Chianucci F, Cutini A (2012). Digital hemispherical photography for estimating forest canopy properties: current controversies and opportunities. iForest 5: 290-295. Review Paper - doi: 10.3832/ifor0775-005 ©iForest – Biogeosciences and Forestry Introduction Accurate and reliable measures of forest canopy are crucial to a wide range of studies including hydrology, carbon and nutrient cycling, and global change (Chen et al. 1997, Macfarlane et al. 2007c). For this rea- son, forest canopy properties are widely used in many long-term research programs and, on the other hand, to monitoring forest eco- systems’ status (Cutini 2003, Macfarlane 2011). In addition, the availability of obser- vations on forest canopy properties such as leaf area index and forest light conditions are essential to calibrate remotely-sensed in- formation based on airborne and satellite data (Rich 1990, Wang et al. 2004). However, direct measurements of forest canopy are particularly challenging to ob- tain, owing to inherent difficulties in making direct measurements of forest, high levels of spatial and temporal variability, and diffi- culty of generalizing local measurements to the landscape scale (Chen & Cihlar 1995, Chen et al. 1997, Macfarlane et al. 2007b). Harvesting of trees for direct measurements in forest is labor intensive, destructive, time and money consuming, and can not be ap- plied to large areas (Chen et al. 1997). Al- ternative and less destructive methods based on tree allometry and litterfall have been de- veloped in order to measure forest canopy properties. Nevertheless, even these methods are labor intensive, time-consuming and not error-free because of their site- and spe- cies-dependency (Breda 2003). By contrast, remotely sensed vegetation indexes have novel potential but still need cross calibra- tion by means of ground-based observations (Wang et al. 2004). As a consequence, indi- rect measures of forest canopy properties using ground-based instruments have long been implemented, as documented by the rich literature (i.e., Chen et al. 1997, Cutini et al. 1998, Breda 2003, Jonckheere et al. 2004, Macfarlane et al. 2007b). Indirect methods enable estimation of fo- rest canopy properties by measurements of the radiation transmission through the ca- nopy, making use of the radiative transfer theory (Ross 1981). However, indirect me- thods, including the LAI-2000 plant canopy analyzer (Li-Cor, Lincoln, Nebraska) and AccuPAR Ceptometer (Decagon Devices, Pullman, WA), two of the most commonly used devices, are hindered by the complexity of forest canopy architecture (Chen et al. 1997) and the high-cost of the instruments (Macfarlane et al. 2007c). Since the first approach provided by Evans & Coombe (1959), hemispherical photo- graphy (also known as fish-eye photography) has long been used for the indirect optical measurement of forest canopy. However, be- cause of significant obstacles involving film cameras (i.e., lack of software, time-consu- ming acquisition and processing procedu- res), film hemispherical photography has been progressively forsaken (Breda 2003). Advances in digital photographic technology have led to renewal of interest in photo- graphic methods for indirectly quantifying forest canopy. So far, hemispherical photo- graphy is the widely used of several photo- graphic techniques. Fish-eye photography enables characterization of forest canopy by means of photographs taken looking upward (or, in some cases, looking downward) through an extreme wide-angle lens (Jonck- heere et al. 2004). The method has many ad- vantages over the other indirect methods. It is rapid, inexpensive and readily available; hemispherical image provides a permanent record of the geometry distribution of gap fraction, which is generally used to calculate forest light regimes and canopy properties such as canopy openness, leaf area index, leaf angle distribution. Hence, hemispherical photography can greatly expands the number of the canopy properties that are possible to estimate, as compared with the other indirect methods. In spite of the recent improvement in digi- tal photography, significant obstacles to the adoption of digital hemispherical photogra- phy still remain; accurate and meaningful es- timates of forest canopy properties with di- gital hemispherical photography are hin- dered by different critical steps, regarding image acquisition and software processing; thus, adequate field collection and image processing procedure is required to achieve the standard of an ideal device (Jonckheere et al. 2004). The purpose of this contribution is to briefly introduce some of the major