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Using the shoot as basic unit in coniferous canopy radiation modeling Pauline Stenberg

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Title: Using the shoot as basic unit in coniferous canopy radiation modeling Pauline Stenberg


1
Using the shoot as basic unit in coniferous
canopy radiation modelingPauline Stenberg
Miina RautiainenUniversity of Helsinki,
Department of Forest Ecology
  • An overwiev of previous work and results from
    three recent papers
  • Smolander Stenberg, 2003. A method to account
    for shoot scale clumping in coniferous canopy
    reflectance models. Remote Sens. Environ.
    88363-373.
  • Smolander Stenberg, 2005. Simple
    parameterizations for the radiation budget of
    uniform broadleaved and coniferous canopies.
    Remote Sens. Environ. 94355-363.
  • Rautiainen Stenberg, 2005. Application of
    photon recollision probability in coniferous
    canopy reflectance simulations. Remote Sens.
    Environ. (accepted).

2
Background
  • In coniferous canopies, needles are densely
    packed (clumped) in shoots with dimensions of
    typically only a few centimeters.
  • Mutual shading of needles in shoots and the small
    scale variation in needle area density (within
    and between the shoots) violate the assumptions
    needed in the definition of the elementary
    volume.
  • Use of the coniferous shoot, or a shoot-like
    leaf, as the basic element (structural unit) is
    proposed as a way to overcome this problem.

3
Visual illustration of the problem
4
What is needed?
  • We need to model and measure shoot structure and
    optical properties of shoots, specifically the
    G-function and the scattering phase function.

5
STAR The G-function of shoots
  • The G-function of coniferous shoots corresponds
    to the ratio of shoot silhouette area to total
    (or hemisurface) needle area.
  • Overlapping of needles in the shoot causes its
    G-value (STAR) to be smaller than that of a
    single leaf or needle.
  • Spherically averaged STAR for Scots pine and
    Norway spruce indicate that the reduction in
    shoot silhouette area (G-value) from needle
    overlapping typically is in the order of 35-50 .

6
Simulated scattering phase function
7
Shoot structural parameter psh
8
Parameterization of canopy radiation budget using
the recollision probability
  • The model canopies

9
Terminology used for the compartments of the
canopy radiation budget
  • Incoming photons

R(l)
albedo
I0
A(l)
absorption
T0
Ts(l)
T(l)
transmittance


10
Canopy absorption and scattering
11
Relationships between leaf (needle) and canopy
albedos
12
Recollision probability p as function of LAI
13
Application of the photon recollision probability
to simulate canopy reflectance
  • A simple parameterization model (PARAS) for
    taking into account the effect of within-shoot
    scattering was tested against empirical data.
  • The model uses a relationship between the canopy
    recollision probability (p) and LAI.
  • The relationship depends on whether the canopy is
    broadleaved or coniferous.

14
Forest data
  • Scots pine dominated study site, 400 plots
  • Stand inventory and LAI-2000 measurements from
    all plots
  • Landsat ETM image from the time of the field
    campaign

15
Results
16
Conclusions
  • 1) Scattering of a vegetation unit (e.g. the
    whole canopy or a single shoot) is well explained
    by the simple formula

where p is the recollision probability between
the elements (scattering centers).
2) A major improvement in simulating canopy
reflectance in NIR is achieved by simply
accounting for the within-shoot scattering. The
low NIR reflectance observed in coniferous areas
is mainly due to within-shoot scattering.
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