Heat pulse probes are used worldwide to quantify sap flow in plants. Probes consisting of a single needle inserted into a plant have a simple physical configuration relative to multi-probe methods. The single probe also minimizes physical damage to plant tissue. Single probe methods are therefore increasingly used for sap flux density measurements. However, at low sap flux density values, existing single probe heat pulse (SPHP) methods cannot be used for accurate measurements. To increase the sensitivity of SPHP to low sap flux densities, this paper describes and validates a simplified expression of sap flow and heat transport under various heat inputs and finite heat pulse time intervals. To test this theoretical development, measurements were collected from SPHP probes inserted into mature poplar forest trees (Populus tomentosa Carrière) during the 2020 growing season. The results show that the new method can successfully monitor low sap flux density: the lowest measured heat pulse velocity was 3 cm h⁻¹, which is an improvement from the original SPHP method (20 cm h⁻¹). The improved SPHP method had a root mean square deviation (RMSD) of 1.63 cm h⁻¹ in comparison with the heat ratio (HR) method. The proposed SPHP method is therefore suitable for quantifying both low and high sap flux densities.

热脉冲探头在全球范围内用于量化植物中的液流。与多探针方法相比，由插入植物的单针组成的探针具有简单的物理配置。单个探针还最大限度地减少了对植物组织的物理损伤。因此，单探针方法越来越多地用于树液通量密度测量。然而，在低 sap 通量密度值下，现有的单探针热脉冲 (SPHP) 方法无法用于准确测量。为了提高 SPHP 对低液流密度的敏感性，本文描述并验证了在各种热输入和有限热脉冲时间间隔下液流和热传输的简化表达式。为了测试这一理论发展，从插入成熟杨树林树的 SPHP 探针中收集了测量值（Populus tomentosa Carrière) 在 2020 年生长季节。结果表明，新方法可以成功监测低液流密度：测得的最低热脉冲速度为 3 cm h ^-1，这是对原始 SPHP 方法（20 cm h ^-1）的改进。与热比 (HR) 方法相比，改进的 SPHP 方法的均方根偏差 (RMSD) 为 1.63 cm h ^-1。因此，建议的 SPHP 方法适用于量化低和高树液通量密度。