Stem Water Storage Dynamics in Amazonian Palms and Dicotyledonous Trees

Abstract Stem water storage plays a key role in buffering plants against drought, yet continuous measurements of stem water content remain rare in tropical forest trees. Using frequency domain reflectometry (FDR) sensors, we continuously monitored stem water storage and mobilisation dynamics in co-occurring palm and dicotyledonous tree species in the eastern Amazon during the extreme 2023 drought. This approach provides, to our knowledge, the first temporal high-resolution dataset of stem hydration in Amazonian palms ( Astrocaryum vulgare Mart., Oenocarpus distichus Mart.) and co-occurring trees. Palms demonstrated substantially higher absolute water storage capacity and greater seasonal and diurnal water mobilisation compared to dicots. Using a novel analysis for critical relative stem water content thresholds revealed striking physiological divergence: palms maintained high relative diurnal discharge capacity, (i.e., the amount of water that can be released from the stem during the day, relative to the maximum capacity), at hydration levels where dicots exhibited significant impairment. In addition, we identify a common soil moisture threshold at 0.19 m 3 m −3 below which stem water declines rapidly, indicating likely hydraulic disconnection from the soil. We demonstrate the usefulness of FDR technology combined with a new threshold analysis to describe how tropical palms and trees respond to increasing drought stress under climate change.