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The study of the temporal-spatial variability of dry sequences and the probability of their occurrence are particularly important in understanding the impact of climate change on droughts. This paper analyses the different properties of dry sequences, focussing on extreme condition, analysing their degree of spatial coherence, and their temporal variability. For the study, daily precipitation data are used for the period 1961-2000 throughout Argentina. The region north of 40°S is divided in two from the meridian of 63°W with highly differentiated 'dry' properties. The eastern region is more homogeneous where mean dry sequences last less than six days and long sequences of about 60 days. The Andean region shows a marked east-west gradient in any of the above properties, with extreme values of over 10 days (mean sequences) and 150 days (the longest sequences). At the seasonal level the above properties reflect a differential pattern according to the time of year under study. In the case of summer, when the dry sequences can be more harmful to crops, the maximum duration in the Pampa region can extend to about 25 days. Owing to the problems that dry sequences of over 30 days can produce in the different ecosystems and based on the above findings, the temporal-spatial variability is analysed, and a variability over the years, together with a progressive decrease in the occurrence of such events, is found. The study of extreme dry events provides useful tools that can be applied to different hydrological and agricultural needs. Furthermore, our climatological findings help validate climate models. Copyright © 2010 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

Microrefugia are important for supporting populations during periods of unfavourable climate change and in facilitating rapid migration as conditions ameliorate. With ongoing anthropogenic climate change, microrefugia could have an important conservation value; however, a simple tool has not been developed and tested to predict which settings are microrefugial., We provide a tool based on terrain ruggedness modelling of individual catchments to predict Andean microrefugia. We tested the predictions using nine Holocene Polylepis pollen records. We used the mid-Holocene dry event, a period of peak aridity for the last 100 000 yr, as an analogue climate scenario for the near future., The results suggest that sites with high terrain rugosity have the greatest chance of sustaining mesic conditions under drier-than-modern climates. Fire is a feature of all catchments; however, an increase in fire is only recorded in settings with low rugosity., Owing to rising temperatures and greater precipitation variability, Andean ecosystems are threatened by increasing moisture stress. Our results suggest that high terrain rugosity helps to create more resilient catchments by trapping moisture through orographic rainfall and providing firebreaks that shelter forest from fire. On this basis, conservation policy should target protection and management of catchments with high terrain rugosity. [ABSTRACT FROM AUTHOR]

Aim To test the niche breadth hypothesis ( NBH), which states that dominant species have broader environmental tolerances than rare species, focusing on oligarchic species distributions (1) along the gradients of edaphic and climatic individual variables, and (2) within the n-dimensional environmental frame defined by all edaphic and climatic variability. Location Amazonian and Andean tropical rain forests along a ca. 3000 m elevation gradient, covering an area of 200 × 200 km in northwestern Bolivia. Methods All woody plants with a DBH ≥2.5 cm were measured in 98 0.1-ha plots. We analysed 18 chemical properties of the soils in each plot. Climatic and topographic variables were obtained from available open-access databases. Three measures were calculated for each of the species found at each forest type: (1) regional-scale dominance based on frequency and local abundance, (2) niche breadth along each of the environmental variables, and (3) total niche size within the whole environment. Results Oligarchic species showed broader niche breadths than the other species that constituted the community assembly in both Amazonian and Andean rain forests. The niche breadth of any species tended to be positively correlated with its degree of dominance. The Amazonian forest showed a stronger oligarchic pattern than the Andean forest, and the Amazonian common species showed larger niches overall. However, this pattern differed for some particular variables: Amazonian oligarchies had narrower niches along the variables related to organic matter and most climatic variables, whereas Andean oligarchies had narrower niches along several micronutrient factors and temperature variables. Conclusions The results provide strong empirical support for the NBH in tropical rain forests. However, different patterns of dominance were found in the two habitats: oligarchic species ranged from narrow-niched species to very broad generalist species. Broad-niched oligarchic species have also been reported in other regions, suggesting an important role of niche size linking commonness at different scales. Oligarchic species exhibit relatively narrow niches with respect to soil factors if measured along wide gradients, regardless of the forest type studied. In contrast, the opposite pattern was found for many climatic variables, indicating higher sensitivity to climate in Amazonia compared to the Andes. Despite these differences, Amazonia has much larger total niche sizes for its common species than the Andes overall. [ABSTRACT FROM AUTHOR]

The Andes are known to have influenced speciation patterns in many taxa, yet whether species diversification occurred simultaneously with their uplift or only after uplift was complete remains unknown. We examined both the phylogenetic pattern and dates of branching in Adelomyia hummingbirds in relation to Andean uplift to determine whether diversification coincides with the chronological phases of the uplift or with recent climatic fluctuations after Andean formation. Results suggest that the genus Adelomyia originated in the central Andes in the Miocene and was found to be comprised of six deeply divergent phylogroups dating between 3.5 and 6 Ma. The most basal splits in the tree, corresponding to the most southerly distributed of the six phylogroups, diverged in the late Miocene, whereas the northern phylogroups originated during the early-to-mid-Pliocene, when the northern Andes reached heights sufficient to support Adelomyia populations. Although we provide evidence for a southern origin for the group, the subsequent diversification of the northern phylogroups did not strictly follow the hypothesized south-to-north orogeny of the Andes. Further genetic structure within phylogroups may have resulted from Pleistocene climate fluctuations after the onset of the six lineages during the Mio-Pliocene. We explore the processes that promoted diversification in the Andes and suggest that in at least some groups, divergence was coupled to Andean orogeny. [ABSTRACT FROM AUTHOR]