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.
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.
| Original language | English |
|---|---|
| Pages (from-to) | 2169-2186 |
| Number of pages | 18 |
| Journal | Hydrology and Earth System Sciences |
| Volume | 25 |
| Early online date | 21 Apr 2021 |
| DOIs | |
| Publication status | Published - 21 Apr 2021 |
Bibliographical note
AcknowledgementsThanks are expressed to the Dorset Environmental Research Centre for meteorological data, Carl Mitchell (University of Toronto Scarborough) for snowmelt lysimeter isotope data, Samantha Evans (Boise State University), Jeff McDonnell and Kim Janzen (University of Saskatchewan) for isotopic analyses, Robert Monico and Ciara Cooke for field assistance and to the editor (Natalie Orlowski) and two anonymous reviewers for constructive comments on an earlier version of this paper.
Financial support
This research has been supported by the Natural Sciences and Engineering Research Council of Canada (grant no. 2015-06116) and the European Research Council, FP7 Ideas: European Research Council (grant no. VEWA (335910)).