Spatial variation in throughfall, soil, and plant water isotopes in a temperate forest

Abstract

Studies of stable isotopes of water in the environment have been fundamental to advancing our understanding of how water moves through the soil–plant–atmosphere continuum; however, much of this research focuses on how water isotopes vary in time, rather than in space. We examined the spatial variation in the δ18O and δ2H of throughfall and bulk soil water, as well as branch xylem and bulk leaf water of Picea abies (Norway spruce) and Fagus sylvatica (beech), in a 1‐ha forest plot in the northern Alps of Switzerland. Means and ranges of water isotope ratios varied considerably among throughfall, soil, and xylem samples. Soil water isotope ratios were often poorly explained by soil characteristics and often not predictable from proximal samples. Branch xylem water isotope values varied less than either soil water or bulk leaf water. The isotopic range observed within an individual tree crown was often similar to that observed among different crowns. As a result of the heterogeneity in isotope ratios, inferences about the depth of plant root water uptake drawn from a two end‐member mixing model were highly sensitive to the soil sampling location. Our results clearly demonstrate that studies using water isotopes to infer root water uptake must explicitly consider how to characterize soil water, incorporating measures of both vertical and lateral variations. By accounting for this spatial variation and the processes that shape it, we can improve the application of water isotopes to studies of plant ecophysiology, ecohydrology, soil hydrology, and palaeoclimatology.

The Demographics of Water: A Review of Water Ages in the Critical Zone

“…To that end, a better knowledge of the tracer signatures of the water that effectively recharges the catchment is needed, because not every tracer signature contributes to the recharge as initially measured in precipitation. Generally, small‐scale variation of the tracer signal can be high, which challenges sampling strategies, as for example, highlighted by the spatial variability of stable isotopic compositions of water in the upper soil layers (Goldsmith et al, ; Yang, Chen, et al, ). Water and therefore tracer signatures can be lost or altered due to interception, sublimation, evaporation, or transpiration.…”

Global Isotope Hydrogeology―Review

“…Comparisons of throughfall and precipitation δ 18 O data demonstrate that most ( n = 17 of n = 22) studies have higher throughfall δ 18 O values than precipitation δ 18 O values (Allen et al, ). Differences arise from evaporative isotope effects, atmospheric moisture exchanges, and mixing in canopies (Allen et al, , ; Goldsmith et al, ). Where interception is important, differences among throughfall versus precipitation δ 18 O values should be considered. Annual precipitation isotope compositions (i.e., δ 18 O P (annual) ) vary spatially.…”

Persistence and Plasticity in Conifer Water‐Use Strategies

“…These aforementioned studies capture a small cross section of new works that have collectively challenged the idea of using stable water isotopes in the xylem as a simple tracer for depth of water uptake (Sprenger et al, ). Despite this evidence of complexity in depth and spatial pattern of soil water isotopes, other recent work has found more homogenous patterns in the isotopic ratio of plant water, suggesting some of the isotopic heterogeneity observable within soil water profiles might be buffered as the signal is transferred to plants (Goldsmith et al, ). The extensive survey of xylem waters across Switzerland by Allen et al (), noted that across a large domain the trees appeared to rely almost exclusively on winter precipitation.…”

“…However, the idealized case where isotopically enriched summer precipitation is layered atop isotopically depleted winter precipitation is often disrupted by processes such as rapid penetration of rain through preferential flow paths, hydraulic redistribution, lateral flow, and waters of distinct seasonal origins being held selectively according to pore size (Berry et al, ; Brooks et al, ; Dubbert & Werner, ; Sprenger et al, ; Thomas et al, ). Furthermore, the presumably simple transfer of the isotopic ratio of precipitation to the surface soil water can be affected by evaporative enrichment of the surface water and precipitation throughflow effects as precipitation interacts with the canopy (Goldsmith et al, ).…”