Co-evolution of xylem water and soil water stable isotopic composition in a northern mixed forest biome

Jenna R Snelgrove,James M Buttle,Dörthe Tetzlaff,Matthew J Kohn

doi:10.5194/hess-25-2169-2021

Jenna R Snelgrove, James M Buttle + Show 2 more

Open Access

https://doi.org/10.5194/hess-25-2169-2021

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Abstract

Abstract. Plant–soil water isotopic dynamics in northern forests have been understudied relative to other forest types; nevertheless, such information can provide insight into how such forests may respond to hydroclimatic change. This study examines the co-evolution of xylem water and soil water stable isotopic compositions in a northern mixed forest in Ontario, Canada. Gross precipitation, bulk soil water and xylem water were sampled from pre-leaf out to post-senescence in 2016 for eastern white cedar, eastern hemlock, red oak and eastern white pine. Near-bole soil water contents and mobile soil water isotopic compositions were measured for the last three species. Mobile soil water did not deviate significantly from the local meteoric water line (LMWL). In contrast, near-surface bulk soil water showed significant evaporative enrichment relative to the LMWL from pre-leaf out to peak leaf out under all tree canopies, while xylem water was significantly depleted in 18O and particularly 2H relative to bulk soil water throughout the growing season. Inter-species differences in deviation of xylem water from the LMWL and their temporal changes emerged during the growing season, with coniferous species xylem water becoming isotopically enriched, while that of red oak became more depleted in 2H and 18O. These divergences occurred despite thin soil cover (generally <0.5 m depth to bedrock) which would constrain inter-species differences in tree rooting depths in this landscape. Isotopic fractionation at the tree root and fractionation of xylem water via evaporation through the tree bark are among the most plausible potential explanations for deviations between xylem and soil water isotopic compositions. Differences in the timing and intensity of water use between deciduous and coniferous trees may account for inter-specific variations in xylem water isotopic composition and its temporal evolution during the growing season in this northern forest landscape.

Highlights

Northern forest landscapes are highly sensitive to climate change (Laudon et al, 2017; Sprenger et al, 2018a) and may experience marked hydrological shifts in the future, such as changes in the amount, form and timing of precipitation (Carey et al, 2010; Hartmann et al, 2013) as well as increases in drought frequency and intensity (Brinkmann et al, 2019)
Environmental isotopes have often been used to study water use by vegetation (e.g. Evaristo et al, 2015), and efforts to account for the isotopic composition of plant water in relation to that of major water pools in forest landscapes has led to the ecohydrological separation or “two water worlds” hypothesis
The purpose of this study is to examine the co-evolution of the isotopic composition of xylem water and soil water from pre-leaf out to post-senescence for some common tree species in Canada’s northern forest landscapes

Summary

Introduction

Northern forest landscapes are highly sensitive to climate change (Laudon et al, 2017; Sprenger et al, 2018a) and may experience marked hydrological shifts in the future, such as changes in the amount, form and timing of precipitation (Carey et al, 2010; Hartmann et al, 2013) as well as increases in drought frequency and intensity (Brinkmann et al, 2019). Alterations in snow accumulation and ablation have important implications for soil water availability to plants at the start of the growing season (Smith et al, 2011; Carey et al, 2013), and vegetation in northern landscapes can exhibit rapid responses to such changes Snelgrove et al.: Co-evolution of xylem water and soil water stable isotopic composition (Brooks et al, 2010; McDonnell, 2014). This hypothesis proposes that a highly mobile pool of soil water similar in isotopic composition to precipitation contributes to groundwater and streamflow, while a less mobile pool of evaporatively enriched soil water supplies plant transpiration (Goldsmith et al, 2012; Knighton et al, 2019; Sprenger and Allen, 2020)

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