The lack of giving attention to the water needs of trees can lead to water wastage or water stresses in trees. Accurate irrigation of trees requires the availability of information on the amount and manner of water consumed in trees. However, for the precise implementation of irrigation, a reliable method of measuring the plant's water requirements is needed to avoid water stress. Existing methods of measuring the water requirement of trees can often be used for a part of the tree (such as stem or leaf) and require the presence of the user and spend time and money, so these methods are often used in research applications.
Materials and Methods
A sap-flow meter device was also developed to make a heat pulse in a tree trunk every 15 minutes. The device consists of measuring probes, a processor unit, and a data logger. In order to evaluate, the device the probes were installed on the trunk of the Ficus Benjamina tree in a controlled environment, at two different heights, and the results of the sap flow through the vascular tissue were compared with the data obtained by the lysimetric method. The Ficus Benjamina tree with a trunk diameter of 3.5 cm and height of 196 cm was prepared in the summer of 2022. By measuring the speed of destruction of the heat pulse and using the theorem of heat transfer, and assuming that heat transfer is done only through the sap flow from vascular tissue, the amount of sap flow was estimated by the Heat ratio method (HRM).
The trunk was triple drilled 1.5 mm in diameter and 25mm in depth. After drilling, the probes were placed in the holes. In order to avoid heat transfer from the probes to the outside, the trunk was covered by Glass wool insulation (Figure 3). To measure the reliability of the device, the lysimetric method was used to measure the tree transpiration. For this purpose, the soil surface of the pot was covered with cellophane so that the evaporation and weight reduction of the pot were done only from the leaves of the tree. Every hour, the weight of the pot was measured by a digital scale. Changes in the weight of the pot indicate the amount of water evaporated. In other words, this water has been transferred through the vascular tissue of the tree trunk.
Results and Discussion
The results showed that the sap-flow meter device measures the amount of water consumption of the tree with a little overestimation compared to the value obtained by the lysimetric method. Sap flow and transpiration follow a similar trend so that it increases during the day and reaches their peak value in the middle of the afternoon. This value reached 17.98 ml.h-1 for sap flow and 16 ml.h-1 for transpiration (by lysimetric method), followed by a rapid decrease in the late afternoon as the air was cooled. In addition, the results of device measurements showed spraying irrigation on the leaves lowers the speed and amount of sap flow. Water spraying on the leaves affects the speed of sap flow, so that when the canopy gets wet, water evaporation from the leaf surface made them colder, and the speed of sap flow decreases significantly.
The v1/v2 ratio is not constant with time, and selecting the appropriate measurement starting time is essential for the working cycle of the device for the data acquisition. It is essential to measure (by the device) the difference between temperature probes 40 seconds after heat pulse generation. The sap flow and transpiration followed a similar trend during the experiments, the sap flow and transpiration increased during the daytime and reached their peak value in the middle of the afternoon, which is 17.98 ( ) for the first day and 19.75 ( ) sap flow in the second day. The related lysimetric method transpirations reached 16 ( ), followed by a rapid decrease in the late afternoon.
According to the obtained results, the following are deduced from the developed device:
Sap flow and transpiration follows a similar trend during the test period, and the estimated value of the sap flow is 30 percent higher than that of the lysimetric method.
The device was able to show the effect of surface irrigation well, irrigation by spray affects the speed of sap flow so that when the canopy gets wet, the speed of sap flow decreases noticeably. Sap flow and transpiration are directly proportional to air temperature, and canopy temperature, and inversely proportional to relative humidity.
The results showed that the heat pulse method after calibration can reliably estimate the amount of sap flow in the vascular tissue of trees.