draw- backs of the digital hemispherical photo- graphy method. Given that different contro- versies of digital hemispherical photography have been usually treated separately, this contribution is aimed to: (i) provide a basic foreground of digital hemispherical photo- graphy, in order to outline strengths and weaknesses of the method; (ii) to give an up- date framework of the main procedure re- cently proposed to overcome the technical problems of digital hemispherical photo- graphy; (iii) to provide an reliable field measurement and images processing pro- tocol for canopy description and analysis, particularly regarding sampling strategy. © SISEF http://www.sisef.it/iforest/ 290 iForest (2012) 5: 290-295 (1) Consiglio per la Ricerca e la Sperimentazione in Agricoltura - Centro di Ricerca per la Selvicoltura, v. S. Margherita 80, I-52100 Arezzo (Italy); (2) Department for Innovation in Biological, Agro-Food, and Forestry systems, University of Tuscia, v. San Camillo de Lellis, I-01100 Viterbo (Italy) @@ Francesco Chianucci (francesco.chianucci@entecra.it) Received: Sep 13, 2012 - Accepted: Nov 13, 2012 Citation: Chianucci F, Cutini A, 2012. Digital hemispherical photography for estimating forest canopy properties: current controversies and opportunities. iForest 5: 290-295 [online 2012-12-17] URL: http://www.sisef.it/iforest/contents? id=ifor0775-005 Communicated by: Roberto Tognetti Digital hemispherical photography for estimating forest canopy properties: current controversies and opportunities Francesco Chianucci (1-2), Andrea Cutini (1) Hemispherical photography has been used since the 1960s in forest ecology. Nevertheless, specific constraints related to film cameras have progressively prevented widespread adoption of this photographic method. Advances in di- gital photographic technology hold great promise to overcome the major draw - backs of hemispherical photography, particularly regarding field techniques and image processing aspects. This contribution is aimed to: (i) provide a basic foreground of digital hemispherical photography; (ii) illustrate the major strengths and weakness of the method; (iii) provide an reliable protocol for im- age acquisition and analysis, to get the most out of using hemispherical photo- graphy for canopy properties extraction. Keywords: Digital Hemispherical Photography, Fisheye Lens, Leaf Area Index, Radiative Transfer, Foliage Clumping mailto: http://www.sisef.it/iforest/contents?id=ifor0775-005 http://www.sisef.it/iforest/contents?id=ifor0775-005 Chianucci F & Cutini A - iForest 5: 290-295 Foreground to Digital Hemispherical Photography The first hemispherical lens was developed by Hill (1924), to study cloud formation. The first approach to fish-eye photography in forestry was then provided by Evans & Coombe (1959), which used hemispherical photography to describe the light environ- ment under forest canopy. Anderson (1964, 1971) used fish-eye photography to calculate the direct and scattered components of solar radiation from visible sky directions. Sub- sequently, film hemispherical photography has been used for a long time to estimate forest canopy properties (Bonhomme et al. 1974, Anderson 1981, Chan et al. 1986, Wang & Miller 1987). However, technical and theoretical obstacles involving many time consuming steps have progressively prevented the wide spread adoption of film hemispherical photography (Breda 2003, Macfarlane et al. 2007c). More recently, advances in digital photo- graphic technology and image processing software have led to a renewal of interest in digital hemispherical photography for indi- rect quantification of forest canopy proper- ties (Breda 2003, Macfarlane et al. 2007c, Jarčuška et al. 2010). Digital cameras have greatly simplified the process of image cap- ture and processing, when compared with film cameras (Macfarlane 2011). In addition, over the last few years, numerous commer- cial software packages, as well as freeware programs for canopy analysis, have been de- veloped (Frazer et al. 1999, Jonckheere et al. 2005, Jarčuška 2008). Recent studies con- firmed the accuracy of digital hemispherical photography in estimating forest canopy properties (Englund et al. 2000, Jonckheere et al. 2005, Leblanc et al. 2005, Macfarlane et al. 2007a, Ryu et al. 2010a). Moreover, new photographic techniques have been tested recently, confirming the high poten- tiality of digital photography (Macfarlane et al. 2007b, Ryu et al. 2010b, Chianucci & Cutini 2013). Theoretical background Hemispherical photography is a method that measures the gap fraction at multiple zenith angles. Gap fraction is computed by applying