Www.helsinki.fi/yliopisto Generating fine resolution leaf area index maps for boreal forests of Finland Janne Heiskanen, Miina Rautiainen, Lauri Korhonen,

www.helsinki.fi/yliopisto

Generating fine resolution leaf area index maps for boreal forests of

Finland

Janne Heiskanen, Miina Rautiainen, Lauri Korhonen,

Matti Mõttus, Pauline Stenberg

IGARSS 2011, 24–29 July 2011, Vancouver, Canada

www.helsinki.fi/yliopisto 2

Leaf area index (LAI)• Key variable in modeling vegetation-atmosphere

interactions, particularly carbon and water cycle

• One half of the total leaf surface area per unit ground surface area

Several global-scale LAI products, but finer spatial resolution (e.g. Landsat and SPOT) is needed to describe the spatial heterogeneity of LAI

Empirical, vegetation index (VI) based methods are typically used in fine resolution mapping, but more physically-based approach could generalize better in space and time, and between sensors

Introduction


Generate fine-resolution forest LAI maps for Finland using satellite image mosaics at 25 m resolution

LAI estimation methods• Empirical model based on reduced simple ratio (RSR)

• Inversion of forest reflectance model (PARAS)

Compare upscaled LAI maps with MODIS LAI (V005)

Objectives


> 1000 field plots measured with LAI-2000 PCA or hemispherical photography (2000–2008)

SPOT HRVIR and Landsat ETM+ images from the same summer (atmospherically corrected)

LAI field measurements


Requires min and max SWIR reflectance factors

Best model fit if values are determined separately for each scene (scene-specific RSR) instead of general values (global RSR)

RSR-Le regression models

min_max_

max_

SWIRSWIR

SWIRSWIR

red

NIRRSR

Le

RSR


PARAS forest reflectance model

L

LLgroundcanopy

p

pifcgfcgfBRF

1202121 )(),()()(

θ1 and θ2: view and Sun zenith anglescgf = canopy gap fractionρground = BRF of the forest background

f= canopy upward scattering phase function i0(θ2 ) = canopy interceptance ωL = leaf albedo

Photon recollision probability (p): the probability by which a photon scattered from a leaf (or needle) in the canopy will interact within the canopy again

Rautiainen & Stenberg 2005, RSE

ground component canopy component

p

p

pp


Can use field measurements of canopy structure and optical properties of foliage and understory

Calculation of p from LAI-2000 PCA data (Stenberg 2007, RSE)

30,000 simulations for training neural networks

• LAI-2000 PCA (cgf, p)

• Leaf (needle) albedo from images

• Mixtures of forest understory spectra (Lang et al. 2001)

• Red, NIR and SWIR

PARAS simulations

Empirical data

BRFred

BR

FN

IR

LAI

DIFNp

1

1

DIFN = ‘diffuse non-interceptance’


Accuracy at an independent validation site

PARAS

RMSE = 0.59 (25.1%)Bias = -0.27 (-11.4%)

r = 0.88

Measured Le

Est

ima

ted

Le

RSR (scene-specific)

Measured Le

Est

ima

ted

Le

RMSE = 0.57 (24.2%)Bias = -0.30 (-12.7%)

r = 0.90

Heiskanen et al. 2011, JAG


Country-wide mosaics (IMAGE2000/2006) produced by Finnish Environmental Institute (SYKE)

• 37 Landsat ETM+ scenes, 1999–2002

• 83 IRS P6 LISS and SPOT-4 HRVIR scenes, 2005 or 2006

Input data for Finnish Corine Land Cover databases (CLC2000/2006)

Images have been atmospherically corrected, but red and SWIR reflectance factors were calibrated using satellite data from the field sites

Satellite image mosaics


RSR

Effective LAI (Le)LAI

estimation methods

Correction for shoot-level clumping

LAI

Validation

Field plots(6 sites)

MODIS LAI Intercomparison

Satellite image mosaics

(2000/2006)

Land cover maps

(2000/2006)

Heiskanen et al. 2011, JAG


Scene-specific RSR

SWIR BRF Forest maskScene-boundaries (2006)

+ +


Data set ρSWIR Range Mean SD

IMAGE2000(n = 37)

Min 0.057–0.118 0.082 0.016

Max 0.208–0.276 0.235 0.012

IMAGE2006(n = 83)

Min 0.063–0.133 0.089 0.019

Max 0.193–0.285 0.221 0.015

Global values based on sample plotsρSWIR_min = 0.063ρSWIR_max = 0.244

Scene-specific RSR:ρSWIR_min, ρSWIR_max


Accuracy at modelling sites

RSR (scene-specific) RSR (global) PARAS

Measured Le

Est

ima

ted

Le

Measurement site Date RSR (scene-specific) RSR (global) PARAS

LAI Image R2 RMSE R2 RMSE R2 RMSE

Puumala (n = 395) 6/2000 2.8.1999 0.64 0.52 0.64 0.54 0.69 0.56

Saarinen (n = 370) 7/2001 27.6.2001 0.61 0.65 0.61 0.72 0.66 0.84

Hirsikangas (n = 24) 5–6/2005 6.8.2006 0.42 0.55 0.42 0.55 0.48 0.57

Rovaniemi (n = 20) 6/2005 2.7.2006 0.70 0.54 0.75 0.29 0.79 0.46

Tähtelä (n = 261) 6/2006 7.6.2006 0.45 0.55 0.45 0.35 0.38 0.38

Hyytiälä (n = 73) 6–7/2008 17.7.2006 0.58 0.73 0.57 0.79 0.63 0.88


LAI maps (global RSR)

2000 2006


LAI≤ 1.0

1.1–2.0

2.1–3.0

3.1–4.0

4.1–5.0

5.1–6.0

> 6.0

LAI 2006 MODIS LAI(IMAGE2006 dates)

MODIS LAI(July average 2002–2010)

White = non-forest (< 50% forest), Black = clouds Good quality (main algorithm with or without saturation)

LAI 2006 and MODIS LAI (V005)


Comparison with MODIS LAI

MODIS LAI includes also understory LAI

Scene-wise averages


Empirical and forest reflectance model based methods for estimating LAI

• Empirical model based on RSR (global) was selected for generating LAI maps for Finnish forests

• Realistic LAI patterns but the highest values are underestimated

Reflectance data and land cover maps• Systematic difference in red and SWIR bands

• Phenological differences between the images

• Clumping correction

Further validation of MODIS LAI (V005)

Conclusions

18www.helsinki.fi/yliopisto

Thank you!

http://www.mm.helsinki.fi/~mxrautia/lai/index.htm

Heiskanen, J, M Rautiainen, L Korhonen, M Mõttus & P Stenberg (2011). Retrieval of boreal forest LAI using a forest reflectance model and empirical regressions. International Journal of Applied Earth Observation and Geoinformation 13: 595–606. doi:10.1016/j.jag.2011.03.005

Www.helsinki.fi/yliopisto Generating fine resolution leaf area index maps for boreal forests of Finland Janne Heiskanen, Miina Rautiainen, Lauri Korhonen,

Documents

canopy reflectance

canopy interceptance

canopy components

resolution forest lai

canopy structural parameter

canopy gap fractionground

total leaf surface area

modis lai v0053objectiveswww