significantly longer. Collection of pollen in the morning despite very low resource availability suggests that pollen may be a limiting resource for A. pauperata.6. In contrast to many existing examples of bimodal activity patterns in highly endothermic bees, the bimodal activity patterns of female A. pauperata appear to be driven not by thermal considerations but by daily patterns of pollen release from its principal food source.

Status of Raso LarkAlauda razaewith notes on threats and foraging behaviour

The Raso LarkAlauda razaeis a globally endangered species that is endemic to the islet of Raso (16°93′ N, 24°38′ W) in the Cape Verde archipelago (Collaret al.1994). Raso is a small (c.7 km2) uninhabited islet with a plain averaging 25 m in elevation to the south and west and small boulder-strewn hills to the north and east rising up to 164 m (Figure 1). The islet comprises mostly rocky desert, with some extensive sandy patches on the western plain. Vegetation is sparse and confined to small patches on the plains and along dry river valleys.

Deep-water massive sands: facies, processes and channel geometry in the Numidian Flysch, Sicily

The sedimentology of the Numidian Flysch in northern Sicily has been little studied in the last twenty years, with most works concentrating on the structural complexities of the area, rather than the facies characteristics. The Numidian basin was intensively deformed during its complex tectonic history and the basin deposits have become segmented into different structural units. The Oligo-Miocene aged Flysch comprises predominantly quartzarenitic sandstones and interbedded mudstones, both of turbiditic affinities and most likely derived from a NW African source. The sediments are well exposed along the coast and in dry river valleys in northern Sicily, and provide excellent examples of deep-water massive sands, i.e. very thick (4–25 m) units of structureless sandstone associated with turbidites and related facies. The overall characteristics of the examined sections indicate that the sandstones were deposited within both isolated channels and larger channel complexes that fed across a mud-dominated slope. The more proximal sections, e.g. Ponte Finale and Pollina have a high proportion of pebble conglomerates, shale-clast conglomerates and very thick-bedded structureless pebbly sandstones, deposited by high-density turbidity currents, debris flows and slumping. Other sections, either more distal or with a more uniform sand-rich source, e.g. Contrada di Romano, are dominated by very thick-bedded and amalgamated structureless sandstones with extensive zones of water-escape structures. The massive sands are thought to originate from the gradual aggradation of sediment beneath steady or near-steady flows, with rapid deposition from final-stage modified grain flows forming the water-escape features.

Field assessment of flood event suspended sediment transport from ephemeral streams in the tropical semi-arid catchments

An assessment of suspended sediment transport was carried out in a number of semiarid catchments during flood events in order to quantify the degradation rates. In order to quantify these, a systematic sampling procedure of the episodic flood events was proposed for representative catchments. The procedure allows for an integration over the whole run-off episode using both the rising and falling limbs of the run-off hydrograph to compute the sediment quantities for each individual flood event.Higher sediment concentrations occurred in the rising limb than those at the recession for any stage of flow. The maximum suspended sediment concentration was observed at the peak of the flood hydrograph. An integration of the sediment concentration over its duration gave the total sediment yield from the flood event. For the ephemeral channels, only a small number of flood events were observed over a three-year experimental period each with a duration of the order of 3-6 h. It is notable that high sediment loads were associated with high flow volumes which were effectively the result of the catchment characteristics and incident rainfall causing the flood events in the respective catchments. A large percentage of the annual sediment yield from a catchment is transported by the ephemeral streams during a small number of flood events. The correct determination of the total sediment yield from any of the flood events depends entirely on the accuracy of the measurements.The understanding of run-off and sediment loss for the representative catchments aims at assisting planning, management and control of water and land resources for sustainable development in the semi-arid parts of the tropics. The sediment rates reveal the degradation of catchments which have repercussions on the crop and pasture production and this has a bearing on the soil and water conservation programmes in the delicate ecological balance of the semi-arid areas. Further, these rates will determine the lifespan of the reservoirs planned for the dry river valleys (ephemeral streams) and existing ones which serve livestock and domestic needs. These occasionally will require costly rehabilitation and scooping to increase effective storage unless conservation measures are taken, and these measures are bound to vary from place to place as per the representative catchments output.

