The forest canopy can play a significant role in modifying the amount and isotopic composition of water during its passage throughout the near-surface critical zone. Here, partitioning of gross rainfall into interception, throughfall, and stemflow and its implications for the amount and isotopic composition of soil water was studied for red oak, eastern white pine, and eastern hemlock trees in a northern hardwood-conifer forest in south central Ontario, Canada. Stemflow production was greatest for red oak as a result of its upward-projecting branches and least for eastern white pine due to its horizontal branches and rougher bark. These stemflow contributions to the near-bole soil surface failed to produce consistently wetter soils relative to distal locations from the bole for all tree species. There was also no consistent evidence of isotopic enrichment of throughfall and stemflow relative to gross rainfall or of stemflow relative to throughfall for red oak or eastern hemlock. However, there was isotopic enrichment of both throughfall and stemflow for eastern white pine with increasing maximum atmospheric vapour pressure deficit, which may reflect the potential for evaporative fractionation as a result of retention and detention of water moving through the canopy by the rougher bark of this species. Dry soil conditions limited sampling of mobile soil water during the study, and there was no consistent evidence that either throughfall or stemflow fluxes controlled temporal changes in the isotopic signature of soil water beneath the tree. Thus, the potential for throughfall and stemflow fluxes in northern hardwood-conifer forests to modify the isotopic composition of water taken up by the tree via transpiration remains an open question.
This work was funded through the Natural Environment and Engineering and Research Council of Canada (2015-06116) and the European Research Council (ERC, Project GA 335910 VeWa). Thanks to Jeff McDonnell and Kim Janzen (University of Saskatchewan) for isotopic analysis, Robert Monico and Ciara Cooke for assistance in the field, and Miriam Coenders and two anonymous reviewers for their constructive comments on an earlier version of this paper.
Data Availability StatementThe data that support the findings of this study are available from the corresponding author upon reasonable request.
- critical zone
- isotopic fractionation
- northern hardwood-conifer forest
- soil water
- stable water isotopes