Nadia Richard Fernandes & Michael Chelle Does Needle Clumping Affect Shoot Scattering and Canopy BRDF ?

Outlook Context Background Smolander Approach Objectives Impact of needle reflectance and transmittance on shoot scattering and canopy BRDF A simple shoot parameterization Impact of simple shoot parameterization on canopy BRDF Conclusion

Outlook Context Context Background Smolander Approach Objectives Impact of needle reflectance and transmittance on shoot scattering and canopy BRDF A simple shoot parameterization Impact of simple shoot parameterization on canopy BRDF Conclusion

Conte o xt  Monitoring the seasonal development and carbon uptake by vegetation in relation with global climatic change.  Northern vegetation (Dominant forests) represent significant carbon sink. VEGETATION MISR Spectral & Directional Information Reflectance or Radiative Transfer Models Empirical approaches (Vegetation Indices) Biophysical variables Leaf Area Index fcover fAPAR albédo Functioning Model PROCESSES Photosynthesis Growth Energy & mass exchanges  Understand & model processes at various scales

LAI Evaluation Reference LAI x10 VGT LAI x10 Watson Lake, YukonKejimikujik, Nova Scoatia / 1.4

Outlok Context Background Background Smolander Approach Objectives Impact of needle reflectance and transmittance on shoot scattering and canopy BRDF A simple shoot parameterization Impact of simple shoot parameterization on canopy BRDF Conclusion

Coniferous Forest Modeling Background Tree scale Shoot scale Structure organization at various scales constrains canopy radiative transfer. Clumping around branch Clumping around Twig Spatial Distribution Stand Scale  Empirical approaches : Implicit taking into account within field measurements  Reflectance & Radiative transfer models:  Turbid media models all structure captured in single element  Geometrical Optical (GO): opaque crowns are described by geometric shapes (cone, spheroid, cylinder …)  Geometric-Optical & Radiative Transfer GORT: Radiative transfer within crowns are considered  Ray Tracing/ Radiosity: explicit placement of elements – how much complexity? -  Shoots are typically assumed the basic element

Needle clumping parameterization Background Tree scale Shoot structure variability : (specie, age, canopy depth) Stand Scale Pinus Bancksiana (Jack Pine)Picea-Mariana (Black Spruce ) Pinus-sylvestris (Scot Pine)  How needle clumping affect shoot scattering ?  Shoot silhouette to total needle area ratio of the Shoot ‘STAR’: (Oker-Blom 1985) Projected Shoot area divided by total needle area Average over spherical shoot orientation : Variability [ ]  Needle to shoot area ratio ‘  ’: (Chen and Cihlar 1995 ) Coefficient factor within GORT models and Optical LAI measurements (TRAC, hemispherical photographs, LAI2000)

Outlook Context Background Smolander’s Approach Smolander’s Approach Objectives Impact of needle reflectance and transmittance on shoot scattering and canopy BRDF A simple shoot parameterization Impact of simple shoot parameterization on canopy BRDF Conclusion

Shoot scattering parameterization Smolander’s Approach Tree scale Impact of STAR variable on Shoot scattering albedo  sh : (Smolander & Stenberg 2003)  N : Needle albedo p sh : probabilty of more than 1 interaction within the shoot  Validation over Scot pine using “constrained” ray tracing simulations Shoot scale - Shoot phase function shows a hot spot in illumination direction - Shoot phase function averaged over all directions is bi-lambertian Canopy scale: - Spherical oriented shoots may be as bi-lambertian flat leaves with G(  )=2STAR. - Good agreement in nadir view

5SCALE simulation on BOREAS Old Black Spruce Is it sufficient to reproduce BRDF ? STAR effect at canopy scale Smolander’s Approach  Is such shoot parameterization sufficient to reproduce Canopy BRDF ?

Outlook Context Background Smolander Approach Objectives Objectives Impact of needle reflectance and transmittance on shoot scattering and canopy BRDF A simple shoot parameterization Impact of simple shoot parameterization on canopy BRDF Conclusion

Objectives 1- What is the impact of Needle reflectance and transmittance on shoot scattering and canopy BRDF? 2- Can we design a simple shoot with same effective optical properties as a detailed shoot ? 3- What is the impact of using simplified shoot on canopy scale BRDF?