the Beer-Lambert law (eqn. 1): where LAI is the leaf area index, θ is the zenith angle of view, G(θ) is named G-func- tion and corresponds to the fraction of fo- liage projected on the plane normal to the zenith direction. In theory, by measuring the gap fraction at multiple zenith angles it is possible to simultaneously determine both LAI and the foliage angle distribution func- tion (Macfarlane et al. 2007b). Forest light environment was also derived from gap frac- tion. The method makes the following assump- tions: • Leaves are randomly distributed within the canopy; • Individual leaf size is small compared with the canopy and thereby with the sensor field of view; • Foliage is black, namely it do not transmit light; • Leaves are azimuthally randomly oriented. Current controversies and corrective strategies Photographic exposure Photographic exposure affects the mag- nitude of the canopy gap fraction (Zhang et al. 2005). The importance of exposure con- trol is well documented, since automatic ex- posure has been demonstrated to prevent ac- curate and reliable estimates of the gap frac- tion (Chen et al. 1991, Macfarlane et al. 2000, Zhang et al. 2005). Images taken with automatic exposure underestimates gap frac- tion in open canopies, while overestimates gap fraction in medium-high density cano- pies (Zhang et al. 2005); as a consequence, exposure needs to be manually set. The optimum exposure for hemispherical photography would be the one which makes the sky appear as white as possible, provi- ding in the meantime the best contrast between canopy and sky (Chen et al. 1991). Adequate exposure can be approximately de- termined in two steps: • reference exposure is measured in a clea- ring (open sky), with aperture set to provi- de adequate depth of field; • subsequently, exposure is set to over- expose image (generally by 1-3 stops of the shutter speed) relative to the open sky reference (Macfarlane et al. 2000, Zhang et al. 2005, Macfarlane et al. 2007c), with the aperture unchanged. This exposure setting makes the sky appears white, providing satisfactory contrast between canopy and sky, and is not influenced by the stand density (Zhang et al. 2005). Gamma function Unlike film cameras, image sensors in di- gital cameras have the advantage of respond linearly to light (Zhang et al. 2005). How- ever, in order to simulate the non-linear be- havior of the human eye, the in-camera soft- ware applies a logarithmic transformation by means of gamma function (Cescatti 2007). The gamma function describes the relation between actual light intensity during photo- graphy and the resulting brightness value of the pixel (Wagner 1998). A gamma value of 1.0 denotes an image that accurately repro- duces actual light intensity (Macfarlane et al. 2007c). Digital cameras typically have gam- ma values between 2.0 - 2.5. The main effect of this correction is to lighten the midtones, thus resulting in worse estimate of canopy light transmittance (Cescatti 2007). Some studies found that gamma correction strongly affects forest canopy properties es- timates in both film and digital cameras (Wagner 1998, Leblanc et al. 2005, Macfar- lane et al. 2007c); consequently, back-cor- recting to 1 the gamma function of the im- ages is recommended. Pixel classification The optimal light intensity (brightness va- lue) from a digital color or grey levels image is generally used as threshold value to distin- guish pixels belonging to sky or canopy, thus producing a binary image (Wagner 1998, Jonckheere et al. 2004, Jonckheere et al. 2005, Cescatti 2007). Some authors sug- gested using the blue channel instead of the grey levels of the RGB image, because the foliage elements have a much lower reflec- tivity and transmittance in the blue region of the visible electromagnetic spectrum (Le- blanc et al. 2005, Zhang et al. 2005, Macfar- lane 2011). Previously, pixel classification was per- formed manually. Some software package such as GLA (Gap Light Analyzer - Frazer et al. 1999) still employs interactive manual thresholding. However, some studies pointed out that manual thresholding could be a re- levant source of error due to its subjectivity (Rich 1990, Jonckheere et al. 2004, Cescatti 2007, Jarčuška et al. 2010). As a con- sequence, different automatic, objective, operator-independent thresholding methods have been proposed to replace manual thre- sholding (Ishida 2004. Jonckheere et al. 2005, Nobis & Hunziker 2005, Macfarlane 2011), while commercial software packages (i.e., Winscanopy) typically have developed automatic pixel classification algorithms (Macfarlane 2011). A detailed analysis of the