Trees are woody perennial plants having a single, usually elongated main stem with few or no branches on their lower part. Trees could be large or mature (DBH > 30 cm), medium-sized or growing (DBH 10-30 cm) or regenerating seedlings (DBH < 10 cm). A plant belonging to a tree species is considered for measurement in tree quadrats if it is taller than 1.5 m and its DBH is greater than 5 cm (a girth of about 15 cm). The height and DBH class to be considered in the tree quadrat will vary with the project type and age of the stand. In the case of old, mature forests with large trees, a tree could be defined to have a DBH of greater than 30 cm. Locate the tree plots in the field and measure the DBH, height and other parameters for all the trees using the procedure described in this section.
Parameters to be measured include species, number of stems, DBH, height, status of regeneration, state (living, dead and standing, dead and fallen) and extent of damage to crown.
Frequency of measurement for trees will vary from 1 year for fast-growing tree species to 5 years for slow-growing naturally regenerating trees (refer to Chapter 4 for details).
DBH DBH is easy to measure and verify. It requires only a measuring tape and a marker. DBH is measured using the following procedure:
• Mark a point 130 cm above the ground on the tree trunk.
• Place the measuring tape around the trunk at the 130 cm mark.
• Measure and record the DBH or GBH in centimetres:
° If a tree has multiple shoots, count and measure GBH/DBH for all shoots. ° If the tree is large, girth is normally measured with a measuring tape.
° If the tree is young and slender, measure the DBH with a slide caliper. ° If the tree is on the boundary, include it for measurement in the sample plot only if more than 50% of its girth is inside the plot.
A tree could have multiple shoots and/or crooked trunks, could be growing at an angle, and could be on a sloping hill. The measurement technique for irregularly shaped trees and different land conditions is illustrated in Fig. 10.6.
Tree height normally refers to total tree height defined as the vertical distance from the ground level to the uppermost point. Tree height is also often referred to as merchantable height since many allometric equations are derived for this height. Height is measured for all the tree stems for which DBH is measured (Commonwealth of Australia 2001). Unlike DBH, measurement of tree height is difficult for tall
trees, especially in a dense forest or plantation with tree close together and overlapping crowns. Height can be measured using different methods.
(i) Measurement using instruments Tree height can be measured using various instruments or even using a measuring tape. However, measuring the height of individual trees with overlapping tree crowns and trees in a dense forest or plantation poses a challenge for measurement even using instruments. Trees taller than 5 m can be measured using a graduated height-stick by holding the stick against the side of the tree. Clinometer is one of the instruments used for measuring the height of trees but it is not suitable for dense vegetation where visibility is limited. Mark out a spot 10 m from the tree from which the tree can be viewed using a clinometer. If necessary, move beyond 10 m. If the plot is located on a steep slope, view the tree from across the slope to maintain the required distance. Sighting the tree through the clinometer, align the centre line with the base of the tree (ground level on the upside slope) and record the reading on the percent scale (base angle %). Next, aim the clinometer at the top of the tree and record the reading on a percentage scale. Calculate the height using the following equation:
(ii) Height classes Trees can be grouped into different height classes (e.g. 0-5, 6-10, 11-15, 16-20 m). These classes can be used as reference height classes to get an approximate estimate of the height of trees in a plot. Trees are observed during fieldwork and categorized into these height classes. Field investigators with a little practice and experience can estimate the height class of a tree by mere observation and place it in the appropriate class.
(iii) Height equation Height for a given tree species is correlated with its DBH. A regression equation can be developed for a given species by measuring the height and DBH of atleast 30 sample trees of different heights. Using the height estimation equation, height can be estimated from DBH data for a given tree species. The equation could be along the following lines.
where D = DBH, a = constant, b = regression coefficient.
For example, Chaturvedi (2007), based on the measurements of more than 4,000 sample trees of teak (Tectona grandis) of different age classes in Western Ghats, India, estimated the relationship between DBH and height, by considering height as the dependent variable and DBH and age as independent variables.
[top angle (%) - base angle (%)] x horizontal distance 100
Height data are required for all the tree species during the project development phase as well as project monitoring phase. The following approach could be adopted for measuring the height of trees during these two phases.
Project development phase During the project development phase, height of trees in the baseline scenario could be measured using a measuring tape or clinometer, since only a few trees may have to be measured. Alternatively, tree height class method could be adopted for forests or plantations with dense tree population. Project monitoring phase Multiple methods could be adopted for measuring tree heights during the monitoring phase:
• During the seedling or younger stage when the trees are shorter than, say, 5 m, all trees in the sample plots could be measured using a simple graduated pole or measuring tape.
• When the trees grow taller, either height-DBH equations could be developed by measuring trees of different heights and DBH or height-measuring instruments could be used.
Regeneration status It is important to know whether a tree has regenerated or grown from a seed or sapling planted deliberately or has grown naturally as a part of the project activity. Such a distinction - trained field staff can make out the difference - enables the contribution of each to the carbon stock to be estimated separately. This information should be recorded in the data format for each of the tree species. Status of trees The following information regarding the health of trees should be recorded, which will be useful in estimating the quantities of biomass from living trees and from deadwood:
• If the crown is damaged, the percentage of damage
• Whether the tree is dead and standing or dead and fallen
Tagging of trees Perennial trees may have to be measured periodically over a number years or even decades in forestry projects. Therefore, it may be desirable to mark or tag them to make it easier to find them and to ascertain their species and number. This purpose could be achieved by fixing aluminium or other metallic tags to the trees.
A suggested format for recording the data is shown in Table 10.4.
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