The AMMA-CATCH Gourma observatory site in Mali: 7The experimental strategy includes deployment of a variety of instruments, from local to 8 meso-scale, dedicated to monitoring and documentation of the major variables characterizing 9 the climate forcing, and the spatio-temporal variability of surface processes and state 10 variables such as vegetation mass, leaf area index (LAI), soil moisture and surface fluxes. 11This paper describes the Gourma site, its associated instrumental network and the research 12 activities that have been carried out since 1984. In the AMMA project, emphasis is put on the 13 relations between climate, vegetation and surface fluxes. However, the Gourma site is also 14 important for development and validation of satellite products, mainly due to the existence of 15 large and relatively homogeneous surfaces. The social dimension of the water resource uses 16 and governance is also briefly analyzed, relying on field enquiry and interviews. 18The climate of the Gourma region is semi-arid, daytime air temperatures are always high and 29Land surface in the Gourma is characterized by rapid response to climate variability, strong

The patterns of the changes in woody plant population densities, size and species composition is documented and discussed for 24 rangeland sites monitored from 1984 to 2006 in Gourma (Mali). The sites are sampled along the North-South bioclimatic gradient on each of the main soils and levels of grazing intensity. Site woody plant populations range from extremely sparse on shallow soils, to scattered on sandy soils, to open forest in temporarily flooded clayed soils, and to narrow thickets on hard pans. Three different methods contributed to assess and monitor woody plant density and canopy cover. In the short term woody populations were struck by the 1983-84 droughts irrespective 1 of their edaphic situation and location along the bioclimatic gradient. Drought induced mortality was not more severe under drier climate within the Sahel gradient but occurred sooner after drought in shallow soils, and with a lag of a year or two on flooded clay soils. No evidences were found of higher mortality rates in stands with history of intense grazing. Although rainfall remained below average for a decade after the drought, active recruitment of woody plants occurred in all sites starting as soon as 1985. Recruitment proceeded by successive cohorts, often with short-living perennial undershrubs and pioneer shrubs settling first. Acacia species were among the first to settle or re-establish, especially on the sites most intensively grazed. The release of competition due to drought induced mortality and to the reduction of herbaceous cover contributed to the success of the recruitment. The species composition change that resulted could first be interpreted as a shift toward a more arid tolerant flora, then some diversification occurred since the mid 90's that could indicate a possible return to previous composition in the long term, confirming the resilience Sahel vegetation

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Core Ideas AMMA‐CATCH is a long‐term critical zone observatory in West Africa. Four sites sample the sharp ecoclimatic gradient characteristic of this region. Combined measurements of meteorology, water, and vegetation dynamics began in 1990. Intensification of rainfall and hydrological cycles is observed. The strong overall re‐greening may hide contrasted changes. West Africa is a region in fast transition from climate, demography, and land use perspectives. In this context, the African Monsoon Multidisciplinary Analysis (AMMA)–Couplage de l'Atmosphère Tropicale et du Cycle eco‐Hydrologique (CATCH) long‐term regional observatory was developed to monitor the impacts of global change on the critical zone of West Africa and to better understand its current and future dynamics. The observatory is organized into three thematic axes, which drive the observation and instrumentation strategy: (i) analyze the long‐term evolution of eco‐hydrosystems from a regional perspective; (ii) better understand critical zone processes and their variability; and (iii) meet socioeconomic and development needs. To achieve these goals, the observatory has gathered data since 1990 from four densely instrumented mesoscale sites (∼104 km2 each), located at different latitudes (Benin, Niger, Mali, and Senegal) so as to sample the sharp eco‐climatic gradient that is characteristic of the region. Simultaneous monitoring of the vegetation cover and of various components of the water balance at these four sites has provided new insights into the seemingly paradoxical eco‐hydrological changes observed in the Sahel during the last decades: groundwater recharge and/or runoff intensification despite rainfall deficit and subsequent re‐greening with still increasing runoff. Hydrological processes and the role of certain key landscape features are highlighted, as well as the importance of an appropriate description of soil and subsoil characteristics. Applications of these scientific results for sustainable development issues are proposed. Finally, detecting and attributing eco‐hydrological changes and identifying possible regime shifts in the hydrologic cycle are the next challenges that need to be faced.