Yoshiko Iida, Takashi Kohyama
The above ground form and size of woody plants can be characterized by the dimensions of the stem and crown. The relatioships between architectural dimensions characterize the size-dependent structural changes of species or whole stands. Such among-species differences in structural chenges are often discussed in terms of “life history strategies” in relation to light capture, biomass distribution/allocation/accumulation and mechanical safety.
Foresters and biologists have developed a variety of techniques and equipments to measure woody plant size and form under field conditions. These measurements usually include estimates of stem diameter, plant height, crown height, and various crown dimensions. The precision and efficiency of these measurements depends on the used techniques and equipment. For most protocols, the traits are best estimated by two or more people.
- Steel measurement tape for stem diameter and crown (5 m and 30 m length may be enough).
- Telescopic measurement pole (e.g. fiberglass splicing pole)
- Rangefinder (laser rangefinder such as a Nikon ProStaff Laser 440, LaserAce 300orLaserAce Hypsometer; MDL, or an ultra sonic measuring system(e.g. Vertex IV, Haglof)
Units, terms, definitions
- Stem diameter of trees is usually measured at “breast height” (diameter at breast height, in short referred to as DBH or D, in centimeters). In most studies, the default breast height is set at 1.3 m above the stem base. This DBH can also be calculated from measurements of stem girth (GBH) , at the same height above the stem base.
- Height (or total height, H, in meters) is defined as the vertical distance from the stem base to the top of the woody plant.
- Height of the lowest living branch (Hb, in meters) is defined as the vertical distance from stem base to the base of the lowest major living branch. We can calculate crown depth (CD) from the difference between H and Hb. Please decide the criteria of threshold size of major living branch before you start the measurement. Tiny branch with few leaves are usually ignored.
- Height of the lowest leaf (or foliage, Hf, in meters) is defined as the vertical distance from stem base to the lowest living leaf. This measure provides a second way of calculating crown depth from the difference between H and Hf. However, also here the same problem arises: sometimes few leaves appear at very low height, while the actual crown with most leaves is at much greater height. There is little consistency across studies to account for such problems, but it is important to set up clear criteria for this when a new study is initiated.
- Crown width (W, in meters) is in most cases provided at two perpendicular directions, for example two wind directions or the widest crown width and the one perpendicular to it. Crown width can be used to calculate the crown area and, in combination with the crown depth, to calculate the crown volume.
Fig. 1. The major architectural dimensions of a woody plant to characterize size and shape. Symbols: Plant height H, foliage height Hf, branch height Hb, and girth at breast height GBH.
According to Sterck et al. (2001), Kohyama et al. (2003), Bohlman & O’Brien (2006), Poorter et al. (2006) and Iida et al. (in press).
– Stem diameter (DBH or D)
The stem diameter can be measured using a steel measurement tape at 1.3 m height above ground level. Please make sure to keep the tape horizontal to avoid overestimations. Calculate the stem diameter at breast height (DBH) from the girth at breast height (GBH): DBH = GBH/ℼ. Alternatively, the stem diameter can be measured using a specialized tape at 1.3 m height above ground, which provides the stem diameter at breast height directly.
Fig. 2. Usage of GBH (or DBH) tape:
Left-hand photo shows the correct and more precise way to measure the stem diameter. The right-hand photo creates error, because it is more difficult to read the scale correctly. So, note to keep the zero at lower position of tape.
– Heights of the top of plant and crown (H, Hb, Hf)
Method 1 (less than 2 m tall)
For seedlings and saplings (less than 2 m tall), a measurement tape is kept vertical from stem base to the upper most apical meristem to measure the plant height (H), to the lowest branch height to provide Hb and to lowest leaf in the crown to provide Hf.
Method 2 (2 m < H < 15 m)
For trees with 2m < H < 15 m tall, a telescopic measurement pole is recommended. Poles are available that extend to a length of ~15 m.
1. One person acts as the height observer and looks for a position from where he/she can see the focal tree and, in particular, the plant top, the lowest major branch and the lowest foliage. This person also records the data that will be provided.