Outlook Context Background Smolander Approach Objectives Impact of needle reflectance and transmittance Impact of needle reflectance and transmittance on shoot scattering and canopy BRDF A simple shoot parameterization Impact of simple shoot parameterization on canopy BRDF Conclusion

Method: Detailed Shoot (S) Impact of needle reflectance and transmittance … Twig length7.7 cm Twig width0.3 cm Needle length2.85 cm Needle width0.092 cm Needle Number190 Total Needle area cm2 Needle-twig angle 40.5  STAR0.104  Shoot structure: Scot pine Main Assumptions Needle as rectangular box defined by its width and length Twig as decahedron with given width and length Same needle number is fixed on each decahedron face Needle-Twig angle is constant  Shoot scattering: Forward ray tracing simulation Optical properties A RED BA NIR B Twig reflectance Needle reflectance Needle transmittance PARCINOPY (Chelle 1997) Scot pine shoot Optical properties Directional scattering Reflectance coefficient Transmittance coefficient Albedo Illumination direction  s Black soil

Shoot Scattering Impact of needle reflectance and transmittance …  Directional Radiation scattering (  N =  N ) NIR  SH =0.74 RED  SH =0.056 Hot spot in principal plane around illumination direction Shoot albedo is quite similar whatever the illumination direction while reflectance and transmittance coefficients show some differences

 N -  N Needle effect (NIR) Impact of needle reflectance and transmittance … Shoot albedo is quite insensitive Reflectance and transmission coefficients are sensitive up +5% & -13% LAI=1.28 LAI=2.56  Canopy scale Random shoot distribution with spherical inclination Increase of backscattering and decrease of forward scattering in the principal plane No effect around nadir view direction  Shoot scale (illumination  s =45  )

 N -  N Needle effect (RED) Impact of needle reflectance and transmittance …  Canopy scale LAI=1.28 Random shoot distribution with spherical inclination Increase of backscattering and decrease of forward scattering in the principal plane No effect around nadir view direction LAI=2.56 Shoot albedo is quite insensitive Reflectance and transmission coefficients are sensitive up  7%  Shoot scale (illumination  s =45  )

Summary Illumination direction NIR(  =  )RED(  =  )  Smolander   REDNIRREDNIR Table4: Reflectance, transmittance and albedo computed for an ‘Accurate’ shoot with illumination directions around (0  45  75  ) considering  N =  N and Smolander’s albedo. Shoot albedo is quite similar whatever the illumination direction while reflectance and transmittance coefficients show some differences Smolander parameterization agrees with ours in the NIR but not in RED (differences in shoot structure design)

Outlook Context Background Smolander Approach Objectives Impact of needle reflectance and transmittance on shoot scattering and canopy BRDF A simple shoot parameterization A simple shoot parameterization Impact of simple shoot parameterization on canopy BRDF Conclusion

Method: Simple Shoot A simple shoot parameterization  Decahedron shoot (SD) Projected shoot silhouette area (As) conserved by reducing width (2.88cm) Convex ‘non self shadowing’ element with surface area A=4*As Decahedron sides are lambertian Optical properties of each side are equal to detailed shoot one averaged on illumination directions (with  N =  N )  Flat leaf (SF) Lambertian Leaf area equals to 4*As. Optical properties equal to detailed shoot one averaged on illumination directions Optical propertiesREDNIR Reflectance Transmittance Illumination angles0; 45; 90

SD Shoot Scattering A simple shoot parameterization  Directional Radiation scattering NIR  SD =0.74;  SD =0.46;  SD =0.28 RED  SD =0.052;  SD =0.032;  SD =0.020 Hot spot in principal plane around illumination direction Albedo is quite close from detailed shoot albedo Some differences in reflectance and transmittance coefficients Backscattering enhancement occurs over a large view directions

