3.6.1 Age
For some conifers, age estimates can be obtained by counting the number of branch whorls, although this method is less accurate in older trees because it can be difficult to determine where branches have been abscised. In some broadleaved trees and tree ferns it is possible to estimate ages by counting the number of leaf scars on the terminal shoot, although again this method is only useful with younger trees and branches.
Fig. 3.6 Researchers are often tempted to climb trees to collect samples of leaves, flowers, or seeds for analysis, but tree climbing should only be attempted by people who have received appropriate training. Use of safety equipment, such as the harness and head protection pictured here, is essential. (Photo by Adrian Newton.)
Age estimates are generally obtained by counting the number of annual rings in a stem cross-section. Annual ring formation depends on the fact that wood formed earlier in the year tends to be more porous and lighter in colour than that formed later in the year. Total tree age is obtained by counting rings at ground level; if measurements are made higher up the stem, then the number of years that the tree takes to grow to this height should be added to the total (Avery and Burkhart 2003). Although this method works well in most temperate forests, it can also be used in some tropical forests, particularly where there is a pronounced dry season or where the trees are seasonally deciduous (Schweingruber 1988).
Ring counts are often taken on sawn sections of a tree trunk, which should be smoothed with a plane or knife and viewed with a hand lens or dissecting micro- scope in order for accurate counts to be obtained. If sawn sections are not available, anincrement borercan be used (Figure 3.7), which consists of a hollow tube with a cutting bit that is screwed into the tree (Husch et al. 2003) and is obtainable from forestry suppliers. A reverse turn snaps the core of wood inside the tube, which is Measuring individual trees | 105
Fig. 3.7 Using an increment borer on a Larix deciduatree during a
dendroecological study of mixed woodlands at the upper timberline of the central Italian Alps. (Photo by John Healey.)
removed with an extractor (Figure 3.8). Accurate determination of tree age by this method requires coring through the centre or pith of the tree, which can be diffi- cult to locate. This process can be assisted by employing two people, one of whom takes the core and the other of whom indicates the perpendicular axis of the tree from a distance of 2–3 m (Schweingruber 1988). Alternatively, a holding device can be used, which can be adjusted in the axial direction by a peg and screw.
The maximum length of an increment borer is around 50 cm, which determines the upper limit to the size of tree that can be aged using this method. Cores in standing trees are typically taken at breast height (1.3 m above ground level). The bore holes should be sealed with grafting wax (available from garden centres and forestry suppliers) to minimize the risk of introducing disease as a result of coring (Schweingruber 1988). Tree species with dense wood can be very difficult to core, and the wood cores or the borer itself can be difficult to extract intact. Extracted cores are fragile and should be stored in a plastic tube, drinking straw, or other appropriate container. They can be labelled with soft pencil when freshly collected.
The cores can be glued into a groove in a block of wood with water-soluble glue, to assist preparation and inspection.
The visibility of tree rings in trunk disks or cored samples can be increased by cutting transverse radial strips with a sharp blade, such as a multiple-snap-off blade knife, or by polishing the sample with sandpaper of different grades. Samples Fig. 3.8 Extracting the stem-core from an increment borer for
dendrochronological analysis of growth rings. Great care is needed when extracting and transporting the cores, as they can be very fragile. (Photo by John Healey.)
displaying little contrast between tree ring boundaries can be mounted between two blocks and cut to a thickness of 0.25 mm, then examined in transmitted light under a microscope. Staining of tree rings with paper dye can also be used to improve visibility, but this is generally not successful (Schweingruber 1988). The surface of prepared samples can best be examined under a stereomicroscope with a spotlight, or a hand lens. A calibrated eye-piece is useful for making measurement of ring widths. Dedicated instruments are also available for detailed analysis of tree ring widths, linked to custom-designed computer software (such as WinDENDRO, 具www.regeninstruments.com/典).
Analysis of tree rings has been widely used in archaeology as a means of dating wood fragments, and has also been widely used to analyse past climate change.
Details of the methods used in dendrochronology are described by Cook and Kairiukstis (1990) and Schweingruber (1988).
A number of problems may be encountered when obtaining ring counts (Husch et al. 2003), namely:
● In slow-growing trees, rings may be very close together and consequently difficult to count.
● In some species rings are indistinct, because there is little difference between wood formed in the spring and in the summer.
● Some species may form more than one ring in a growing season, for example during a period of dry weather or as a result of defoliation caused by insect attack. False rings often do not extend around the entire circumference of the tree, however.
● Tree ring counts are generally very difficult to obtain from tropical trees, except in areas with a pronounced annual dry season.
Where ring counts are not possible, radiocarbon dating can potentially be used, although this technique is limited to trees of great age (500 years old) (Martínez- Ramos and Alvarez-Buylla 1999). It is also possible to estimate tree age using models of growth increment, for those species where reliable long-term growth data are available (Chambers and Trumbore 1999).
