area (in units of mol m–2s–1), referred to as the photosynthetic photon flux density (PPFD). Alternatively, sensors may be used that measure solar energy, in units of watts per square metre (W m–2). Such sensors (referred to as solarimeters or pyranometers) may either measure total solar energy, or energy within the photo- synthetically active 400–700 nm waveband, depending on the use of filters. This type of sensor is preferred if information is needed on the energy relations of plants (relation between solar radiation and leaf temperature, for example). Note that it is not possible to convert from radiometric measures of solar energy to quantum measures without knowledge of the spectral composition of the vegetation (Jennings et al. 1999).
Other types of light sensor include:
● Tube solarimeters, which consist of a glass tube enclosing a strip thermophile of alternating black and white surfaces. Such solarimeters have been used very widely in crop science; they provide a measure of solar energy flux, and must be used with filters if measurements of the photosynthetically active part of the spectrum is required. As a result of their glass construction, solarimeters are relatively fragile.
● Ceptometers, such as the LP80 device manufactured by Decagon 具www.
decagon.com/lp80/典, which consists of a linear probe 86.5 cm long that contains 80 quantum sensors sensitive to the PAR waveband. The instrument is particularly used for measurements of LAI (see section 3.7.3).
● Plant canopy analyserssuch as the LAI-2000 manufactured by LI-COR Inc.
具www.licor.com典, which is similarly used widely for measuring LAI (see section 3.7.3). The instrument consists of a near-hemispherical lens (148 field of view) held in front of five concentric silicon ring detectors, which enable canopy light interception at five angles to be measured. Radiation intercepted by the canopy is computed by dividing the above-canopy detector outputs by the below- canopy detector outputs (LI-COR Inc., 1992). The diffuse non-interceptance (DIFN) calculated by this instrument is similar to the instantaneous diffuse light transmission obtained on overcast days and has been found to be closely related to daily temporal variation of light (Hanan and Bégué 1995).
A further type of sensor is used to analyse the spectral composition of solar radiation. Options include spectral radiometers or the Red/Far-Red sensor marketed by Skye Instruments 具www.skyeinstruments.com典. This has a basic design similar to a quantum sensor, but with filters enabling ratio of red to far-red light (R : FR, 660:
730 nm) to be measured, a ratio that has a major influence on the development and growth of some plant species. It is important to calibrate such sensors regularly, as their performance can change over time.
Such sensors can be used with handheld meters for spot readings. Some are equipped with memory storage (‘integrators’) enabling measurements to be recorded over time. Alternatively, sensors may be attached to programmable data loggers, which enable readings from multiple sensors to be stored over prolonged periods. Note that luxmeters should not be used in ecological investigations, as
the measurements that they provide (illuminance or brightness as perceived by the human eye, given in units of lumens, lux, or foot candles) are not relevant to tree growth and survival (Jennings et al. 1999).
Estimation of the light environment beneath a forest canopy canopies is very challenging because of the high spatial and temporal variability typically encoun- tered. Underneath a forest canopy, a single point receives both direct and diffuse light. Direct light comes from the solar disc, and varies according to the time of day and year as the solar altitude changes. Diffuse light comes from all parts of the sky and is much more uniform, both spatially and temporally, than direct light under a forest canopy (Anderson 1964).
Although PAR sensors can provide accurate measurements, large numbers of sensors are needed to adequately characterize the light environment within a forest stand. For example, Baldocchi and Collineau (1994) estimated that sample plots in many tropical forests would require over 270 sensors for a representative description. It is important that the sensors are maintained in a level position, at a fixed height above ground level, and careful consideration is given to where they are located. Typically, if multiple sensors are available, they are located by using stratified random approaches or on points of a grid. Ideally, light should be measured continuously for several days in order to take account of temporal variation. The sensors should be kept clean and horizontal, or else significant errors can be introduced (Jennings et al. 1999).
Characterizing the light environment via instantaneous measurement of light transmission on clear, sunny days around noon has been very popular among researchers (Gendron et al. 1998). It is possible to take measurements at numerous locations in the understorey during such a period, assuming that irradiance above the canopy is similar for all measurements. Often, the amount of light recorded is expressed as a percentage of radiation incident at the top of the canopy (%PPFD, usually measured in an open area such as a large forest clearing) (Gendron et al.
