the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
The seasonal origins and ages of water provisioning streams and trees in a tropical montane cloud forest
Gregory R. Goldsmith
Roxanne M. Cruz-de Hoyos
Adan Julian Ccahuana Quispe
A. Joshua West
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 lysimeter and stream waters, 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) was estimated to be 12 % for branch xylem waters, while there was no significant evidence for new water in streams. 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 tropical montane cloud forest adds to understanding of the mechanisms that govern the partitioning of water moving through different ecosystems.
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Emily I. Burt et al.
Status: open (until 29 Jun 2023)
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RC1: 'Comment on hess-2023-75', Anonymous Referee #1, 15 May 2023
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Using a long time-series of precipitation, stream, branch, and lysimeter water isotope data, the authors explore seasonality and age fractions of water within each pool over a two-year period in a we tropical montane ecosystem in Peru. They find that tree xylem water were the most isotopically variable reflecting more recent precipitation inputs, whereas lysimeter water at 1m and stream water are isotopically stable indicating older stored water. I found this study very interesting and novel in that it includes all these pools within one study, allowing a more complete understanding of how water was moving within this ecosystem. In temperate ecosystems, isotopic studies have indicated that trees use less mobile, older water within the soil that generally originated from wet winters. However, in this wet tropical forest, trees are using recent precipitation, such that once precipitation percolates through to 1 m (lysimeter data), the seasonal variation is gone, indicating a long temporal mixing of the water at 1 m depth, and in the stream water. I found the data and presentation convincing. I particularly liked the various isotopic metrics they calculated to illustrate the water dynamics: seasonality, young water fraction and new water fraction. Overall, I think this work will be a strong contribution to the literature, and provide ideas for similar studies in other ecosystems.
I had a lot of comments and clarifications that I think would make a clearer and cleaner story. In addition, I think more details on soil moisture, water table depth, and precipitation amounts would help make their case in many instances. Please see the detailed comments directly on the pdf.
Emily I. Burt et al.
Emily I. Burt et al.
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