The Excavation of Winklebury Camp, Basingstoke, Hampshire

Winklebury Camp (SU61355290), an Iron Age 'plateau fort’, now of some 7.6 hectares (19 acres), is situated in the parish of Basingstoke, approximately 1 mile north-west of the town centre of Basingstoke, (fig. 1), though it is now surrounded by housing estates. It occupies a hill of Upper Chalk which has been isolated from the main mass of the North Hampshire Downs by two dry river valleys, one trending south-east, the other north-east. Both are tributary to the R. Loddon. The hill rises to a maximum height of 126 m a.s.l., near the projected line of the western rampart and slopes off gradually to north and south, and more steeply to the south-east. About half a mile to the south of the fort, a tributary of the R. Loddon, now culverted, would have provided the nearest source of water.Winklebury Camp has suffered from gradual encroachment by the surrounding housing estates, losing the ditch and rampart face to the north and east, and ditch, rampart and a small part of the interior, to the west. Only to the south does any of the bank and ditch survive. In the south-west corner of the fort, in an area protected by the remains of a small copse, a 90 m length of eroded rampart remains to an approximate height of 2 m. There is no ditch fronting this rampart now, but immediately east of the end of the stretch of rampart is the only remaining length of ditch. This extends for some 200 m along the remainder of the south side of the fort, averaging 2 m deep and 5 m wide. It rises in the south-east corner at a causeway, beyond which is another short stretch of ditch which is rapidly lost in house gardens. This causeway is faced by rampart, but there are indications, for example a slight change in the direction of the rampart at this point, that this is in fact a blocked entrance.

Pleistocene Changes in the Fauna and Flora of South America

In recent years, the view that Pleistocene climatic events played a major role in the evolution of the biotas of southern, primarily tropical continents has begun to displace the previously held conviction that these areas remained relatively stable during the Quaternary. Studies of speciation patterns of high Andean plant and avian taxa (7-14) have led to the conclusion that Pleistocene climatic events were the factors that ultimately shaped the patterns now observed in the paramo-puna and the related Patagonian flora and fauna. The final uplift of the Andes at the end of the Tertiary automatically limits the age of the high Andean habitats and their biotas to the Quaternary. Within this period, the number of ecological fluctuations caused by the glaciations could easily have provided the mechanism behind the patterns now present in these habitats (Appendix, 1; Figs. 1 and 2; Table 1). In glacial periods, when vegetation belts, were lowered, organisms in the paramo-puna habitat were allowed to expand their ranges. In interglacial periods, these taxa were isolated on disjunct peaks, where differentiation could occur. At times of ice expansion, glacial tongues and lakes provided local barriers to gene exchange, whereas in warm, interglacial times, dry river valleys were a major deterrent to the interbreeding of populations on different mountains (Fig. 2; Table 2). A preliminary analysis of about 10 to 12 percent of the total South American avifauna (14), subsequent to the study of the high Andean biota, suggested that the birds of all the major habitats of the continent possess, with about equal frequency, similar stages of speciation. This correspondence in levels of evolution indicated that the avifauna of vegetation zones which were thought to have been more stable (for example, tropical rainforests) are as actively speciating as are those of the more recent paramo-puna habitats. More intensive work on lowland tropical taxa (16, 19-21) and recent work on montane forest elements (40) now justify the conclusion that the floras and faunas of these areas were also greatly affected by Pleistocene climatic shifts. In the broad region of South America that lies within the tropics, a series of humid-arid cycles (Appendix, 6, 8-10) drastically and repeatedly altered vegetation patterns during the Quaternary. Both montane and lowland rainforests were fragmented during dry periods and were able to reexpand during humid phases. Speciation of forest elements was initiated-and sometimes completed-in isolated patches of the fragmented forest. Secondary contact, with hybridization or reunition of populations that did not become reproductively isolated, occurred in periods of reexpansion. These biological data, combined with supportive geological evidence (Appendix, 1-11), show that climatic events during the last million or so years have affected the biota of South America as much as the Pleistocene glacial changes affected the biotas of Eurasia and North America. Since most of South America lies within tropical latitudes, it is suggested here that part of the diversity of species in the tropical areas of this continent is due to two historical factors: the lack of wholesale elimination of species (compared with northern and high latitudes), and ample opportunity for speciation in successive periods of ecological isolation. The apparent paradox of the wealth of species in the "stable tropics" is partially explained by the fact that the tropics have probably been quite unstable, from the point of view of their biotas, during the Pleistocene and perhaps part of the Tertiary.

Exploring for Plants in Southern Tibet

HAVING left Tezpur, an ancient Assamese town, on the right bank of the Brahmaputra, towards the end of April last year, and crossed the outer range of the Assam Himalaya by the Pankim La at 10,000 ft., I arrived in the dry river valleys early in May. There is only one possible route over the great range in a distance of three hundred miles, between the Bhutan frontier and the gorge of the Brahmaputra; east of the Bhareli River savage tribes Daflas, Akas, Abors and others bar the way. Travelling leisurely northwards, the Tibetan frontier was crossed by the Se La (14,000 ft.) early in June and Monyul was reached. The rainy season had begun, and this part of the journey, over a series of moderately high passes, was cold and wet.