Sensors and instrumentation to measure Sap flow in small stem plants

Heat-based sap flow sensors are widely used in research to analyse plant-water relationships and to study water flow dynamics of plants to determine the water requirements for optimum growth. To date, the majority of field studies of sap flow have been conducted in large plants, with only a few methods being suitable for small stems of less than 10 mm in diameter. This paper discusses the suitability of sensors and electronic designs for a heat-based method to monitor sap flows over the velocity range 2-150 cm/hr, in a small plant stems of 5 to 10 mm in diameter. Three heat-pulse methods (Compensated Heat Pulse, Heat Ratio and Tmax) were tested using both internal and external sensor designs. Based on preliminary findings, two alternative methods of analysis were developed and tested in the laboratory. A short field experiment was carried out to compare heat pulse velocity and global solar radiation. We found the external sensor was not suitable for small stems (<; 20 mm diam.) with thick (> 3mm) bark. A small internal sensor showed more promising results for resolving a wider range of sap flows and tracking the influence of global solar radiation.

I. Introduction

Sap is the fluid that transports water and nutrients through a plant. Sap flux density (cm/hr) directly relates to the amount of water the plant is transpiring (L/hr) divided by the conducting cross sectional (cm²) and is a metric that can be used to analyze the plant-water relationships and water flow dynamics within and around a plant. This information is widely used by research scientists to determine plant water use and irrigation requirements for a range of woody-stemmed species [2] [4] [5].