Tracing plant source water dynamics during drought by continuous transpiration measurements: An in‐situ stable isotope approach

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.