Aridity

Aim  There remains some uncertainty concerning the causes of extinctions of Madagascar’s megafauna. One hypothesis is that they were caused by over-hunting by humans. A second hypothesis is that their extinction was caused by both environmental change and hunting. This paper systematically addresses the second hypothesis through examination of two new pollen records from south-eastern Madagascar alongside other published records across the island.Location  South-eastern Madagascar.Methods  We reconstructed past vegetation and fire dynamics over the past 6000 years at two sites in south-eastern Madagascar (Ste-Luce) using fossil pollen and charcoal contained in sedimentary sequences. We investigated drivers of vegetation changes and how these, in turn, influenced faunal species in the south-east, using published climatic, archaeological and faunal records. Further, we also used published records to provide a synthesis of environmental changes on the whole island.Results  Vegetation reconstructions indicate that the mosaic vegetation in the region of Ste-Luce was highly dynamic in response to climatic changes. The open woodland, surrounding the littoral forest, transformed into an ericoid grassland between c. 5800 and 5200 cal. yr bp, possibly in response to a moderate drought recorded during this period. The littoral forest was more stable between c. 5100 and 1000 cal. yr bp, with only some minor compositional changes c. 2800 cal. yr bp and between c. 1900 and 1000 cal. yr bp. Significant forest decline, however, is observed at c. 950 cal. yr bp, coinciding with a drought and a marine surge. A comparison of these results with a synthesis of published vegetation records across the island shows asynchronous vegetation changes in response to various droughts during the Holocene, except for the 950 cal. yr bp drought event, with evidence of widespread vegetation transformations and fires across the island.Main conclusions  Pronounced climatic desiccation between 1200 and 700 cal. yr bp may have been the slow driver framing and triggering vegetation transformations and decline in megafaunal populations. In addition, hunting by drought-impacted human inhabitants and competition with newly introduced cattle would have amplified the impacts on megafaunal populations, leading to numerous extinctions in this period.

A survey of C4 plants in Europe was performed with 216 species based on information in the literature and new studies. C4 species were found in 10 families: the eudicots Amaranthaceae, Chenopodiaceae, Euphorbiaceae, Molluginaceae, Nyctaginaceae, Polygonaceae, Portulacaceae and Zygophyllaceae and the monocots Cyperaceae and Poaceae. The majority of the C4 species belong to four families, Amaranthaceae (23), Chenopodiaceae (65), Cyperaceae (27) and Poaceae (88). In central and southern Europe, the abundance of native C4 plants varied between 44 and 88% of total C4 plants occurring, the rest being invasive C4 species. The occurrence of total C4 species, C4 monocots and C4 Chenopodiaceae was assessed for five major phyto-geographical regions of Europe (north-west, north-east, central, south-west, and south-east). The abundance of C4 plants of total C4 dicots, C4 Chenopodiaceae, total C4 monocots, C4 Poaceae and C4 Cyperaceae was related to the climatic variables of annual mean daily temperature, annual precipitation and DeMartonne's aridity index. The abundance of total C4 plants decreases with increasing temperature and expression of aridity (decreasing aridity index) and is not correlated with precipitation. The abundance of total C4 dicots and C4 Chenopodiaceae is correlated with precipitation and aridity but not temperature, whereas the abundance of total C4 monocots, C4 Poaceae and C4 Cyperaceae is correlated with temperature and aridity but not precipitation. © 2010 The Linnean Society of London, Botanical Journal of the Linnean Society, 2010, 163, 283–304.

