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| Titel |
Comparison of vertical resolved leaf area index measurements in an open canopy savannah-type forest |
| VerfasserIn |
Arndt Piayda, Matthias Cuntz, Maren Dubbert, Christiane Werner, João S. Pereira |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250076633
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| Zusammenfassung |
| Leaf area index (LAI) is a very important vegetation parameter in soil-vegetation-atmosphere
exchange modeling. To represent the structure of ecosystems in vertically distributed
modeling, vertical resolved LAI distributions as well as vertically and angular gap
fraction (Pgap) distributions are needed, but rarely available. Additionally, former
studies neglect woody plant components when using light interception or digital
photography based methods for LAI or Pgap observations. This can lead to significantly
biased results, particularly in semi-arid savannah-type ecosystems with low LAI
values.
The objective of this study is to compare three non-destructive LAI measurement
techniques in a sparse savannah-type cork oak canopy in central Portugal in order to derive
vertically resolved LAI as well as vertically and angular resolved Pgap.
Since established canopy analyzers, such as the LAI-2000, rely on diffuse light
conditions, which are rarely realized in semi-arid regions, we also employed fast, digital
cover photography (DCP) working independently from diffuse light conditions. We used
vertical and angular distributed DCP and applied object-based image analysis techniques to
exclude woody plant components from Pgap estimation and LAI determination. We compared
the results with vertically distributed LAI-2000 measurements, and additionally with vertical
estimates based on easily measurable forest canopy parameters. We employed bootstrap
resampling methods to determine the accuracy of all measurements depending on sample
size.
Leaf inclination measurements indicate planophile leaf orientation. Thus LAI was
calculated with Pgap and the leaf inclination information. This led to a spatial averaged LAI
of 0.52 +- 0.06 for DCP while LAI-2000 measurements resulted in 0.67 +- 0.07. Uncertainty
bounds of LAI converge much faster with increasing sample size for the DCP than
for the LAI-2000. This allows a more efficient sampling design, which is of great
importance in heterogeneous canopies. Both methods show comparable vertical LAI and
Pgap distributions. Furthermore, the vertical distribution of LAI derived from the
simple, canopy parameter based model matches very well with the directly measured
distribution. However, Pgap of digital cover photographs shows a stronger dependence on
zenith angle compared to LAI-2000. The latter follows more theoretical expectations
because it uses a smaller viewing angle. Future analyses of DCP need, therefore, to
determine optimal aperture for simultaneous estimate of LAI and angular-dependent
Pgap.
In summary, we developed a fast method for measuring vertical LAI and vertical and
angular dependent Pgap profiles within forest canopies using digital cover photography for
the purpose of vertical resolved photosynthesis or radiative transfer modeling, in particular
suitable for deployment in sparse, savannah-type ecosystems. A simple stand parameter
based model is able to approximate the vertical profile of LAI if vertically distributed
measurements are not feasible. However, the vertical profile of Pgap measured with digital
cover photography needs further investigation. |
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