Scaling Biomass Measurements for Examining MODIS Derived Vegetation Products

Matthew C. Reeves and Maosheng Zhao

Numerical Terradynamic Simulation GroupUniversity of Montana

Missoula, MT

Objectives…

Part 1: Discuss conversion of plot level biomass measurements to regional scales

Part 2: Characterize effectiveness of MODIS products for monitoring grassland vegetation

Part 1

Scaling Biomass Measurements

Focuses on Little Missouri National Grasslands

Climate: continental/semi-arid

Vegetation: mixed grass prairie common in the northern Great Plains

C3 : C4 ~ 70:30

Sampling was constrained to Federal Lands comprised of rolling prairie

Study Area

Little Missouri National Grasslands

Focuses on Little Missouri National Grasslands

Climate: continental/semi-arid

Vegetation: mixed grass prairie common in the northern Great Plains

C3 : C4 ~ 70:30

Sampling constrained to Federal grasslands

Study Area

Woody Stringer

Methods…Collecting and Processing Field Measurements

Data were collected at 2,200 plots (473 transects) during 2001 growing season Sampling Periods

May 26 – 30 June 13 – 17 July 13 – 17 August 9 – 13

Measurements included: Beginning and ending GPS locations Clipping herbaceous biomass within 0.5m2 quadrat Species composition Bare ground estimate Percentage of living vegetation estimated for each plot

All biomass dried at 65°C for 48 hours

ETM+ (30 m spatial resolution) approximately corresponding to each period of sampling was acquired (all image data clipped to grasslands)

All four periods of ETM + imagery converted to NDVI(NIR – Red)/(NIR + Red)

Spatial relations between NDVI and observed biomass explored included: 3*3, 5*5, 7*7 zonal mean Average NDVI by allotment Pt. In cell extraction 90, 150, 500 meter buffers around transects

Average NDVI by zone of met. Influence

Methods…scaling from plot frame to pixel

ETM+ Imagery

Methods…scaling from plot frame to pixelMeteorological Data

Weather station data chosen within and adjacent to the LMNG

Met data were screened

Thiessen polygons were created around retained met. stations

Information derived from meteorlogical data included: Summation of precipitation across varying time frames Water balance (Ppt. - Pet)

Growing Degrees Days (Tavg – Tbase)

NCDC Weather Station Distribution Surrounding the Little Missouri National Grasslands

Montana North Dakota

Spatial Arrangement of Thiesson Polygons

Montana North Dakota

Montana North Dakota

Vegetation Mosaic

~ 182,000 ha

Biomass modeled as a function of ETM NDVIPrecipitation Growing degree days

NDVI = ETM+ Normalized difference vegetation index

PPTsum = Summation of Precipitation from day 0 – time of sampling midpoint

GDD = Summation of growing-degree day

GDDopt =number of growing degree days required for peak of greenness

Methods…scaling from plot frame to pixelBuilding the scaling model

Methods…scaling from plot frame to pixelValidating the scaling model

Biomass Prediction Cross Validation

R2 = 0.7839

0

10

20

30

40

50

60

0 10 20 30 40 50 60

Observed

Pre

dic

ted

MAE = 4.22

Bias = -0.08

Results…Regional Biomass prediction

Mean Biomass in the Little Missouri National Grasslands

550

650

750

850

950

1050

1150

148 167 197 223

Sampling period midpoint

Ab

ov

e g

rou

nd

ph

oto

sy

nth

es

izin

g

(bio

ma

ss

(k

g h

a-1

)

Influence of C4 species

Results…Zonal Biomass prediction

Modeled Biomass for the 2001 Growing Season in the Little Missouri National Grasslands

400

500

600

700

800

900

1000

1100

1200

1300

1 2 3 4 5 6 7 8 9 10 11

Water Balance Zone

Abo

ve g

roun

d ph

otos

ynth

esiz

ing

biom

ass

(Kg

ha-

1 )

28-May16-Jun16-Jul11-Aug

Part 2

Comparing and Characterizing MODIS Data

250, 500, or 1000 meter spatial resolution

GLOBAL COVERAGE EVERY 1-2 DAYS (Landsat 16 days)

on-board calibration + 36 spectral channels

( AVHRR 5, TM 7, ETM+ 8)

More accurate geo-location (within 0.1 pixels)

Unprecedented processing and quality assurance tests before distribution

DATA ARE FREE!

