Daigo, M; Kajiwara, K; Honda, Y

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1 Nara Women's University Digital I Title Author(s) Citation An improved tree-height measurement production in a larch forest on Mt. Thanyapraneedkul, Juthasinee; Ikega Daigo, M; Kajiwara, K; Honda, Y J.Thanyapraneedkul 他 : 人間文化研究科年報 ( 奈良間文化研究科 ), 第 26 号, pp Issue Date Description URL Textversionpublisher This document is downloaded

2 An improved tree-height measurement method for calculating net primary production in a larch forest on Mt. Yatsugatake, Japan J. Thanyapraneedkul 1), K. Ikegami 1), K. Muramatsu 2), N. Soyama 3), M. Daigo 4), K. Kajiwara 5), Y. Honda 5) 1) Graduate School of Humanities and Science, Nara Women s University 2) Kyousei Science Center for Life and Nature, Nara Women s University 3) Faculty of Economics, Doshisha University 4) Center for Research and Development of Liberal arts Education, Tenri University 5) Center for Environmental Remote Sensing, Chiba University 1. Introduction Tree height, together with diameter at breast height (DBH), is an important parameter used to calculate net primary production (NPP). It is used to validate estimates of NPP obtained from satellite sensor data. Tree height is defined as the vertical distance from ground level to the highest green point on a tree (referred to hereafter as the tip of the tree) 1) and can be measured directly or indirectly. Direct measurement techniques can only be used on felled or small trees. Taller trees are normally measured indirectly using a hypsometer, a tool that measures angles and distances and uses geometric and trigonometric principles to calculate tree height. Using these devices in close proximity to a subject tree results in significant error in height measurement, and measurements should be made from a location where the measurer can observe the tree tip. However, in forests, it is generally not possible to measure a tree s height from an appropriate distance because line of sight is interrupted. The Japan Aerospace Exploration Agency (JAXA) recently constructed an observation tower in a larch forest on Mt. Yatsugatake for the purposes of observing phenology, weather conditions, and forest radiation and collecting data to validate satellite sensor data analysis. Because tree tips can be better observed from the observation tower than from the ground, in this study, we focused on using the tower to measure tree heights with the goal of improving the accuracy of tree-height measurements. 2. Study site The study site is located in a larch forest on Mt. Yatsugatake, in Yamanashi Prefecture, Japan (Fig. 1), at 1260 m above mean sea level. The site encompasses 1200 m 2 and has a slope of around 5.8. The site includes 77 larch trees. The meteorological and ecological tower shown in Fig. 1 is located at * School of Interdisciplinary Research of Scientific Phenomena and Information 261

3 N, 'E. The tower is about 25 m tall and has five floors. The relative locations of the tower and larches are shown in Fig. 1. Leaf-out begins in May for the larches, and leaves begin to fall in November. Snowfall generally occurs from November to April in the study area. Fig. 1 Flux tower site (left), larch forest surrounding the tower (middle), and locations of trees relative to the tower (right). 3. Methodology 3.1 Equipment We used an ultrasonic digital hypsometer (Vertex III, Haglö, Sweden) 2) to measure tree height. The Vertex III measures the angle from the horizontal to the tree tip and the distance from the observer to the tree. Two methods are available for measuring the distance component, one using an ultrasonic transponder and the other using a laser. To measure the distance between the observer and the tree with the ultrasonic transponder, the transponder is placed in front of the tree, and the ultrasonic wave is detected and the distance calculated automatically by the Vertex III. Alternatively, the distance can be measured by laser. However, it is easier to use the former method in the forest because of the numerous obstacles, i.e., leaves and branches that get in the way of the laser beam. Therefore, all distance measurements in this study were made using an ultrasonic transponder. 3.2 Tree height measurement methods We compared three different methods of measuring tree height using the Vertex III. The first method is a generally used technique, and the two other methods utilized the tower. 1) Method 1: Measuring from ground level The ultrasonic transponder was placed in front of a tree, and the observer measured the distance to the tree and the angle to the tree tip using the height measurement mode of the Vertex III, as shown in Fig. 2-A. 262

