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The development of near-surface remote sensing requires the accurate extraction of leaf area index (LAI) from networked digital cameras under all illumination conditions. The widely used directional gap fraction model is more suitable for overcast conditions due to the difficulty to discriminate the shaded foliage from the shadowed parts of images acquired on sunny days. In this study, a new LAI extraction method by the sunlit foliage component from downward-looking digital photography under clear-sky conditions is proposed. In this method, the sunlit foliage component was extracted by an automated image classification algorithm named LAB2, the clumping index was estimated by a path length distribution-based method, the LAD and G function were quantified by leveled digital images and, eventually, the LAI was obtained by introducing a geometric-optical (GO) model which can quantify the sunlit foliage proportion. The proposed method was evaluated at the YJP site, Canada, by the 3D realistic structural scene constructed based on the field measurements. Results suggest that the LAB2 algorithm makes it possible for the automated image processing and the accurate sunlit foliage extraction with the minimum overall accuracy of 91.4%. The widely-used finite-length method tends to underestimate the clumping index, while the path length distribution-based method can reduce the relative error (RE) from 7.8% to 6.6%. Using the directional gap fraction model under sunny conditions can lead to an underestimation of LAI by (1.61, 55.9%), which was significantly outside the accuracy requirement (0.5, 20%) by the Global Climate Observation System (GCOS). The proposed LAI extraction method has an RMSE of 0.35 and an RE of 11.4% under sunny conditions, which can meet the accuracy requirement of the GCOS. This method relaxes the required diffuse illumination conditions for the digital photography, and can be applied to extract LAI from downward-looking webcam images, which is expected for the regional to continental scale monitoring of vegetation dynamics and validation of satellite remote sensing products.

Leaf area index L is a key vegetation parameter that can be used in soil–vegetation–atmosphere exchange modeling. To represent the structure of ecosystems in vertically distributed modeling, vertically resolved L distributions as well as vertical and angular gap probability P gap distributions are needed, but they are rarely available. On the experimental side, studies often neglect woody plant components when using indirect methods for L or P gap observations. This can lead to significantly biased results, particularly in semi-arid savannah-type ecosystems with low L values. The objective of this study is to compare three non-destructive leaf area index measurement techniques in a sparse savannah-type cork oak canopy in central Portugal in order to derive vertically resolved L as well as vertically and angularly resolved P gap . We used the established LAI-2000 device as well as fast digital cover photography (DCP), which was vertically and angularly distributed. We applied object-based image analysis to DCP to exclude woody plant components. We compared the results with vertically distributed LAI-2000 measurements and with vertical estimates based on easily measurable crown parameters. Height and angularly distributed DCP was successfully applied here for the first time. It delivers gap probability P gap and effective leaf area index L e measurements that are comparable to the established LAI-2000. The height and angularly dependent leaf clumping index Ω could be determined with DCP, which led to a 30% higher total leaf area index L for DCP compared to LAI-2000. The exclusion of woody tissue from DCP yields on average a 6.9% lower leaf area index L. Including Ω and excluding woody tissue, the L of DCP matched precisely with direct measurements using litter traps. However, the set-up and site-specific adjustment of the image analysis algorithm remains challenging. We propose a special filter for LAI-2000 to enhance data quality when used in open canopies. Finally, if height-dependent observations are not feasible, ground-based observations of crown parameters can be used to derive very reasonable L height distributions from a single, ground-based L observation.

Leaf Area Index (LAI) is an important biophysical parameter necessary to infer vegetation vigour, seasonal vegetation variability and different physiological and biochemical processes of vegetation. Gap fraction analysis has been carried out to estimate plot‐wise LAI. Tropical dry deciduous forests of the study area can be categorized into two prominent phases—growing and senescent phases. Efforts have been made to observe the relationship between ground based LAI values and satellite derived parameters, such as radiance values and also different vegetation indices, namely the Normalized Difference Vegetation Index (NDVI), maximum, minimum, amplitude, sum and radiance NDVI values for both the phases. Multi‐temporal IRS 1C WiFS data have been used. WiFS provides information in two bands: red (0.62–0.68 µm) and near‐infrared (0.77–0.86 µm). All the images from the representative months have been corrected radiometrically and geometrically. NDVI values have been derived for all the representative months. Amo...

A correct exposure is of crucial importance for accurate retrieval of canopy parameters using hemispherical photograph techniques. Digital hemispherical photographs were collected under different sky brightness conditions using a Nikon CoolPix 4500 camera with an FC-E8 fish-eye lens for canopies of different species and openness. Different exposure schemes were employedto investigatethe effects ofphotographic exposure ontheestimations ofthe effective leaf area index(Le)and gap fraction. The contrast between the sky and foliage under each exposure scheme was calculated to determine the correct exposure under different weather conditions. The results demonstrated that digital hemispherical photographs taken with automatic exposure are not reliable, causing Le underestimations by 16‐71% for medium and high density canopies (Le = 3.2‐4.8) and corresponding gap fraction overestimations by 18‐72%. While for open canopies with Le < 1.26, Le was overestimated by 11‐29%, and the corresponding gap fraction was underestimated by 4‐28%. Studies showed that increasing one stop of exposure results in 3‐28% differences inLe for canopies with different openness. Based on the analysis, we determined the optimum exposure and developed a protocol for acquiring digital hemispherical photographs. The protocol requires first measuring reference exposure for the open sky using a built-in camera light meter, and then take photographs inside the canopy using the same camera with two stops of more exposure than the reference exposure in order to make the sky appear white and consequently also maximize the contrast between theskyandfoliage.Thisprotocol isapplicablefordifferentskybrightnessandfordifferentcanopyopenness.Indensecanopies,this procedure requires much less exposure than automatic exposure, but in very open canopies, this procedure requires more exposure than automatic exposure. Using the exposure determined with this procedure rather than the automatic exposure, the comparison of Le values from the LAI-2000 and digital photographs is greatly improved, with R 2 increasing from 0.77 to 0.95, and RMSE

Thresholding is a central part of the analysis of hemispherical images in terms of gap fraction and leaf area index (LAI), and the selection of optimal thresholds has remained a challenge over decades. The need for an objective, automatic, operatorindependent thresholding method has long been of interest to scientists using hemispherical photography. This manuscript deals with the comparison of a wide variety of different well-known automatic thresholding techniques against the subjective manual method, using high-dynamic range digital hemispherical photographs. The performance of the different thresholding methods was evaluated based on: (1) visual inspection by means of a multi-criteria decision system and (2) quantitative analysis of the methods’ sensitivity to an overall performance criterium. The automatic Ridler clustering method proved to be the most robust thresholding method for various canopy structure conditions. This automatic method might be the best solution for a fast, reliable and objective use of hemispherical photographs for gap fraction and LAI estimation in forest stands, given that the threshold setting is no longer manually performed. The fine-tuning potential of local thresholding methods to better address particular photographic limitations (e.g. over-exposure in a certain image region) is also presented. # 2005 Elsevier B.V. All rights reserved.