different classi- fication methods falls out of the scope of this contribution (for a more complete descrip- tion, see Wagner & Hagemeier 2006, Mac- farlane 2011). However, Macfarlane et al. (2007c) noted that correcting images for the camera’s gamma function and correcting the gap fraction distribution for foliage clum- ping are more important on leaf area index derived from digital hemispherical photo- graphy than the classification method cho- sen. In addition, Macfarlane (2011) even found that none of the more complicated classification methods available for image processing (also from remotely-sensed ima- gery classification procedures) yielded re- sults that greatly differed from a simple global binary threshold classification. Leaf area index estimates Three main sources of discrepancy are iForest (2012) 5: 290-295 291 © SISEF http://www.sisef.it/iforest/ P (θ )=exp(−G (θ )LAIcos(θ ) ) Digital hemispherical photography for forest canopy estimation commonly recognized when digital hemi- spherical photography is used to estimate forest leaf area index. (a) Digital hemispherical photography es- timates a plant area index, rather than actual leaf area index, due to the contribution of woody elements (Breda 2003). Deciduous forests allow the estimation of woody area index from optical sensors, which can be es- timated from gap fraction during leafless (Cutini et al. 1998, Leblanc 2008). For ever- green broadleaved and coniferous species, the woody material could be estimated from destructive sampling (Leblanc 2008), or from tabled woody to total area ratio (Chen et al. 1997). (b) Another source of discrepancy is the clumping of foliage (Breda 2003, Macfar- lane et al. 2007c). Foliage clumping (Ω) strongly affect the canopy gap fraction, ac- cording to the Beer-Lambert law (eqn. 2, as modified by Nilson 1971): To overcome the limit of a non-random distribution of foliage within the canopy, some commercial software for hemispherical image analysis (i.e., Winscanopy) calculates clumping indexes from an analysis of the gap size distribution (Chen & Cihlar 1995) or from the gap fraction distribution of a number of azimuth segments for each annu- lus of the hemisphere (Lang & Xiang 1986, Van Gardingen et al. 1999), or by combining these two approaches (Leblanc et al. 2005). Van Gardingen et al. (1999) demonstrated that correcting for foliage clumping can re- duce the underestimation of up to 15%, com- pared with conventional analysis of hemi- spherical photography, which can results in an underestimate of 50% of the leaf area in- dex derived from harvesting. Another ad- vantage of fish-eye photography is that the instrument enables assessment of both wi- thin and between crowns clumping effects, which results in greater accuracy in LAI re- trieval in dense canopies, when compared with LAI-2000 PCA (Chianucci & Cutini 2013). (c) Even though an apparent advantage of fisheye photography is that LAI and the ex- tinction coefficient (k) are simultaneously es- timated [G-function is related to extinction coefficient by G(θ) = k · cos(θ)], previous studies found that the foliage angle distribu- tion calculated from hemispherical photo- graphy appeared sensitive to canopy struc- ture (Chen & Black 1991, Macfarlane et al. 2007a). As such, the foliage angle distribu- tion calculated from fish-eye images should be treated with caution. To overcome this limit, an alternative is measuring the gap fraction at a single zenith angle of θ = 57.5°, given that the extinction coefficient at this angle was largely independent of the foliage angle distribution (Bonhomme & Chartier 1972). Some software packages allow the 57.5 degree analysis of fish-eye images (i.e., Winscanopy and CAN-EYE). Protocol for image acquisition and hemispherical software image analysis In order to provide clear and concise sug- gestions to get the most out of using digital cameras for forest canopy properties estima- tion, an hypothetical application of digital hemispherical photography is illustrated, with an example of the compact camera Nikon CoolPix 4500, equipped with the FC- E8 fish-eye lens converter, and the Winscan- opy software. Camera setup and software analysis was set according to Macfarlane et al. (2007c). The reason for choosing a com- pact camera is motivated mainly because the Nikon CoolPix models have been very popu- lar in this field, and the performance of these cameras, as well as other compact camera models, have been deeply investigated. For instance, Frazer et al. (2001) compared film photography with the 2.1 Megapixel Coolpix 950, Inoue et al. (2004) compared the effect of quality and image size in