MODISMODERATE RESOLUTION IMAGING

SPECTRORADIOMETER

MOD 09 Surface Reflectance

MOD 11 Land Surf. Temp. / Emissivity

MOD 12 Land Cover / Change

MOD 13 Vegetation Indices

MOD 14 Thermal Anomalies / Fire

MOD 15 Leaf Area Index / FPAR

MOD 17 Net Primary Production MOD 43 BRDF / Albedo

MOD 44 Vegetation Continuous Fields

MODIS LAND PRODUCTS

Mod 15 Leaf Area Index (LAI)

Conceptualized as a spatially continuous photosynthetically active layer

Measures vertical density of projected leaf area

Example: LAI of 2 = Two meters of vertically distributed leaf area per unit of land.

Relating Temporal Trends of Leaf Area Index to Modeled Biomass

Com paring 2001 Tem poral Patterns of LAI to Modeled Biom ass at the Little Missouri National

Grass lands

0.51.01.52.02.53.03.54.04.55.05.5

480 680 880 1080 1280

Above ground photosynthesizing biomass (Kg ha-1)

Leaf

Are

a In

dex

(MO

DIS

)

MayJuneJulyAugust``

`

LAI appears insensitive to small changes in biomass

3 3 6

2667

9 83

13985

222

16847

19 26 1740

2000

4000

6000

8000

10000

12000

14000

16000

18000F

req

ue

ncy

Some Zones Include High Proportion of Agricultural land

Comparing 2001 Spatial Patterns of LAI to Modeled Biomass at the Little Missouri National Grasslands

(ALL zones)

r2 = 0.04May

r2 = 0.22June r2 = 0.12

August

r2 = 0.41July

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

480 680 880 1080 1280

Above ground photosynthesizing biomass (Kg ha-1)

Leaf

Are

a In

dex

(MO

DIS

)

MayJuneJulyAugust

Improving the Spatial Relations Between MODIS LAI and Modeled Biomass

Improving 2001 Spatial patterns of LAI to Biomass at the Little Missouri National Grasslands

r2 = 0.78July

r2 = 0.35August

r2 = 0.22June

r2 = 0.11May

0.6

1.1

1.6

2.1

2.6

3.1

3.6

4.1

4.6

5.1

400 600 800 1000 1200Above Ground Photosynthesizing Biomass (Kg ha -1)

Le

af A

rea

Ind

ex

(MO

DIS

)

MayJuneJulyAugust

Agricultural zones removed

NOTE:…relationship is strongest when biomass is at peak

LAI and Biomass Change Through Time

Percentage Change in Biomass and MODIS LAI for 2001 at the Little Missouri National Grasslands

-100

0

100

200

300

400

1

Zones through time

% C

ha

ng

e L

AI

-30

-20

-10

0

10

20

30

40

50

60

% C

ha

ng

e a

bo

ve g

rou

nd

b

iom

ass

% Change LAI

% Change Biomass

May - June

June - July

July - August

LAI

Biomass

250 m footprint

1 km footprint

Approximate location of Road

MODIS Receives a More Spatially Averaged Spectral Response

Than ETM

Conclusions

1. Reliable conversion of plot level measurements to landscape scales possible

2. Spatial patterns of MODIS LAI are tightly linked with biomass

3. Temporal Trajectory of LAI with biomass is reliable

4. Comparing LAI with biomass is inherently difficult

5. Success of smaller regional studies depends on: intimate local knowledge Relatively large differences in biomass in a given time frame