1998). Messier and Puttonen (1995) proposed a new method to estimate light environments in the understorey, by measuring instantaneous diffuse light trans- mission on overcast days. This is based on the fact that under an overcast sky,
%PPFD at any particular microsite tends to be very stable throughout the day (Messier and Puttonen 1995). Evidence presented by these researchers suggests that instantaneous percent above-canopy PPFD ( (PPFD in understorey/PPFD above canopy) 100) under completely overcast conditions, measured with PAR quantum sensors, can provide an accurate estimate of the mean daily percentage above-canopy PPFD over the course of a day and under all sky conditions (Messier and Puttonen 1995, Parent and Messier 1996). This result was supported by Gendron et al. (1998), who found that 10 minute averages taken on overcast days provided a more accurate assessment of %PPFD over a growing season than instantaneous measurements taken on sunny days around noon.
Given their relatively high cost and long cable lengths, quantum sensors are expensive and cumbersome for multiple-point sampling. For this reason, a number of simpler indirect methods are widely used, some of which are considered below.
Measuring light environments | 169
4.5.2 Hemispherical photography
Hemispherical or ‘fish-eye’ photography has a long history of use in plant ecology, dating back to the pioneering efforts of Anderson (1964), Becker (1971), and Evans and Coombe (1959). As a result of the development of high-resolution digital cameras and advances in image-processing software, there has been a recent renewal of interest in this method (Bréda 2003). The technique is described in detail by Cannell and Grace (1993) and Rich (1989, 1990), and examples of its application are provided by Rich et al. (1993) and Whitmore et al. (1993).
The technique involves taking a photograph of a forest canopy by using a conventional camera with an unconventional lens, which has a very wide field of view (180 , hence the common name ‘fish-eye’). It is important to note that a true ‘fish-eye’ lens should have a 180 field of view for accurate measure- ment of light environments; some hemispherical lenses do not have such a wide field of view and therefore do not capture the full range of incident light.
The camera should be mounted so that it is level (Figure 4.4), and it is important
Fig. 4.4 The HemiView system used for taking hemispherical photographs of forest canopies. The system comprises a 180 fisheye lens with a high resolution digital camera, mounted in a self-levelling camera mount to ensure that it is held horizontally. (Photo courtesy of Delta-T Devices Ltd.)
Measuring light environments | 171
Fig. 4.5 A hemispherical photograph on to which solar paths have been
superimposed, using HemiView software. This enables the occurrence of sunflecks and associated solar irradiance to be calculated on any day of the year. (Photo courtesy of Delta-T Devices Ltd.)
Fig. 4.6 An example of a hemispherical photograph of a forest canopy, taken using the Hemiview system. (Photo courtesy of Delta-T Devices Ltd.)
that orientation of the camera (due north, for example) is indicated on the image, to facilitate subsequent analysis. Photographs can be analysed manually or with computer software to determine the geometry and position of canopy openings and the path of the sun at various times, and to indirectly estimate the characteristics of light environments beneath plant canopies as well as properties of the canopies themselves (Roxburgh and Kelly 1995) (Figures 4.5, 4.6). This enables light transmission to be estimated for any specified period (daily, growing season etc.). Also, both diffuse and direct light components transmitted through the canopy can be estimated (often presented as ‘site factors’).
A number of commercial instruments are now available (Table 4.2) that enable hemispherical photographs to be analysed. In addition, a number of individual researchers have developed software for analysing hemispherical photographs, including Solarcalc (Chazdon and Field 1987), HEMIPHOT/WINPHOT (ter Steege 1994) 具www.bio.uu.nl/~boev/staff/personal/htsteege/htsteege.htm典 and Gap Light Analyser (GLA) (Frazer et al. 1999) 具www.rem.sfu.ca/forestry/index.htm典;
具www.ecostudies.org/典. Whichever analysis system is used, hemispherical photographs with both digital and film cameras must be taken under uniform sky conditions, such as those encountered just before sunrise or sunset or when the sky is evenly overcast.
During analysis, the different parts of the digitized image are classified as either black (completely blocked by foliage) or white (clear sky). The most critical step in image processing is determining the threshold between the sky and canopy elements (Bréda 2003). Small changes in the threshold value selected can result in relatively large changes in estimates of canopy closure, particularly beneath dense canopies; yet a consistent threshold value can be difficult to find (Jennings et al.
