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Drought‐induced tree mortality is expected to increase globally due to climate change, with profound implications for forest composition, function and global climate feedbacks. How drought is experienced by different species is thought to depend fundamentally on where they access water vertically below‐ground, but this remains untracked so far due to the difficulty of measuring water availability at depths at which plants access water (few to several tens of metres), the broad temporal scales at which droughts at those depths unfold (seasonal to decadal), and the difficulty in linking these patterns to forest‐wide species‐specific demographic responses. We address this problem through a new eco‐hydrological framework: we used a hydrological model to estimate below‐ground water availability by depth over a period of two decades that included a multi‐year drought. Given this water availability scenario and 20 year long‐records of species‐specific growth patterns, we inversely estimated the relative depths at which 12 common species in the forest accessed water via a model of water stress. Finally, we tested whether our estimates of species relative uptake depths predicted mortality in the multi‐year drought. The hydrological model revealed clear below‐ground niches as precipitation was decoupled from water availability by depth at multi‐annual scale. Species partitioned the hydrological niche by diverging in their uptake depths and so in the same forest stand, different species experienced very different drought patterns, resulting in clear differences in species‐specific growth. Finally, species relative water uptake depths predicted species mortality patterns after the multi‐year drought. Species that our method ranked as relying on deeper water were the ones that had suffered from greater mortality, as the zone from which they access water took longer to recharge after depletion. Synthesis. This research changes our understanding of how hydrological niches operate for trees, with a trade‐off between realized growth potential and survival under drought with decadal scale return time. The eco‐hydrological framework highlights the importance of species‐specific below‐ground strategies in predicting forest response to drought. Applying this framework more broadly may help us better understand species coexistence in diverse forest communities and improve mechanistic predictions of forests productivity and compositional change under future climate.

International audienceA groundwater recharge process of heterogeneous hard rock aquifer in the Moole Hole experimental watershed, south India, is being studied to understand the groundwater flow behaviour. Significant seasonal variations in groundwater level are observed in boreholes located at the outlet area indicating that the recharge process is probably taking place below intermittent streams. In order to localize groundwater recharge zones and to optimize implementation of boreholes, a geophysical survey was carried out during and after the 2004 monsoon across the outlet zone. Magnetic resonance soundings (MRS) have been performed to characterize the aquifer and measure groundwater level depletion. The results of MRS are consistent with the observation in boreholes, but it suffers from degraded lateral resolution. A better resolution of the regolith/bedrock interface is achieved using electrical resistivity tomography (ERT). ERT results are confirmed by resistivity logging in the boreholes. ERT surveys have been carried out twice - before and during the monsoon - across the stream area. The major feature of recharge is revealed below the stream with a decrease by 80% of the calculated resistivity. The time-lapse ERT also shows unexpected variations at a depth of 20 m below the slopes that could have been interpreted as a consequence of a deep seasonal water flow. However, in this area time-lapse ERT does not match with borehole data. Numerical modelling shows that in the presence of a shallow water infiltration, an inversion artefact may take place thus limiting the reliability of time-lapse ERT. A combination of ERT with MRS provides valuable information on structure and aquifer properties respectively, giving a clue for a conceptual model of the recharge process: infiltration takes place in the conductive fractured-fissured part of the bedrock underlying the stream and clayey material present on both sides slows down its lateral dissipation

International audienceBiogeochemical and hydrological cycles are currently studied on a small experimental forested watershed (4.5 km(2)) in the semi-humid South India. This paper presents one of the first data referring to the distribution and dynamics of a widespread red soil (Ferralsols and Chromic Luvisols) and black soil (Vertisols and Vertic intergrades) cover, and its possible relationship with the recent development of the erosion process. The soil map was established from the observation of isolated soil profiles and toposequences, and surveys of soil electromagnetic conductivity (EM31, Geonics Ltd), lithology and vegetation. The distribution of the different parts of the soil cover in relation to each other was used to establish the dynamics and chronological order of formation. Results indicate that both topography and lithology (gneiss and amphibolite) have influenced the distribution of the soils. At the downslope, the following parts of the soil covers were distinguished: i) red soil system, ii) black soil system, iii) bleached horizon at the top of the black soil and iv) bleached sandy saprolite at the base of the black soil. The red soil is currently transforming into black soil and the transformation front is moving upslope. In the bottom part of the slope, the chronology appears to be the following: black soil > bleached horizon at the top of the black soil > streambed > bleached horizon below the black soil. It appears that the development of the drainage network is a recent process, which was guided by the presence of thin black soil with a vertic horizon less than 2 in deep. Three distinctive types of erosional landforms have been identified: 1. rotational slips (Type 1); 2. a seepage erosion (Type 2) at the top of the black soil profile; 3. A combination of earthflow and sliding in the non-cohesive saprolite of the gneiss occurs at midslope (Type 3). Types 1 and 2 erosion are mainly occurring downslope and are always located at the intersection between the streambed and the red soil-black soil contact. Neutron probe monitoring, along an area vulnerable to erosion types 1 and 2, indicates that rotational slips are caused by a temporary watertable at the base of the black soil and within the sandy bleached saprolite, which behaves as a plane of weakness. The watertable is induced by the ephemeral watercourse. Erosion type 2 is caused by seepage of a perched watertable, which occurs after swelling and closing of the cracks of the vertic clay horizon and within a light textured and bleached horizon at the top of black soil. Type 3 erosion is not related to the red soil-black soil system but is caused by the seasonal seepage of saturated throughflow in the sandy saprolite of the gneiss occurring at midslope

International audienceIt is commonly accepted that forest plays role to modify the water cycle at the watershed scale. However, the impact of forest on aquifer recharge is still discussed: some studies indicate that infiltration is facilitated under forest while other studies suggest a decrease of recharge. This paper presents an estimate of recharge rates to groundwater in a humid forested watershed of India. Recharge estimates are based on the joint use of several methods: chloride mass balance, water table fluctuation, geophysics, groundwater chemistry and flow analysis. Two components of the recharge (direct and indirect) are estimated over 3 years of monitoring (2003-2006). The direct and localized recharges resulting from rainfall over the entire watershed surface area is estimated to 45 mm/yr while the indirect recharge occurring from the stream during flood events is estimated to 30 mm/yr for a two km-long stream. Calculated recharge rates, rainfall and runoff measurements are then combined in a water budget to estimate yearly evapotranspiration which ranges from 80 to 90% of the rainfall, i.e. 1050 mm/y as an average. This unexpected high value for a deciduous forest is nevertheless in agreement with the forest worldwide relationship between rainfall and evapotranspiration. The large evapotranspiration form the forest cover contributes to decrease the recharge rate which leads to a lowering of the water table. This is the reason why the stream is highly ephemeral