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Alien invasive species represent a severe risk to biodiversity. Such is the case of buffel grass (Cenchrus ciliaris L.), a native species of Southern Asia and East Africa, which was introduced to the United States and Mexico for use in improved pasture. Here we present a coarse-grain approach to determine areas where buffel grass can potentially invade in Mexico. Potential species distributions, suitable for an invasion by buffel grass, were obtained through genetic algorithms. We generated the algorithms with databases of herbaria specimens; environmental digital covers of climate, soil texture, and vegetation; and the program called Genetic Algorithm for Rule-Set Prediction. This spatial modeling approach was validated with a case study for the state of Sonora, Mexico, where the occurrence of buffel grass has been proven. The most threatened vegetation types for the specific case of Sonora were desert scrub, mesquite woodlands, and tropical deciduous forest. The model prediction agreed with the field observations recorded in Sonora and allowed us to apply the same procedure to produce a map of the potential sites of buffel grass invasion for Mexico. The areas at risk of invasion mostly occurred in desert scrub, located in the arid and semiarid regions of northern Mexico. This methodology provides an initial baseline for assessment, prevention, and management of alien species that may become invasive under certain environmental conditions. Additionally this modeling approach provides a tool for policy makers to use in making decisions on land-use management practices when alien species are involved. Resumen: Las especies exóticas invasoras representan un riesgo para la biodiversidad. Tal es el caso deCenchrus ciliaris, una especie nativa del sureste de Asia y este deÁfrica, que fue introducida a los Estados Unidos y México como un pasto mejora de pasturas. Presentamos un método de grano grueso para determinaráreas que potencialmente pueden ser invadidas por C. ciliaris en México. Las distribuciones potenciales deáreas susceptibles de invasión por C. ciliaris, se obtuvieron por medio de algoritmos genéticos. Generamos los algoritmos con bases de datos de especímenes de herbario, coberturas ambientales digitales (i. e. clima, textura del suelo y vegetación) y el programa Genetic Algorithm for Rule-Set Prediction. Este método de modelado espacial fue validado con un estudio de caso para el estado de Sonora, México, donde se había probado la ocurrencia de C. ciliaris. Para el caso específico de Sonora, los tipos de vegetación más amenazados

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Riparian Zones are considered biodiversity and ecosystem services hotspots. In arid environments, these ecosystems represent key habitats, since water availability makes them unique in terms of fauna, flora and ecological processes. Simple yet powerful remote sensing techniques were used to assess how spatial and temporal land cover dynamics, and water depth reflect distribution of key land cover types in riparian areas. Our study area includes the San Miguel and Zanjon rivers in Northwest Mexico. We used a supervised classification and regression tree (CART) algorithm to produce thematic classifications (with accuracies higher than 78%) for 1993, 2002 and 2011 using Landsat TM scenes. Our results suggest a decline in agriculture (32.5% area decrease) and cultivated grasslands (21.1% area decrease) from 1993 to 2011 in the study area. We found constant fluctuation between adjacent land cover classes and riparian habitat. We also found that water depth restricts Riparian Vegetation distribution but not agricultural lands or induced grasslands. Using remote sensing combined with spatial analysis, we were able to reach a better understanding of how riparian habitats are being modified in arid environments and how they have changed through time.

Ecological processes are centered to water availability in drylands, however less known nutrient stoichiometry can help explain much of their structure and ecological interactions. Here we look to the foliar stoichiometry of carbon (C), nitrogen (N) and phosphorus (P) of 38 dominant plant species from the Sonoran Desert, grouped in four different functional types to describe ecological characteristics and processes. We found that foliar N, C:N, C:P and N:P stoichiometric ratios, but not P, were higher than those known to most other ecosystems and indicate P but not N limitations in leaves. Biological N fixers (BNF) had even higher leaf N concentrations, but bio-elemental concentrations and stoichiometry ratios were not different to other non-N fixing legume species which underscores the need to understand the physiological mechanisms for high N, and to how costly BNF can succeed in P-limiting drylands environments. Stoichiometry ratios, and to lesser extent elemental concentrations, were able to characterize BNF and colonizing strategies in the Sonoran Desert, as well as explain leaf attribute differences, ecological processes and biogeochemical niches in this dryland ecosystem, even when no direct reference is made to other water-limitation strategies.

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RESUMENPara entender los procesos de los ecosistemas desde un punto de vista funcional es fundamental entender las relaciones entre la variabilidad climática, los ciclos biogeoquímicos y las interacciones superficie-atmósfera. En las últimas décadas se ha aplicado de manera creciente el método de covarianza de flujos turbulentos (EC, por sus siglas en inglés) en ecosistemas terrestres, marinos y urbanos para medir los flujos de gases de invernadero (p. ej., CO 2 , H 2 O ) y energía (p. ej., calor sensible y latente). En diversas regiones se han establecido redes de sistemas EC que han aportado información científica para el diseño de políticas ambientales y de adaptación. En este contexto, el presente trabajo delimita el marco conceptual y técnico para el establecimiento de una red regional de medición de flujos de gases de efecto invernadero en México, denominada MexFlux, cuyo objetivo principal es mejorar nuestra comprensión de la forma en que la variabilidad climática y la transformación ambiental influye en la dinámica de los ecosistemas mexicanos ante los factores de cambio ambiental global. En este documento se analiza primero la importancia del intercambio de CO 2 y vapor de agua entre los ecosistemas terrestres y la atmósfera. Después se describe brevemente la técnica de covarianza de flujos turbulentos para la medición de éstos, y se presentan ejemplos de mediciones en dos ecosistemas terrestres y uno urbano en México. Por último, se describen las bases conceptuales y operativas a corto, mediano y largo plazo para la continuidad de la red MexFlux. 325-336 (2013) Atmósfera 26(3), 326 R. Vargas et al. ABSTRACTUnderstanding ecosystem processes from a functional point of view is essential to study relationships among climate variability, biogeochemical cycles, and surface-atmosphere interactions. Increasingly during the last decades, the eddy covariance (EC) method has been applied in terrestrial, marine and urban ecosystems to quantify fluxes of greenhouse gases (e.g., CO 2 , H 2 O) and energy (e.g., sensible and latent heat). Networks of EC systems have been established in different regions and have provided scientific information that has been used for designing environmental and adaptation policies. In this context, this article outlines the conceptual and technical framework for the establishment of an EC regional network (i.e., MexFlux) to measure the surface-atmosphere exchange of heat and greenhouse gases in Mexico. The goal of the network is to improve our understanding of how climate variability and environmental change influence the dynamics of Mexican ecosystems. First, we discuss the relevance of CO 2 and water vapor exchange between terrestrial ecosystems and the atmosphere. Second, we briefly describe the EC basis and present examples of measurements in terrestrial and urban ecosystems of Mexico. Finally, we describe the conceptual and operational goals at short-, medium-, and long-term scales for continuity of the MexFlux network.