Nadia Rochdi@nrcan.gc Richard Fernandes & Michael Chelle

Does Needle Clumping Affect Shoot Scattering and Canopy BRDF ? Nadia Rochdi@nrcan.gc.ca Richard Fernandes & Michael Chelle

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

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

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

Outlok • 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

Background Structure organization at various scales constrains canopy radiative transfer. Clumping around branch Spatial Distribution Clumping around Twig Shoot scale Stand Scale Tree scale Coniferous Forest Modeling • 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

Background Pinus Bancksiana (Jack Pine) Picea-Mariana (Black Spruce) Pinus-sylvestris (Scot Pine) • 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 [0.09 0.22] • Needle to shoot area ratio ‘ ’: (Chen and Cihlar 1995 ) Coefficient factor within GORT models and Optical LAI measurements (TRAC, hemispherical photographs, LAI2000) Needle clumping parameterization Shoot structure variability : (specie, age, canopy depth) Stand Scale Tree scale • How needle clumping affect shoot scattering ?

Outlook • Context • Background • 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

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

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

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?

Optical properties Twig length A RED B 7.7 cm A NIR B Impact of needle reflectance and transmittance … Twig width Twig reflectance 0.05 0.05 0.3 cm 0.45 0.45 Needle reflectance Needle length 0.05 0.062 2.85 cm 0.45 0.65 Needle transmittance Needle width 0.05 0.028 0.092 cm 0.45 0.25 Needle Number 190 Total Needle area 199.28 cm2 Needle-twig angle 40.5  STAR 0.104 Method: Detailed Shoot (S) • 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 Scot pine shoot • Directional scattering • Reflectance coefficient • Transmittance coefficient • Albedo PARCINOPY (Chelle 1997) Illumination direction s Optical properties Black soil

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

Impact of needle reflectance and transmittance … 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 N-NNeedle effect (NIR) • Shoot scale (illumination s=45) • Shoot albedo is quite insensitive • Reflectance and transmission coefficients are sensitive up +5% & -13%

Impact of needle reflectance and transmittance … 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 N-NNeedle effect (RED) • Shoot scale (illumination s=45) • Shoot albedo is quite insensitive • Reflectance and transmission coefficients are sensitive up 7%

Illumination direction NIR(=) RED(=)  Smolander      RED NIR RED NIR 0 0.49 0.28 0.036 0.021 0.057 0.77 0.044 0.79 45 0.38 0.33 0.026 0.03 0.056 0.71 75 0.42 0.33 0.03 0.024 0.054 0.75 Summary Table4: Reflectance, transmittance and albedo computed for an ‘Accurate’ shoot with illumination directions around (0 45 75 ) considering N=Nand 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)

A simple shoot parameterization Method: Simple Shoot • 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

A simple shoot parameterization SD Shoot Scattering • Directional Radiation scattering NIR SD=0.74; SD=0.46; SD=0.28 RED SD=0.052; SD=0.032; SD=0.020 • Hot spotin 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

A simple shoot parameterization Scattering comparison • Directional scattering intercomparison (NIR) • Detailed shoot ( Red line) • Decahedron shoot (Black line) • Lambertian leaf (Dashed line) s=75, incidence plane s=75, perpendicular plane • Both detailed and SD shoots show quite similar scattering profiles s=0, incidence plane s=0, perpendicular plane • 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

Illumination direction NIR RED      NIR RED 0 0.51 0.25 0.0357 0.0187 0.76 0.054 45 0.447 0.275 0.0306 0.0202 0.72 0.0508 75 0.416 0.317 0.0295 0.023 0.73 0.0525 Average 0.458 0.28 0.032 0.0206 0.738 0.052 Summary Table7: Reflectance, transmittance and albedo computed for ‘Decahedron’ shoot with illumination directions around (0 45 75 ).

Surface opt. properties  RED   NIR  Impact of simple shoot parameterization on BRDF Detailed (S) 0.05 0.05 0.45 0.45 Decahedron (SD) 0.031 0.025 0.43 0.31 Flat (SF) 0.031 0.025 0.43 0.31 Shoot Canopy 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 Shoot canopy • Directional scattering • Reflectance coefficient • Transmittance coefficient • Albedo PARCINOPY (Chelle 1997) Illumination direction (45) Optical properties Black soil

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

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

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

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

Impact of simple shoot parameterization on BRDF (SD) and (S) comparison • Upward and downward hemispherical canopy fluxes • NIR • RED • 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

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

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

Conclusion 1-N-N effect • At shoot scale,no effect in albedobut 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 lambertianflat leaf show significant discrepancies (using ray tracing over a simplified structure is not realistic :Models assessment).

Lab measurements

Nadia Rochdi@nrcan.gc Richard Fernandes & Michael Chelle