4 Fig. 2-A Height measurement method 1, where the observer with the Vertex III is positioned on the ground. T3: ultrasonic transponder; H: tree height. 2) Method 2: Measuring from the 4 th floor of the flux tower This method was the same as method 1, but the observer s position was changed from the ground to the 4th floor of the tower, as shown in Fig. 2-B. Fig. 2-B Height measurement method 2, where the observer is located on the 4 th floor of the tower. 3) Method 3: Measuring from the 5th floor of the flux tower In this method, the angle and the distance were measured from the top of the tower. If the study area were flat, we could just add the tower height to the angle measurement height. However, the area is not flat and has a slope; thus, we needed to determine the differential height from the tower to the base of the tree. To calculate the differential height from the tower base, the distance between the base of the tree (x1) and the tower base, and the angle to the tree s roots (1) were determined, as shown in Fig. 2-C (left). In this case, the height of the transponder is the same as the height of the observer s eye. Using this setting, the measured angle to the transponder from the tower base was the same as the angle from the base of the tree to the tower s base. In Fig. 2-C, x1 is the diagonal distance from the corner of the tower s base to the base of the tree. 1 is the slope from the corner of the tower base to the tree base. The height difference (h1) is calculated as h1 = SIN 1*x1, and the horizontal distance (d1) is calculated as d1 = COS 1*x1. On the 5 th floor, the angle (2) to the tree tip is measured, and the height difference (h2) between eye height and the tree tip is calculated as h2 = TAN 2*d2. Finally, tree height (H) is calculated as H = h1 + Htower + Heye + h2. Here, Htower is the tower s height of 25 m, and Heye is the height of the observer s eye from the 5 th floor of the tower. 263

5 Fig. 2-C Height measurement method 3, where the observer is located on the 5 th floor of the tower (right). Parameters measured to calculate differential height: observer on the ground (left) and on the 5 th floor of the tower (right). Fig. 3 Corner number (left), and top view of tower with dimensions in meters (right). 3.3 Tree height measurements The heights of all trees at the study site were measured using method 1 on 13 December 2008, 17 March 2009, and 26 March A limited number of trees were measured seasonally using method 2 on 17 April 2009 and 26 March 2010 and using method 3 on 26 March 2010 and 29 April

6 4. Checking the accuracy of in situ tree height measurements made using the Vertex III To better understand the error associated with tree height measurements, two experiments were carried out with objects of known height. The first experiment involved measuring the height of a pole, the top of which could be clearly observed, and finding the minimum distance between observer and object. The second experiment involved a Christmas tree, which had the shape of a tree and could be used to test the importance of transponder location. In the forest, the transponder often cannot be placed in the most appropriate location due to interference from leaves and branches. 4.1 Pole measurements An extendable pole, with a fully extended height of exactly 10 m, was used for this experiment. The distance between the observer and the pole varied from 5 to 10 m (results not shown). We determined that the angle from tree tip to the Vertex III should be no less than 51 and no more than 79, otherwise error greater than 10% and 15%, respectively, occurs 3) (results not shown). Slope had no effect on this measurement. 4.2 Christmas tree measurements A Christmas tree m in height was placed on an 8.20-m-tall building, for a total height of 9.49 m. The transponder and the Vertex III were at ground level. In the first experiment, the distance between the observer and the tree varied from 15.5 to 7.5 m. Next, the distance between the transponder and the observer was kept at a constant 15.5 m while we changed the transponder location and angle. The results are shown in Table 1. A leaning transponder (so that the transponder was not directly under the tree tip) resulted in an error of around one to three percent. Table 1 Christmas tree height and transponder location 265

7 5. Results and discussion 5.1 Tree heights measured using methods 1 and 2. The heights of all 77 trees at the study site can be measured using method 1 if the observer can find a suitable position to observe each tree tip. Thus, winter is a good season to conduct measurements using method 1 because the trees have lost their needles. However only a limited number of trees can be measured using method 2 because the tree tip must be visible from the tower. If trees or needles overlap, and the tree is too close to the tower, the tree tip is not easily seen. The number of trees measurable using method 2 was 45 at this site under the best, defoliated, conditions. The measurement results for these 45 trees using methods 1 and 2 are summarized in Table 2. The mean and standard deviation of the height measurements using method 1 were ± 2.27 m for March 2009 and ± 2.16 m for March The mean and standard deviation of heights measured using method 2 were ± 2.70 m in April 2009 and ± 2.71 m in March ) Stability of method 1 and 2 measurements Tree height was measured at the beginning and the end of the winter season with methods 1 and 2, respectively. Because trees do not grow during the winter season, the measurements using methods 1 and 2 should be the same for each tree. We compared the measurement heights for methods 1 and 2. The results are shown in Fig. 4 for method 1 and in Fig. 5 for method 2. The average difference between the measurements at the beginning and end of the winter season was 0.65 ± 0.71 m for method 1 and 0.18 ± 0.21 m for method 2. The difference was smaller for method 2 than for method 1, indicating that method 2 was more stable than method 1. The difficulty of viewing tree tips from the ground resulted in the large difference in method 1 observations. For both methods, strong winds, which moved tree tips, resulted in observation error, but the effect was greater using method 1. 2) Differences between method 1 and method 2 tree-height measurements Measurements made using the two methods were compared using data from March 2009 and March The results are shown in Fig. 6. The average difference in value between the two methods was 1.1 ± 0.87 m. Method 1 measurements tended to be larger than method 2 measurements. 266