Chiang, J-M., M. A. Arthur, and B. A. Blankenship (Department of Forestry, University of Kentucky, Lexington, KY 40546-0073). The effect of prescribed fire on gap fraction in an oak forest understory on the Cumberland Plateau. J. Torrey Bot. Soc. 132: 432–441. 2005.—Excessive shade in the understory has been widely cited as the most important factor causing poor oak regeneration in eastern deciduous forests. We examined the effect of single and multiple fires on forest stand structure and the consequent impacts on the gap fraction measured in the understory. Differences in midstory stem density (stems 2–15 cm diameter at breast height; DBH) before and after prescribed burning were highly correlated with fire frequency (R2 = 0.86, P < 0.01). On average, each additional burn reduced midstory stem density by ∼550 stems per hectare. Hemispherical photography showed an increase of 3.8 % in understory gap fraction in the first growing season after burning. This increase in gap fraction, which was likel...

The leaf area index (LAI), was estimated in deciduous forests of southern Sweden using different estimation techniques. The effective LAI (L-e) was estimated optically by measurements with the gap fraction instrument LAI-2000 and corrections for the aggregation of leaves were achieved by using the TRAC (Tracing Radiation and Architecture of Canopies) instrument. Two methods for correcting the contribution of the woody material were tested. The aim was to find the most reliable estimation technique, and therefore the measurements were compared with LAI estimated from litter fall collection. Optically estimated L-e was correlated with litter trap LAI, indicating that L-e can be used as an estimate of LAI in deciduous forests of the same characteristics as tested here. The litter fall collection technique involves knowledge about the specific leaf area (SLA). Since, we found that SLA varied between leaves picked from top positioned branches and leaves picked from low positioned branches as well as between different species, we suggest some caution when interpreting the values. (Less)

The theoretical background of modeling the gap fraction and the leaf inclination distribution is presented and the different techniques used to derive leaf area index (LAI) and leaf inclination angle from gap fraction measurements are reviewed. Their associated assumptions and limitations are discussed, i.e., the clumping effect and the distinction between green and non-green elements within the canopy. Based on LAI measurements in various canopies (various crops and forests), sampling strategy is also discussed.

We compared estimates of leaf area index obtained by the LAI-2000 plant canopy analyzer (LAIPCA) to direct estimates of LAI ("true" LAI) obtained through allometric relationships. This was done for two Scots pine (Pinus sylvestris L.) stands, where LAI was gradually reduced by removing branches and whole trees. LAI (defined on a hemisurface area basis) decreased from 2.24 to 0.50 in the branch removal experiment and from 1.58 to 0.29 in the tree removal experiment. The aim of the study was to analyse the variation in the ratio of the LAI-2000 estimate to the true LAI (LAIPCA/LAI) with changes in stand structure and total leaf area. In the tree removal plot, which had a smaller proportion of woody (branch) area, LAIPCA/LAI remained fairly stable (0.63–0.69) and was smaller than that in the branch removal plot, where LAIPCA/LAI increased from 0.76 to 1.16 along with the decrease in leaf area and a subsequent increase in woody (stem) area. The ratio of LAIPCA to the plant area index (PAI) differed less betwe...

This study took place in an evergreen scrub oak ecosystem in Florida. Vegetation reflectance was measured in situ with a laboratory-made sensor in the red (640-665 nm) and near-infrared (750-950 nm) bands to calculate the normalized difference vegetation index (NDVI) and derive the leaf area index (LAI). LAI estimates from this technique were compared with two other nondestructive techniques, intercepted photosynthetically active radiation (PAR) and hemispherical photographs, in four contrasting 4 m 2 plots in February 2000 and two 4 m 2 plots in June 2000. We used Beer's law to derive LAI from PAR interception and gap fraction distribution to derive LAI from photographs. The plots were harvested manually after the measurements to determine a "true" LAI value and to calculate a light extinction coefficient (k). The technique based on Beer's law was affected by a large variation of the extinction coefficient, owing to the larger impact of branches in winter when LAI was low. Hemispherical photographs provided satisfactory estimates, slightly overestimated in winter because of the impact of branches or underestimated in summer because of foliage clumping. NDVI provided the best fit, showing only saturation in the densest plot (LAI = 3.5). We conclude that in situ measurement of NDVI is an accurate and simple technique to nondestructively assess LAI in experimental plots or in crops if saturation remains acceptable. Resume. A l'aide d'un capteur realise au laboratoire, nous avons mesure in situ la reflectance d'un peuplement bas de chenes a 655 et 825 nm pour calculer la NDVI et estimer le LAI. Ces estimations de LAI ont ete comparees a celles obtenues par deux autres techniques non destructrices dans six placettes de 4 m 2 , en fevrier et juin 2000 : la mesure du PAR intercepte (rayonnement photosynthetiquement actif) et la technique des photographies hemispheriques. Pour calculer le LAI, nous avons utilise la loi de Beer a partir du PAR intercepte et la fraction de trouees pour exploiter les photographies. Les feuilles de chaque placette ont ete recoltees a la main apres les mesures afin d'obtenir un LAI de reference et de calculer un coefficient d'extinction ( k). La technique du PAR intercepte a ete affectee par une importante variation du coefficient d'extinction, due a l'interception plus forte des branches en hiver lorsque le LAI etait peu eleve. Les photographies hemispheriques ont fourni des valeurs de LAI satisfaisantes, legerement surestimees en hiver a cause des branches et sous-estimees en ete a cause de l'agregation du feuillage. Le NDVI a fourni les meilleurs resultats, montrant toutefois une tendance a la saturation dans la placette la plus dense.

We investigated ground and canopy damage and recovery following conventional logging and reduced-impact logging (RIL) of moist tropical forest in the eastern Amazon of Brazil. Paired conventional and RIL blocks were selectively logged with a harvest intensity of approximately 23 m 3 ha −1 (geometric volume) in the dry seasons (July–December) of 1996 and 1998. Ground damage (roads+skid trails+log decks) in the conventional logging treatments occupied 8.9–11.2% of total operational area. In contrast, ground damage in RIL treatments ranged from 4.6 to 4.8% of the total area. Forest canopy damage was assessed using gap fraction measurements collected with an automated optical canopy analyzer (LAI-2000; Licor Inc.) in March 1999. Canopy opening varied by time since logging. The recently logged (1998) blocks had integrated canopy gap fractions of 21.6 and 10.9% of total area for conventional and RIL blocks, respectively. The blocks logged in 1996 had more closed canopies with 16.5 and 4.9% gap fraction for conventional and RIL blocks, respectively. For comparison, undisturbed forest had a canopy gap fraction of 3.1%. Measurements of ground disturbance and gap fraction using the Licor LAI-2000 generally agree with other field evaluations of RIL and conventional logging. Detailed understanding of canopy structural changes resulting from different logging intensities are critical to the prospect of logging damage estimation using current and future remote sensing products.

