Plants can have positive effects on each other. For example, the accumulation of nutrients, provision of shade, amelioration of disturbance, or protection from herbivores by some species can enhance the performance of neighbouring species. Thus the notion that the distributions and abundances of plant species are independent of other species may be inadequate as a theoretical underpinning for understanding species coexistence and diversity. But there have been no large-scale experiments designed to examine the generality of positive interactions in plant communities and their importance relative to competition. Here we show that the biomass, growth and reproduction of alpine plant species are higher when other plants are nearby. In an experiment conducted in subalpine and alpine plant communities with 115 species in 11 different mountain ranges, we find that competition generally, but not exclusively, dominates interactions at lower elevations where conditions are less physically stressful. In contrast, at high elevations where abiotic stress is high the interactions among plants are predominantly positive. Furthermore, across all high and low sites positive interactions are more important at sites with low temperatures in the early summer, but competition prevails at warmer sites.

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It is generally assumed that tree growth in the upper limit of a forest is mainly controlled by summer temperature. This general statement is mostly based on studies from extra‐tropical mountains and has been rarely evaluated in subtropical latitudes frequently characterized by drier climates. In the subtropical mountains from Northwestern Argentina (∼23° S), annual precipitation decreases with elevation from >1500 mm at 1200– 1500 m, to <200 mm above 4000 m. In consequence, tree growth at high elevations in the region may be seriously limited by water supply. In order to assess the influence of precipitation on tree growth, we evaluated the relationships between climatic variations and radial growth in four species growing at different altitudinal zones: Juglans australis from the montane cloud forest at 1800 m; Alnus acuminata from the montane savanna‐like woodland at 2700 m; Prosopis ferox from the subalpine dryland at 3500 m; and Polylepis tarapacana from the high‐elevation alpine dryland at 4750 m. Dendrochronological techniques were used to relate variations in annual ring width with instrumental climatic records. Growth rings were correctly dated to the year of formation, and the cross‐dated series standardized using autoregressive models to reduce non‐climatic signals present in the records. Tree‐ring chronologies, ranging from 117 to 341 years, were compared, during the common period, with instrumental climatic records using correlation‐function analysis. In spite of the remarkable differences in elevation and environmental conditions among the sampling sites, correlation functions with climate indicated that the radial growth of the four species is largely controlled by precipitation. In most cases, increased precipitation during the previous and current growing seasons favors tree growth, while temperatures are negatively correlated with radial growth, likely due to the negative effect on water availability. These results indicate that the generalized idea of upper‐treeline growth limited by summer temperatures should be carefully evaluated in low‐latitude environments and does not apply to subtropical areas with severe water deficits or strong moisture seasonalities.

Radiometric corrections serve to remove the effects that alter the spectral characteristics of land features, except for actual changes in ground target, becoming mandatory in multi-sensor, multi-date studies. In this paper, we evaluate the effects of two types of radiometric correction methods (absolute and relative) for the determination of land cover changes, using Landsat TM and Landsat ETM + images. In addition, we present an improvement made to the relative correction method addressed. Absolute correction includes a crosscalibration between TM and ETM + images, and the application of an atmospheric correction protocol. Relative correction normalizes the images using pseudo-invariant features (PIFs) selected through band-to-band PCA analysis. We present a new algorithm for PIFs selection in order to improve normalization results. A post-correction evaluation index (Quadratic Difference Index (QD)), and post-classification and change detection results were used to evaluate the performance of the methods. Only the absolute correction method and the new relative correction method presented in this paper show good postcorrection and post-classification results (QD index < 0; overall accuracy .80%; kappa .0.65) for all the images used. Land cover change estimations based on uncorrected images present unrealistic change rates (two to three times those obtained with corrected images), which highlights the fact that radiometric corrections are necessary in multi-date multi-sensor land cover change analysis.

Landsliding is a complex process that modifies mountainscapes worldwide. Its severe and sometimes long-lasting negative effects contrast with the less-documented positive effects on ecosystems, raising numerous questions about the dual role of landsliding, the feedbacks between biotic and geomorphic processes, and, ultimately, the ecological and evolutionary responses of organisms. We present a conceptual model in which feedbacks between biotic and geomorphic processes, landslides, and ecosystem attributes are hypothesized to drive the dynamics of mountain ecosystems at multiple scales. This model is used to integrate and synthesize a rich, but fragmented, body of literature generated in different disciplines, and to highlight the need for profitable collaborations between biologists and geoscientists. Such efforts should help identify attributes that contribute to the resilience of mountain ecosystems, and also should help in conservation, restoration, and hazard assessment. Given the sensitivity of mountains to land-use and global climate change, these endeavors are both relevant and timely.

We analyzed changes in land cover in the Sierra de San Javier and its surroundings, an area of ca. 70 000 ha near San Miguel de Tucumán, an urban center of ca. 1 million people in subtropical Argentina. The analysis covered two periods: 1949-1972 and 1972-2006 using remote sensing techniques. For the year 2001, we mapped the patterns of distribution of secondary forests dominated by the most abundant exotic tree species (Ligustrum lucidum). Based on land-cover maps, we estimated sediment yield as an index of watershed condition. Urban area was growing during the whole study period. Between 1949 and 2006, forest area increased approximately 1400 ha, mostly over abandoned agriculture and grasslands; this expansion was accelerated between 1972 and 2006. Increased forest cover resulted in a reduction in erosion and sediment yield that was disproportionately large, as most new forests are located in areas of steep slopes and high rainfall. By 2001, Ligustum-dominated forests had expanded to more than 500 ha, in the southern portion of the sierra only. Overall, the analysis quantifies a process of Neotropical periurban forest transition, likely associated with socioeconomic changes related to population urbanization, that promotes improvements of some environmental services, such as watershed and biodiversity conservation. However, natural communities are strongly affected by past land use and neighboring urban areas, which have promoted a growing importance of exotic species with mostly unknown ecological consequences.

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