Agricultural and Forest Meteorology 154–155 (2012) 1–8 Contents lists available at SciVerse ScienceDirect Agricultural and Forest Meteorology jou rn al h om epa g e: www.elsevier.com/locate/agrformet Analyzing forest canopies with ground-based laser scanning: A comparison with hemispherical photography Dominik Seidel a,∗ , Stefan Fleck b,c , Christoph Leuschner b a Chair of Remote Sensing and Forest Inventory, Burckhardt Institute, University of Göttingen, Büsgenweg 5, 37077 Göttingen, Germany b Plant Ecology, Albrecht von Haller Institute of Plant Sciences, University of Göttingen, Untere Karspüle 2, 37073 Göttingen, Germany c Nordwestdeutsche Forstliche Versuchsanstalt, Grätzelstraße 2, 37079 Göttingen, Germany a r t i c l e i n f o Article history: Received 8 March 2011 Received in revised form 14 October 2011 Accepted 17 October 2011 Keywords: TLS Canopy structure Hemispherical photography Voxel approach a b s t r a c t We tested ground-based high resolution laser scanning as a tool for analyzing the complex canopy struc- ture of temperate broad-leaved forests. The canopies of 35 groups of trees were analyzed by laser scans from various positions inside a mixed stand to generate three-dimensional (3D) point clouds of the axes and leaves. The scan data was used to produce hemispheric views of the canopy at various locations that were compared to synchronously taken hemispherical photographs of the same part of the canopy. We conclude that terrestrial laser scanning (TLS) in mature forests can overcome several of the methodolog- ical problems inherent to conventional canopy analysis with optical methods and thus may soon offer a promising tool for functional research in complex forest canopies. Certain limitations of the TLS approach, mostly caused by hardware limitations that will probably soon be overcome, are encountered. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The structure of tree canopies exerts a major control on the energy and mass exchange between forests and the atmosphere. The distribution of light and photosynthetic activity in the canopy and the source strength for water vapour depend not only on total leaf area but also on the spatial distribution and exposure of leaves and needles in the canopy. Competition for light and canopy space is influenced by the branching patterns of the trees and the invest- ments in terms of new leaves and structural organs necessary to occupy canopy volume (Reiter et al., 2005). Thus, a deeper understanding of tree crowns and canopy inter- actions in forests requires profound knowledge of the spatial structure of tree canopies. However, precise data on the distribu- tion of leaf area and axes in the crown, leaf clumping and canopy gaps is difficult to obtain for adult trees, simply because of the sheer size of the plants and difficulties in canopy access. In the past, analyses of the spatial structure of tree canopies or the associated light climate were based on a variety of techniques and instruments (Seidel et al., 2010). Hemispherical photographs with wide-angle (fish-eye) lenses taken from the ground vertically upwards that allowed calculation of the fraction of diffuse and direct radiation reaching the camera viewpoint have been among the most frequently used approaches (Evans and Coombe, 1959; ∗ Corresponding author. Tel.: +49 551 39 9572; fax: +49 551 39 9787. E-mail address: dseidel@gwdg.de (D. Seidel). Anderson, 1964; Frazer et al., 2001; Fiala et al., 2006). Such pho- tographs may also be used to characterise the light climate along a height gradient inside the canopy. A major shortcoming of this approach is that it is nearly impossible (or at least extremely time consuming) to perform this kind of measurement along a dense grid of camera positions in the canopy as a lift or a large and expandable tripod would be needed in the field. In addition, there is an on- going discussion on the accuracy of the information obtained with canopy photography and on necessary improvements of the tech- nique. Most problematic are the effects of different sky conditions on the images and subjective interventions in the processing of the colour images which are to be converted into black-and-white images based on a threshold that is sometimes to be chosen in a sub- jective way (Anderson, 1964; Zhang et al., 2005; Guevara-Escobar et al., 2005). 3D laser scanner measurements conducted on the forest floor (terrestrial LiDAR; TLS) offer opportunities to overcome most of these problems. Terrestrial LiDAR has recently been employed in attempts to calculate canopy openness and LAI in forest stands. When compared to conventional hemispheric photos taken from the ground, a good agreement was found (Danson et al., 2007; Lovell et al., 2003). It should be stated here that the LiDAR instru- ments used are not able to separate woody from photosynthetically active plant material and the derived parameter should therefore be named ‘plant area index’ (PAI) instead of ‘leaf area index’. However, a major advantage of calculating the desired structural parameters (such as openness, PAI) from scanner data is the non-subjective character of the data processing which would represent a large step 0168-1923/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.agrformet.2011.10.006