1. Modelling of light transmission under heterogeneous forest canopy
Da SILVA David, BALANDIER Philippe, BOUDON Frédéric,
André MARQUIER, Christophe PRADAL,
Christophe GODIN, SINOQUET Hervé

2. Light influence Transmited irradiance Understorey microclimate :
Light, temperature,
air humidity, …
Flora, wildlife, regeneration saplings
Soil water, soil temperature
Soil biological activity, humus

3. Understorey developement
Maximum
biodiversity
Regeneration
Classical
forest flora
Relative Biodiversity
Invasion by
competitives
species
Bare soil
Transmittance (%)‏
Larch stands, France and Belgium
[Balandier et al., Forestry, 2006]

4. Objectives
Determine a minimal set of simple measurements to obtain satisfactory light transmission classes
Basal area
Height
Closest neighbour distance
General model
Coarse or detailed parametrization
Mono or multi-scale
Output : light transmission classes

5. Method Model developement
Starting from detailed description of stand at tree scale
Sensitivity analysis
spatial distribution, clustering
size of tree crown
shape and orientation of tree crown
Simplify the measures requirement

6. Light interception model
Porous envelope based
Fraction of light intercepted is related to the crown projection area and its envelope projection area in the direction =
x 0.93

7. Light interception model
Crown discretization using regularly spaced beams
Beam opacity evaluation according to components distribution hypothesis
Known positions
Uniform distribution
[Sinoquet et al., 2005]

8. Experimental unit Light measurements : PAR sensors
Tree height
Buffer zone, no light measurement
64 PAR
sensor grid
Interest zone
Light measurements :
PAR sensors
Hemispherical photos
Dendrometric measures :
X, Y positions
Total height
Base crown height
DBH (diameter at breast height)‏
4 to 8 crown radii

9. Light measurements Grid of PAR Sensor Full light reference
[Chaboche K., 2006]
Full light reference

10. Crowns opacity Thresholding and pixel count using PiafPhotem
[UMR Piaf, INRA, Clermont-Ferrand]
Tree opacity = 93,5%

11. Crown reconstruction using Skinned Surfaces
Constructed from different profiles

12. 3D tree reconstruction using
Stand reconstruction
Fitting the hulls
to measured radii
3D tree reconstruction using
height informations

13. Computing direct & diffuse incident light
Direct light depending on latitude, longitude, julian day, time step, start and stop hour
Diffuse light simulated by 46 directions of Turtle Sky [Den Dulk 89]

14. Simulation steps

15. Simulation steps Weighted merging according to elevation
[Campbell G.S. and Norman J.M., 1998]

16. Combining direct and diffuse
Image are merged using PDIF ratio
(diffuse PPFD / diffuse+direct PPFD)‏

17. Extracting interest zone information
Tree opacity = 93.5%
Simulation
Mean :
Std : 8.071

18. Calibration Ongoing work First results for one stand Unsuficient edge
Tree opacity = 93.5%
Simulation
Mean :
Std : 8.071
Ongoing work
First results for one stand
Measures
Mean :
Std : 9.455
Unsuficient edge
Unique opacity

19. Stand replication Simulation Tree opacity = 93.5% Mean : 42.68
Std : 12.08
Measures
Mean :
Std : 9.455

20. Calibration options
Duplicates positioning and/or introducing randomness
Use of porous wall as calibrating device
LAD determination to take beam travel distance in crown into account
Usage of the multi-scale possibilities of the model

21. Multi-scale light interception model
Known positions
Uniform distribution
[Sinoquet et al., 2005]

22. Perspectives (after calibration…)
Stand type caracterization [Marie-Ange NgoBieng, 2007]
Simulation of n stands
Variability caracterization
Sensitivity analysis
Spatial distribution
Crown shape
Intra and inter-tree clustering
Determination of descriptors of main interest

23. Acknowledgment Virtual Plants team UMR Piaf ANR Project ECOGER
CHAUBERT Florence
CHOPARD Jérôme
STOMA Szymon
UMR Piaf
DONES Nicolas
ANR Project ECOGER
“Bases of mixed stands sustainable management of mixed stands:
ecophysiology, growth and demogenetics of constitutive species.”
  Collaboration between INRA, the French National forestry commission and CEMAGREF

24. Existing models Simple : [Monsi & Saeki,1953 ; Ross, 1981]
Output = mean an std
Layers : [Kimes & Smith, 1980 ; Hanan 2001 ]
Not adapted for heterogeneous stands
3D cells : [Kimes & Kirchner, 1982 ; Sinoquet et al., 2005]
Difficulty to characterize canopy cells in the field
Tree crown : [Cescatti, 1997 ; Brunner, 1998 ; Canham et al., 1999]
Different level of shape complexity
High and fixed requirement