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

Whats is the source of transpiration at this wet system?

Maimai (New Zealand)

While streamflow sources have been heavily studied and traced at Maimai, we still know very little about the source of transpiration at this well-studied research site. Does transpiration source the same old water as streams? Our ongoing project is funded by NSERC DG.

Turkey Lakes Watersheds (Ontario, Canada)

We are using long-term data and ongoing hydrometric data monitoring at two hillslopes to develop a process understanding of the mechanisms that drive patterns of tree water use and its connectivity with stream water sources. Our ongoing project is funded by NSERC DG and the Canadian Forest Service, Great Lakes Forest Research Centre.

Hillslope Ecohydrology and Transpiration Phenology (Ontario, Canada)

We are using a hillslope to look at the connectivity between the source of transpiration and streamflow during snowmelt. We are also investigating the transpiration phenology of the different species (student-led analysis). 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.

What is the source of transpiration in the Amazon Forest? (Tapajos, Brazil)

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.

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

SPIKE II

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. Ecohydrology. doi: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 Research. doi: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 Processes. doi: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 Processes. doi: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 Research. doi:10.1029/2020WR029049

Snowmelt water use at transpiration onset

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).

Paper from this project:

Nehemy, M.F., Maillet, J., Perron, N., Pappas, C., Sonnentag, O., Baltzer, J. L., Laroque, C. P., and McDonnell, J.J. (2022). Snowmelt water use at transpiration onset: Phenology, isotope tracing and tree water transit time. Water Resources. doi:10.1029/2022WR032344

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 transpiration: The stem-temp approach for determining start and end of season. Agricultural and Forest Meteorology.

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

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.