Search

Abstract: 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, operator-independent 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. [Copyright &y& Elsevier]

Rapid, reliable and objective estimations of leaf area index (LAI) are essential for numerous studies of atmosphere–vegetation interaction, as LAI is very often a critical parameter in process-based models of vegetation canopy response to global environmental change. This paper reviews current knowledge concerning the use of direct and indirect methods for LAI determination. The value of optical LAI measurements by means of hemispherical photography has already been demonstrated in previous studies. As clumping seems to be the main factor causing errors in indirect LAI estimation, we suggest that the use of a digital camera with high dynamic range has the potential to overcome a number of described technical problems related to indirect LAI estimation. Further testing and defining of a standardised field protocol for digital hemispherical photography is however needed to improve this technique to achieve the standards of an ideal device. [Copyright &y& Elsevier]

Abstract: The objectives of this research were to investigate (1) the influence of a geometric laser measurement pattern and (2) the shadow effect on the accuracy of a quantitative mathematical description of individual tree structure using a terrestrial laser system. The commercially available SICK Laser-Measurement-System-200 (LMS200, Sick, A.G.) was used in this study to obtain 3D structural datasets with high point densities. An artificial tree was constructed in an experimental set-up, thereby creating the possibility to arrange the structural key elements (leaves and branches) according to predetermined patterns. The multidirectional accuracy and quality of the laser measurement set-ups (lateral sideway, lateral bottom-up, hemispherical bottom-up) subsequently were compared based on the development of a mathematical description of each case. Vertical plant profiles were computed for the different experimental set-ups showing the direct influence of the vertical distribution pattern of the laser beams on the estimation of plant material in a certain height bin. The main measurement direction proved to have the biggest impact on the quality of the dataset, since the derived vertical profile followed the main trend of this laser beam distribution pattern. The use of probability-based statistics was necessary to correct the vertical profiles in order to obtain an acceptable estimation of the amount of plant material per height bin of 0.05m. [Copyright &y& Elsevier]