Importance of tree diameter and species for explaining the temporal and spatial variations of xylem water δ¹⁸O and δ²H in a multi‐species forest

Abstract

Identifying the vegetation and topographic variables influencing the isotopic variability of xylem water of forest vegetation remains crucial to interpret and predict ecohydrological processes in landscapes. In this study, we used temporally and spatially distributed xylem stable water isotopes measurements from two growing seasons to examine the temporal and spatial variations of xylem stable water isotopes and their relationships with vegetation and topographic variables in a Luxembourgish temperate mixed forest. Species‐specific temporal variations of xylem stable water isotopes were observed during both growing seasons with a higher variability for beeches than oaks. Principal component regressions revealed that tree diameter at breast height explains up to 55% of the spatial variability of xylem stable water isotopes, while tree species explains up to 24% of the variability. Topographic variables had a marginal role in explaining the spatial variability of xylem stable water isotopes (up to 6% for elevation). During the drier growing season (2020), we detected a higher influence of vegetation variables on xylem stable water isotopes and a lower temporal variability of the xylem water isotopic signatures than during the wetter growing season (2019). Our results reveal the dominant influence of vegetation on xylem stable water isotopes across a forested area and suggest that their spatial patterns arise mainly from size‐ and species‐specific as well as water availability‐dependent water use strategies rather than from topographic heterogeneity. The identification of the key role of vegetation on xylem stable water isotopes has critical implications for the representativity of isotopes‐based ecohydrological and catchments studies.

Characterizing the heterogeneity of eastern hemlock xylem water isotopic compositions: Implications for the design of plant water uptake studies

“…Significant correlations between δ XYLEM , elevation, and distance from the stream (Figure S6) may be related to soil moisture heterogeneity across the stand (Goldsmith et al, 2019;Oerter & Bowen, 2019); however, we did not observe significant correlations between δ XYLEM and TWI (Figure S6), often a stronger predictor of soil moisture content (Buchanan et al, 2014), and potentially of shallow soil moisture isotopic compositions (Knighton, Souter-Kline, et al, 2019) compared to elevation. In contrast, a study in a mixed beech/oak forest reached a contrasting conclusion that elevation was not a very strong predictor of δ XYLEM (Fresne et al, 2023), possibly due to greater across-tree variations in rooting depths than existed across the hemlock in this study.…”

“…During July, a linear model based on elevation and SBH explained a substantial amount of δ 18 O variation (Figure 5), similar to the 54% of δ 18 O variance explained by genus, DBH, and soil depth in a mixed hardwood forest (Gaines et al, 2016) and in general agreement with the importance of DBH during the dry season in a beech/oak forest (Fresne et al, 2023). Environmental conditions (i.e., elevation) had a stronger relative influence on δ XYLEM variations than plant structure (i.e., sapwood depth) (Figure 5).…”

“…Prior studies have shown that sampling season, soil textures (Goldsmith et al, 2019;Oerter & Bowen, 2019), topographic wetness index (Knighton et al, 2020;Knighton, Souter-Kline, et al, 2019), and elevation (Allen et al, 2019;Tetzlaff et al, 2021) can explain some δ XYLEM variability. There is also evidence that water uptake depths vary with rooting depths (Couvreur et al, 2020), tree age, diameter, and height (Fresne et al, 2023;Gaines et al, 2016;Song et al, 2018;Tao et al, 2021;Wu et al, 2019). A study of trees in a mixed-hardwood forest showed that δ XYLEM variations were well predicted (54% variance explained) by plant genus, diameter at breast height (DBH), and local soil depth (Gaines et al, 2016).…”

“…The study also considered δ XYLEM variability across tree species, which can be considerable (Barbeta & Peñuelas, 2017;Evaristo & McDonnell, 2017;Knighton et al, 2021). A study of beech and oak trees found that tree diameter and species were significant predictors of δ XYLEM variations, particularly during the dry season, with limited influence of elevation (Fresne et al, 2023). Based on these prior studies, we hypothesized that a significant amount of δ XYLEM variation within a given species could be explained by variations in tree and landscape characteristics.…”

Challenges in studying water fluxes within the soil-plant-atmosphere continuum: A tracer-based perspective on pathways to progress

“…Applications that encompass a range of different plant species (including non-woody species), ecosystems, functional diversity levels with different vessel and wood anatomies leading to different hydraulic traits and applications of in-situ measurements over longer time periods (> months to years) are needed to e.g., assess species ecohydrological niche or complementarity (e.g., Fresne et al, 2023) and to assess feedback mechanisms from individual plants to ecosystems. Figure 2 highlights future research directions and shows interdisciplinary potential in conducting experiments, applying methods and models.…”