two different Coolpix models (990 vs. 900), Leblanc et al. (2005) used both Coolpix 990 and 5000 in boreal forests, Englund et al. (2000) tested the effect of image quality using the Coolpix 950. These researchers found that little or no differences exists between TIFF and JPEG images from the same camera, but that image size can influence canopy properties estima- tes. Recently, DSLR (Digital Single Lens Re- flex) cameras have become much more af- fordable and their resolution has greatly in- creased (Pekin & Macfarlane 2009), but tho- rough appraisals using DSLR cameras are still poorly documented; hence, generaliza- tion over canopy measurement procedures using DSLR cameras can not be achieved so far. However, we refers to the work of Pekin & Macfarlane (2009) for a detailed analysis of the effect of quality, image size, file format, ISO in both Coolpix 4500 and DSLR Nikon D80. Sampling strategy Sampling strategy is a key issue when per- forming ground measurements that need to be representative of the whole canopy (Weiss et al. 2004). Number of images and spatial location of shots define the sampling strategy. Canopy and vegetation type, spatial variability, plot area, sensor angle of view and distance to the edge of the stand can greatly influence the accuracy of sampling design (Chason et al. 1991). It is best to consider a sampling protocol designed for the canopy type which is being measured. Canopy height is the first factor which should be considered. As a rule-of- thumb, the distance between the sensor and the nearest leaf should be at least four times the width of the leaf. As a consequence, the use of upward pointing fish-eye images in short canopies such as grassland and agricul- tural crops should be carefully evaluated (Leblanc 2008). The distance between the lens and the canopy may be too short, and the resulting canopy covered by the field of view of the camera may be not representative of the spatial distribution of the canopy (Liu & Pattey 2010), When this situation occurs, the use of downward looking camera orien- tation comes as a reliable and practical al- ternative for agricultural crops and grassland (Demarez et al. 2008, Garrigues et al. 2008, Liu & Pattey 2010). Downward pointing camera can also be used to separate under- story vegetation and top canopy vegetation in a forest stand. Canopy spatial variability is a major factor affecting sampling strategies. For closed and randomly distributed canopies, a grid of sample points is usually a suitable strategy (Law et al. 2001), even though predeter- mined sample location may require several adjustments, in that the presence of leaves immediately above the sensor may block the entire view at low zenith angles. By contrast, Leblanc et al. (2002) proposed the sampling along a 70 m transects over boreal and tem- perate forests, with measurements every 10 m. In the case of regular tree distributions, e.g., plantations of tree in evenly spaced rows, the adoption of a crisscross array scheme is recommended to ensure sampling under trees, thus avoiding bias from in- ter-row gaps sampling (Chen et al. 1997); the sample distance should be proportional to the range of distances between rows (Weiss et al. 2004). Accurate samplings in open and heteroge- neous canopies are more challenging to ob- tain. Gap fraction is greatly influenced by clumping, especially in heterogeneous cano- pies (see eqn. 2). Moreover, clumping occurs at different scales, from shoot level (within crown) to stand level (between crowns). This multiscale nature makes it hard to quantify foliage clumping (Ryu et al. 2010a). Irrespective of the method used to estimate gap fraction, in most applications gap frac- tion is given only in term of zenith angle, since an assumption of azimuthal symmetry is generally used (Van Gardingen et al. 1999, Leblanc 2008). This assumption im- plies that such standard techniques should be limited to homogeneous canopies. It is well known that conifer needles are not randomly arranged in space, and radiation penetration models assuming homogeneous canopy will underestimate the transmittance of a conifer canopy. Hemispherical photography enables assessment of both within and between © SISEF http://www.sisef.it/iforest/ 292 iForest (2012) 5: 290-295 P (θ )=exp(−G (θ )Ω LAIcos(θ ) ) Chianucci F & Cutini A - iForest 5: 290-295 crowns clumping (for more details, see the section “Leaf area index estimates”. As such, the incorporation of clumping is strongly re- commended, when available from software outputs (Tab. 1). Again, heterogeneous canopies require more repetitions (images) than homogeneous canopies to