Limitations of a compensation heat pulse velocity system at low sap flow: implications for measurements at night and in shaded trees.

Unlike an ideal system, the return time to thermal balance (t(b)) between upstream and downstream thermistors, as measured by the (compensation) heat pulse velocity method, effectively depends on the heat input and the water content of the wood at zero and low sap flow. Even when these factors were held constant and ambient temperature was stabilized, a twofold variation in t(b) at zero flow was observed within and among Greenspan Technology sensors implanted in wooden posts, making it impossible to distinguish zero flow from low sap velocities (< 0.01-0.02 mm s(-1)). This limitation has serious consequences because the contribution of low flow rates to water movement is important during both daytime and nighttime in tropical understory and overstory trees. Measurements in an artificial flow system showed that this technical limitation is exacerbated by erratic variation in sensor response at both zero and low flow rates. The limited sensitivity of the tested sap flow sensors may be caused by their poor thermal contact with wood. Interim procedures are suggested for estimating minimum detectable sap flow and delimiting the hydroactive zone until the sensitivity and interchangeability of sap flow probes are improved.