Forest leaf area index (LAI) is a key characteristic affecting a field canopy microclimate. In addition to traditional professional measuring instruments, smartphone-based methods have been used to measure forest LAI. However, when smartphone methods were used to measure conifer forest LAI, very different performances were obtained depending on whether the smartphone was held at the zenith angle or at a 57.5° angle. To further validate the potential of smartphone sensors for measuring conifer LAI and to find the limits of this method, this paper reports the results of a comparison of two smartphone methods with an LAI-2000 instrument. It is shown that the method with the smartphone oriented vertically upwards always produced better consistency in magnitude with LAI-2000. The bias of the LAI between the smartphone method and the LAI-2000 instrument was explained with regards to four aspects that can affect LAI: gap fraction; leaf projection ratio; sensor field of view (FOV); and viewing zenith angle (VZA). It was concluded that large FOV and large VZA cause the 57.5° method to overestimate the gap fraction and hence underestimate conifer LAI. For the vertically upward method, the bias caused by the overestimated gap fraction is compensated for by an underestimated leaf projection ratio.

In this study, a novel method was proposed for estimation of effective plant area index (PAI) from Terrestrial Laser Scanner (TLS) data. Integrating vertical and tilt scanning from Terrestrial laser scanner, discrete directional canopy gap fraction could be calculated and effective PAI could be estimated from canopy gap fraction. Finally, the effective PAI estimated from Terrestrial LiDAR are validated with effective PAI from TRAC. Results show effective PAI from Terrestrial LiDAR and TRAC have a certain relation, and the R 2 of regression relation of them is 0.5036. This experiment highlights the potential Terrestrial LiDAR for measuring forest structure parameters in plot and could be as a new tool for field measurements combining with traditional methods.

Hemispherical photographs were taken in a range of Sitka spruce ( Picea sitchensis (Bong.) Carr.) stands in northern Britain. These were used to estimate the canopy gap fraction, and thereby estimate the transmission of light to the ground within each stand. Stands under routine management systems, whether thinned or unthinned, generally had too low a gap fraction to allow sufficient light through for seedling growth. The only stand with a gap fraction substantially higher than the others was a widely spaced (8 m) stand from a spacing experiment. The observations showed good agreement with a theoretical curve of canopy transmittance calculated using the Beer–Lambert law. The results indicate that a Sitka spruce stand should be thinned to a basal area 2 ha −1 , which is below management table stocking, if the gap fraction is to be increased to levels suitable for seedling growth.

Based on inversion of gap fraction data (Poisson model of foliage distribution), three optical methods using the Demon, the Plant Canopy Analyzer LAI-2000 (PCA) and hemispherical photographs, have been compared to estimate canopy openness (CO) and leaf area index (LAI) in a mature, neutrophil, oak-beech-hornbeam forest on mull in eastern France. Mean CO over the whole hemisphere was similar for PCA (7.9 %) and hemispherical photographs (8.0 %). The needle method, a vertical point quadrat method, applied to the litter after leaf fall has served as a reference to LAI (4.7). The Demon provided the estimate (4.9) closest to the reference value. The PCA and hemispherical photographs underestimate mean LAI by 30 % (3.3) and 19 % (3.8), respectively, if used without correction. Based on fish-eye sensors, LAI estimates can be improved if 3 annuli (4.2) or 2 annuli (4.5) are used in place of 5 with the PCA, or by means of logarithmic averaging of gap fractions over azimuth at an appropriate angular resolution (180°: 4.6, or 120°: 5.2) with hemispherical photographs. Not taking into account azimuthal variation in gap fraction distribution generates a more important error than the error induced by light scattering near horizon. In order to improve LAI estimates, an original iterative procedure is presented, which allows the simultaneous calculation of LAI over a broad range of angular azimuthal resolutions.

Gaps are unavoidable when compositing small or narrow plant parts (e.g., conifer needles, twigs, narrow leaves, and leaflets) on sample holders in preparation for measuring spectral optical properties. The Daughtry et al. (1989) (A new technique to measure the spectral properties of conifer needles. Remote Sens. Environ. 27:81–91.) method of measuring conifer needle optical properties utilizes a relatively large gap fraction (approximately 0.3–0.6) and needles painted black on one surface of the sample from which the gap fraction of the sample is indirectly determined. Following this protocol typically results in distortions in optical properties, including underestimates in transmittance (sometimes negative values), and only one surface of the sample can be measured. The objectives of this article are to: 1) evaluate the influence of gaps between sample elements (conifer needles, twigs, narrow leaves and leaflets) on optical properties calculated with the published equations from Daughtry et al. (1989) and 2) revise the original Daughtry et al. method for optical property measurements by using an image-analysis to directly measure the gap fraction and use both surfaces of the sample. We achieve these objectives by reviewing the theory and investigating the effects of gaps by measurements of an inert photographic film material, fir needles, and mesquite leaflets. Tests to estimate the transmittance of film samples (film) and foliage (fir needles, mesquite leaflets) indicate that a relatively small gap fraction (less than 0.20) reduces the occurrence of computed negative transmittance values, reduces the variation in computed values, and yields values expected for the “true” or “nongap” transmittance. Employing the image analysis along with reduced gap fractions decreased the variance of measurements and permitted measurements of both surfaces per sample, thus reducing the time required by making half as many samples as originally required by Daughtry et al.

The aim of this study was to validate an indirect method of estimating leaf area index (LAI) in cordon-trained spur-pruned grapevines. Indirect estimates were made using the LAI-2000 instrument, based on the principles of gap fraction theory. Results indicate that using an adjustment coefficient factor, this method could provide rapid and accurate estimates of LAI.

We compared leaf area index (LAI) estimates of a broad-leaf tropical hardwood, Metrosideros polymorpha Gaud (‘Ōhi’a), using a optical method (LI-COR LAI-2000) and direct determination (harvest and allometry). There was a strong correlation between LAI estimates by the two methods, but direct estimates were higher than the optical estimates by a factor of 2.44. The ratio of harvest leaf area to projected leaf area within twigs was similar (2.42) to that of whole plots, suggesting that aggregation of leaves at this scale of branching may account for most of the underestimate by the optical method. The within-branch ratio of actual to projected leaf area did not differ among three sites on three islands of varying land surface age but similar climate, suggesting that a correction factor determined by harvest could be used to adjust optical estimates of LAI in other M. polymorpha forests.

In the past few years, a number of systems for making indirect canopy structural estimates have become commercially available. These include linear sensors, such as the DEMON, line quantum sensors, and hemispherical sensors, such as the LAI-2000, the Leaf Laser, and the CI-100. The performance of these instruments as reported in the literature is reviewed for forests, row crops, and individual trees.

In this study, the Effective Plant Area Indices (PAI e ) for areas of Korean Pine (Pinus koraiensis) and Oaks (Quercus spp.) were estimated by calculating the ratio of intercepted and penetrated LIDAR laser pulses for the canopies of the three forest types. Initially, the canopy gap fraction (G LiDAR ) was generated by extracting the LiDAR data reflected from the canopy surface, or inner canopy area. The LiDAR-derived PAI e was then estimated by using G LIDAR with the Beer-Lambert law. A comparison of the LiDAR-derived and field-derived PAI e revealed the coefficients of determination for Korean Pine and Oak to be 0.82 and 0.59, respectively. These differences between field-based and LIDAR-based PAI e for the different forest types were attributed to the amount of leaves and branches in the forest stands. The probability that the LiDAR pulses are reflected from bare branches is low as compared to the reflection from branches with a high leaf density.

