Modelling of light transmission under heterogeneous forest canopy

Da SILVA David, BALANDIER Philippe, BOUDON Frédéric,André MARQUIER, Christophe PRADAL,

Christophe GODIN, SINOQUET Hervé

FSPM 07 - Napier

Light influence

Understorey microclimate :Light, temperature,

air humidity, …

Transmited irradiance

Soil water, soil temperature

Soil biological activity, humus

Flora, wildlife, regeneration saplings

FSPM 07 - Napier

Understorey developement

0 10 20 30 40 50

Rel

ativ

e B

iod

iver

sity

Transmittance (%)

Bare soil

Classicalforest flora

Maximum biodiversity

Invasion by competitives

speciesRegeneration

Larch stands, France and Belgium

[Balandier et al., Forestry, 2006]

FSPM 07 - Napier

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

FSPM 07 - Napier

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

FSPM 07 - Napier

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

FSPM 07 - Napier

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]

FSPM 07 - Napier

Light measurements :

• PAR sensors

• Hemispherical photos

Experimental unit

Dendrometric measures :

• X, Y positions

• Total height

• Base crown height

• DBH (diameter at breast height)

• 4 to 8 crown radii Tree height

Tree height

Buffer zone, no light measurement

64 PAR sensor grid

Interest zone

FSPM 07 - Napier

[Chaboche K., 2006]

Full light reference

Light measurementsGrid of PAR Sensor

FSPM 07 - Napier

Crowns opacity

Thresholding and pixel countusing PiafPhotem[UMR Piaf, INRA, Clermont-Ferrand]

Tree opacity = 93,5%

FSPM 07 - Napier

Crown reconstruction using Skinned Surfaces

Constructed from different profiles

FSPM 07 - Napier

Stand reconstruction

Fitting the hulls to measured radii

3D tree reconstruction using height informations

FSPM 07 - Napier

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]

FSPM 07 - Napier

Simulation steps

FSPM 07 - Napier

Simulation steps

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

FSPM 07 - Napier

Combining direct and diffuse

Image are merged using PDIF ratio(diffuse PPFD / diffuse+direct PPFD)

FSPM 07 - Napier

Extracting interest zone information

Tree opacity = 93.5%Simulation

Mean : 48.972Std : 8.071

FSPM 07 - Napier

Calibration

• Unsuficient edge

• Unique opacity

Measures

Mean : 38.081Std : 9.455

Tree opacity = 93.5%Simulation

Mean : 48.972Std : 8.071• Ongoing work

• First results for one stand

FSPM 07 - Napier

Stand replication

Tree opacity = 93.5%Simulation

Mean : 42.68Std : 12.08

Measures

Mean : 38.081Std : 9.455

FSPM 07 - Napier

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

FSPM 07 - Napier

Multi-scale light interception model

Known positions

Uniform distribution

[Sinoquet et al., 2005]

FSPM 07 - Napier

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

FSPM 07 - Napier

Acknowledgment

Virtual Plants team

• 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