# Effect of Ambient Temperature on Impedance-Based Physiological Activity Evaluation of Zelkova Tree
## 1 Introduction
[PERSON] measured and published the correlation between the nitrogen content and electrical impedance in lettuce [7]. Two 0.75 cm long stainless steel needles were used as electrodes, and the impedance was measured using an LCR meter. Since the contact area between the needle and the lettuce leaf is small, the contact impedance  
Figure 1: Appearance of zelkova trees according to the seasons (a) summer and (b) fall. is expected to be relatively large compared to the impedance of the lettuce leaf. Since the two-contact measurement method was used, a large error due to contact impedance would have been included in the measurement results. [PERSON]. [PERSON] utilized impedance measurements to monitor the healing process after grafting [8]. The impedance was measured using two nickel sheet electrodes that can cover tree skin up to 45 mm in diameter. Because the electrodes were placed around the outside of the tree bark, the contact impedance of the electrodes would have been very large, and because the two-contact measurement method was used, measurement errors are presumed to have occurred. Since the previously mentioned preliminary studies all used a two-contact impedance measurement method, they could not in principle overcome the error in the contact resistance. There are also examples of the four-point electrical impedance measurement method being applied to plant applications research. [PERSON] measured the impedance of leaves cut into 5 cm \(\times\) 6 cm pieces using a four-point measurement method six times over two days while they were drying [9]. [PERSON] used a four-point impedance measurement method using an impedance analyzer (Impedance Interface 1294, Solartron Analytical, Farnborough, UK) to examine the relationship between the phosphoric acid content and impedance of tomato leaves [10]. This study was conducted in an indoor laboratory using devices or instruments and performed four-contact measurements based on intermittent measurements. The previous four-point measurement systems are insufficient in the real-time, long-term impedance monitoring of trees growing in actual outdoor environments, so no continuous long-term impedance monitoring data has yet been reported. To monitor the physiological activity of outdoor trees, a tree physiological activity monitoring system (TPAM) capable of continuous, long-term, four-point impedance monitoring was developed. The developed TPAM system was mounted on a tree, and the tree impedance was measured continuously from summer to winter. The TPAM system simultaneously measures not only the impedance but also the ambient temperature and illuminance, and the measured data are accumulated in the hub system through Zigbee wireless communication. TPAM is a useful research tool that can obtain biometric and ambient information of trees growing outdoors on a long-term continuous remote basis. TPAM makes it possible to acquire and interpret long-term biological and environmental data on plants in areas where this was previously impossible. Section 2 explains the difference between the two-contact impedance measurement method and the four-contact impedance measurement method, and Section 3 describes the hardware and software that make up the TPAM system. Section 4 presents the results of the tree impedance long-term monitoring using the developed TPAM system and discussions on the results, and Section 5 concludes.