achieve good spatial sampling. Image-processing software also allows to mask some part of the hemisphere, in order to reduce the field of view, which may im- prove spatial representation in heterogen- eous canopies, i.e., to include dense and sparse regions of a heterogeneous canopy in separate images. The masking procedure could also be used in mixed forests, in order to sample clusters of different species in different images. Masking can also be used to prevent some undesired part of the image from being ana- lyzed (i.e., sun glint, operator, etc). As previously outlined, use of downward pointing camera enables analysis of under- story, and even allows separating this com- ponent from top of canopy elements. In the case of taller understory, Rich et al. (1999) suggested using tall-folding monopod with self-leveling mount set-up to sample top of canopy, or, alternatively, using a ladder. Use of a ladder also enable measuring canopy at different heights, which could be useful in tropical wet forests. Other sampling difficulties arise from mea- surement on single trees, because indirect methods are poorly suitable for single plants (Cutini & Varallo 2006). However, Hemi- view software proposes specific options for measurements on single trees (Rich et al. 1999). The LAI-2000 user’s manual pro- poses similar suggestions, which can also be suited to hemispherical photography. Specific precautions should be adopted for slope, such as holding the lens normal to the ground; e.g., self-leveling tripod is provided with Winscanopy equipment. Some authors even suggested corrective methods to intro- duce slope effect in the analysis (Walter & Torquebiau 2000, Schleppi et al. 2007). Image acquisition Hemispherical images should be collected in summer, under fully developed canopy conditions, and under uniform overcast sky, or alternatively close to sunrise or sunset (Leblanc et al. 2005); both these sky condi- tions enable a perfect diffuse sky, thus avoiding the interference of direct sunlight, which can cause errors of up to 50% (Welles & Norman 1991). Images should be collected as fine quality and at maximum resolution JPEG, with the lens set to F1, which enables circular ima- ges. Lens set to F2 enables full-frame fish- eye image instead of circular image, with the former having a better resolution than the circular format. However, only recent re- leases of Winscanopy software (since 2006a version) have implemented analysis of full- frame image; so far, canopy analysis has been usually performed only on circular images (Macfarlane et al. 2007b). Camera must be aligned to magnetic north and poin- ted upward by means of a self-leveling tri- pod. The aperture must be set to minimum (5.3) and, with the camera in aperture-prio- rity (A) mode, the exposure must be recor- ded in an adjacent clearing. Subsequently, the mode must be changed to manual (M) and the shutter speed must be lowered by two stops in comparison to the exposure metered in the clearing (Zhang et al. 2005). Exposure should be measured regularly be- neath the canopy using a spot light meter, in order to check possible changes in sky con- ditions during image acquisition (Macfarlane et al. 2007c). Different exposures can be promptly col- lected by setting exposure bracketing, which automatically adjust the shutter speed from the starting exposure, which is set by the operator (i.e., the open sky reference). On the other hand, digital cameras which can save image files in RAW format, such as DSLR allows varying the exposure after image acquisition. Software image analysis Gamma function of the images needs back- correction to 1 prior to hemispherical soft- ware image analysis. Given that Nikon CoolPix 4500 has a gamma function of ap- proximately 2.2 (Leblanc et al. 2005), the original images must be adjusted with the gamma correction set to 0.45 (1/2.2), using a standard image manipulation program such as Irfanview (Macfarlane et al. 2007b). In the blue band of the electromagnetic spectrum, the foliage appears darker than in the other bands, thus minimizing the inter- ference of multiple scattering in the canopy and chromatic aberration (Zhang et al. 2005). In addition, in diffuse sky conditions, the sky is saturated in the blue band, and thus appears white in 8-bit blue channel (Leblanc 2008). As such, the blue channel of the images should be used in the canopy analysis to achieve optimal brightness value (thresholding). Image must be sharpened (medium), to enhance the contrast between sky and canopy, and then analyzed using iForest (2012) 5: 290-295 293 © SISEF http://www.sisef.it/iforest/ Tab. 1 - Main characteristics