1999). Other shortcomings of the method are that the canopy is assumed to be a single layer of leaves; the presence of any leaves is assumed to completely block the passage of light. Furthermore, hemispherical analysis systems currently do not have the ability to assess reflection from leaves, or layers of leaves; reflection and transmission may be affected by leaf orientation relative to sun angle, which is not considered by the technique (Roxburgh and Kelly 1995). The method also assumes that there are no significant seasonal changes in the canopy throughout the growing season. However a number of authors have found close correspondence between direct measurements of PPFD using quantum sensors and estimates derived from hemispherical photography (Easter and Spies 1994, Rich et al.
1993), although the technique appears to be less reliable in shaded sites (Chazdon and Field 1987, Roxburgh and Kelly 1995).
One of the main drawbacks of hemispherical photography is the high cost, not only of complete analytical systems but also of the lens required. This has stimu- lated interest in using relatively low-cost, ‘consumer’ digital cameras, some of which offer the capacity to take ‘fish eye’ photographs. Following a comparison of such cameras with conventional systems, Frazer et al. (2001) caution against using consumer cameras for scientific applications, because of distortion detected in the
Measuring light environments|173 Table 4.2 Comparison of three commercial systems for analysis of hemispherical photographs, used for characterizing forest light
environments (updated from Bréda 2003).
Measurements Field Resolution Company Website
of view
WinSCANOPY LAI, leaf-angle distribution, 180 Depends on choice Regent Instruments Inc., www.regent.qc.ca
and mean leaf angle, of camera; typically Quebec, Canada
angular distribution of gap 6–12 megapixels frequencies, sunfleck
distribution, total radiation and site factors (direct, diffuse, and global)
HemiView LAI, leaf-angle distribution 180 25921944 pixels Delta-T Devices Ltd., www.delta-t.co.uk
and mean leaf angle, angular Cambridge, UK
distribution of gap frequencies, sunfleck distribution, total radiation and site factors (direct, diffuse, and global)
CI-110 Imager LAI, sky view factor, mean 150 768494 pixels CID Inc., Vancouver, www.cid-inc.com
foliage inclination angle, USA
foliage distribution and extinction coefficient of the canopy
Note: The WinSCANOPY and HemiView devices are canopy analysis systems based on analysis of colour hemispherical images; the standard systems include a digital camera, a calibrated fish-eye lens, and a self-levelling system. Images are taken in the field and processed externally using specific software. Outputs are available by sky sector or aggregated into a single overall whole-sky value. The digital plant canopy imager CI-110 is different, because it is designed to capture and processes colour hemispherical images that can be analysed either in real-time in the field, or subsequently in the laboratory. The hemispherical lens is mounted on an auto-levelling design on the tip of a handle connected to a portable computer dedicated to the equipment. Note that the latter system has a relatively low field of view (150 ).
imagery. However it is probable that this will become less of a problem as the quality of consumer cameras continues to improve, and acceptable results have been obtained by other authors (Englund et al. 2000).
4.5.3 Light-sensitive paper
Friend (1961) described a simple technique for measuring light based on the use of light-sensitive diazo paper, which was recently re-evaluated by Bardon et al.
(1995), on which this account is based. Stacks of diazo (ozalid) paper are constructed into booklets of 20 sheets. These are then placed in Petri dishes for protection, maintained in position against the inner surface of the lid by a piece of sponge. A piece of black paper with a central hole of approx 0.95 cm2is placed inside the lid, allowing light to reach the surface of the booklet of paper. The hole is kept covered and the Petri dishes stored in the dark until the commencement of measurements. Once exposed, the amount of light received is estimated from the number of layers of paper that are bleached after dry development with ammonia vapour, with a development time of 20–25 min.
The assumption made by this technique is that the number of diazo sheets exposed is related to the total quantity of radiation received (i.e., duration intensity). Bardon et al. (1995) tested whether PPFD measured with a quantum sensor correlates with the number of exposed sheets of diazo paper under a variety of conditions. A stronger linear relationship was found between the number of layers of exposed diazo paper and maximum instantaneous PPFD than the number of layers of exposed diazo paper and accumulated PPFD or log10accumulated PPFD. Under field conditions, full sunlight resulted in exposure of no additional layers of diazo paper after about noon. The authors concluded that diazo paper seems to record irradiance at a low rate, giving the impression that it is recording accumulated PPFD, whereas in fact it is not. Bardon et al. (1995) therefore recommend that diazo paper should not be used to measure accumulated PPFD under field conditions, especially for periods that include a significant amount of time after noon or under conditions with light flecks or varying irradiance. However the method can provide an indication of the maximum intensity of solar radiation received during the period of observation. According to Jennings et al. (1999), however, this method can only provide an approximate estimate of PAR.