La sécheresse et la salinité agissent simultanément dans la tolérance et l’acclimatation sous conditions salines. Par conséquent, les plantes soumises à ces types de stress devraient avoir développé des adaptations structurelles spécifiques dès le stade de germination en passant par la croissance de la racine, les feuilles et la tige. La solution à ces problèmes environnementaux est de rechercher des espèces relativement économes en eau et résistantes à des épisodes récurrents de divers stress abiotiques tel que le stress salin. L’effet de la sécheresse s’est manifesté au niveau de la distribution des espèces d’acacia le long du territoire algérien, caractérisé beaucoup plus par la pluviométrie. En effet, La répartition géographique du genre Acacia Mill. (Fabales Mimosaceae) en Algérie a été déterminée après la prospection, la localisation et la description des populations de différentes espèces sur l’ensemble du territoire algérien. Vingt-quatre régions ont été prospectées en fonction de l'abondance relative des espèces. Selon les données climatiques des années 2000-2016, les espèces suivantes sont adaptées à une pluviométrie annuelle inférieure à 100 mm : A. albida, A. ehrenbergiana, A. laeta, A. nilotica, A. seyal et A. tortilis. Trois espèces (A. karroo, A. farnesiana et A. saligna) se trouvent dans des zones de pluviométrie annuelle comprise entre 250 et 500 mm). A. decurrens est situé dans une zone où les précipitations annuelles dépassent 1000 mm. L’étude de l’effet des prétraitements sur les dix espèces recensées en Algérie a été nécessaire afin d’améliorer la régénération de ces espèces. Ainsi, le taux de germination passe de 5-10% dans la nature à plus de 90% après traitement à l’acide sulfurique pendant une durée bien définie. Les résultats approuvent aussi que le type de dormance chez les graines de ce genre est de type physique. L'objectif de l’étude de l’effet de la salinité sur la germination contribue aussi à déterminer le seuil de tolérance des semences et de leur régénération dans un milieu salin. Seulement trois parmi les dix espèces répertoriées seront étudiées. Cette sélection est basée surtout sur la distribution géographique et la densité de chaque espèce choisie. A. saligna représente le Nord, A. tortilis pour le Sud, et A. karroo qui est une espèce qui est présente dans le Nord et dans le Sud en même temps. Le classement des espèces étudiées dans un ordre de tolérance décroissant, sur la base de leur capacité germinative, est alors le suivant : A. karroo > A. tortilis > A. saligna. La croissance, l'homéostasie ionique, l'osmoprotection, l’indice de tolérance à la salinité, ainsi que les ajustements anatomiques d'A. karroo et d’A. saligna cultivés à diverses concentrations de NaCl pendant 21 jours ont été évalués. Les résultats ont montré que le sel a entrainé des changements remarquables au niveau de certains paramètres liés à la croissance, du RWC et du RWL, des pigments photosynthétiques, des protéines et des glucides, en fonction du niveau de sel. Les teneurs en Na+, en Ca2+ ainsi que le rapport Na+/K+ dans les feuilles a augmenté avec les degrés de salinité, tandis que les teneurs en K+ a été réduit de manière significative par rapport aux plantes témoins chez ces deux espèces d’acacia. Les teneurs en proline, en acides aminés libres totaux et en sucres réducteurs se sont considérablement accumulées au niveau des feuilles. Les examens anatomiques ont révélé diverses modifications anatomiques dans les tissus des tiges et des racines vis-à-vis du stress salin, telles que l'augmentation/réduction de l’épaisseur, cortex, de la bande vasculaire ou de la moelle. A. karroo s’est montré plus tolérante à la salinité qu’A. saligna. En effet, les valeurs de l’indice de tolérance des plantes au stress salin suggèrent qu’A. karroo a efficacement adopté certains mécanismes afin de s’acclimater à des niveaux de salinité plus élevés. Nos résultats montrent que l'adaptabilité d’une espèce à la salinité est étroitement liée à la sélectivité en ions, à l'accumulation d'osmoprotecteurs, aux ajustements anatomiques et à la production de la biomasse sèche. Nos résultats confirment qu'A. karroo et A. saligna sont deux espèces d'arbre appropriée pour la conservation du sol et de l'écologie, ce qui favoriserait la restauration des terres dans les zones arides et semi-arides tout en profitant économiquement des intérêts de ces espèces.

