Data from: The roots of the drought: hydrology and water uptake strategies mediate forest-wide demographic response to precipitation

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 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 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, 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 multi-year drought. The hydrological model revealed clear belowground 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. 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.

受气候变化影响，全球范围内干旱诱导的树木死亡率预计将呈上升趋势，这对森林群落组成、生态系统功能及全球气候反馈（global climate feedbacks）均将产生深远影响。学界普遍认为，不同物种对干旱的感知本质上取决于其地下垂直取水的深度，但此前这一问题始终未能得到有效观测与解析，原因主要有三：其一，难以直接测量植物取水土层（通常为数米至数十米）的水分可利用性；其二，深层土壤干旱的发生时间尺度跨度极大，从季节尺度到十年尺度不等；其三，难以将上述观测模式与森林内物种特异性的种群动态响应（demographic responses）建立关联。
为此，本研究提出了一套全新的生态水文框架（eco-hydrological framework）：我们利用水文模型（hydrological model），对包含一场多年期干旱事件的20年时间序列内，不同深度的地下水分可利用性进行了估算；结合该水分可利用情景与20年的物种特异性生长记录，通过水分胁迫（water-stress）模型，反推了该森林中12个常见物种的相对取水深度；最后，我们检验了物种相对取水深度能否有效预测多年干旱后的树木死亡率。
研究结果显示，水文模型清晰揭示了地下水文生态位（hydrological niche）的存在：在多年尺度下，降水与不同深度土层的水分可利用性出现解耦；不同物种通过分化取水深度来划分水文生态位，因此在同一林分中，不同物种所经历的干旱模式存在显著差异，进而导致物种特异性的生长表现出现明显分化。最终，物种相对取水深度能够有效预测多年干旱后的树木死亡率模式：本研究方法判定为依赖更深层水源的物种，其死亡率更高，原因在于这类物种取水的土层在水分耗尽后，需要更长时间才能得到补给。
本研究革新了我们对树木水文生态位运作机制的认知，揭示了在十年一遇的干旱事件中，树木的潜在生长能力与抗旱存活能力之间存在权衡（trade-off）。该生态水文框架强调了物种特异性地下取水策略在预测森林干旱响应中的关键作用。将该框架推广应用，或有助于我们更深入地理解多样森林群落中的物种共存机制，并改进未来气候变化背景下森林生产力与群落组成变化的机理预测模型。