Aim The primary objectives of this study were (1) to assess, in the light of palaeoecological reconstruction, the climate stability hypothesis used by evolutionary biologists to explain high diversity in historically stable areas, and (2) to identify the response mechanisms of a tropical rain forest microrefugium to climatic variability. Location North-eastern Brazil, Serra de Maranguape. Methods Vegetation and climatic changes were reconstructed using a pollen record in a sediment core from a forest hollow, and the chronology was based on accelerator mass spectrometry radiocarbon analyses. Results Past vegetation dynamics consisted of three main forest types, shown by major compositional changes in rain forest assemblages between 5000 and 1000 cal. yr bp. Dense ombrophilous forest was abruptly replaced by heliophilous early successional tree taxa at 4275 cal. yr bp. These early successional tree taxa were established over a period of c. 100 years, and their dominance lasted for c. 750 years and was associated with dry conditions until 3525 cal. yr bp. Subsequently, the expansion of secondary successional tree taxa over a period of c. 550 years enabled the recovery of ombrophilous forest. Main conclusions The vegetation changes in the Serra de Maranguape provide evidence for the high sensitivity of this rain forest microrefugium to climatic variability on a multidecadal to millennial time-scale during the mid- to late Holocene. Despite the substantial compositional and climatic changes, this microrefugium apparently was continuously forested and responded to climatic instability by recruiting key species to its highly diverse stock. This evidence helps to address the joint concerns of evolutionary biologists and palaeoecologists regarding how forests can persist during periods of climatic variability by showing that some tropical regions can remain continuously forested despite reorganization during abrupt and short-term climatic changes. (Résumé d'auteur

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-Water from small alluvial aquifers constitutes an attractive and low-cost option for irrigation and rural development in Northeastern Brazil. Based on piezometric measurements, geochemical analyses and electrical conductivity estimates, the present case study identified the main processes determining the hydrosaline dynamics of an alluvial aquifer in a small watershed inserted in the crystalline bedrock of a semi-arid region in Ceará and evaluated the availability of water for irrigation. Accumulation of salts in soil are related to evaporative flux from the aquifer and is increased by irrigation from the groundwater of the alluvial aquifer. The water in these aquifers may be used for irrigation, but represents a risk of soil salinization and alkalinization. Integrated management of surface and underground water resources in the Forquilha watershed may help control irrigation water quality (salinity and residual alkalinity), thereby rationalizing the use of local reservoirs and minimizing losses from evaporation. It has to take into account the complex dynamic of salts and water between the reservoirs, release of water into the river, floods and irrigations.Key words -Salinity. Groundwater. Irrigation.Resumo -A agricultura irrigada a partir da água subterrânea dos pequenos aqüíferos aluviais é uma alternativa interessante e de baixo custo para o desenvolvimento do meio rural no Nordeste brasileiro. A partir de um estudo de caso numa micro-bacia no centro da área cristalina do semiárido cearense é analisada a contribuição de características físicas (piezometria), geoquímicas e de modelos (de balanço hidrológico e de massa) para identificar a origem e os principais processos que governam a dinâmica da salinidade das águas de um pequeno aqüífero aluvial e avaliar a disponibilidade de água para irrigação. A irrigação conduz a uma redistribuição dos sais da zona saturada para a zona não saturada do aqüífero, podendo ocorrer acumulação. Devido as suas características, as águas do aqüífero aluvial podem ser usadas para a irrigação, entretanto, existe certo grau de risco de salinização e sodificação dos solos. O manejo integrado das águas superficiais e subterrâneas da bacia permite controlar a qualidade da água (salinidade e sodicidade residual) disponibilizada para a irrigação, permitindo ainda um melhor aproveitamento das águas dos açudes e limitando as perdas por evaporação. Ele deve levar em conta a dinâmica complexa da água e dos sais entre os reservatórios, as liberações de água no rio, as cheias e as irrigações.Palavras-chave -Salinidade. Água subterrânea. Irrigação. * Autor para correspondência 1 Recebido para publicação em 31/08/2010; aprovado em 31/01/2011 Trabalho submetido e selecionado no primeiro Simpósio Brasileiro de Salinidade realizado de 12-15/10/2010 em Fortaleza, Ceará, Brasil; Financial support received from FUNCEME, CNPQ and CAPES-COFECUB for the doctorate study in water resources of Julien