El presente artículo representa una revisión y un complemento de la literatura existente, así como una motivación para el estudio de las zonas áridas en un contexto ingenieril y de solución de problemas comunes en hidrología e hidráulica. Se plantean las recomendaciones y criterios para el análisis hidrológico en este tipo de regiones, que no han sido suficientemente estudiadas, sabiendo que la mayor parte de la normativa internacional y local se basan en teorías y experimentaciones en regiones húmedas. Se proponen métodos actuales para el tratamiento de información hidrometeorológica escasa y la predicción de caudales en cuencas sin medición, a través de casos aplicados a la vertiente del Pacífico peruano.

Desertification, the process of land degradation in dry areas, is one of the most complex environmental and socio-economical threatening events. Although Nigeria is a party to the United Nations Convention to combat Desertification, the extent and severity of desertification in Nigeria has not been fully established, neither the rate of its progression properly documented. Nevertheless there is a general consensus that desertification is by far the most pressing environmental problem in the northern parts of the country. The desertification processes are identifiable from the gradual shift in vegetation of trees and bushes to more grasses, and bushes to desert-like sand along the past decades. Moreover, this agricultural area serves as the food basket for the entire country making the region highly vulnerable to the impact of these phenomena. Increasing aridity is an important cause of desertification, which is a major challenge experienced in northern Nigeria. It is therefore important to quantitatively determine the level of aridity being experienced in this region. This study assessed the spatio-temporal patterns of a measurement of aridity, named Aridity Intensity Index (AII), to determine the extent of this phenomenon over the period from 1998 to 2013, using daily precipitation data from the Tropical Rainfall Measuring Mission (TRMM) satellite. These data were grouped into two 5-year periods (1998–2002, and 2009–2013), and we produced interpolated surfaces of the AII using Inverse Distance Weighting and Ordinary Kriging for each year within the two groups, as well as for averaged values of the AII for both groups. Additionally, the corresponding error prediction maps were produced. The results showed that the most northern region of Nigeria is threatened by high aridity and dryness with a general small decline in dryness in the south most tip of the study area itself.

Arid regions in the Old World Dry Belt are assumed to be marginal regions, not only in ecological terms, but also economically and socially. Such views in geography, archaeology, and sociology are—despite the real limits of living in arid landscapes—partly influenced by derivates of Central Place Theory as developed for European medieval city-based economies. For other historical time periods and regions, this narrative inhibited socio-economic research with data-based and non-biased approaches. This paper aims, in two arid Graeco-Roman landscapes, to show how far approaches from landscape archaeology and social network analysis combined with the “small world phenomenon” can help to overcome a dichotomic view on core places and their areas, and understand settlement patterns and economic practices in a nuanced way. With Hauran in Southern Syria and Marmarica in NW-Egypt, I revise the concept of marginality, and look for qualitatively and spatially defined relationships between settlements, for both resource management and social organization. This ‘un-central’ perspective on arid landscapes provides insights on how arid regions functioned economically and socially due to a particular spatial concept and connection with their (scarce) resources, mainly water.

Keywords:
aridity; marginality; landscape archaeology; Marmarica (NW-Egypt); Hauran (Syria/Jordan); Graeco-Roman period; spatial scales in networks; network relationship qualities; interaction; resource management

How well the ecology, zoogeography and evolution of modern biotas is understood depends substantially on knowledge of the Pleistocene1, 2. Australia has one of the most distinctive, but least understood, Pleistocene faunas. Records from the western half of the continent are especially rare3. Here we report on a diverse and exceptionally well preserved middle Pleistocene vertebrate assemblage from caves beneath the arid, treeless Nullarbor plain of south-central Australia. Many taxa are represented by whole skeletons, which together serve as a template for identifying fragmentary, hitherto indeterminate, remains collected previously from Pleistocene sites across southern Australia. A remarkable eight of the 23 Nullarbor kangaroos are new, including two tree-kangaroos. The diverse herbivore assemblage implies substantially greater floristic diversity than that of the modern shrub steppe, but all other faunal and stable-isotope data indicate that the climate was very similar to today. Because the 21 Nullarbor species that did not survive the Pleistocene were well adapted to dry conditions, climate change (specifically, increased aridity) is unlikely to have been significant in their extinction.

