Estimating forest canopy bulk density using six indirect methods

Abstract

Canopy bulk density (CBD) is an important crown characteristic needed to predict crown fire spread, yet it is difficult to measure in the field. Presented here is a comprehensive research effort to evaluate six indirect sampling techniques for estimating CBD. As reference data, detailed crown fuel biomass measurements were taken on each tree within fixed-area plots located in five important conifers types in the western United States, using destructive sampling following a series of four sampling stages to measure the vertical and horizontal distribution of canopy biomass. The six ground-based indirect measurement techniques used these instruments: LI-COR LAI-2000, AccuPAR ceptometer, CID digital plant canopy imager, hemispherical photography, spherical densiometer, and point sampling. These techniques were compared with four aggregations of crown biomass to compute CBD: foliage only, foliage and small branchwood, foliage and all branchwood (no stems), and all canopy biomass components. Most techniques had the best performance when all canopy biomass components except stems were used. Performance dropped only slightly when the foliage and small branchwood canopy biomass aggregation (best approximates fuels available for crown fires) was employed. The LAI-2000, hemispherical photography, and CID plant canopy imager performed best. Regression equations that predict CBD from gap fraction are presented for all six techniques.Résumé : La densité apparente de la canopée (DAC) est une caractéristique importante de la cime qui est nécessaire pour prédire la propagation d'un feu de cime mais qui est cependant très difficile à mesurer sur le terrain. Les auteurs présentent ici un travail de recherche exhaustif dont le but était d'évaluer six techniques indirectes d'échantillonnage pour estimer la DAC. Les données de référence proviennent de mesures détaillées de la biomasse des combustibles dans la cime prises sur chaque arbre dans des placettes à superficie fixe situées dans cinq types importants de forêt résineuse de l'ouest des États-Unis en utilisant une approche destructrice après avoir procédé à une série d'échantillonnages en quatre étapes pour mesurer la distribution verticale et horizontale de la biomasse dans la canopée. Les six techniques indirectes de mesure sur le terrain comprenaient les instruments suivants : le LAI-2000 de LI-COR, le ceptomètre d'AccuPAR, l'imageur digital du couvert végétal CID, la photographie hémisphérique, le densiomètre sphérique et l'échantillonnage par point. Ces techniques ont été comparées à quatre regroupements de la biomasse de la cime pour calculer la DAC : feuillage seulement, feuillage et petites branches, feuillage et toutes les branches (sans la tige) et toutes les composantes de la biomasse de la cime. La plupart des techniques offraient la meilleure performance lorsque toutes les composantes de la biomasse de la canopée, à l'exception de la tige, étaient utilisées. La performance diminuait juste un peu avec l'utilisation du regroupement de la biomasse de la cano...

Canopy fuel characteristics and potential crown fire behavior in Aleppo pine (Pinus halepensis Mill.) forests

“…In view of the above, CBD values in the present study are realistic. The maximum CBD value of the 4-m running mean of the 1-m canopy layers that was used in this study is within the limits of the CBD values needed to model crown fire behavior [43,70,79]. CBD was computed using only the available canopy biomass (needles and branches < 0.63 cm).…”

“…Alexander et al [9] distributed the canopy fuel weight vertically for each crown fuel component using the fraction of the total canopy fuel weight by crown segment as a function of the total crown height. Keane et al [43] estimated CBD using six groundbased methods with several optical instruments, estimating Leaf Area Index (LAI). LAI was converted to an estimate of crown fuel biomass using specific leaf area factors.…”

The spatial and temporal evolution of contributing areas

“…The evapotranspiration point measurement from the eddy flux tower was disaggregated across the watershed on an hourly time step based on a linear relationship between height and leaf area index as suggested for these coniferous forests [Keane et al, 2005], similar to the derived distribution approach of Emanuel et al [2011]. The eddy flux tower footprint has been estimated to cover the majority of the LSC subwatershed [Emanuel et al, 2010;Mitchell et al, 2015].…”

Review of ground-based methods to measure the distribution of biomass in forest canopies

“…A type of hemispherical photography with similar characteristics but with an included software that processes the images directly is the digital plant canopy imager (CI-110, CID Bioscience, WA, USA). It is not treated as an extra method here as it is basically identical to hemispherical photography in the manner of generating the data, but doing the analysis in real-time (Bréda 2003;Keane et al 2005).…”