An improved estimate of leaf area index based on the histogram analysis of hemispherical photographs

Juárez, R. I. Negrón; Rocha, Humberto Ribeiro da; Figueira, A.M.S.; Goulden, Michael L.; Miller, S. D.

doi:10.1016/j.agrformet.2008.11.012

Cited by 40 publications

(16 citation statements)

References 25 publications

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“…The suggestion that MODIS LAI changes are not representative of actual changes in leaf area is without basis because the MODIS LAI algorithm explicitly accounts for changes in SZA [ Knyazikhin et al , 1998] so as to preclude spurious LAI changes. Moreover, a large body of literature presents evidence of dry season leaf area increase [ Asner et al , 2004; Carswell et al , 2002; Pinto‐Junior et al , 2010; Malhado et al , 2009; Negrón Juárez et al , 2009; Doughty and Goulden , 2008]. In addition, Galvão et al [2011] did not examine the influence of leaf flush—leaf spectral changes—on NIR reflectance changes, which is presented here.…”

Section: Discussionmentioning

confidence: 98%

“…It has also been argued by Myneni et al [2007] that a gradually increasing leaf area enhances the evapotranspiratory water vapor flux into the atmosphere during the dry season, which would facilitate convection and increase the probability of rainfall during the late dry season, factors that influence a transition to the wet season [ Li and Fu , 2004; Fu and Li , 2004]. Field‐based studies support these ideas through reports of enhanced leaf area: 5.5 to 6.5 [ Carswell et al , 2002], 3.32 to 4.25 [ Pinto‐Junior et al , 2010], 8% increase [ Malhado et al , 2009], a small increase [ Negrón Juárez et al , 2009] but consistent with work of Myneni et al [2007], and a moderate increase in LAI [ Doughty and Goulden , 2008]. Besides, higher (on average 30%) evapotranspiration fluxes have been observed during the dry season [ Juárez et al , 2007, 2008].…”

Section: Introductionmentioning

confidence: 99%

“…This brings us to the heart of the debate: are the observed seasonal changes in NIR reflectance of Amazon forests (Figure 1) due to changes in leaf area, or to changes in leaf optical properties, or both? The proposition that changes in forest canopy reflectances are due alone to changes in leaf optical properties, as argued by Doughty and Goulden [2008] and Asner and Alencar [2010] and recently discussed by Brando et al [2010], does not acknowledge ground‐based measurements of seasonal leaf area changes [ Asner et al , 2004; Carswell et al , 2002; Pinto‐Junior et al , 2010; Malhado et al , 2009; Negrón Juárez et al , 2009; Doughty and Goulden , 2008] and litterfall [ Xiao et al , 2005; Chave et al , 2010], and emerging evidence regarding the role Amazon forests play in the transition from dry to wet season [ Li and Fu , 2004; Fu and Li , 2004; Juárez et al , 2007, 2008], all of which support the interpretation proposed by Myneni et al [2007]. On the other hand, the argument that only leaf area changes explain changes of forest canopy reflectance ignores the very obvious changes in leaf optical properties between younger and older leaves and between healthy and epiphyll infested leaves.…”

Section: Introductionmentioning

confidence: 99%

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Seasonal changes in leaf area of Amazon forests from leaf flushing and abscission

