Dry season rainfall as a source of transpired water in a seasonal, evergreen forest in the western Amazon region inferred by water stable isotopes

Abstract

The present work aimed to investigate the potential sources of water for plants in an area of evergreen forest located in western Amazonia (Rebio Jaru). We used a natural abundance of water isotopes—δ2H and δ18O—to trace the main source of water to plants at the beginning of the dry period (May 2016) and at the end of the dry period/transition to the wet period (October 2016) following a severe El Niño drought (ENSO 2015/16). Soil samples were collected in a soil profile up to 4 m depth. Plant samples from 18 trees (14 species) were collected in May and in October 2016. Rainwater and river water samples were collected between September 2015 and February 2017. We found that, at the end of the dry period/transition to the wet period (i.e., October 2016), the average plant xylem signal was more enriched (δ2H: −20.0 ± 8.1‰; δ18O: −1.13 ± 1.88‰) than in May 2016 (δ2H: −36.7 ± 5.6‰; δ18O: −3.50 ± 1.30‰), the onset of the dry period. The averaged isotopic soil signal in May 2016 (δ2H: −35.4 ± 5.90‰; δ18O: −5.19 ± 0.70‰) is slightly more depleted than in October (δ2H: −27.6 ± 13.8‰; δ18O: −4.35 ± 1.73‰) and, in general, more depleted than the xylem signal. In the dual isotope space, the xylem signal at the beginning of the dry period follows the rainfall signal of the wet period, while the xylem signal at the end of the dry period/transition to the wet period follows the signal of the dry season rainfall, suggesting that plants mostly transpire recent rainwater. Contrary to what was expected, we did not find evidence in the xylem signal of the water stored in the soil pores, which suggests that to meet to the water demands of the dry period, plants do not use the water from past periods stored in the soil layers.

Deep roots mitigate drought impacts on tropical trees despite limited quantitative contribution to transpiration

“…Globally, Miguez-Macho and Fan (2021) found that 70 % of plant transpiration is sourced from current precipitation but highlighted the plasticity of RWU, with deep-water contributing 83 % in winter-dry tropics during the driest month. In contrast, Borma et al (2022) found a strong reliance of the studied tropical evergreen forest on recent precipitation during both wet and dry season and concluded that this challenges the role of deep roots as a central drought-mitigation mechanism.…”

“…By comparing plant water stable isotope values to those of potential water sources, researchers have determined uptake percentages using simple mixing equations, statistical methods and modeling ( Rothfuss and Javaux, 2017 ). This has greatly advanced our understanding of trees' contribution to tropical ecosystem water cycling, including the interplay of seasonal dynamics of RWU depth and niche segregation ( Schwendenmann et al, 2015 ; Sohel et al, 2021 ), the occurrence of hydraulic lift ( Jackson et al, 1999 ) and the dependency on incoming precipitation ( Borma et al, 2022 ).…”

“…Additionally, stable isotope studies investigating RWU have traditionally relied on destructive sampling, which only represents discrete points in time ( Beyer and Penna, 2021 ) and thus misses dynamic shifts in RWU arising from changes in stomatal control, atmospheric demand and water availability across soil depths ( Rothfuss and Javaux, 2017 ). Isotope studies specifically investigating RWU of tropical trees at soil depth >1 m are so far limited to one ( Brum et al, 2019 ; Sohel et al, 2021 ) or few sampling time points surrounding a labeling event ( Stahl et al, 2013 ) or focused on differences between wet and dry season ( Romero-Saltos et al, 2005 ; Borma et al, 2022 ). Additionally, experiments were only conducted in ecosystems with pronounced seasonality (dry period >3 months) and to the authors' best knowledge, there is only one study from outside the Amazon ( Sohel et al, 2021 ).…”