In situ monitoring of tree water uptake depths, storage and transport reveals different strategies during drought and recovery

<p><span>Due to</span><span> ongoing and likely intensifying climate change impacts, ecosystem water availability </span><span>is</span><span> altered across the globe</span><span>.</span><span> Humid tropical forests, which evolved under conditions of abundant water, might be particularly vulnerable to water stress. One important factor in a tree's resilience to a less reliable water supply from precipitation is a root system that reaches deep into the ground. However, accessing deep soil regions as well as observing active deep root water uptake</span><span> is challenging. Consequently</span><span>, the occurrence, functioning and importance of deep roots are not well understood.</span></p><p><span>The Biosphere 2 Tropical Rainforest</span><span> in Arizona, USA</span><span> offers a unique possibility to further investigate this knowledge gap as environmental conditions can be controlled and soil </span><span>can be </span><span>accessed from below. Within the interdisciplinary B2 WALD project, we imposed a two</span><span>-</span><span>month drought on the enclosed ecosystem. To identify deep water uptake, </span><span>water labeled with </span>^{<span>2</span>}<span>H isotopes</span><span> was supplied through a drainage system in 2-3 m soil depth before </span><span>the drought ended</span><span>. To investigate tree reactions to the manipulations in water supply, we closely monitored atmospheric conditions, soil water content and isotopic composition as well as tree</span><span> sap flow, stem water content and the isotopic composition of tree xylem and transpired water.</span><span> Only few data sets exist, combining water stable isotope information with </span><span>different</span><span> hydrometric measurements within the same experiment. </span><span>Additionally, w</span><span>e used novel </span><span><em>in situ</em></span><span> approaches to monitor the isotopic composition in soils, tree xylem and transpiration in </span><span>high</span><span> temporal resolution.</span></p><p><span>Combining all measurements in 10 tree individuals of 5 different species, we found contrasting reactions to the added deep water. </span><span>Except of</span><span> two understory trees, all canopy trees had access to </span><span>it, </span><span>suggest</span><span>ing</span><span> that deep root</span><span>s</span><span> could be a common feature also in tropical tree species</span><span>. </span><span>T</span><span>rees</span><span> did not use </span><span>deep water</span><span> in the same way</span><span>. We observed differences in the speed and timing of the reaction as well as in within-tree water dynamics. While some individuals first refilled their stem water storage, others used the deep water source to preserve their sap flow and transpiration stream.</span><span> This</span><span> not only</span> <span>impacted the time course of tree water isotop</span><span>es</span><span> but knowledge of those </span><span>different behaviors </span><span>is pivotal in better understanding and predicting tree</span><span> performance,</span><span> survival and ecosystem water cycling</span><span>.</span> <span>In summary, </span><span>our</span><span> data</span><span> illustrat</span><span>es</span><span> the need for an extensive network combining different measurements to correctly interpret tree water isotope dynamics</span><span>, tree water use strategies and</span><span> to further uncover the </span><span>functioning</span><span> of deep roots and assess their importance for ecosystem resilience in a changing climate.</span></p>