Title: Using the shoot as basic unit in coniferous canopy radiation modeling Pauline Stenberg
1Using 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).
2Background
-
- 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.
3Visual illustration of the problem
4What is needed?
- We need to model and measure shoot structure and
optical properties of shoots, specifically the
G-function and the scattering phase function.
5STAR 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 .
6Simulated scattering phase function
7Shoot structural parameter psh
8Parameterization of canopy radiation budget using
the recollision probability
9Terminology used for the compartments of the
canopy radiation budget
R(l)
albedo
I0
A(l)
absorption
T0
Ts(l)
T(l)
transmittance
10Canopy absorption and scattering
11Relationships between leaf (needle) and canopy
albedos
12Recollision probability p as function of LAI
13Application 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.
14Forest 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
15Results
16Conclusions
- 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.