Correction of anisotropy effects on penta-needle heat-pulse probe sap-flux density and thermal property measurements

Abstract

Growing interest in methods for estimating plant stem/trunk sap-flux density and thermal properties include the use of heated needles inserted into the plant. A penta-needle heat-pulse probe (PHPP) coupled with an on-chip integrated INV-WATFLX algorithm was newly developed for inverse estimation of isotropic porous media thermal-diffusivity, κ, -conductivity, λ, and heat velocity, Vh (converted to water-flux density, J), thus heat capacity, C (=λ/κ), and water content could also be derived. This integrated sensor, however, has yet to be applied in anisotropic sapwood sensing. Here, we conducted a numerical simulation of the PHPP heat pulse and a deviation analysis when using an INV-WATFLX code developed by Yang and Jones [Comput. Geosci.—UK. 35 (2009) 2250] in anisotropic porous media. Deviations in J were up to +40% and as low as -30%, and within 12% in κ, λ and C at static conditions for varied PHPP installation angles, α, in sapwood. We developed a correction of anisotropy effects, and followed up with a field test of the sensors installed on standing poplar (Populus simonii Carr.) trees using α = 0°, 15° and 30°. Field tests showed the corrected J estimated using PHPPs at α = 15° and 30° both agreed well with J from thermal dissipation probes (TDPs) in 1:1 line (R2 = 0.87 and 0.83, P < 0.01). The corrected J at α = 0° showed an apparent 30% underestimate (R2 = 0.87, P < 0.01), which was assumed to be due to wound effects. All PHPP estimates exhibited similar and stable κ, λ and C at night, but showed a diurnal fluctuation in J to varying extents likely due to the flow turbulence by inserted needles.