A terrestrial laser scanner can be used to measure canopy directional gap fraction distribution in forest stands. But some equipment only measure parts of gap fraction once on a time. A tilt mount of terrestrial laser scanner is able to enlarge the field of view of the scanner by scanning several times on the same position but with different tilt angles. Gap fraction were estimated from integration of the vertical and tilt mount of terrestrial laser scanner. Directional gap fraction distributions from hemispherical photographs were compared with distributions estimated from terrestrial laser scanner. The results showed that the directional gap fraction distributions from the two methods are similar.

Gap fraction, which is the probability that a light penetrates through the canopy unintercepted and reaches the surface under the canopy, is characteristic geometric parameter of canopy's structure, and has great influence on radiative transfer of the vegetation. In this article, a photographic method was introduced to efficiently and accurately calculate the gap fraction of three-dimensional (3D) canopies. The 3D structures generated by the Clumped Architecture Model of Plants (CLAMP). The key parameters of CLAMP model include sowing scheme, clumping index, average leaf angle and leaf area index. By analyzing the sensitivity of these parameters to gap fraction, the changing features of gap fraction were found. The research of this article shows that photographic method is a powerful way to quantitatively study vegetation by remote sensing technology, and has potential of wider applications.

It has been possible to acquire precise point clouds of surface features from terrestrial laser scanner data. And it is a research hot spot to figure up the LAI of a single tree or wood combined data clouds model of single tree with gap fraction model. We used the point clouds of a single tree from field measurements combined with gap fraction model to estimate the LAI. And we found that a reasonable result could be computed in this way.

The paper investigates methodology comparisons for retrieving effective leaf area index (LAI) using digital hemispherical photograph (DHP) in rice canopy. A set of self-making DHP instrument equipped with a fish-eye Len is utilized to acquire DHP in rice canopy, and some self-developing DHP processing procedures are utilized to pre-process DHP and extract effective LAI from DHP (LAIDHP) rapidly. Based on Beer-Lambert’s law and gap fraction that computed from DHP, four methods of single zenith angle (SZA), Lang, Mill formula(Mf) and iterative formula(IF) are used to derive LAIDHP(effective LAI from them are called LAISZA, LAILang LAIMf and LAIIF ,respectively). LAISZA, LAILang, LAIMf and LAIIF are inter-compared and are also compared with from AccuPAR LP-80(LAIAPAR) and direct manual method (LAIdirect). It is found that, in general, LAISZA, LAILang LAIMf and LAIIF are similar to LAIAPAR, but slightly lower than LAIdirect. During their intercomparsion, LAILang is more similar to LAIAPAR than other three and LAIMfis more similar to LAIdirect, while LAISZA and LAILang are almost the same. It is implied that Lang method outperformed the other three when compared with AccuPAR LP-80 and Mill formula method outperformed the other three when compared with direct manual measure.

We estimated leaf area index (LAI) in a needle-leaved forest and a broad-leaved orchard using four instruments which measure fractional light penetration through the canopy. Gap fraction data were analyzed by a one-dimensional inversion model or by using the Beer-Lambert Law. Instrument and analytical technique both had a strong influence on calculated LAI. There was no consistent pattern of LAI results among instruments so no simple cross-calibration can be offered. Three of the four instruments accurately estimated orchard LAI when used with one or the other analytical technique. In addition to performance, practical considerations including cost, sampling, and data analysis were also compared among the instruments. Because the instruments do not provide consistently accurate LAI estimates, LAI should be independently corroborated before use in any particular situation. These instruments may be most useful for relative LAI comparisons in a specific canopy type when a single combination of instrument and analytical technique is employed.

Physically-based remote sensing methods have progressively become more attractive for monitoring the vertical and horizontal structure of vegetation. A relatively recent development in modeling the radiation field of a vegetation canopy is the spectral invariants theory. The theory states that the radiation budget of a vegetation canopy can be parameterized using only spectrally invariant parameters which depend on canopy structure in a complex manner. In this paper, we briefly review how spectral invariants can be estimated from hyperspectral remote sensing data or in situ vegetation canopy gap fraction measurements.

This study investigates the retrieval performances of LAI using radiative transfer model inversion. The SAIL and GEOSAIL models were used here to evaluate the impact of the complexity of canopy architecture on LAI estimation. The inversion of SAIL or GEOSAIL models was carried out using a look up table technique. Test data sets were generated with SAIL and GEOSAIL to evaluate the performances. Results indicate first that the way the solution is selected within the LUT appeared to be very important. Comparing the inversion using SAIL or GEOSAIL over test cases simulated with SAIL or GEOSAIL show that consistency between the RTM used in the inversion process and the test cases improves the estimation. However, estimation of LAI for complex canopy architecture with non random distribution of leaves (clumping) is difficult when no prior information is available on the targets. However, it is demonstrated that the effective LAI, i.e. the LAI retrieved from the gap fraction measurements assuming. random leaf distribution, is estimated with much better accuracy.

