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| Titel |
Modelling spatio-temporal variations in leaf chlorophyll content for broadleaf and needle forest canopies |
| VerfasserIn |
H. Croft, J. M. Chen, Y. Zhang, A. Simic |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250070108
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| Zusammenfassung |
| Foliar chlorophyll content in forested ecosystems plays a fundamental role in plant
photosynthesis, determines plant productivity and can indicate vegetation stress and
disturbance. Obtaining accurate measurements of leaf chlorophyll content across a range of
spatial and temporal scales is crucial for monitoring vegetation productivity and providing
inputs to photosynthesis and carbon cycle models. However, leaf chlorophyll retrieval is
complicated as canopy reflectance in the visible and near-infrared wavelengths is affected not
only by leaf pigment concentration but also by leaf area index (LAI), canopy architecture,
illumination and viewing geometry and understory vegetation. Consequently, empirical
indices, often developed at leaf-level, are species, site and time specific. In order to
investigate the potential of monitoring chlorophyll dynamics over a growing season at the
canopy scale, a process modeling approach is needed to account for the variation of
other variables affecting canopy reflectance. Canopy radiative transfer models use
physical laws to describe the interaction of solar radiation inside the canopy between
scattering elements, which could provide a more accurate estimate of chlorophyll
content over multiple vegetation species, time-frames and across broader spatial
extents.
This study used a coupled canopy (4Scale) and leaf (PROSPECT) model approach to
investigate the ability of radiative transfer models to estimate foliar chemistry for multiple
vegetation types and species (broadleaf and needle) from optical remote sensing data. Canopy
reflectance data was acquired from the Medium Resolution Imaging Spectrometer
(MERIS), from 390–1040 nm in 15 wavebands at a spatial resolution of 1200 m, and
inverted using a look up table (LUT) approach. Twenty sites were selected in Ontario,
Canada representing different dominant vegetation species (Picea mariana, Pinus
banksiana and Acer saccharum), and a variety of canopy closures and structures.
These sites were sampled over multiple time-frames, where a number of ground
measurements were conducted. Ground data include LAI, measured using Tracing
Radiation and Architecture of Canopies (TRAC), leaf reflectance spectra from ASD
Spectroradiometer (400-2500 nm) and leaf samples were taken for laboratory chlorophyll
determination. Leaf chlorophyll content ranged from 19-62 μg/cm2 and LAI from 1.15 to
8.4, providing a large dynamic range to test the approach. The model results show
that MERIS derived chlorophyll content demonstrated a good relationship with
measured leaf chlorophyll content, particularly from denser canopies at higher LAI
values.
This research provides an empirical and theoretical basis for the future retrieval of
spatially distributed assessments of leaf chlorophyll content over different spatial and
temporal scales. The ability of this technique to characterise variations in chlorophyll content
across different vegetation species and canopy structures is important for making the method
operational across coarser spatial extents, and for its inclusion in photosynthesis and carbon
cycle models. |
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