Influence of woody tissue and leaf clumping on vertically resolved leaf area index and angular gap probability estimates Arndt Piayda a,⇑ , Maren Dubbert b , Christiane Werner b , Alexandre Vaz Correia c , Joao Santos Pereira c , Matthias Cuntz a a Deptartment Computational Hydrosystems, UFZ Helmholtz Centre for Environmental Research, Permoserstr. 15, 04318 Leipzig, Germany b Agroecosystem Research Department, BayCEER Center, University of Bayreuth, Universitätsstr. 30, 95447 Bayreuth, Germany c Department of Forestry, Instituto Superior de Agronomia, Technical University of Lisbon, Tapada da Ajuda, 1349-017 Lisbon, Portugal article info Article history: Received 15 October 2014 Received in revised form 19 December 2014 Accepted 22 December 2014 Keywords: Leaf area index Gap fraction Clumping index Woody tissue LAI-2000 Digital cover photography abstract 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 tech- niques 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 X 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 X 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. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction Leaf area index L is defined as the one-sided leaf area per unit of ground area (Watson, 1947). It is an important structural parame- ter of plants, canopies and ecosystems and strongly influences amounts of carbon uptake (e.g. Bunce, 1989) and transpiration (e.g. Monteith, 1965), radiative energy absorbed and reflected by the canopy (Monteith, 1959) and the maximum capacity of rainfall interception (Rutter et al., 1971). The structural parameter quanti- fying the probability of a direct beam of radiation passing through the canopy without being intercepted by the foliage is the gap probability P gap (Monsi and Saeki, 1953, 2005), which depends on L, tree density and other stand attributes. It controls the energy dis- tribution between plant surfaces and the soil surface as well as within the plant (Chen and Black, 1992; Nilson, 1971) and, thus, the ecosystem albedo. Multiple techniques exist and have been widely used to mea- sure L and P gap . Direct techniques include destructive sampling or litter traps (Jonckheere et al., 2004) and are not suitable for mea- suring P gap . In general, they deliver the most precise results but are very labor intensive, and multiple observations during the year are often not feasible. Indirect techniques include the inclined point quadrat method (Warren Wilson, 1960, 1965), the http://dx.doi.org/10.1016/j.foreco.2014.12.026 0378-1127/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: arndt.piayda@ufz.de (A. Piayda). Forest Ecology and Management 340 (2015) 103–113 Contents lists available at ScienceDirect Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco