Forest Aerial View

We investigate how forests exploit subsurface water storage, the source of transpiration, and the connectivity between transpiration sources and streamflow. To do this, we use field-based research and quantitative analyses combining stable isotope tracing with tree hydraulic monitoring and hydrometric measurements across a broad range of spatial and temporal scales.

Ongoing research

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Hillslope ecohydrology

We are using a hillslope to look at the connectivity between the source of transpiration and streamflow. We leverage the shallow soils and impermeable bedrock (i.e. known boundary conditions) to trace tree water sources in high-temporal resolution. The research site is the Harp 4 watershed, near Dorset, Ontario.

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What is the source of transpiration in the Amazon Forest?

We are investigating the source of transpiration at a large hillslope in the Amazon, where we contrast tree water use across the plateau and the river valley. The research site is the Floresta Nacional dos Tapajos (FLONA), located in the State of Pará, in Brazil.

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Stable isotope analysis of plant water

The stable isotope compositions of hydrogen and oxygen in water have been widely used to investigate plant water sources. Still, traditional isotopic measurements of plant waters are expensive and labor intensive. We are looking for ways to sidestep methodological limitations in isotope analysis in plants.

For more details, check out publications on to this project:

Nehemy, M. F., Millar, C., Janzen, K., Gaj, M., Pratt, D. L., Laroque, C. P. and McDonnell, J.J. 2019. 17O- excess as a detector for co-extracted organics in vapor analyses of plant isotope signatures. Rapid Communications in Mass Spectrometry. doi: 10.1002/rcm.8470.

Millar, C., Janzen, K., Nehemy, M. F., Koehler, G., Hervé-Fernández, P., McDonnell, J. J. 2021. Organic contamination detection for isotopic analysis of water by laser spectroscopy. Rapid Communications in Mass Spectrometry. doi:10.1002/rcm.9118

Past projects

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SPIKE II

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The stable isotopes of hydrogen and oxygen have shed new light on patterns of tree water use in time and space. However, the use of isotopes alone has not been enough to provide a mechanistic understanding of source water partitioning. Here, we combine isotope data in xylem and soil water with measurements of tree's physiological information including tree water deficit (TWD), fine root distribution, and soil matric potential, to investigate the mechanism driving tree water source partitioning. We used a large vegetated lysimeter located at EPFL (ECHO Lab), in Switzerland.

For more details, check out publications on this project:

Nehemy, M. F., Benettin, P., Allen, S. T., Steppe, K., Rinaldo, A., Lehmann, M. M., and McDonnell, J. J. 2022. Phloem water isotopically different to xylem water: Potential causes and implications for ecohydrological tracing. Ecohydrologydoi:10.1002/eco.2417.

Asadollahi, M., Nehemy, M. F., McDonnell, J. J., Rinaldo, A., Benettin, P. Towards a closure of catchment mass balance: Insight on the missing link from a vegetated lysimeter. Water Resources Researchdoi:10.1029/2021WR030698.

Nehemy, M. F., Benettin, P., Asadollahi, M., Pratt, D., Rinaldo, A., and McDonnell, J. J. 2021. Tree water deficit and dynamic source water partitioning. Hydrological Processesdoi:10.1002/hyp.14004.

Benettin, P., Nehemy, M. F.,  Cernusak, L. A., Angar, K., McDonnell, J. J. 2021. On the use of leaf water to determine plant water source: A proof of concept. Hydrological Processesdoi:10.1002/hyp.14073

Benettin, P., Nehemy, M. F., Asadollahi, M., Pratt, D., Bensimon, M., McDonnell, J. J., and Rinaldo, A. 2021. Tracing and closing the water balance in a vegetated lysimeter. Water Resources Researchdoi:10.1029/2020WR029049

Snowmelt water use at transpiration onset

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Are trees thirsty for snowmelt when they wake up in the spring? Most studies have investigated the source of transpiration in the summer. Here we investigate the water source for transpiration prior, during and after snowmelt in the boreal forest. We combined stable isotope tracing, tree hydraulic monitoring and phenological changes. We used two long-term research sites within the Boreal Ecosystem Research and Monitoring Sites (BERMS).

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Transpiration phenology

Field-based assessment of transpiration phenology is a significant challenge in northern ecosystems. Here we develop a simple and objective metric that uses stem radius change correlations with sapwood temperature to determine the timing of phenological changes in transpiration in the boreal forest. 

For more details, check out publications on to this project:

Pierrat Z., Nehemy, M. F., Roy, A., Magney, T., Parazoo, N., Laroque, C. P., Pappas, C., Sonnentag, O.,

Grossmann, K., Bowling, D. R., Seibt, U., Ramirez, A., Johnson, B., Helgason, W., Barr, A., Stutz, J. 2021. Tower-based remote sensing reveals mechanisms behind a two-phased spring transition in a mixed-species boreal forest. Biogeoscience. doi:10.1029/2020JG006191

Nehemy, M.F., Maillet, J., Perron, N., Pappas, C., Sonnentag, O., Baltzer, J. L., Laroque, C. P., and McDonnell, J.J. Phenological assessment of boreal forest transpiration: A new approach using stem diurnal cycles.

The relationship between water-table level, insect defoliation events, and tree-radial growth across two boreal species

Eastern larch (Larix laricina) and black spruce (Picea mariana) are the most dominant tree species in peatlands in Canada, however interactions between peatland hydrology and species-specific radial growth are poorly understood. This study investigates the relationships between the growth/hydrological response of eastern larch and black spruce across a topographical gradient within a peatland in Saskatchewan

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For more details, check out publications on to this project:

Nehemy, M. F. and Laroque, C. P. 2018. Tree-ring analysis of larch sawfly (Pristiphora erichsonii (Hartig)) defoliation events and hydrological growth suppression in a peatland. Dendrochronologia. doi:10.1016/j.dendro.2018.06.006.