Arid regions in the Old World Dry Belt are assumed to be marginal regions, not only in ecological terms, but also economically and socially. Such views in geography, archaeology, and sociology are—despite the real limits of living in arid landscapes—partly influenced by derivates of Central Place Theory as developed for European medieval city-based economies. For other historical time periods and regions, this narrative inhibited socio-economic research with data-based and non-biased approaches. This paper aims, in two arid Graeco-Roman landscapes, to show how far approaches from landscape archaeology and social network analysis combined with the “small world phenomenon” can help to overcome a dichotomic view on core places and their areas, and understand settlement patterns and economic practices in a nuanced way. With Hauran in Southern Syria and Marmarica in NW-Egypt, I revise the concept of marginality, and look for qualitatively and spatially defined relationships between settl...

از گذشته هاي دور كشاورزان مناطق خشك كويري از روش هاي مختلفي براي استفاده بهينه تر از منابع آب استفاده مي نمودند. ولي با گذشت زمان كشاورزان خود را محدود به روش هاي مورد استفاده پيشينيان نكرده  بلكه با استفاده از تجربيات خود روش هاي جديدي ابداع نموده اند. در اين مقاله برخي از روش هاي نويني كه كشاورزان استان اصفهان براي استفاده بهينه تر از منابع آبی خود به کار می گیرند مانند استفاده از روش آبياري جويي به جاي غرقابي، تغيير نوع محصولات زير كشت، كشت زيرپلاستيك، تغيير اندازه كرت ها، تغيير الگوي استفاده از كود معرفي خواهد شد.

In this study, we present grain-size distributions of the terrigenous fraction of two deep-sea sediment cores from the SE Atlantic (offshore Namibia) and from the SE Pacific (offshore northern Chile), which we "unmix" into subpopulations and which are interpreted as coarse eolian dust, fine eolian dust, and fluvial mud. The downcore ratios of the proportions of eolian dust and fluvial mud subsequently represent paleocontinental aridity records of southwestern Africa and northern Chile for the last 120,000 yr. The two records show a relatively wet Last Glacial Maximum (LGM) compared to a relatively dry Holocene, but different orbital variability on longer time scales. Generally, the northern Chilean aridity record shows higher-frequency changes, which are closely related to precessional variation in solar insolation, compared to the southwestern African aridity record, which shows a remarkable resemblance to the global ice-volume record. We relate the changes in continental...

Paleo Lake Bungunnia covered more than 40 000 km2 of southern Australia during the Plio-Pleistocene, although the age and origin of the lake remain controversial. The Blanchetown Clay is the main depositional unit and outcrop at Nampoo Station in far-western New South Wales provides the most continuous lacustrine section preserved in the basin. Here the Blanchetown Clay represents the maximum lake fill and comprises: (i) a basal well-sorted sand with interbedded clay (Chowilla Sand), representing initial flooding at the time of lake formation; (ii) a thick sequence of green-grey clay comprised dominantly of kaolinite and illite, with the apparently cyclic occurrence of illite interpreted to represent cool and dry glacial climatic intervals; and (iii) a 2.6 m-thick sequence of finely laminated silt and silty clay, here defined as the Nampoo Member of the Blanchetown Clay. New magnetostratigraphic data constrain the age of the oldest lake sediments to be younger than 2.581 Ma (Matuyam...