Scattering comparison A simple shoot parameterization  Directional scattering intercomparison (NIR) Detailed shoot ( Red line) Decahedron shoot (Black line) Lambertian leaf (Dashed line)  s=75 , incidence plane  s=0 , incidence plane  s=75 , perpendicular plane  s=0 , perpendicular plane Both detailed and SD shoots show quite similar scattering profiles Existence of less shadowing for decahedron shoot induces slow decrease far away from illumination direction less anisotropy in perpendicular plane due to the uniformity of shadow distribution

Summary Illumination direction NIRRED   NIRRED Average Table7: Reflectance, transmittance and albedo computed for ‘Decahedron’ shoot with illumination directions around (0  45  75  ).

Outlook Context Background Smolander Approach Objectives Impact of needle reflectance and transmittance on shoot scattering and canopy BRDF A simple shoot parameterization Impact of simple shoot parameterization on Impact of simple shoot parameterization on canopy BRDF Conclusion

Shoot Canopy BRDF Impact of simple shoot parameterization on BRDF  Canopy Characteristics 3 types: detailed shoot (S), Decahedron shoot(SD), Flat shoot (SF) Scene dimensions: 0.5mx0.5mx1m deep (infinite boundary) LAI=[0.64, 1.28, 2.56, 5.12, 10.24]; Random distribution Spherical inclination distribution (SD) canopy have the same positions and inclination than (S) canopy (SF) shoot is described by tree surfaces randomly distributed (3D structure )  Radiative transfert simulation Surface opt. properties  RED  NIR  Detailed (S) Decahedron (SD) Flat (SF) PARCINOPY (Chelle 1997) Shoot canopy Optical properties Directional scattering Reflectance coefficient Transmittance coefficient Albedo Illumination direction (45  ) Black soil

( SD) and (S) BRDF comparison (NIR) Impact of simple shoot parameterization on BRDF Detailed shoot ( symbol) Decahedron shoot (line)

scattering contributions (NIR) Impact of simple shoot parameterization on BRDF Multiple ( Bleu) Single (Red) Detailed ( symbol) Decahedron (line) Multiple ( Bleu) Single (Red)

(SD) and (S) BRDF comparison (RED) Impact of simple shoot parameterization on BRDF Detailed shoot ( symbol) Decahedron shoot (line)

Scattering contribution (RED) Impact of simple shoot parameterization on BRDF Detailed ( symbol) Decahedron (line) Multiple ( Bleu) Single (Red)

(SD) and (S) comparison Impact of simple shoot parameterization on BRDF RED NIR Multiple scattering regime is quite different Compensation between multiple and single scattering in (SD) canopy induces canopy reflectance and transmittance similar to (S) canopy in NIR Discrepancies remains in RED due to the absence of this compensation  Upward and downward hemispherical canopy fluxes

(SF) and (S) BRDF comparison Impact of simple shoot parameterization on BRDF Detailed shoot ( symbol) Decahedron shoot (line) NIR RED NIR RED

Scattering contribution (RED) Impact of simple shoot parameterization on BRDF Multiple ( Bleu) Single (Red) Detailed shoot ( symbol) Decahedron shoot (line)

Outlook Context Background Smolander Approach Objectives Impact of needle reflectance and transmittance on shoot scattering and canopy BRDF A simple shoot parameterization Impact of simple shoot parameterization on canopy BRDF Conclusion Conclusion

Conclusion 1-  N -  N effect At shoot scale, no effect in albedo but some differences in directional scattering and reflectance and transmittance coefficients At canopy scale,  N -  N has significant effect around hot spot direction (Implication on LAI and clumping index estimation). 2- Can we design a simple shoot? Convex volume albedo is quite close with some differences in transmittance and reflectance coefficients Convex volume describes directional scattering better than flat leaf 3- Impact of a simple shoot on canopy BRDF? Convex volume: - is inconvenient in RED due to a change in canopy clumping. - doesn’t well describe the canopy radiative regime. Simplification of shoot structure as lambertian flat leaf show significant discrepancies (using ray tracing over a simplified structure is not realistic :Models assessment).

Lab measurements