In situ monitoring of tree water uptake depths, storage and transport reveals different strategies during drought and recovery
Authors: Kühnhammer Kathrin , van Haren Joost , Kübert Angelika , Dubbert Maren , Ladd Nemiah , Meredith Laura , Werner Christiane , Beyer Matthias 
Published: March 2022

&lt;p&gt;&lt;span&gt;Due to&lt;/span&gt;&lt;span&gt; ongoing and likely intensifying climate change impacts, ecosystem water availability &lt;/span&gt;&lt;span&gt;is&lt;/span&gt;&lt;span&gt; altered across the globe&lt;/span&gt;&lt;span&gt;.&lt;/span&gt;&lt;span&gt; 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&lt;/span&gt;&lt;span&gt; is challenging. Consequently&lt;/span&gt;&lt;span&gt;, the occurrence, functioning and importance of deep roots are not well understood.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;The Biosphere 2 Tropical Rainforest&lt;/span&gt;&lt;span&gt; in Arizona, USA&lt;/span&gt;&lt;span&gt; offers a unique possibility to further investigate this knowledge gap as environmental conditions can be controlled and soil &lt;/span&gt;&lt;span&gt;can be &lt;/span&gt;&lt;span&gt;accessed from below. Within the interdisciplinary B2 WALD project, we imposed a two&lt;/span&gt;&lt;span&gt;-&lt;/span&gt;&lt;span&gt;month drought on the enclosed ecosystem. To identify deep water uptake, &lt;/span&gt;&lt;span&gt;water labeled with &lt;/span&gt;&lt;sup&gt;&lt;span&gt;2&lt;/span&gt;&lt;/sup&gt;&lt;span&gt;H isotopes&lt;/span&gt;&lt;span&gt; was supplied through a drainage system in 2-3 m soil depth before &lt;/span&gt;&lt;span&gt;the drought ended&lt;/span&gt;&lt;span&gt;. 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&lt;/span&gt;&lt;span&gt; sap flow, stem water content and the isotopic composition of tree xylem and transpired water.&lt;/span&gt;&lt;span&gt; Only few data sets exist, combining water stable isotope information with &lt;/span&gt;&lt;span&gt;different&lt;/span&gt;&lt;span&gt; hydrometric measurements within the same experiment. &lt;/span&gt;&lt;span&gt;Additionally, w&lt;/span&gt;&lt;span&gt;e used novel &lt;/span&gt;&lt;span&gt;&lt;em&gt;in situ&lt;/em&gt;&lt;/span&gt;&lt;span&gt; approaches to monitor the isotopic composition in soils, tree xylem and transpiration in &lt;/span&gt;&lt;span&gt;high&lt;/span&gt;&lt;span&gt; temporal resolution.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;Combining all measurements in 10 tree individuals of 5 different species, we found contrasting reactions to the added deep water. &lt;/span&gt;&lt;span&gt;Except of&lt;/span&gt;&lt;span&gt; two understory trees, all canopy trees had access to &lt;/span&gt;&lt;span&gt;it, &lt;/span&gt;&lt;span&gt;suggest&lt;/span&gt;&lt;span&gt;ing&lt;/span&gt;&lt;span&gt; that deep root&lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt; could be a common feature also in tropical tree species&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;T&lt;/span&gt;&lt;span&gt;rees&lt;/span&gt;&lt;span&gt; did not use &lt;/span&gt;&lt;span&gt;deep water&lt;/span&gt;&lt;span&gt; in the same way&lt;/span&gt;&lt;span&gt;. 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.&lt;/span&gt;&lt;span&gt; This&lt;/span&gt;&lt;span&gt; not only&lt;/span&gt; &lt;span&gt;impacted the time course of tree water isotop&lt;/span&gt;&lt;span&gt;es&lt;/span&gt;&lt;span&gt; but knowledge of those &lt;/span&gt;&lt;span&gt;different behaviors &lt;/span&gt;&lt;span&gt;is pivotal in better understanding and predicting tree&lt;/span&gt;&lt;span&gt; performance,&lt;/span&gt;&lt;span&gt; survival and ecosystem water cycling&lt;/span&gt;&lt;span&gt;.&lt;/span&gt; &lt;span&gt;In summary, &lt;/span&gt;&lt;span&gt;our&lt;/span&gt;&lt;span&gt; data&lt;/span&gt;&lt;span&gt; illustrat&lt;/span&gt;&lt;span&gt;es&lt;/span&gt;&lt;span&gt; the need for an extensive network combining different measurements to correctly interpret tree water isotope dynamics&lt;/span&gt;&lt;span&gt;, tree water use strategies and&lt;/span&gt;&lt;span&gt; to further uncover the &lt;/span&gt;&lt;span&gt;functioning&lt;/span&gt;&lt;span&gt; of deep roots and assess their importance for ecosystem resilience in a changing climate.&lt;/span&gt;&lt;/p&gt;