Tracing plant source water dynamics during drought by continuous transpiration measurements: An in‐situ stable isotope approach
Angelika Kübert, Maren Dubbert, Ines Bamberger, Kathrin Kühnhammer, Matthias Beyer, Joost van Haren, Kinzie Bailey, Jia Hu, Laura Meredith, S. Nemiah Ladd, Christiane Werner
Abstract
Abstract The isotopic composition of xylem water (δ X ) is of considerable interest for plant source water studies. In‐situ monitored isotopic composition of transpired water (δ T ) could provide a nondestructive proxy for δ X ‐values. Using flow‐through leaf chambers, we monitored 2‐hourly δ T ‐dynamics in two tropical plant species, one canopy‐forming tree and one understory herbaceous species. In an enclosed rainforest (Biosphere 2), we observed δ T ‐dynamics in response to an experimental severe drought, followed by a 2 H deep‐water pulse applied belowground before starting regular rain. We also sampled branches to obtain δ X ‐values from cryogenic vacuum extraction (CVE). Daily flux‐weighted δ 18 O T ‐values were a good proxy for δ 18 O X ‐values under well‐watered and drought conditions that matched the rainforest's water source. Transpiration‐derived δ 18 O X ‐values were mostly lower than CVE‐derived values. Transpiration‐derived δ 2 H X ‐values were relatively high compared to source water and consistently higher than CVE‐derived values during drought. Tracing the 2 H deep‐water pulse in real‐time showed distinct water uptake and transport responses: a fast and strong contribution of deep water to canopy tree transpiration contrasting with a slow and limited contribution to understory species transpiration. Thus, the in‐situ transpiration method is a promising tool to capture rapid dynamics in plant water uptake and use by both woody and nonwoody species.