8 Fig. 4 Tree height measurements at the beginning (Dec, 2008) and end (Mar, 2009) of the winter season (measured on the ground, method 1). Fig. 5 Tree-height measurements at the beginning (Nov, 2009) and end (Mar, 2009) of the winter season (measured from the 4th floor of the tower, method 2). Fig. 6 Difference in tree height using the measurement data for March 2009 and March

9 3) Tree-height growth We studied the effects of tree-height measurement on the estimation of tree growth. Tree growth or change in stem timber volume (V [m 3 ]) was calculated from DBH (d [cm]) and height (h [m]) data using the stem timber volume method 4) : The total volume of a tree is calculated based on the proportion of the stem 5) to the whole tree. Therefore, it is important to have an accurate measurement of tree height. Figure 7 shows tree growth calculated using the values measured by the two methods for one year. Method 1 resulted in some estimates of tree growth that were too large or too small. Thus, for estimating tree growth, method 2 was more stable. Fig. 7 Tree-height growth based on method 1 and 2 measurements. 268

10 Table 2. A comparison of tree heights measured using methods 1 and 2 (m). 269

11 5.2 Tree heights measured using method 3 Methods 1 and method 2 produced height measurements directly from the Vertex III. Method 3, however, required post-measurement calculations to produce tree-height measurements. Distance (x1) and angle (1) from the ground and angle (2) from the 5 th floor were used to calculate tree height using principles of trigonometry. The results and calculations of tree height are summarized in Table 3. Table 3 Tree height determined using method 3, and the parameters measured on the ground (distance (x1) and angle (1)) and on the 5 th floor of the tower (angle (2)). We then calculated the differences for all pairs of data (5th floor vs. 4 th floor; 5 th floor vs. ground; 4 th floor vs. ground) as: 270

12 The percent difference (%diff) (equation 1) was evaluated for all trees. The results are shown in Figure 8. The lowest error percentage was consistently between the 5 th and 4 th floor measurements, indicating that methods 2 and 3 were relatively stable and produced very similar results for any given tree. Figure 8 shows that in most cases, the percent difference between the 4 th and 5 th floor measurements was less than 5% (error ± 1 m.) Fig. 8 Percent difference of tree height measurement for two pairs of data (5 th floor vs. 4 th floor and 4 th floor vs. ground) Fig. 9 Relationship between tree heights measured using methods 2 (4 th floor) and 3 (5 th floor) (r 2 = ). 271

