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RESUMENLas tierras áridas y semiáridas representan el 60% del territorio mexicano. Además de albergar a diversas especies endémicas, concentran una gran proporción de la población y ofrecen a sus habitantes servicios ecosistémicos cruciales. Sin embargo, los impactos de actividades productivas han generado procesos de degradación y desertización, acentuados por la alta sensibilidad de estos sistemas a la variabilidad climática. La información sobre los procesos de cambio en el uso del suelo y los impactos de la variabilidad climática en las regiones áridas y semiáridas de México se encuentra dispersa en diferentes documentos y es necesario hacer un ejercicio de integración para entender las sinergias en el territorio y brindar información más completa para la toma de decisiones. En ese sentido, este estudio integra la información sobre cambios en el uso del suelo y cobertura vegetal en cinco ecorregiones áridas y semiáridas de México (sonorense, chihuahuense, tamaulipeca, Altiplanicie mexicana y valle de Tehuacán-Cuicatlán) para el periodo 2002-2011, y su exposición actual a sequías, bajas temperaturas, inundaciones y ciclones tropicales. Discutimos estos resultados en términos de las tendencias climáticas regionales documentadas en la literatura, lo que nos sirve de marco de referencia para analizar las interacciones entre esos factores. Encontramos que el cambio de uso de suelo y pérdida de cobertura vegetal difiere entre ecorregiones, siendo la sonorense y la chihuahuense las menos afectadas. Por otra parte, a excepción de la ecorregión del valle de Tehuacán-Cuicatlán, todas las ecorregiones presentan grados de exposición altos a eventos extremos, aunque cada una presenta un perfil distinto. Las tendencias climáticas observadas en estas ecorregiones apuntan hacia un aumento tanto de temperaturas máximas como mínimas; las tendencias para precipitación son menos claras. Nuestros resultados son un punto de partida para ubicar las áreas con procesos antrópicos de cambio de uso de suelo y eventos climáticos extremos, con el fin de identificar posibles sinergias entre estos dos estresores y plantear un mejor manejo de estas áreas en un contexto de cambio global. ABSTRACTArid and semi-arid lands in Mexico dominate 60% of the national territory. They harbor many endemic species, as well as a large proportion of the population, and offer diverse ecosystem services. These regions are subjected to anthropogenic impacts derived from agriculture and livestock farming, which have caused land degradation and desertification. In addition, these ecosystems are highly sensitive to climate variability and projected global environmental change. However, information about how land use/land cover change and This is an open access article under the CC BY-NC License (http://creativecommons.org/licenses/by-nc/4.0/). 356 P.A. Pontifes et al.climate variability processes overlap and synergize each other in these ecoregions is scattered in different publications, and it is desirable to integrate it to provides more complete information f...

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Abstract. Ombrotrophic raised peatlands provide an ideal archive for integrating late Holocene records of variations in hydroclimate and the estimated stable isotope composition of precipitation with recent instrumental measurements. Modern measurements of mean monthly surface air temperature, precipitation, and δD and δ 18 O-values in precipitation from the late twentieth and early twenty-first centuries provide a short but invaluable record with which to investigate modern relationships between these variables, thereby enabling improved interpretation of the peatland palaeodata. Stable isotope data from two stations in the Global Network for Isotopes in Precipitation (GNIP) from southern South America (Punta Arenas, Chile and Ushuaia, Argentina) were analysed for the period 1982 to 2008 and compared with longer-term meteorological data from the same locations (1890 to present and 1931 to present, respectively). δD and δ 18 O-values in precipitation have exhibited quite different trends in response to local surface air temperature and precipitation amount. At Punta Arenas, there has been a marked increase in the seasonal difference between summer and winter δ 18 O-values. A decline in the deuterium excess of summer precipitation at this station was associated with a general increase in relative humidity at 1000 mb over the surface of the Southeast Pacific Ocean, believed to be the major vapour source for the local precipitation. At Ushuaia, a fall in δ 18 O-values was associated with an increase in the mean annual amount of precipitation. Both records are consistent with a southward retraction and increase in zonal wind speed of the austral westerly wind belt. These regional differences, observed in response to a known driver, should be detectable in peatland sites close to the GNIP stations. Currently, insufficient data with suitable temporal resolution are available to test for these regional differences over the last 3000 yr. Existing peatland palaeoclimate data from two sites near Ushuaia, however, provide evidence for changes in the late Holocene that are consistent with the pattern observed in modern observations.

High-altitude páramo grasslands are important for their biodiversity and the ecosystem services that they provide to Andean people, but they are sensitive to disturbances, such as fire. Understanding the ecological impacts of disturbance is critical for the effective management of páramos. Indicator species studies can provide a relatively efficient way to gain such understanding. Puya hamata is a flagship giant rosette plant and has potential as an indicator of recent páramo fire history. To determine population size structure, mortality, recruitment and growth rates of Puya hamata rosettes, all Puya plants in 400 m 2 plots were surveyed in 2008 and again one year later. Sixteen plots were recorded in both years, containing exactly 1000 plants. Mortality was very low during this period (0.6%). Only 27 new plants were recruited. Three different size distribution patterns were observed in the plots: (1) low plant numbers across all size ranges; (2) a single dominant peak in numbers at a particular size; (3) two dominant peaks in numbers at distinct sizes. Estimated life span of Puya hamata was 28 years based on growth rates, and growth rate declined beyond the size at which most rosettes reproduce. To investigate the impact of different fire intensities on Puya hamata mortality, 400 m 2 plots within a mosaic of unburned and burned patches of different fire intensities were surveyed one month after the fire. Fire mortality was low in the medium and high intensity plots, and fires selectively killed smaller plants rather than larger ones. No mortality was observed in the unburned and low intensity fire plots. It is proposed that Puya responds to burning with pulses of seedling recruitment during periods of open vegetation after fires and very little recruitment at other times. Therefore, surveys of Puya plants can reveal past fire events in their population size structure. The combination of sensitivity to fire at recruitment, low fire mortality rates afterwards, and a 28-year lifespan makes Puya hamata an ideal indicator species of recent fire history in páramos.Key words. Bromeliaceae, burning, Ecuador, giant puya, mortality, páramo, population dynamics, seedling recruitment, semelparity.Puya hamata demography and páramo fire history 54 RESUMEN Los páramos son importantes por su biodiversidad y los servicios ecosistémicos que proporcionan a los pueblos andinos, pero son sensibles a los disturbios como las quemas. El entendimiento de los impactos ecológicos de los disturbios es crucial pare al manejo efectivo de los páramos. El estudio de especies indicadoras puede contribuir de manera eficiente a este entendimiento. Puya hamata es una roseta gigante, considerada como especie bandera que tiene el potencial de actuar como indicador de la historia reciente de quema dentro de los páramos. Para determinar la estructura de tamaño de la población, la mortalidad, reclutamiento y tasa de crecimiento de Puya hamata, se midieron todas las plantas de Puya dentro de cuadros de 400 m 2 en 2008 y un año más tarde. Se re...