In a field study, cost-benefit relationships of aboveground resource allocation were analysed in branches of Norway spruce (Picea abies [L.] Karst.) and European beech (Fagus sylvatica L.). The study identified response patterns in allocation of resources under different light conditions in both species. It was postulated that resource investment and gains based on crown volume have the potential to quantitatively describe the plant’s competitive ability (i.e. competitiveness). Three cost-benefit ratios (efficiencies) were defined to compare trees of contrasting growth form and leaf type, thereby considering metabolic processes involved in C allocation (Grams et al. 2002): (1) Efficiency of space sequestration (occupied aboveground or belowground space per unit of resource investment), (2) efficiency of space exploitation (resource gain per unit of aboveground or belowground space) and (3) efficiency of “running costs” (in terms of occupied aboveground or belowground space per unit of respiration or transpiration). This study was conducted within the framework of a collaborate research program with the title “Sonderforschungsbereich 607: Growth and Parasite Defense - Competition for Resources in Economic Plants from Agronomy and Forestry” funded by the ‘Deutsche Forschungsgemeinschaft’, project SFB 607-B4. Ten spruce and ten beech trees within a mixed forest stand “Kranzberger Forst” north of Munich/ Germany were investigated for two years (1999-2000). In each tree a study branch from the upper sun and from the lower shade crown were chosen to cover the range of morphological and physiological variability within individual trees. The crown volume occupied by a branch was approximated based on a frustrum model enveloping the foliage of the branch. Assessment of crown volume was readily performed, low?cost and uncomplicated to calculate. Biomass of foliage and axes of each study branch was monitored non?destructively by means of allometric relationships, derived and validated on comparable harvested trees. CO2 and H2O leaf gas exchange was analysed with a portable infrared gas exchange system throughout the annual course by measuring light & CO2 dependencies and respiration. Gas exchange data was used to parameterise a leaf gas exchange model. The annual gas exchange was calculated in ten-minute intervals based on microclimate (light from four sensors per branch, temperature, humidity, and CO2 concentration), and was scaled biometrically to the branch level. A custom made system was developed to continuously measure respiration of axes. Respiration was scaled dependent on temperature to the surface area of the whole branch. Ozone was applied in the second year of investigation as an experimental tool to chronically disturb the homeostasis of resource allocation within branches. Whole crowns of five spruce and five beech study trees were fumigated with twice ambient ozone. Leaf area and mass distribution was derived from optical measurements of leaf area index in the vertical profile within the stand. Growth form and foliage type are contrasting in the coniferous and evergreen spruce and the deciduous and broadleaved beech. Still, it could be shown that the annual gross carbon gain was very similar in adult trees of spruce and beech when related to units of foliated space. Running costs of transpiration and respiration were also rather similar in this aspect. Apparently, differences in leaf level characteristics can vanish when relating physiological performance to space. The annual gross carbon gain scaled from branch to the stand level of spruce and beech showed good agreement to studies of gross primary production in forest stands. The annual carbon balance was negative in all investigated shade branches of beech and in part of the shade branches of spruce. Some of these branches were sustained by the trees over years. This contradicts the ‘theory of branch autonomy’ and is not a common finding, as it probably occurs in branches within the lower shade crown of shade-tolerant species only. The light compensation point of the carbon balance was at lower light availability in beech than in spruce branches, which is of advantage for growth and persistence of shade branches and subordinate individuals of beech. On the stand level, the annual carbon balance was deteriorated in spruce due to negative carbon balances. In beech, the fraction of shade branches with a negative carbon balance was small and therefore the carbon balance was affected to a minor extent compared to spruce at the stand level. Predominantly, spruce and beech were different in their efficiencies of space sequestration. Sun branches of spruce compared to beech sequestered crown volume with lower carbon investments of foliage and axes. This seems more advantageous to spruce during undisturbed growth of a stand as by sequestration of the same amount of space and inherently a similar carbon gain, a higher proportion of carbon than in beech sun branches remains that can be allocated to stem and roots. This was in line with published findings, where spruce was reported to dominate over beech through faster growth in southern Germany including the study site ‘Kranzberger Forst’. However, the relative annual increment of crown volume was larger in sun branches of beech than in spruce. This can be of advantage to beech e.g. in disturbed environments with new gap formation in the canopy. However, losses of invested carbon through the direct interaction of swaying tree crowns were considerably higher in beech compared to spruce trees. The lost carbon mass was equivalent to a loss of a similar amount of space in both species. It appears that larger gap size with decreasing potential of crown abrasion is important for beech to benefit through a rapid volume increment. The disturbance of spruce and beech through elevated ozone concentrations had not taken effect on the efficiency of space sequestration within one season of fumigation, which confirms the findings of other studies. Different strategies of space sequestration determined competitiveness in this study. This was also concluded in a study of hedgerow-species differing in successional status and in a phytotron study with competition among young trees. In general, space-related efficiencies have the power to address competitiveness quantitatively, which allows comparison of contrasting species. It is encouraged to include space-related resource gains and investments in competition studies as space appears to be a resource itself and object of competitive interactions. The transfer and the expansion of space?related analysis to studies of e.g. responses in different environments, interactions in herbaceous and woody plants, belowground interactions or invading neophytes, is very promising, as new insights and understanding of the processes can be expected that determine competitiveness of species and individual plants. In einem Mischbestand wurden Kosten-Nutzen Bilanzen der oberirdischen Ressourcen-Allokation analysiert. An Zweigen von Fichte (Picea abies [L.] KARST.) und Rotbuche (Fagus sylvatica L.) wurden Muster in der Veränderung der Ressourcen-Allokation unter verschiedenen Lichtbedingungen ermittelt. Der Versuchsansatz basierte darauf, dass Ressourcen-Investitionen und -gewinne im Bezug auf einen besetzten Kronenraum, das Konkurrenzverhalten von Pflanzen quantitativ wiedergeben. Dazu wurden drei Qutotienten (Effizienzen) gebildet in welche die metabolischen Prozesse eingehen, die an der Kohlenstoff-Allokation beteiligt sind, um Bäume von unterschiedlichem Wuchs- und Laub-Typus vergleichen zu können (Grams et al. 2002): (1) Effizienz der Raumbesetzung (besetzte Raumeinheit pro investierter Ressource), (2) Effizienz der Raumausnutzung (Ressourcengewinn pro Raumeinheit) und (3) Effizienz der ‚laufenden Kosten’ (Raum pro Einheit veratmeten Kohlenstoffs oder transpirierten Wassers). Diese Studie war Bestandteil des Sonderforschungsbereichs 607 mit dem Titel „Wachstum und Parasitenabwehr – Wettbewerb um Ressourcen in Nutzpflanzen aus Land- und Forstwirtschaft“, finanziert von der Deutschen Forschungsgemeinschaft. Je zehn Buchen und Fichten wurden für zwei Jahre (1999-2000) im Mischbestand „Kranzberger Forst“ nördlich von München untersucht. Je Baum wurde ein Zweig aus der oberen Sonnenkrone und ein Zweig aus der unteren Schattenkrone ausgewählt, um die Spannweite morphologischer und physiologischer Variabilität zu erfassen. Der Kronenraum, der von einem Zweig eingenommen wurde, wurde durch einen das Laub umhüllenden Körper aus Kegelstümpfen beschrieben. Die Volumenvermessung war kostengünstig, leicht durchzuführen und das Volumen unkompliziert zu berechnen. Die Biomasse jedes Messzweiges wurde am Anfang und Ende des Jahres non-destruktiv mittels allometrischer Beziehungen erhoben, welche an vergleichbaren geernteten Bäumen abgeleitet und validiert worden waren. Der CO2- und H2O-Gaswechsel von Blättern und Trieben wurde im Jahresverlauf mit einem tragbaren Infrarot-Gaswechselmessgerät durch Messungen von Atmung, Licht- und CO2- Abhängigkeiten analysiert. Mit den Gaswechseldaten wurde ein Blattgaswechselmodell parametrisiert und der Jahresverlauf des Gaswechsels in 10-minütigen Schritten anhand des Mikroklimas (Licht von vier Sensoren pro Zweig, Temperatur, relative Feuchte und CO2-Konzentration) berechnet und über biometrische Beziehungen auf Zweigebene hochgerechnet. Zur kontinuierlichen Messung der Achsenatmung wurde eine Gaswechselmessanlage entwickelt. Die Atmungsrate der Achsen wurde aus Verläufen der Messung mittels der Temperatur und der Oberfläche der Achsen auf den ganzen Zweig hochgerechnet. Im zweiten Jahr der Untersuchung wurde Ozon als experimentelles Werkzeug zur chronischen Belastung und damit zur Störung der Ressourcen-Allokation eingesetzt. Dazu wurde herrschende Ozonkonzentrationen in der gesamte Krone von jeweils fünf Fichten und fünf Buchen (Messbäume) über ein neuartiges Freiland-Begasungssystem verdoppelten. Anhand von optischen Blattflächenindex-Messungen im vertikalen Profile des Bestandes wurden die Blattflächen- und Blattmassenverteilungen in Buchen- und Fichtenkronen bestimmt. Trotz des unterschiedlichen Wuchs- und Laub-Typus des immergrünen Nadelbaums Fichte und des wechselgrünen Laubbaums Buche war die Jahres-Brutto-Kohlenstofffixierung in ausgewachsenen Bäumen sehr ähnlich, wenn die Fixierung auf das besetzte Astvolumen bezogen wurde. Das gilt weitgehend auch für die „laufenden Kosten“ der Transpiration und Atmung. Offensichtlich können bestehende Unterschiede in der Physiologie bei Bezug auf die Biomasse oder Oberfläche durch Bezug auf den Raum aufgehoben werden. Die Hochrechnung der Jahres-Brutto-Kohlenstoffixierung des Bestandes zeigte gute Übereinstimmung mit Werten die für die Brutto-Kohlenstoffaufnahme von Wäldern der gemässigten Zone angegeben werden. Die Jahres-Kohlenstoffbilanz war in allen Schattenzweigen der Buche und in einem Teil der Schattenzweige der Fichte negativ. Einige dieser Zweige wurden über Jahre hinweg von den Bäumen beibehalten. Über solche Befunde, die im Widerspruch zur ‚Theorie der Ast-Autonomie’ stehen, wurde bisher selten berichtet, da negative Jahres-Bilanzen wahrscheinlich nur in dem unteren Kronenbereich von schattentoleranten Arten vorkommen. Der Lichtkompensationspunkt der Jahres-Kohlenstoffbilanz der Zweige lag in Buche verglichen mit Fichte bei einem niedrigeren Lichtangebot. Das ist sowohl für Schattenzweige als auch für unterständige und unterdrückte Individuen der Buche ein Vorteil. Auf Bestandesebene wurde die Jahres-Kohlenstoffbilanz der Fichten durch die negativen Bilanzen der Schattenzweige stärker herabgesetzt als bei Buche. Bei Buche war der Anteil an Ästen mit negativen Bilanzen so gering, dass sich nur eine geringe Auswirkung auf die Jahres-Kohlenstoffbilanz zeigte. Der hauptsächliche Unterschied zwischen Buche und Fichte trat in der Raumbesetzung auf. Sonnenzweige der Fichte besetzten im Vergleich mit Buche den Kronenraum mit niedrigeren jährlichen Kohlenstoff-Investitionen für Laub und Achsen. Das erscheint vorteilhaft für Fichte bei störungsfreien Bestandeswachstum, da bei gleichem besetztem Volumen und damit gleichen Kohlenstoffgewinn wie bei Buche mehr Kohlenstoff zur Allokation in den Stamm und die Wurzeln bleibt. Diese Annahmen stimmen mit veröffentlichten Ergebnissen überein, wonach in Süddeutschland die Fichte wegen besserem Wachstum über Buche dominiert. Der jährliche Zuwachs an besetzen Kronenraum war in Sonnenzweigen der Buche grösser als bei Fichte, was für die Buche beispielsweise bei Lückenbildung durch Störungen im Bestand von Vorteil sein kann. Jedoch waren die Kohlenstoff-Verluste durch Astabbrüche in den Kontaktzonen zu den Nachbarbäumen bei Buche wesentlich höher als bei Fichte. Der durch Astbrüche eingebüsste Kronenraum war bei beiden Baumarten wiederum ähnlich. Es scheint, dass grössere Bestandeslücken mit geringerer Häufigkeit von direkten Kroneninteraktionen für die Buche mit ihrem raschen Raumzuwachs von Vorteil sind. Die Störung der Allokation in Buche und Fichte durch Erhöhung der Ozonkonzentration hat sich nach einem Jahr der Begasung nicht erkennbar auf die Raumbesetzung ausgewirkt. Unterschiede in der Strategie der Raumbesetzung bestimmten in dieser Studie die Konkurrenzkraft. Das war auch die Folgerung einer Heckenstudie mit Holzpflanzen unterschiedlicher sukzessionaler Stellung und in einem Konkurrenzexperiment mit jungen Pflanzen in Phytotronen. Zusammenfassend kann man sagen, dass die raumbezogenen Kosten-Nutzen-Bilanzen das Konkurrenzverhalten quantitativ beschreiben, wodurch man sehr unterschiedliche Arten vergleichen kann. Es wird empfohlen raumbezogene Gewinne und Investitionen in Konkurrenzstudien künftig mit einzubeziehen, da Raum an sich als eine Ressource aufgefasst werden kann und zum Gegenstand der Konkurrenzinteraktionen wird. Die Übertragung und Erweiterung von raumbezogenen Analysen, beispielsweise auf Studien über Reaktion auf veränderten Umweltbedingungen, über Interaktionen von krautigen und holzigen Pflanzen, über unterirdische Wechselwirkungen oder über Interaktionen mit Neophyten, scheint vielversprechend, da neue Erkenntnisse über Prozesse, die das Konkurrenzverhalten von Arten und Einzelpflanzen steuern, und ihr besseres Verständnis erwartet werden können.

