Abstract:
Key message: The woody plant life forms in a tropical dry forest differ largely in their diurnal and seasonal patterns of root
water uptake and the related atmospheric and edaphic drivers.
Abstract: As a consequence of global climate change, the frequency and intensity of drought events in tropical forest ecosystems is expected to increase. Woody species in tropical dry forests (TDFs) exhibit diverse strategies to withstand drought such as leaf-shedding or stem succulence that have mainly been studied from an aboveground perspective. Much less is known about the functioning of small and fine roots during the dry season and their response to soil re-wetting. We measured sap flux in small-diameter roots (2–6 mm) of four woody Ecuadorian TDF species using calibrated miniature sap flow gauges based on the heat-ratio method to unravel seasonal and diurnal patterns in root water uptake in dependence on phenology, soil moisture, and climatic drivers. Continuous data from 17 months showed that water uptake in the deciduous trees Eriotheca ruizii, Ceiba trischistandra, and Vachellia macracantha was closely linked to leaf phenology, soil moisture, and, in wet soil, vapor pressure deficit (VPD), while the tall, stem-succulent cactus Armatocereus laetus revealed a highly opportunistic water uptake pattern with rapid responses to small rainfall amounts in the dry season and decoupling from
atmospheric drivers. While root sap flux density was on average highest in C. trischistandra with largest foliage area, the cactus roots were able to absorb water all day round without daytime dormancy and thus outperformed the other two species even during wetter periods. Deeper insights into small-root functionality across seasons is essential for a more mechanistic understanding of the water relations of TDF perennials with different drought response strategies.
