A note on upscaling coniferous needle spectra to shoot spectral albedo Miina Rautiainen a, ⁎, Matti Mõttus b , Lucia Yáñez-Rausell c, d , Lucie Homolová c, d , Zbyněk Malenovský c , Michael E. Schaepman c a Department of Forest Sciences, PO BOX 27, FI-00014 Univ. of Helsinki, Finland b Department of Geosciences and Geography, PO BOX 64, FI-00014 Univ. of Helsinki, Finland c Department of Geography, Univ. of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland d Laboratory of Geo-Information Science and Remote Sensing, Wageningen University, PO BOX 47, 6700 AA Wageningen, The Netherlands abstract article info Article history: Received 16 June 2011 Received in revised form 19 October 2011 Accepted 21 October 2011 Available online 23 November 2011 Keywords: Photon recollision probability Spectral invariants STAR Pinus sylvestris Mutual shading of needles in coniferous shoots and small-scale variations in needle area density both within and between shoots violate conventional assumptions used in the definition of the elementary volume in ra- diative transfer models. In this paper, we test the hypothesis if it is possible to scale needle spectral albedo up to shoot spectral albedo using only one structural parameter: the spherically averaged shoot silhouette to total area ratio (STAR). To test the hypothesis, we measured both structural and spectral properties of ten Scots pine (Pinus sylvestris) shoots and their needles. Our results indicate that it is possible to upscale from needle to shoot spectral albedo using STAR. The upscaling model performed best in the VIS and SWIR regions, and for shoots with high STAR values. As STAR is linearly related to photon recollision probability, it is also possible to apply the upscaling model as integral part of radiative transfer models. © 2011 Elsevier Inc. All rights reserved. 1. Introduction In coniferous canopies, needles are densely packed in shoots with dimensions of typically only a few centimeters. Multiple scattering occurring within shoots is a long-known optical phenomenon (e.g. Norman & Jarvis, 1975). Mutual shading of needles in shoots and small scale variations in needle area density both within and between shoots also violate the traditional assumptions made in the definition of elementary volume in radiative transfer (RT) models. Thus, the use of a coniferous shoot (sometimes referred to as ‘shoot-like leaf’) as the basic scattering element or structural unit has been proposed to solve this problem (Nilson & Ross, 1997). Forest reflectance simula- tions have also highlighted the importance of accounting for within- shoot scattering; within-shoot scattering may be the single most im- portant structural effect causing the reflectance of coniferous forests to be lower than that of broadleaved forests (Rautiainen & Stenberg, 2005). The G-function, also called ‘mean projection of unit foliage area’ was originally defined for flat leaves (Nilson, 1971). For coniferous shoots, it is conceptually analogous to the ratio of shoot silhouette area to total (or hemisurface, defined as half of the total) needle area. Overlapping of needles in the shoot causes the shoot's G-value (defined as the spherically averaged silhouette to total needle area ratio, abbreviated as STAR) to be smaller than that of a single needle (Stenberg, 2006). The overlap can be quantified by a needle clumping index (Nilson, 1999) or shoot shading factor (Stenberg et al., 1994). The STAR for a shoot with no-within shoot shading is 0.25 (Stenberg, 1996), because the spherically averaged projection area of a needle is precisely one fourth of its total surface area (Lang, 1991). From extensive empirical measurements we know that, for example for Scots pine, the reduction in shoot silhouette area resulting from nee- dles overlapping is typically over 40% (Oker-Blom & Smolander, 1988). This results in considerable differences between the approaches needed for RT modeling in broadleaved and coniferous canopies. Consequently, a method that would unify the mathematical treatment of the basic ele- ments in RT modeling in both broadleaved and coniferous species is needed. An elegant theory connecting STAR to the scattering properties of a shoot or a canopy was put forward by Smolander and Stenberg (2003, 2005), and later applied to a forest reflectance model based on the photon recollision probability theory by Rautiainen and Stenberg (2005). This theory states that the scattering of a vegetation unit (i.e. its spectral albedo) is approximated by the formula (Eq. 1): ω unit λ ð Þ¼ ω element λ ðÞ 1-p 1-pω element λ ðÞ   ð1Þ where ω is the spectral albedo and p is the photon recollision proba- bility between the ‘elements’ (scattering centers). The photon recolli- sion probability p is defined as the probability that a photon scattered from a leaf surface will interact with the canopy again. (Note that the photon recollision probability p as shown in Eq. (1) is conceptually Remote Sensing of Environment 117 (2012) 469–474 ⁎ Corresponding author. Tel.: + 358 919158191; fax: + 358 919158100. E-mail address: miina.rautiainen@helsinki.fi (M. Rautiainen). 0034-4257/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.rse.2011.10.019 Contents lists available at SciVerse ScienceDirect Remote Sensing of Environment journal homepage: www.elsevier.com/locate/rse