International audience; This study describes a technique to estimate green area index (GAI) of row crops from gap fraction measurements at 57.5° perpendicular to the row using downward looking digital photos. This particular directional configuration makes the gap fraction independent from leaf angle distribution and minimizes leaf clumping when plants overlap within the row and when rows overlap from this particular direction which is the case for several crops including wheat, maize, sorghum, sunflower and soybean. This was demonstrated from generic row crop canopy architecture models. Additional simulations over realistic 3D scenes of wheat crop allowed to calibrate the following equation relating the gap fraction (Po(57.5°)) to GAI for this particular directional configuration: Po(57.5°) = e−0.824·GAI. This relationship appears very robust across development stages, cultivars and variations due to environmental conditions. When comparing with the situation where leaves are randomly distributed, performances degrade significantly, demonstrating that some residual clumping (Ω = 0.89) has to be accounted for. Field experiments were conducted over wheat crops using colour digital photos taken at 57.5° zenith angle from above in a compass direction perpendicular to the rows. The corresponding gap fraction was computed after image segmentation based on the three colours. The equation derived from wheat architecture model simulations was then used to estimate GAI. Comparison with destructive GAI field measurements shows very good performances with a relative RMSE of 12%. GAI values estimated with this technique were also showing a good consistency with LAI2000 PAI (plant area index) estimates. However, systematic biases between the two estimates were observed, due to canopy elements at the bottom of the canopy not sampled by the instrument because of the height of the LAI2000 sensor, as well as accounting for the residual clumping in the proposed method. These results suggest that this GAI estimation method is very efficient over wheat crops from emergence up to flowering independently from possible architecture variation due to genotype or environmental condition differences. Possible extension to other crops is discussed

A Poisson model is developed to describe sunfleck or gap size distributions beneath clumped plant canopies. This model is based on the assumption that foliage clumps are randomly distributed in space and foliage elements are randomly distributed within each clump. Using this model, the foliage clumping index, leaf area index ( L ), clump area index, element area index in each clump, and element and clump widths were successfully derived for two artificial canopies and a thinned and pruned Douglas-fir forest stand. It is shown that existing theories for deriving L from measurements of canopy gap fraction have limitations, and the use of canopy architectural information derived from canopy gap size distribution can substantially improve the technique for indirectly measuring L of plant canopies.

