Sampling gap fraction and size for estimating leaf area and clumping indices from hemispherical photographs

Abstract

Hemispherical photography is becoming a popular technique for gap fraction measurements to characterize biophysical parameters and solar radiation in plant canopies. One of the crucial steps in the measurement of canopy gap fraction using hemispherical photography is determining the resolution of the sampling grid. In this work, the effects of varying resolutions of sampling grids by modifying the angle widths of zenithal annuli and azimuthal sectors were evaluated for leaf area and clumping indices computations. Sensitivity analysis was performed to test these effects using artificial photographs simulating ideal canopies with varying leaf area index and aggregation levels of foliage elements. Contrasting forest types, including natural tropical cloud forest and exotic plantations, were tested as real canopies. Results indicate that leaf area and clumping indices estimates are significantly affected by the variation of sampling grids. A new approach to solve the problem of null-gap segments, obscured completely by foliage, is proposed. However, the determination of optimal combinations of zenithal annuli and azimuthal sector angular widths that suit all canopy types remains a difficult practical problem that is often overlooked. Finally, theoretically sound gap fraction and size sampling regions were demonstrated for reliable estimates of canopy biophysical parameters.

Canopy Structural Changes Following Widespread Mortality of Canopy Dominant Trees

“…Standard errors thus represent within-treatment variation. Clumping index was based on a modified method for calculating clumping index for hemispherical canopy photographs [35] and was calculated as natural log of the average gap fraction at each canopy height (z) [across the length of the transect (x)] divided by the average natural log of gap fraction at each height (across the length of the transect; see Equation 3 in Gonsamo et al [35]). We used paired t-tests assuming unequal variance (JMP 9.0; SAS Institute, Cary, NC, USA) to evaluate differences between treatments and years.…”

Introducción a la fotografía hemisférica en ciencias forestales

“…Todos los parámetros estructurales relacionados directamente con los claros pueden ser considerados de primer orden, mientras que los obtenidos a partir de estos mediante adopción de supuestos podrán ser considerados de segundo orden (Gonsamo et al, 2010).…”

“…Éste puede ser medido con la fotografía hemisférica bajo el supuesto de que los elementos del dosel se comportan como objetos oscuros (i.e. no transmiten luz) en la banda del azul (Gonsamo et al, 2010). Se calcula con las ecuaciones (2) y (3).…”

“…Sin embargo, el LAI es proporcional al -Ln(GF) y éste no tiene solución para GF=0, por lo tanto, la resolución angular debe ser suficientemente gruesa como para que ningún segmento de cielo quede solamente con pixeles negros. Como ha sido indicado en numerosos estudios, este aspecto del procesamiento tiene un efecto significativo en la determinación del LAI (Gonsamo et al, 2010). Es posible que en la captura de la fotografía se produzcan los mayores errores ya que, dependiendo de la región, es muy difícil satisfacer las condiciones ideales de iluminación.…”

Aboveground-Biomass Estimation of a Complex Tropical Forest in India Using Lidar

“…Similar voxel-based approaches were also successfully used to retrieve forest parameters at stand, plot and tree levels using total frequency of discrete return sampling along height bins of canopy columns [23,[30][31][32]. Generally speaking, [33] indicated that structural parameters that combine height and gap fraction, i.e., the fraction of open sky not obstructed by canopy elements [34], improve biomass estimations. Both [35] and [33] also noted that Lidar might not be adapted to quantifying biomass variations in deciduous forests due to saturation effects.…”