of the most diffuse hemispherical image processing software packages. Software Company Pixel classification Availability LAI methods Clumping index Winscanopy Regent Instruments Inc., Quebec, Canada Automatic and interactive (manual) Commercial 57.5 ° (Bonhomme & Chartier 1972) Chen & Cihlar (1995) LAI 2000 (Miller 1967) Lang & Xiang (see Van Gardingen et al. 1999) Generalized LAI 2000 Hybrid (see Leblanc et al. 2005) Ellipsoidal (Norman & Campbell 1989) - GLA Cary Institute of Ecosystems studies, Millbrook, New York, US Manual Free LAI 2000 (Miller 1967) No CAN-EYE INRA (French Natio- nal Institute of Agro- nomical research) Automatic and interactive (manual) Free 57.5 ° (Bonhomme & Chartier 1972) Lang & Xiang (see Van Gardingen et al. 1999)LAI 2000 (Miller 1967) HemiView Delta-T Device Ltd. Cambridge, UK Manual Commercial LAI 2000 (Miller 1967) No Hemisfer WLS Swiss Federal Institute for Forest, Snow and Landscape Research Automatic and interactive (manual) Commercial 57.5 ° (Bonhomme & Chartier 1972) Chen & Cihlar (1995) LAI 2000 (Miller 1967) Lang & Xiang (see Van Gardingen et al. 1999)Generalized LAI 2000 Ellipsoidal (Norman & Campbell 1989) Digital hemispherical photography for forest canopy estimation hemispherical image analysis software. Win- scanopy enables automatic pixel classifica- tion of the images, thus avoiding human in- put. In addition, Wincanopy enables correction for clumping foliage, which can significantly reduce the underestimation of leaf area index in clumped canopies (Lang & Xiang 1986, Van Gardingen et al. 1999, Jonckheere et al. 2004, Chianucci & Cutini 2013). A zenithal angle range of 0-70° and 8 azi- muth segments should be adequate for the image analysis (Macfarlane et al. 2007c). Comparison of software packages The more popular commercial software packages are Winscanopy and Hemiview. Their standard systems include a digital ca- mera, a calibrated fish-eye lens and a self- leveling tripod. Free software packages are available for hemispherical image analysis such as GLA (Gap Light Analyzer - Frazer et al. 1999) and CAN-EYE. Most of the scientific studies concerning hemispherical photography use method ba- sed on the determination of optimal thre- shold (Hemiview, GLA, Winscanopy). Mo- reover, most of these studies focused on forest canopies (Demarez et al. 2008). CAN- EYE is also widely used in agricultural en- vironments, because of its ability to perform different pixel classification procedures, as compared with thresholding method, thereby allowing analysis of downward-looking images (Demarez et al. 2008). Tab. 1 lists the main characteristics of some of the most widely used software packages. Conclusive considerations Despite uncertainties due to image acquisi- tion and processing steps, digital photo- graphy holds great promise for estimating forest canopy properties, on account of its speed, ready availability and low-cost, which enables widespread use of the method. Pho- tography even shows good potential to re- place other indirect methods, due to its abi- lity to provide simultaneously several para- meters characterizing solar radiation and forest canopy properties (Chen et al. 1997). In addition, unlike other methods, hemi- spherical photograph can be interpreted as a map of canopy openings (or, on the contrary, of canopy closure) relative to the locations from which image is taken, which can be in- spected to provide insight into heterogeneity within a canopy and to compare different canopies at different sites (Rich et al. 1999). Last, but not least, digital photography en- ables widespread use of the method. Aside from scientific purposes, photography can be suitably applied for management and moni- toring issues, i.e., routine canopy properties estimation. Recent advances in digital photographic equipments such as higher resolution came- ras and better quality lenses, combined with robust and efficient image processing routines and software packages, are bringing digital photography to a mature stage, where the field techniques and image processing steps are no longer significant obstacle limi- ting its application (Macfarlane 2011). Acknowledgements This research was supported by RI.SELV.ITALIA Research Program 3.1 - “Silviculture, productivity and conservation of forest ecosystems” research project and by the Research Program D.M. 19477/7301/08 - “Maintenance of collections, databases, and other activities of public interest” fun- ded by the Italian Ministry of Agriculture and Forest Policies. References Anderson MC (1964). Studies of the woodland light climate I. 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