International audience; The objective of this study was to determine the impact of lowering nitrogen supply from 12 to 6 or 4 mM NO3− on tomato fruit yield and quality during the growing season. Lowering nitrogen supply had a low impact on fruit commercial yield (−7.5%), but it reduced plant vegetative growth and increased fruit dry matter content, improving consequently fruit quality. Fruit quality was improved due to lower acid (10−16%) and increased soluble sugar content (5−17%). The content of some phenolic compounds (rutin, a caffeic acid glycoside, and a caffeic acid derivate) and total ascorbic acid tended to be higher in fruit with the lowest nitrogen supply, but differences were significant in only a few cases (trusses). With regard to carotenoids, data did not show significant and univocal differences related to different levels of nitrogen supply. Thus, reducing nitrogen fertilization limited environmental pollution, on the one hand, and may improve, on the other hand, both growers’ profits, by limiting nitrogen inputs, and fruit quality for consumers, by increasing tomato sugars content. It was concluded that primary and secondary metabolites could be affected as a result of a specific response to low nitrogen, combined with a lower degree of vegetative development, increasing fruit irradiance, and therefore modifying fruit composition

International audience; Validation of Leaf Area Index (LAI) derived from moderate resolution remote sensing observations generally involves optical technique to measure ground LAI. As the current validation datasets are derived using multiple optical retrieval techniques, assessment of the consistency between these techniques is required. in this study the effective Plant Area Index (PAI(eff)) retrievals by three major optical instruments, LAI-2000, AccuPAR, and Digital Hemispherical Photographs (DHPs), were analyzed over 10 crops (soybean, com, alfalfa, sorghum, peanut and pasture) at Manfredi site in Cordoba province, Argentina. The focus of research was on quantifying PAI(eff) sensitivity to the type of instrument, retrieval parameters and gap fraction inversion methods as well as environmental conditions (canopy heterogeneity, senescent vegetation, illumination conditions). Results indicate that sensitivity of DHP method to illumination conditions is low (14% compared to 28% and 86% for LAI-2000 and AccuPAR, respectively). The intercomparison of PAI(eff) retrievals indicates large discrepancies between optical techniques for short canopy over which downward-pointing DHP technique performs better than LAI-2000 and AccuPAR. Better agreement was found for tall canopy without senescent vegetation and low spatial heterogeneity. Overall, discrepancies in PAI(eff) between instruments are mainly explained by differences in spatial sampling of transmittance between instruments (over short and heterogeneous canopies) caused by variations in instrument footprint, azimuthal range, and zenith angle spatial resolution (coarser for LAI-2000 than DHP). Our results indicate that DHP is the most robust technique in terms of low sensitivity to illumination conditions, accurate spatial sampling of transmittance, ability to capture gap fraction over short canopy using downward-looking photographs, independence from canopy optical ancillary information, and potential to derive clumping index. It can thus be applied to a large range of canopy structures, and environmental conditions as required by validation protocols.

The spatial pattern was characterized in five experimental plots of 0.5 ha established in Scots pine stands located in the Central Mountain Range of Spain with different ecological conditions and in which different silvicultural practises are employed. A new method is proposed to asses the spatial pattern from hemispherical photographs, based on the variance between the gap fractions in sky sectors, which was calculated as a function of the angular distance. The results were compared with the spatial pattern analysis based on the second moment measure, and with the analysis of the structure of the tree number semivariogram. The three methods give a very similar scale of the pattern for most of the plots. The mature plot with more intensive silviculture showed a regular pattern at scales below 6 m. The mature plot in which the silviculture was less intensive showed a cluster pattern at scales around 10 m, and clustering at similar scales was detected during regeneration. Cluster patterns at scales around 5 m were found at the upper limit of the altitudinal range of Scots pine as well as at the lower limit, where Pyrenean oak stands merge with the pinewood. The method proposed allows the scale of the pattern of the canopy to be determined from hemispherical photographs, and can be easily implemented in forest inventories. © 2007 Elsevier B.V. All rights reserved.

International audience; Digital hemispherical photography poses specific problems when deriving leaf area index (LAI) over sloping terrain. This study proposes a method to correct from the slope effect. It is based on simple geometrical considerations to account for the path length variation within the canopy for cameras pointing vertically. Simulations over sloping terrain show that gap fraction increases up-slope while decreasing down-slope. As a consequence of this balance between up- and down-slope effects, effective LAI estimates derived from inversion of the Poisson model are marginally affected for low to medium slopes (

(1990). Characterizing plant canopies with hemispherical photographs. Remote Sensing Reviews: Vol. 5, Instrumentation for studying vegetation canopies for remote sensing in optical and thermal infrared regions, pp. 13-29.

In this study a stand level approach is proposed for estimating leaf area index (LAI) in even-aged Scots pine stands using gap fraction data derived from hemispherical photographs. The approach includes both the effect of the spatial distribution of the foliage elements as well as the slope of the terrain. Two chronosequences consisting of 11 plots were established in two Scots pine (Pinus sylvestris L.) forests in the Central Mountain Range of Spain. 12 hemispherical photographs were taken in each plot and variables related to the stage of development, density, relative spacing and site index were calculated. The gap frequency Poisson model was used to estimate the leaf area index. A new function is proposed to assess the effect of the foliage inclination on light transmittance for Scots pine, based on the assumption that the foliage elements show circular horizontal cross-sections. A new model was also developed to restrict the clumping effect on the vertical component in the Poisson model. The effect of the slope on LAI estimates was determined and included in the model. The results indicate that both the clumping effect correction and the slope correction improve the fitting of the Poisson model to the gap frequency experimental curves, and that the function proposed for the inclination angle correction may provide an acceptable approach for some coniferous species. The relationship between the parameters of the gap fraction model (the foliage inclination angle and the clumping coefficient) and stand level variables, that is, the stage of development, density, spacing of the stand and the site index, is analysed. © 2007 Elsevier B.V. All rights reserved.

