Research | Magali Nehemy

SPIKE II: Tree water status drives source water partitioning (PhD)

In collaboration with the ECHO Lab at the École Polytechnique Fédérale de Lausanne (EPFL), we used a 2 m3 lysimeter with willow trees (Salix viminalis) planted within, to conduct a high spatial-temporal resolution experiment - The SPIKE II. 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. TWD provided an integrated response of plant water status to water supply and demand. The combined isotopic and TWD measurement showed that short-term variation (within days) in source water partitioning is determined mainly by plant hydraulic response to changes in soil matric potential. We observed changes in the relationship between soil matric potential and TWD that are matched by shifts in source water partitioning. Our results show that tree water use is a dynamic process on the time scale of days. These findings demonstrate tree’s plasticity to water supply over days can be identified with high-resolution measurements of tree water status. More info: Nehemy et al .

BERMS: Tree water use in the cold Critical Zone (PhD)

I am investigating plant water use at two long-term study sites within the Boreal Ecosystem Research and Monitoring Sites (BERMS) in Saskatchewan, Canada. This work uses vapor water fluxes (eddy covariance), soil water conditions, tree sap flow velocities, and the longest band-dendrometer dataset in the boreal forest in Canada. Besides these hydrometric measurements, we collected weekly-resolution stable isotopes measurements of xylem water of three different tree species (Picea mariana, Larix laricina and Pinus banksiana), soil, precipitation, groundwater and stream. The project leverages these high-temporal resolution monitoring of water fluxes along well defined phenological phases to understand the effect of phenology (e.g. stem rehydration) and tree water status on patterns of tree water use and its impact on hydrological processes in northern ecosystems.

Preliminary results from our investigation can be found below:

17O-excess: Novel approach on detecting spectral contamination on vapor analysis (PhD)

Traditional isotopic measurements of plant waters are expensive and labor intensive. Recent work with direct vapor equilibration (DVE) on laser spectroscopy has shown potential to side step limitations imposed by traditional methods. Here, we evaluate DVE analysis of plants with a focus on spectral contamination introduced by organic compounds. We present 17O‐excess as a way of quantifying organic compound interference in DVE. 17O‐excess can be a useful tool to identify spectral contamination and improve DVE plant and soil analysis in the laboratory and in situ. More info: Nehemy et al. 2019

Tree stories about a peatland (MSc)

Water table dynamics and tree radial growth in a minerotrophic fen:

Hydrological processes are the main drivers of plant community composition and productivity in peatlands. However, interactions between peatland hydrology and species-specific radial growth are poorly understood in undisturbed environments. This research showed eastern larch and black spruce growing under the same hydrological regime have an opposite response to water-table level. Black spruce responds positively to water-table rise, while eastern larch responded negatively to the same conditions. The differing responses were associated with species-specific physiological characteristics and micro-site conditions.

Tree-ring analysis of larch sawfly defoliation events and hydrological growth suppression in a peatland:

Insect defoliation events such as larch sawfly are a major forest disturbance in the boreal forest in Canada. Previous defoliation events can be detected in the radial growth of trees. After collecting tree cores at field sites that are heavily inﬂuenced by hydrological conditions, we found that the use of other lines of evidence such as pale rings and long-term hydrological records are crucial to understand radial growth dynamics of trees in peatlands. Our analysis stresses the necessity of using upland sites less influenced by the hydrological regime to accurately reconstruct larch sawfly defoliation events. Our work introduces a novel case study to advance the understanding of complex peatland landscapes where larch sawﬂy defoliation events take place. More info: Nehemy & Laroque, 2018

Tree radial growth: microcores and band dendrometers

We investigates the onset of xylogenises across major tree species (Pinus banksiana, Picea mariana, Larix laricina, Populus tremuloides) in the boreal forest in Canada. We combined measurements of band dendrometer with weekly histological microcore measurements to investigate timing and main environmental factors driving radial growth across this five different species. Result to come.

Edge effect in a tropical seasonal forest fragment (BSc)

In collaboration with other members of the Laboratory of Ecological and Forest Restauration (LERF - UNESP, Botucatu - Brazil), I investigated the edge effect in a tropical seasonal forest fragment. Edge effects can result in complex modifications to the dynamics of forest communities and undergo a wide range of biotic and abiotic changes in forest ecosystems. Despite the importance of identifying the distance of edge influence (DEI) for forest fragments management and conservation, there is still a lack of methodological consensus to estimate its magnitude and extension. We combined biotic and abiotic variables to assess DEI in a seasonal semideciduous Atlantic forest fragment in Brazil. The combination of biotic and abiotic variables indicated the edge effect distance DEI as 50 m, comprising 22.5% of the forest fragment. We believe that the use of multiple parameters might improve assessment of edge effect, and provide a more precise and assertive definition to guide landscape managers and decision makers regarding forest conservation. More info: Sampaio et al.