2. The second person takes his position below the focal tree and gradually lengthens the telescopic pole. At a sign of the observer, he stops extending the pole and checks and calls the length indicated by the pole. The observer usually registers the data.
3. In most cases, the three different height measurements can thus be provided from the same location, if not, the appropriate location is searched for each measurement.
Method 3 (H > 15 m)
For trees > 15 m, a clinometer and distance measurement (e.g. tape, laser rangefinder) or an ultra sonic measuring system can be used. When telescopic poles are not available, these methods can also be used for trees < 15m but at lower precision than using a pole. Here we provide 2 alternative methods.
Method 3-1: Clinometer and distance measurement (by tape or laser rangefinder)
- The observer walks to a position from where the stem base and the height of plant, foliage and/or major branch can be observed.
- The observer measures the angle from the observer’s eye to the tree top using a clinometer or laser rangefinder (Fig.3, ) and the distance from the observer’s eye to tree top (Fig.3, hypotenuse, L1). Alternatively, the observer may measure the distance from the observers eye to stem in horizontal direction (Fig.3, adjacent, L2), using a rangefinder or a measurement tape. In the case the tree stands on the slope, the angle from the observer’s eye to the stem base should also be measured (Fig. 3b).The later method (Fig. 3b) is more precise than former method (Fig. 3a) even at flat sites, so it is recommended to search (if possible) for location where one can see the top of tree, the trunk and the stem base at the same time.
- From these measurements, the height can be calculated as, for example,
H =sin ( ) x L1 + observer height
Or, H = tan( ) x L2 + observer height
In the case that a focal tree is on the slope, additionally we measure the second angle to the stem base (Fig. 3b).
H = tan( ) x L2 + tan( ) x L2 .
Method 3-2 Vertex or laser rangefinder
- Ultra sonic measuring systems are often used nowadays, because they are relatively easy to work with. Here, a protocol is provided for using a vertexIV. After calibration, the transponder T3 can be installed on the stem at 1.3 m height (this height however depends on the settings).
- Move to the position where you can see the top of height.
- Press ON to start the Vertex and aim at the transponder. Keep pressing ON until the cross hair sight goes out momentarily (if it works, you can hear a sound.). Now release ON. (The vertex has measured the distance, the angle and the horizontal distance to the transponder).
- Aim at the top of tree height with the sight cross blinking.Press ON until the cross hair disappears. Now, the tree height is locked and displayed. Then, if you continue to aim at different heights, for instance to measure foliage or major branch height, and press ON without moving your position, you can obtain the required different heights after each other. If you have to move, please repeat the whole procedure from point 2 onwards.
Fig. 3. Height Measurement
– Crown width
We have several types of crown measurements. Crown area can be calculated using crown radii or width, assuming an ellipsoid shape.
1. One person stands at the stem base and keeps the measurement tape fixed.
2. The other person unrolls the measurement tape to the crown edge. To make sure that the edge is reached, this person could use a level or clinometer to position herself/himself underneath the crown edge. Additionally, the person who is standing near the tree may, if possible, swing the focal tree to make the other person find the edge more easily. The distance is than recorded as one radius (often the wind direction is also recorded).
3. This measurement can be repeated in different directions. Most often, crown radii from the stem base to the edge of the crown are measured in 4 or even 8 wind directions (e.g. N, NE, E, SE, S, SW W and NW) with a measurement tape (Fig. 4a, b).
4.Alternatively, the crown widths can be measured in different directions. In this case, the wind directions can also be used, but often two perpendicular horizontal widths are measured, i.e. the longest crown width and the crown width perpendicular to the longest crown width) (Fig. 4c, d).
Fig.4 crown radii and width
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Bohlman S, O’Brien S (2006) Allometry, adult stature and regeneration requirement of 65 tree species on Barro Colorado Island, Panama. Journal of Tropical Ecology 22, 123-136.
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Iida, Y., Kohyama, T., Kubo, T., Kassim, A. R., Poorter, L., Sterck, F. & Potts, M. (in press) Tree architecture and life-history strategies across 200 co-occurring tropical tree species. Functional Ecology