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 meters), the broad temporal scales at which droughts at those depths unfold (seasonal to decadal), and the challenges of linking these patterns to forest-wide species-specific demographic responses.) We addressed this problem through a new an eco-hydrological framework: we used a hydrological model to estimate belowground water availability by depth over a period of two decades that included a multi-year drought. Given this water availability scenario and 20yr long-records of species-specific growth patterns in a seasonally dry tropical forest in Western Ghat, India, we inverse estimated—via a model of water-stress—the depths at which twelve common species in the forest accessed water. Finally we tested, whether our estimates of species relative uptake depths predicted mortality in the multi-year drought.
The hydrological model revealed clear water uptake niches as precipitation was decoupled from water availability by depth at multi-annual scales. 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. This research changes our understanding of how hydrological niches operate for trees with a trade-off between realised growth potential and survival under drought with decadal scale return time. This eco-hydrological framework highlights the importance of species-specific below ground strategies in predicting whole 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.