Samanta¹,

Knyazikhin²,

Xu³

et al. 2012

J. Geophys. Res.

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.[1] A large increase in near-infrared (NIR) reflectance of Amazon forests during the light-rich dry season and a corresponding decrease during the light-poor wet season has been observed in satellite measurements. This increase has been variously interpreted as seasonal change in leaf area resulting from net leaf flushing in the dry season or net leaf abscission in the wet season, enhanced photosynthetic activity during the dry season from flushing new leaves and as change in leaf scattering and absorption properties between younger and older leaves covered with epiphylls. Reconciling these divergent views using theory and observations is the goal of this article. The observed changes in NIR reflectance of Amazon forests could be due to similar, but small, changes in NIR leaf albedo (reflectance plus transmittance) resulting from the exchange of older leaves for newer ones, but with the total leaf area unchanged. However, this argument ignores accumulating evidence from ground-based reports of higher leaf area in the dry season than the wet season, seasonal changes in litterfall and does not satisfactorily explain why NIR reflectance of these forests decreases in the wet season. More plausibly, the increase in NIR reflectance during the dry season and the decrease during the wet season would result from changes in both leaf area and leaf optical properties. Such change would be consistent with known phenological behavior of tropical forests, ground-based reports of seasonal changes in leaf area, litterfall, leaf optical properties and fluxes of evapotranspiration, and thus, would reconcile the various seemingly divergent views.

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Section: Discussionmentioning

confidence: 98%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Seasonal changes in leaf area of Amazon forests from leaf flushing and abscission

Samanta¹,

Knyazikhin²,

Xu³

et al. 2012

J. Geophys. Res.

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“…Amazonia covers about 5.3 million km 2 , and has many environmental and functional gradients [89,90]. For instance, western Amazonia maintains higher rainfall and wood productivity compared with the long dry-season and less productive eastern Amazonia [89,91]. The environmental variability across the basin made it necessary to divide it into regions (e.g., [10,92,93]).…”

Section: Discussionmentioning

confidence: 99%

Windthrow Variability in Central Amazonia

et al. 2017

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Windthrows are a recurrent disturbance in Amazonia and are an important driver of forest dynamics and carbon storage. In this study, we present for the first time the seasonal and interannual variability of windthrows, focusing on Central Amazonia, and discuss the potential meteorological factors associated with this variability. Landsat images over the 1998-2010 time period were used to detect the occurrence of windthrows, which were identified based on their spectral characteristics and shape. Here, we found that windthrows occurred every year but were more frequent between September and February. Organized convective activity associated with multicell storms embedded in mesoscale convective systems, such as northerly squall lines (that move from northeast to southwest) and southerly squall lines (that move from southwest to northeast) can cause windthrows. We also found that southerly squall lines occurred more frequently than their previously reported~50 year interval. At the interannual scale, we did not find an association between El Niño-Southern Oscillation (ENSO) and windthrows.

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“…Programs now exist for estimating total cover and LAI from hemispherical or aerial images (e.g. Negrón Juárez et al 2009, in tropical forest and pasture in Brazil). Assumptions must be made on leaf distribution (clumping) and leaf angle.…”

Section: Measurement Of Covermentioning

confidence: 99%

Cover plus: ways of measuring plant canopies and the terms used for them

Wilson

2010

Journal of Vegetation Science

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Aim: To clarify the concepts and terms that have been used in recording plant canopies and the methods used for measuring them. Concepts: The concepts considered are cover, top cover, repeated cover, number of layers, proportion of repeated cover and leaf area index (LAI). Variants include canopy cover, basal area and leaf area density. Other terms have been used in the past, some are now ambiguous and some are counter‐intuitive. Measurement: Cover and its related concepts can be measured by point intercept, line intercept, angle sampling, charting and image analysis, canopy transmittance, complete enumeration, clipping and allometry. The common procedure of adding the cover of individual species to calculate a “total cover” is invalid: a nonsense number is produced. Other measures of abundance: Cover‐abundance, density, frequency, biomass and calorific values can also be used to measure abundance. They are related to cover. Especially, shoot frequency at a point is cover (the point intercept method). Subjective estimates: There are severe problems with cover‐abundance scales, such as that of Braun‐Blanquet. Estimation of abundance “by eye”, by “informed guess”, etc. has too often compromised results in vegetation science.

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An improved estimate of leaf area index based on the histogram analysis of hemispherical photographs

Cited by 40 publications

References 25 publications

Seasonal changes in leaf area of Amazon forests from leaf flushing and abscission

Seasonal changes in leaf area of Amazon forests from leaf flushing and abscission

Windthrow Variability in Central Amazonia

Cover plus: ways of measuring plant canopies and the terms used for them

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