Aim Geographical, climatic and soil factors are major drivers of plant beta diversity, but their importance for dryland plant communities is poorly known. The aim of this study was to: (1) characterize patterns of beta diversity in global drylands; (2) detect common environmental drivers of beta diversity; and (3) test for thresholds in environmental conditions driving potential shifts in plant species composition. Location Global. Methods Beta diversity was quantified in 224 dryland plant communities from 22 geographical regions on all continents except Antarctica using four complementary measures: the percentage of singletons (species occurring at only one site); Whittaker’s beta diversity, b(W); a directional beta diversity metric based on the correlation in species occurrences among spatially contiguous sites, b(R2); and a multivariate abundance-based metric, b(MV). We used linear modelling to quantify the relationships between these metrics of beta diversity and geographical, c...

El objetivo general de este trabajo fue generar datos para el armado de un marco de referencia adecuado para el estudio paleodietario humano, mediante el analisis de isotopos estables, en la ecorregion de Puna, Noroeste argentino. Con tal fin, se presentan los primeros resultados del analisis isotopos estables del carbono y el nitrogeno (δ13C y δ15N) en vegetales, principalmente Solanum tuberosum (n = 24). Los especimenes se adquirieron en el mercado local y en huertas familiares, estas ultimas sin recibir ni abonos ni plaguicidas sinteticos, los cuales pueden incidir en las relaciones isotopicas del nitrogeno. La metodologia propuesta consistio en el relevamiento de informacion entre productores locales, la caracterizacion del suelo en cuanto a sus propiedades quimicas (PH, conductividad electrica, carbono organico y nitratos) y la medicion isotopica de los vegetales muestreados. Los datos obtenidos demuestran una escasa variacion en los valores δ15N, aunque tres especimenes de la ...

The Murray Basin is a low-lying but extensive intracratonic depocentre in southeastern Australia, preserving an extraordinary record of Late Neogene sedimentation. New stratigraphic and sedimentologic data allow the long-term evolution of the basin to be re-evaluated and suggest a significant role for: (1) tectonism in controlling basin evolution, and (2) progressive and step-wise climatic change beginning in the early Pleistocene. Tectonic change is associated with regional uplift, occurring at approximately the same rate from the early Pliocene until the present day, and possibly associated with changing mantle circulation patterns or plate boundary processes. This uplift led to the defeat and re-routing of the Murray River, Australia’s major continental drainage system. Key to our interpretation is recognition of timing relationships between four prominent palaeogeographic features e the Loxton-Parilla Sands strandplain, the Gambier coastal plain, palaeo megalake Bungunnia and th...

Optically stimulated and thermoluminescence ages from relict shorelines, along with accelerator mass spectrometer 14C ages from freshwater molluscs reveal a record of variable moisture sources supplied by northern and southern river systems to Lake Mega-Frome in southern central Australia during the late Quaternary. Additional lacustrine, palynological and terrestrial proxies are used to reconstruct a record that extends back to 105 ka, confirming that Lakes Mega-Frome and Mega-Eyre were joined to create the largest system of palaeolakes on the Australian continent as recently as 50–47 ka. The palaeohydrological record indicates a progressive shift to more arid conditions, with marked drying after 45 ka. Subsequently, Lake Mega-Frome has filled independently at 33–31 ka and at the termination of the Last Glacial Maximum to volumes some 40 times those of today. Further sequentially declining filling episodes (to volumes 25–10 those of today) occurred immediately prior to the Younger Dryas stadial, in the mid Holocene and during the medieval climatic anomaly. Southern hemisphere summer insolation maxima are a poor predictor of palaeolake-filling episodes. An examination of multiple active moisture sources suggests that palaeolake phases were driven independently of insolation and at times by some combination of enhanced Southern Ocean circulation and strengthened tropical moisture sources.► Relict lake shorelines show that Australia has become more arid over the last 100 ka (85 characters). ► Lake Mega-Frome drained to Lake Eyre for the last time between 50 and 47 ka (75 characters). ► It then filled 33 - 31 ka, post-LGM, in the late Pleistocene and mid Holocene (77 characters). ► We also record lake filling occurringduring the medieval climatic anomaly (74 characters). ► Southern ocean and tropical moisture sources fill these inland Australian lakes (80 characters).