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

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

Understorey developement Maximum biodiversity Regeneration Classical forest flora Relative Biodiversity Invasion by competitives species Bare soil 0 10 20 30 40 50 Transmittance (%)‏ Larch stands, France and Belgium [Balandier et al., Forestry, 2006]

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

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

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

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]

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

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

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

Crown reconstruction using Skinned Surfaces Constructed from different profiles

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

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]

Simulation steps

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

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

Extracting interest zone information Tree opacity = 93.5% Simulation Mean : 48.972 Std : 8.071

Calibration Ongoing work First results for one stand Unsuficient edge Tree opacity = 93.5% Simulation Mean : 48.972 Std : 8.071 Ongoing work First results for one stand Measures Mean : 38.081 Std : 9.455 Unsuficient edge Unique opacity

Stand replication Simulation Tree opacity = 93.5% Mean : 42.68 Std : 12.08 Measures Mean : 38.081 Std : 9.455

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

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

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

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

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