4.5.4 Measuring canopy closure
Canopy closure is the proportion of the sky hemisphere obscured by vegetation when viewed from a single point (Jennings et al. 1999). Note the difference between this term and canopy cover, which refers to the proportion of the forest floor covered by the vertical projection of the tree crowns (Figure 4.7) (Jennings et al. 1999). Methods for estimating the latter are presented in section 3.6.4.
Canopy closure can be measured by using hemispherical photography, or by a number of other techniques detailed below.
Measuring light environments | 175
(a)
(b)
Fig. 4.7 The difference between canopy closure (a) and canopy cover (b). (From Jennings et al. 1999.)
Table 4.3 Crown position indices presented by Clark and Clark (1992).
Class Description
1.0 No direct light (crown not lit directly either vertically or laterally) 1.5 Low lateral light (crown lit only from the site; no large or medium
openings)
2.0 Medium lateral light (crown lit only from side: several small or one medium opening)
2.5 High lateral light (crown lit only from side: exposed to at least one major or several medium openings)
3.0 Some overhead light (10–90% of the vertical projection of the crown exposed to vertical illumination)
4.0 Full overhead light (90% of the vertical projection of the crown exposed to vertical light, lateral light blocked within some or all of the 90 inverted cone encompassing the crown)
5.0 Crown fully exposed to vertical and lateral illumination within the 90 inverted cone encompassing the crown)
Simple visual assessment
It is possible to produce a rapid, visual estimate of canopy closure by comparing the area of canopy with a standard scale. For example, Clark and Clark (1992) presented a simple index based on the crown illumination of individual trees (Table 4.3), which
was found to be significantly correlated with measures derived from hemispherical photographs. The main limitation of this method is the potential for lack of repeatability, particularly between different observers. The magnitude of this error can be estimated by taking repeat measurements. Brown et al. (2000) found that repeatability could be improved by assessing the size of a hole in the canopy by comparing it with a series of ellipses of different size (ranging from 10.3 to 41.0 cm2in area) printed on a transparent Perspex screen, which is held at a fixed distance from the eye by attaching a cord 20 cm long. The score is determined by the size of the ellipse that fits entirely into the largest canopy opening visible in the canopy, whilst standing at the point of measurement (Brown et al. 2000).
An alternative approach was described by Lieberman et al. (1989), to provide a quantitative index of canopy closure based on the three-dimensional stereogeometry of trees at a specific point. The index is based on the following measures: (1) the horizontal distance between the focal tree and each taller neighbour within some given radius, (2) the height difference between the two trees, and (3) the distance from the top of the focal tree to the top of its neighbour, calculated from the height difference and the horizontal distance between the focal tree and the neighbour.
The ratio of (2) to (3) is the sine of the included angle . The index of canopy closure, G, is defined as the sum of these ratios for all italler neighbours within some specified radius:
The index is lowest for large trees with the fewest crowns above their own. The index can be calculated for any point within the forest volume, and therefore can be used to capture the three-dimensional characteristics of a forest canopy.
Spherical densiometer
Aspherical crown densiometeris a relatively simple instrument for estimating crown closure. The instrument is described by Lemmon (1956), and consists of a convex or concave hemispherical mirror etched with a grid of 24 squares. The observer scores canopy cover by assessing whether sky or foliage is visible at four equally spaced points within each square. Strickler (1959) suggested that four readings be taken at each point, one for each of the cardinal directions. Potential problems with the technique are systematic differences between observers (Vales and Bunnell 1988), although this can potentially be estimated by taking repeat measurements by different observers. Bunnell and Vales (1990) and Cook et al. (1995) both reported that instruments that measure wide sky angles, such as the densiometer, underesti- mate canopy cover compared with methods that measure narrow angles such as the Moosehorn (Garrison 1949) considered below. Although the instrument is portable and robust, Jennings et al. (1999) conclude that spherical densiometers do not give a highly accurate measure of canopy closure; as the reflection of the canopy is small, they suffer from poor resolution. A further problem is that obser- vations have to be made by viewing the instrument from the side rather than from
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