Adrien Guyot

Using electrical resistivity tomography to differentiate sapwood from heartwood: application to conifers

Estimating sapwood area is one of the main sources of error when upscaling point scale sap flow measurements to whole-tree water use. In this study, the potential use of electrical resistivity tomography (ERT) to determine the sapwood-heartwood (SW-HW) boundary is investigated for Pinus elliottii Engelm var. elliottii × Pinus caribaea Morelet var. hondurensis growing in a subtropical climate. Specifically, this study investigates: (i) how electrical resistivity is correlated to either wood moisture content, or electrolyte concentration, or both, and (ii) how the SW-HW boundary is defined in terms of electrical resistivity. Tree cross-sections at breast height are analysed using ERT before being felled and the cross-section surface sampled for analysis of major electrolyte concentrations, wood moisture content and density. Electrical resistivity tomography results show patterns with high resistivities occurring in the inner part of the cross-section, with much lower values towards the outside. The high-resistivity areas were generally smaller than the low-resistivity areas. A comparison between ERT and actual SW area measured after felling shows a slope of the linear regression close to unity (=0.96) with a large spread of values (R(2) = 0.56) mostly due to uncertainties in ERT. Electrolyte concentrations along sampled radial transects (cardinal directions) generally showed no trend from the centre of the tree to the bark. Wood moisture content and density show comparable trends that could explain the resistivity patterns. While this study indicates the potential for application of ERT for estimating SW area, it shows that there remains a need for refinement in locating the SW-HW boundary (e.g., by improvement of the inversion method, or perhaps electrode density) in order to increase the robustness of the method.

The contribution of MRS and resistivity methods to the interpretation of actual evapo-transpiration measurements: a case study in metamorphic context in north Benin

A quantitative budget estimate of actual evapo-transpiration is a key issue for enhanced hydrological modelling in northern Benin. Actual evapo-transpiration is estimated using large aperture scintillometer equipment, devoted to sensible heat flux measurements. However, a previous study reported that the actual evapo-transpiration cycle is not fully understood. Indeed, the actual evapo-transpiration depends strongly on several factors such as climate, vegetation pattern, soil water storage and human activities. The respective contributions of the aquifer and vadose zone to the actual evapo-transpiration budget are not known. When using piezometric variations of the water table, the aquifer contribution is not easy to quantify since the specific yield may vary in the investigated area, located in a metamorphic rock environment. In the present study, we investigate whether significant differences in the aquifers specific yield could exist within the large aperture scintillometer measurement area, leading to different actual evapo-transpiration water losses. We use joint frequency electromagnetic resistivity mapping, geological surveys and magnetic resonance sounding (MRS) to delineate the effective porosity of the regolith around the scintillometre measurement area. Thirteen MRS soundings implemented in key areas reveal a clear classification of the main geological units on the basis of their water content. The MRS water content varies between 1.5-3% for amphibolite and micaschists formations to more than 12% for quartzitic fractured formations, whereas the MRS relaxation time T1 is less discriminating (150-250 ms), indicating a small variation in pore size. Then, as a first modelling exercise, we assumed that the MRS water content (the effective porosity) maximizes the specific yield. The actual evapo-transpiration budget given by a previous study (Guyot et al. 2009) is then re-interpreted using geophysical data: we found that a) the measured water table depletion can explain the actual evapo-transpiration value providing enough water for the transpiration process and b) the significant discrepancies in actual evapo-transpiration signals observed between the eastern and western parts of the watershed can be explained by the respective effective porosity of the geological units. Even if further research is needed to link MRS water content to the specific yield and to evaluate a possible role of the deep vadose zone, the hydrogeophysical mapping presented in this study highlights the role of the MRS method for providing relevant information to understand hydrological processes in this complicated geological context of north Benin.

Direct uptake of canopy rainwater causes turgor-driven growth spurts in the mangrove Avicennia marina

Mangrove forests depend on a dense structure of sufficiently large trees to fulfil their essential functions as providers of food and wood for animals and people, CO2 sinks and protection from storms. Growth of these forests is known to be dependent on the salinity of soil water, but the influence of foliar uptake of rainwater as a freshwater source, additional to soil water, has hardly been investigated. Under field conditions in Australia, stem diameter variation, sap flow and stem water potential of the grey mangrove (Avicennia marina (Forssk.) Vierh.) were simultaneously measured during alternating dry and rainy periods. We found that sap flow in A. marina was reversed, from canopy to roots, during and shortly after rainfall events. Simultaneously, stem diameters rapidly increased with growth rates up to 70 μm h-1, which is about 25-75 times the normal growth rate reported in temperate trees. A mechanistic tree model was applied to provide evidence that A. marina trees take up water through their leaves, and that this water contributes to turgor-driven stem growth. Our results indicate that direct uptake of freshwater by the canopy during rainfall supports mangrove tree growth and serve as a call to consider this water uptake pathway if we aspire to correctly assess influences of changing rainfall patterns on mangrove tree growth.

Mobile X-Pol Radar: A New Tool for Investigating Pyroconvection and Associated Wildfire Meteorology

CapsuleThe Bushfire Convective Plume Experiment examines the ability of portable, dual polarized X-band radar to quantify the kinematics of pyroconvection through three cases studies examining the radar moments relevant to fire behaviour.