3.6.2 Stem diameter
Measurements of stem diameter are widely used in both forest ecology and management to characterize the size distribution of forest stands and to estimate timber volumes. Diameter measurements are usually taken at a standard height, thediameter at breast height(dbh), which is defined as 1.3 m above ground level (or 4.5 ft in the USA). Measurements are complicated by the fact that tree stems are often not circular in cross-section, and may be leaning or surrounded by prominent buttresses, making them difficult to measure. The following standard procedure is recommended by Husch et al. (2003):
● When the tree is on a slope, measure dbh on the uphill side of the tree.
● When a tree is leaning, measure dbh on the high side of the tree, in a way that is perpendicular to the longitudinal axis of the stem.
Measuring individual trees | 107
● When the tree has a bulge, limb or some other abnormality at breast height, measure dbh above the abnormality; attempt to measure the dbh that the tree would have had if the abnormality were not present.
● When a tree is multistemmed at breast height, measure each stem separately;
when a tree forks above breast height, measure it as a single stem. If the fork occurs at breast height, measure the dbh below the enlargement of the stem caused by the fork.
● When a tree has a buttress than extends higher than 1 m, measure the stem at a fixed distance (30 cm) above the top of the buttress.
● When the breast height point has been marked on the tree with paint, assume the point of measurement to be the top of the paint mark. Use of such paint marks can greatly improve accuracy when repeated measurements are made.
● If the tree stem is elliptical in cross-section, then measure the major and minor diameters separately, and produce an overall figure by calculating the mean of the two values.
The two most commonly used instruments for measuring tree stem diameters are calipers and diameter tapes. Calipersare usually used when the trees are less than 60 cm dbh; although larger calipers are available, they can be difficult or unwieldy to use in the field. Calipers are usually constructed out of metal, wood, or plastic, and enable the diameter to be read directly off a scale when the arms of the caliper are placed around the tree stem. The caliper arms should be pressed firmly against the tree stem with the main beam of the caliper placed perpendicu- lar to the axis of the tree stem, and the arms parallel and perpendicular to the beam (Husch et al. 2003).
Standard measurement tapescan be used to measure stem diameter by placing the tape around the circumference of the tree at breast height (Figure 3.9).
Measurements of circumference taken in this way can be converted to diameters (assuming a circular cross-section) by dividing the values by . Diameter tapes can be obtained, however, that are graduated at intervals of units (in cm or inches) enabling diameter to be measured directly. Care should be taken to ensure that the tape is positioned correctly: it should be in a plane perpendicular to the trunk of the tree, and pulled taut around the trunk so that accurate measurements are obtained (Husch et al. 2003). Although more accurate results can be obtained with calipers, measurements with tape tend to be more consistent if repeated measure- ments are made, because caliper measurements are more sensitive to the position- ing of the instrument (Husch et al. 2003). A review of different methods for measuring tree diameters is provided by Clark et al. (2000).
Bark thickness can be determined by using a bark gauge, which consists of a steel shaft that is pushed through the bark. The thickness of the bark can be read directly off a scale with the instrument in place. A minimum of two readings should be taken (Avery and Burkhart 2003). When measurements of diameter are made, whether or not the bark was included in the measurement should be recorded.
A number of instruments are available for obtaining measurements of upper stem diameters, which can be useful for assessing the form of the tree or extent of stem taper. Options include optical forks, optical calipers, and fixed-based or fixed- angle rangefinders (Clark et al. 2000). Many of these instruments are expensive and are prone to inaccuracy (Avery and Burkhart 2003). Most commonly, a relas- copeis used, which is a form of optical rangefinder (Husch et al. 2003) (see also Section 7.2). However, calipers or diameter tapes provide more accurate measure- ments of upper stem diameters, if the upper parts of the tree can be accessed through use of climbing ropes or ladders.
3.6.3 Height
Total tree heightcan be defined as the distance along the axis of the tree stem from ground level to the top of the canopy. Other terms commonly used by foresters Measuring individual trees | 109
Fig. 3.9 Measuring stand structure in the New Forest National Park, southern England. The student in the centre is measuring diameter at breast height of an oak tree (Quercus robur) using a diameter tape. The others are carrying a laser rangefinder (left) and a hypsometer (right). (Photo by Adrian Newton.)
includebole height, which refers to the distance between ground level and the first crown-forming branch, and crown length, which is the distance on the axis of the tree stem between the first crown-forming branch and the top of the canopy.
The heights of relatively short trees can be readily measured by using a graduated pole. Height measurements of tall trees are generally made by means of hypsometers, which use trigonometric relations to estimate height (Figure 3.10). The user sights the top of the canopy of the tree being measured and takes a reading, then sights to the base of the tree and takes a second reading. Many hypsometers are scaled accord- ing to appropriate units, enabling the height to be calculated directly as the sum of the two readings. However, if measurements are made on a slope, and the observer’s position lies below the base of the tree, tree height is derived by taking the difference between the two readings (Avery and Burkhart 2003) (Figure 3.11). Some hyp- someters are graduated in degrees, and require the use of basic trigonometry for con- version to height measurements. In both cases, the distance between the point of measurement and the tree should be measured, typically with a measuring tape.
