sapflow

Sap flow, or sapflow, refers to the movement of sap through the vascular tissues of plants, principally the xylem and phloem. In trees and many shrubs, sap flow includes the ascent of water and dissolved minerals from roots to leaves and the distribution of photosynthates from sources to sinks via the phloem. The rate and pattern of sap flow are influenced by environmental conditions such as light, temperature, humidity, soil moisture, and atmospheric demand, as well as by plant physiology and developmental stage.

Measurement of sap flow employs several methods, often using probes inserted into stems. The Granier thermal dissipation method (thermal dissipation probe, TDP) estimates sap flux density by comparing temperatures between a heated and a reference needle placed in the sapwood. The heat ratio method (HRM) uses a short heat pulse and measures the ratio of temperature rises to infer sap velocity and, with calibration, sap flux. Stem heat balance (SHB) methods model the balance of heat into a stem section to derive total sap flow. Each technique has specific assumptions, calibration requirements, and suitability depending on species, stem size, and environmental conditions.

Applications of sap flow measurements span ecohydrology, forestry, and plant physiology. They are used to estimate transpiration at the scale of individual trees and to upscale to stand or ecosystem water use. Sap flow data contribute to studies of drought responses, stomatal regulation, growth-water relations, and hydrological modeling, aiding water resource management and climate-vegetation research.

Limitations include sensor installation effects on stems, species- and size-dependent calibration needs, spatial variability within a plant, and the need for careful interpretation when upscaling to larger scales.