A new paper, led by Dr. Emily Burt during her doctoral work with Josh West (USC), describes the seasonal origins and ages of water moving through the cloud forests near Wayqecha Biological Station in the Andes mountains of Peru. While our group has been working hard to use the seasonal origin index (SOI) developed by Dr. Scott Allen (University of Nevada Reno to understand the spatial variation in tree and stream water sources (Allen et al. 2019 in HESS), this new paper applies SOI to study temporal variation in tree, soil lysimeter, and stream waters. Moreover, it applies the calculation of young water fractions, developed by our collaborator James Kirchner (ETH Zurich; Kirchner 2016 in HESS), to provide real depth of understanding with respect to how water is moving through this remarkable ecosystem. Perhaps the most striking result is that in this very wet system, we observe that plants are taking up wet-season precipitation during the wet season and dry-season precipitation during the dry season. This is a real contrast to what we have observed in the temperate forests of Switzerland (Goldsmith et al. 2022 in GRL) to date and adds a nice new piece to the puzzle:
Burt, E. I., Goldsmith, G. R., Cruz-de Hoyos, R. M., Ccahuana Quispe, A. J., and West, A. J.: The seasonal origins and ages of water provisioning streams and trees in a tropical montane cloud forest, Hydrol. Earth Syst. Sci., 27, 4173–4186, https://doi.org/10.5194/hess-27-4173-2023, 2023.
Abstract: Determining the sources of water provisioning streams, soils, and vegetation can provide important insights into the water that sustains critical ecosystem functions now and how those functions may be expected to respond given projected changes in the global hydrologic cycle. We developed multi-year time series of water isotope ratios (δ18O and δ2H) based on twice-monthly collections of precipitation, lysimeter, and tree branch xylem waters from a seasonally dry tropical montane cloud forest in the southeastern Andes mountains of Peru. We then used this information to determine indices of the seasonal origins, the young water fractions (Fyw), and the new water fractions (Fnew) of soil, stream, and tree water. There was no evidence for intra-annual variation in the seasonal origins of stream water and lysimeter water from 1 m depth, both of which were predominantly comprised of wet-season precipitation even during the dry seasons. However, branch xylem waters demonstrated an intra-annual shift in seasonal origin: xylem waters were comprised of wet-season precipitation during the wet season and dry-season precipitation during the dry season. The young water fractions of lysimeter (< 15 %) and stream (5 %) waters were lower than the young water fraction (37 %) in branch xylem waters. The new water fraction (an indicator of water ≤ 2 weeks old in this study) was estimated to be 12 % for branch xylem waters, while there was no significant evidence for new water in stream or lysimeter waters from 1 m depth. Our results indicate that the source of water for trees in this system varied seasonally, such that recent precipitation may be more immediately taken up by shallow tree roots. In comparison, the source of water for soils and streams did not vary seasonally, such that precipitation may mix and reside in soils and take longer to transit into the stream. Our insights into the seasonal origins and ages of water in soils, streams, and vegetation in this humid tropical montane cloud forest add to understanding of the mechanisms that govern the partitioning of water moving through different ecosystems.
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