Sapflow+: a four‐needle heat‐pulse sap flow sensor enabling nonempirical sap flux density and water content measurements

Abstract

Summary• To our knowledge, to date, no nonempirical method exists to measure reverse, low or high sap flux density. Moreover, existing sap flow methods require destructive wood core measurements to determine sapwood water content, necessary to convert heat velocity to sap flux density, not only damaging the tree, but also neglecting seasonal variability in sapwood water content.• Here, we present a nonempirical heat-pulse-based method and coupled sensor which measure temperature changes around a linear heater in both axial and tangential directions after application of a heat pulse. By fitting the correct heat conduction-convection equation to the measured temperature profiles, the heat velocity and water content of the sapwood can be determined.• An identifiability analysis and validation tests on artificial and real stem segments of European beech (Fagus sylvatica L.) confirm the applicability of the method, leading to accurate determinations of heat velocity, water content and hence sap flux density.• The proposed method enables sap flux density measurements to be made across the entire natural occurring sap flux density range of woody plants. Moreover, the water content during low flows can be determined accurately, enabling a correct conversion from heat velocity to sap flux density without destructive core measurements.

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

“…Four of the seven theoretical methods have been extensively tested and include the compensation heat pulse method (CHPM, [10]), heat ratio method (HRM, [11,12]), T-max method [13], and Sapflow+ method [14]. The methods have been reviewed extensively elsewhere [9,12,[15][16][17][18].…”

“…The parameters ρ s , C w , and C s are constants whereas ρ b and m c need to be measured and are commonly measured once at the end of a campaign (e.g., [39]). Although ρ b does not vary throughout the measurement period, m c can change up to 70% on a daily and seasonal basis [14,40,41]. An incorrect measurement of m c can lead to either an over or underestimation of transpiration.…”

“…However, in recent years greater attention has been given to accurate and dynamic measurements of m c [44]. Importantly, it is increasingly recognized that HPV sensors can simultaneously measure sap flow and m c (e.g., [14,40]). …”

“…Researchers test the accuracy of HPV sensors against an independent method of measuring plant water use including weighing lysimeters (e.g., [14,19]), leaf or canopy gas exchange chambers (e.g., [20][21][22]), or potometers (e.g., [23,24]). Several published papers have tested the accuracy of HPV sensors and a meta-analysis of this literature was conducted.…”

“…The CHPM and T-max methods cannot measure slow or reverse flows [16] and the HRM cannot measure high flows [25]. The Sapflow+ and Dual Pulse methods can measure a wide range of flows [14,19].…”