13 Heights measured using methods 2 and 3 were compared, as shown in Fig. 9. A clear linear relationship was found, with a correlation coefficient (r 2 ) of We found several possible sources of error in tree-height measurements. First, the distance between the observer and the tree can be too short (less than 20% of tree height). For example, trees no. 27, 34, 47, and 48 were too close to the tower, and thus method 1 and 2 measurements had large error components. Second, tree branches can interfere with accurately locating the tip of a tree, especially in method 1. Some trees affected by this were located in areas with dense foliage where it became difficult to see the tree tip area (trees no. 23, 24, and 25). Moreover, the tip of tree no. 7 was bending, which showed that an observer cannot always determine the highest point of a tree. Third, error may result if the transponder cannot be clearly seen from the tower in method 2, because when the transponder location is adjusted to make it visible, it is no longer directly under the tree tip. Fourth, wind and rain are obstacles to accurate height measurement. Tree tips move in the wind and are therefore difficult to accurately identify, and rain weakens the transponder signal, making it difficult for the Vertex III to read the signal. Finally, large trees near the tower (e.g., tree no. 48) block the views from the tower to the tips of other trees (tree nos. 13 and 14). For this reason, method 3, measuring from the 5th floor of the tower, is better for finding tree tips further from the tower, such as tree no. 11,12, 15, and 16 (trees located southeast of the tower; Fig. 1). Therefore, the tower was useful for improving the stability and accuracy of tree height measurements. 6. Conclusion In this study, we compared three methods of measuring tree height with a Vertex III ultrasonic digital hypsometer. The first method involved measuring from ground level, and the other two methods involved measuring tree height from the 4 th and 5 th floors of an observation tower. We compared measurements from ground level and from the 4 th floor taken at different times of the year but in the same tree height season, and found that the measurements from the 4 th floor were more stable than those from the ground level. Tree height measured from the ground tended to be 1 m greater than that measured from the 4 th floor. Moreover, measuring tree height from the 5 th floor was preferable because it reduced the time needed to find a tree tip. The measurement results from the 5 th floor showed a high linear correlation with those from the 4 th floor. These results indicate that measurements from the 4 th and 5 th floors are useful for determining tree height. For future tree-height measurements at the study site, we only need to measure the angle (2) from the 5 th floor because distance and angle (X1 and 1) on the ground will remain the same. Thus, this method is efficient because the time required to resurvey the study trees is reduced. Thus, this method is useful for NPP calculations where changes in tree height need to be determined. Acknowledgements Thank you Dr. Supanika Potithep (JAMSTEC) and her sister for helping in field survey. References 1) Philip W. West, 2003, Tree and Forest Measurement, Springer, pp

14 2) Haglö Sweden AB, Users guide Vertex III and Transponder T3, php. 3) J. Thanyapraneedkul, K. Muramatsu, K. Ikegami, M. Daigo, Tree height measurement in Mt. Yatsugatake, Yamanashi, Japan, 2009, DOSHISHA UNIVERSITY World Wide Business Review Vol.10 Special Issue of Environmental Observation, pp ) J-FIC, The formula of the stem timber table ver. east area of Japan, ) K. Ikegami, et al., Estimating and Validating the Net Primary Production of Vegetation Using ADEOS-II/GLI 250 m Spatial Resolution Data around Yatsugatake Mountain Area, Japan, 2009, DOSHISHA UNIVERSITY World Wide Business Review Vol.10 Special Issue of Environmental Observation, pp Appendix The T3 transponder in Figure 10(A) is an ultrasonic transmitter/receiver that communicates with the Vertex III (B) instrument. For first use calibration is needed. Use a measuring tape to measure the exact distance of 10.0 m between the transponder and the Vertex front. - Press ON to start the Vertex instrument. Then menu CALIBRATE and press ON. - The instrument will calibrate to 10 m and automatically turn off when ready. (It is important to give the instrument approximately 10 minutes to set to the correct temperature before calibrating.) - Start the transponder and place it on 360 degree pole. Then put in front of tree. - From Setup display setting P.OFFSET as 0.3 and T.HEIGHT (observer and t r a n s p o n d e r s height are the same) as 1.5 m - Another observer selects the position that can see tip of tree clearly and not so much angle between tree tip and VertexIII. - Press ON. Aim towards the transponder and press ON then cross hair will blink. - Move blinking cross hair aim towards the height to measure. - Press ON at tip of tree until the cross hair sight goes out. Then record height. Fig degree - T3 transponder (A) and Vertex III (B) 273

15 An improved tree-height measurement method for calculating net primary production in a larch forest on Mt. Yatsugatake, Japan J. Thanyapraneedkul 1), K. Ikegami 1), K. Muramatsu 2), N. Soyama 3), M. Daigo 4), K. Kajiwara 5), Y. Honda 5) Observation towers in forests provide necessary data for satellite image product validation, such as net primary production and biomass. One of the key parameters in these calculations is tree height. Here we compared tree heights calculated using three different non-destructive measurement methods, using a Vertex III ultrasonic digital hypsometer in each case. Method 1 involved measuring tree height conventionally, from the ground. Methods 2 and 3 involved measuring tree height from the 4 th and 5 th floors, respectively, of an observation tower. Tree height measurements from the 4 th floor were more stable than those from the ground. Tree height measured from the ground was greater than that measured from the 4 th floor. The 4 th and 5 th floor measurements showed a good relationship (r 2 = 0.88). From these results, we decided to measure tree height from the 5 th floor of the tower because tree tips can be seen more clearly, thus reducing measurement time, and because future measurements require only remeasuring the angle on the 5 th floor, further reducing the time required to gather tree-height data. Therefore, our measurement method is useful for application to field surveys and observations of tree-height change. 274