The leaf area index (LAI) is a key variable involved in many biophysical processes. Indirect measurements methods provide LAI estimations from in situ gap fraction measurements. Inversion of reflectance models provides LAI estimations from satellite data with high repetitivity on large areas; the models usually used are one-dimensional because few entry parameters are needed. In both cases, these models assume that the foliage elements are randomly distributed, that is not the case in most of the canopies.The main objective of this work is to evaluate if the use of a clumping index in radiative transfer models could improve the simulations for heterogeneous canopies, in order to provide better LAI estimations.In the first part of the work, we evaluate LAI estimations provided by the analysis of hemispherical photographs with the CAN_EYE software; it provides LAI estimations assuming leaves randomly distributed or taking into account the canopy heterogeneity with a clumping index. The evaluation is done by comparison with destructive measurements carried out over wheat, maize and sunflower crops. The main results show that the LAI estimations are improved when using the clumping index, but its calculation with the Lang and Xiang method (1986) must be done with great care according to the species. Possible improvements for its calculation are discussed.The second part of the work concerns the analysis of BRDF simulations at field and landscape scales. Taking BRDF simulations with a 3D model as references (the DART model, Gastellu et al., 1996), we show first that the use of a clumping index in a one dimensional reflectance model generally improves field BRDF simulations for a maize canopy in the red spectral band. In the near infra red, we have best results when the canopy is considered as homogeneous. At the landscape scale, the changes in agricultural land use seem to be the main factor of heterogeneity and this heterogeneity could be partially taken into account with a clumping factor used to simulate BRDF in the visible spectral band. A preliminary study allows us to conclude that LAI estimations from reflectance models could be improved.; L'indice foliaire (LAI) est une variable clé pour l'étude du fonctionnement des surfaces végétales car elle conditionne les échanges de carbone et d'eau avec l'atmosphère. Les méthodes de mesures indirectes fournissent des estimations de LAI à partir de mesures de la fraction de trou in situ. Les méthodes d'estimation du LAI par inversion de modèle de réflectance à partir de données satellitaires utilisent généralement des modèles unidimensionnels car ils nécessitent peu de paramètres d'entrée. Dans les deux cas, les modèles reposent sur l'hypothèse que les éléments sont distribués de façon aléatoire au sein du couvert, ce qui est rarement le cas en réalité.Dans ce contexte, le travail réalisé a pour objectif principal d'évaluer l'apport de l'utilisation d'un coefficient d'agrégation dans la simulation du transfert radiatif de couverts végétaux hétérogènes, en vue d'améliorer les estimations de LAI par inversion de modèles turbides.Dans la première partie du travail, nous avons évalué les estimations de LAI fournies par le logiciel de traitement de photographies hémisphériques CAN_EYE, dont l'intérêt principal est d'estimer le LAI avec ou sans prise en compte de l'agrégation des feuilles. L'évaluation a été réalisée par comparaison avec des mesures destructives effectuées sur des cultures de blé, maïs et tournesol. Les résultats montrent que l'utilisation d'un coefficient d‘agrégation permet d'améliorer sensiblement les estimations de LAI. Toutefois, l'analyse suggère que le calcul du coefficient d'agrégation dans CAN_EYE avec la méthode de Lang et Xiang (1986) doit être amélioré.Dans la deuxième partie du travail, nous avons analysé si l'introduction d'un coefficient d'agrégation dans un modèle de réflectance unidimensionnel permettait d'améliorer les simulations de la réflectance bidirectionnelle (FDRB), à l'échelle de la parcelle et du paysage. Pour cela, nous avons pris comme référence des simulations de FDRB issues d'un modèle 3D, le modèle DART (Gastellu et al., 1996). A l'échelle de la parcelle, nous avons montré l'intérêt du coefficient d'agrégation pour simuler la FDRB d'une parcelle de maïs dans la bande spectrale du rouge. Dans le PIR, l'utilisation d'un modèle unidimensionnel donne de meilleurs résultats. A l'échelle d'un paysage agricole, l'hétérogénéité sub-parcellaire semble être le facteur primordial et sa prise en compte avec un coefficient d'agrégation dans le rouge permet d'améliorer les simulations de la FDRB avec un modèle unidimensionnel. Une étude préliminaire a permis de mettre en évidence que les estimations de LAI par inversion pourraient être sensiblement améliorées si ce coefficient est introduit pour simuler la FDRB dans le visible.

A method for computing leaf area of isolated trees from perspective photographs was developed. The method is based on gap fraction inversion. Photographs are discretized into picture zones where gap fraction is computed from image processing. Canopy volume and leaf area density associated with each picture zone are computed from geometrical considerations and inversion of gap fraction equations. Total leaf area and the vertical profile of leaf area are computed from the product of associated volume and its density. The method has been implemented in software called Tree Analyser, written in C++. The method has been tested by comparison with direct estimation of leaf area of three-dimensional (3D) digitized trees of walnut, peach, mango, olive and rubber. Estimated leaf area was sensitive to picture discretization, individual leaf size and leaf inclination distribution. Optimal size of picture discretization was 17 times projected leaf size. Total leaf area was estimated by using a set of eight photographs taken around the tree in the main horizontal directions: deviation ranged from -11% in peach tree to +5% in rubber tree. The method allows fast and nondestructive monitoring of leaf area of individual tree canopies. The next version of the method will include the estimation of 3D leaf area distribution within the canopy.

We evaluate the potential of deriving fractional cover (fCover) and leaf area index (LAI) from discrete return, small footprint airborne laser scanning (ALS) data. fCover was computed as the fraction of laser vegetation hits over the number of total laser echoes per unit area. Analogous to the concept of contact frequency, an effective LAI proxy was estimated by a fraction of first and last echo types inside the canopy. Validation was carried out using 83 hemispherical photographs georeferenced to centimeter accuracy by differential GPS, for which the respective gap fractions were computed over a range of zenith angles using the Gap Light Analyzer (GLA). LAI was computed by GLA from gap fraction estimations at zenith angles of 0–60°. For ALS data, different data trap sizes were used to compute fCover and LAI proxy, the range of radii was 2–25 m. For fCover, a data trap size of 2 m radius was used, whereas for LAI a radius of 15 m provided best results. fCover was estimated both from first and last echo data, with first echo data overestimating field fCover and last echo data underestimating field fCover. A multiple regression of fCover derived from both echo types with field fCover showed no increase of R2 compared to the regression of first echo data, and thus, we only used first echo data for fCover estimation. R2 for the fCover regression was 0.73, with an RMSE of 0.18. For the ALS LAI proxy, R2 was lower, at 0.69, while the RMSE was 0.01. For LAI larger radii (∼ 15 m ) provided best results for our canopy types, which is due to the importance of a larger range of zenith angles (0–60°) in LAI estimation from hemispherical photographs. Based on the regression results, maps of fCover and LAI were computed for our study area and compared qualitatively to equivalent maps based on imaging spectrometry, revealing similar spatial patterns and ranges of values.

We developed a method for reconstructing tree crown volume from a set of eight photographs taken from the N, S, E, W, NE, NW, SE and SW. This photographic method of reconstruction includes three steps. First, canopy height and diameter are estimated from each image from the location of the topmost, rightmost and leftmost vegetated pixel; second, a rectangular bounding box around the tree is constructed from canopy dimensions derived in Step 1, and the bounding box is divided into an array of voxels; and third, each tree image is divided into a set of picture zones. The gap fraction of each picture zone is calculated from image processing. A vegetated picture zone corresponds to a gap fraction of less than 1. Each picture zone corresponds to a beam direction from the camera to the target tree, the equation of which is computed from the zone location on the picture and the camera parameters. For each vegetated picture zone, the ray-box intersection algorithm (Glassner 1989) is used to compute the sequence of voxels intersected by the beam. After processing all vegetated zones, voxels that have not been intersected by any beam are presumed to be empty and are removed from the bounding box. The estimation of crown volume can be refined by combining several photographs from different view angles. The method has been implemented in a software package called Tree Analyzer written in C++. The photographic method was tested with three-dimensional (3D) digitized plants of walnut, peach, mango and olive. The 3D-digitized plants were used to estimate crown volume directly and generate virtual perspective photographs with POV-Ray Version 3.5 (Persistence of Vision Development Team). The locations and view angles of the camera were manually controlled by input parameters. Good agreement between measured data and values inferred from the photographic method were found for canopy height, diameter and volume. The effects of voxel size, size of picture zoning, location of camera and number of pictures were also examined.