Bole height and crown length can be measured, as well as total tree height, by meas- uring the heights of the appropriate locations on the tree stem. Commonly used types of hypsometer include the Abney level and the Suunto clinometer (Husch et al. 2003). More recently, electronic hypsometers have become commercially available, which use lasers to measure horizontal distances and calculate tree heights Fig. 3. 10 Measuring tree height using a hypsometer. The top of the tree is sighted through the instrument, and the distance from the tree is measured. Note that in deciduous forests, it is much easier to make measurements when the trees are leafless. (Photo by Adrian Newton.)
from angular measurements. However, these are often bulkier than traditional hyp- someters, and much more expensive.
The main challenge to obtaining accurate height measurements, regardless of the type of hypsometer used, is the difficulty of sighting the top of the tree canopy, particularly in closed forest stands. Large, flat-crowned trees are particularly difficult to measure, simply because it is not easy to see the crown apex. As a rule of thumb, tree heights should be measured at a distance approximately equivalent to the height of the tree (Husch et al. 2003). Leaning trees are also difficult to measure; in this case, height should be measured for the point on the ground that is vertically below the canopy apex. Accuracy of measurements can also vary between different users, so, ideally, repeated measurements should be made.
3.6.4 Canopy cover
Jennings et al. (1999) distinguish two basic types of measurement of forest canopies: canopy cover, which is the area of the ground covered by a vertical Measuring individual trees | 111
α1
α1
α2
α2
D
D C
A B
A B
C (a)
(b)
Fig. 3.11 Measuring tree height with a hypsometer, based on tangents of angles.
(After Husch et al. 2003): (a) The total height of the tree may be determined as ABD(tan1tan2). (b) On steep ground, where the tree may be viewed from below, the height of the tree BCCAis given by ABD(tan1tan2).
projection of the canopy, and canopy closure(orcanopy density), which is the pro- portion of the sky hemisphere obscured by vegetation when viewed from a single point. These two terms are often confused in the literature. Canopy cover is an important variable for estimating stand variables from remote sensing data (see section 2.5), and in young forest stands may correlate closely with basal area; how- ever, the relation between these variables is often less pronounced in more mature forest stands (Jennings et al. 1999). Canopy closure is likely to be more closely related to light regime and microclimate, as well as plant growth and survival, at the point of measurement (Jennings et al. 1999).
As noted in Chapter 2, crown diameters can be measured from high-resolution aer- ial photographs. Field measurements of tree crowns are complicated by their inaccessibility and irregularity. The commonest method to measure the size and shape of a tree crown is to project the perimeter of the crown vertically down, then measure it at ground level. Estimates of crown area are typically obtained by measuring the crown diameter at its widest point, then again at right angles to this measurement (Husch et al. 2003). Hand-held or pole-mounted mirrors, prisms, or pentaprisms may be used to achieve the vertical projection. Appropriate instruments and associ- ated methods for this purpose have been described by Cailliez (1980) and Tallent- Halsell (1994). Crown area is estimated from these measurements by using the formula for calculating the area of a circle, from either the mean value of the two meas- urements made or the mean value of minimum and maximum crown diameters.
The method for estimating crown cover of a forest stand is described by Jennings et al. (1999). At each point of measurement, the observer looks vertically upwards and records whether or not the forest canopy obscures the sky. An estimate of forest canopy cover can be produced by calculating the proportion of points where the sky is obscured. Observations can be made without use of any instrumentation, although both accuracy and repeatability can be improved by doing so. Examples of instruments designed to ensure that sightings are truly vertical include the gimbal balance (Walter and Soos 1962) or the sighting tube, which often has an internal crosshair (Johansson 1985). Commercial versions of the latter incorporate bubble levels to ensure that the tube is positioned vertically, and 45 mirrors to ensure that the head posture is horizontal during use. Random or stratified random sampling approaches should be used when taking such measurements according to Jennings et al. (1999), who present formulae for cal- culating the confidence limits of such measurements by using a binomial distribu- tion. Accurate estimates require large sample sizes; these authors suggest that at least 100 observations should be made in any forest area being surveyed.
Measures of canopy closure, rather than canopy cover, are generally to be preferred in ecological studies. This requires estimation of the light received at a particular point, including both direct solar radiation and the indirect radiation that arrives from all parts of the sky. The entire hemisphere surrounding the sample point should therefore be assessed, rather than just the sky immediately above the sample point (Jennings et al. 1999). Methods for assessing canopy closure are presented in section 4.5.4.