Contribution of sapwood traits to uncertainty in conifer sap flow as estimated with the heat-ratio method

Abstract

Inferring whole-tree sap flow rates (Q) with thermometric sap flow sensors requires specification of physiological and structural attributes of trees. Using sap temperature measurements to estimate Q with the heat-ratio method (HRM) requires quantification of the water content (m c), basic density (ρ b), and depth (R s) of sapwood. Values of m c and ρ b serve to estimate sapwood thermal diffusivity (k), a necessary variable in the calculation of heat-pulse velocity (V h) that is often set to a nominal value (k_nom); m c and ρ b are also used to convert V h to sap velocity (V s). The sapwood area across which V s is integrated is often estimated on the basis of R s. Because m c and ρ b are correlated and influence Q through estimation of k and the conversion of V h to V s , we sought to quantify the potential error introduced when Q is calculated with k_nom rather than with k as estimated from measurements of m c and ρ b in five coniferous species. We also examined how variation in m c , ρ b , and R s across sampling scales may contribute to uncertainty in estimated Q. Across the observed range of m c and ρ b , the two traits contributed to a net decline in the process of estimating V s from V h (with k_nom). This suggests that the use of k_nom rather than a calculated k may result in overestimation of Q when m c and ρ b co-vary as they did in our study. Variability in m c and ρ b across sampling scales could induce errors greater than 10% in V s (and hence Q), while within-and among-tree variability in R s could impart even greater errors (up to 130%). We propose that this uncertainty be represented in the expression of error in whole-tree sap flow estimates and in statistical analyses involving those estimates.

How Reliable Are Heat Pulse Velocity Methods for Estimating Tree Transpiration?

“…Currently, existing and newly proposed methods have not improved the accuracy of transpiration estimates. Greater accuracy will be achieved by improving dynamic measurements of wounding, stem moisture content and wood thermal diffusivity [9,35]. The appropriate placement of sensors in sapwood is also critical.…”

“…Other studies that do measure thermal diffusivity only do so once typically at the end of a measurement campaign (e.g., [34]). However, thermal diffusivity can vary significantly from the default value and throughout the measurement period [35]. Consequently, miscalculations of thermal diffusivity can lead to errors in heat and sap velocity calculations and, subsequently, either an over or underestimation of transpiration.…”

“…For example, the T-max method calculates thermal diffusivity when there is zero sap flow [13] and the method has been improved upon [36]. Looker et al [35] performed an in-depth review of thermal diffusivity methods and recommended the method of Vandegehuchte and Steppe [37] for HPV studies.…”

Tree Water Dynamics in a Semi-Arid, Pinus brutia Forest

“…We multiplied the J p by the sapwood area of each concentric ring. For trees with sapwood depth greater than 30 mm, we assumed that J p declined linearly with sapwood depth, such that J p became zero in the heartwood [59]. We conducted Heat Field Deformation (HFD) measurements at sapwood depths between 2 to 9 cm (8 observation points) to verify this assumption (see Section 2.5).…”

Diurnal and seasonal coupling of conifer sap flow and vapour pressure deficit across topoclimatic gradients in a subalpine catchment

“…Mean m c and ρ b were estimated from sapwood samples extracted from trees co-occurring with sap flow trees, with at least three replicate trees per species per landscape position; site-and species-level mean values of m c and ρ b were then used in Equation (7)). As we have noted elsewhere (Looker, Martin, Jencso, & Hu, 2016), the assumption of equivalence in m c and ρ b in co-occurring trees contributes error to the conversion of V h to V s ; nonetheless, greater uncertainty is implicit in V h due to the choice of method for estimating sapwood thermal diffusivity, k, on the basis of m c and ρ b .…”