Integrating clumping effects in forest canopy structure: an assessment through hemispherical photographs

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Methods for analysing foliage nonrandomness in forest canopies by means of hemispherical photographs are assessed. These methods involve calculation of the canopy element clumping factor, at scales coarser than that of the shoot, to adjust for clumping effects on leaf area index (LAI) estimates derived from gap fraction measurements. Two approaches are presented. The first is based on a gap size accumulation method (the Chen and Cihlar clumping index), whereas the second relies on a gap size distribution method (the Pielou coefficient of segregation). Both methods take advantage of hemispherical photographs, allowing measurement of gap size and gap fraction from sequences of black (foliage) and white (sky) pixels along circular transects over the whole range of angles. Clumping factors generated by hemispherical photographs have been analysed and compared using (i) hemispherical photographs of simulated forest stands with fixed LAI (2, 4, and 6) and three-dimensional spatial distribution of foliage, ranging from complete randomness to full clumping; and (ii) in situ hemispherical photographs from forest canopies with known architecture. The following conclusions are drawn based on our experimental data set: (i) overall clumping factors can be obtained by integrating values over the same range of angles as that used to derive LAI, therefore ensuring consistency between estimations of LAI and clumping factors; (ii) the Chen and Cihlar clumping index tends to underestimate clumping in highly clumped canopies, whereas the Pielou coefficient of segregation tends to overestimate clumping in poorly clumped canopies; and (iii) hemispherical photographs provide an efficient tool for describing the degree of canopy nonrandomness in all directions of the hemisphere and to adjust clumping effects on LAI estimates derived from gap fraction analysis. However, clumping factors derived from hemispherical photographs need to be further tested in real canopies and compared with other methods to define their merits and limitations.