Quantifying Biome specific relationships and
Monsoon Event responses
Using LAI , NDVI, PRI, and net ecosystem exchange in desert
grassland and shrubland on the Sevilleta
Maxine Paul
Marcy Litvak, Mentor
Photo credit: Laura Sanchez Jardon
The “Big” Picture: Remote Sensing of
Ecosystems…
• Satellites measure NDVI
(Normalized Difference Vegetation
index) and LAI (Leaf Area Index)
• Measurements relied upon in
ecosystem modeling
• We find it questionable that they are
looking at factors about an
ecosystem on such a large scale (1
pixel=1km!)
http://svs.gsfc.nasa.gov/stories/drought/images/na_ndvi0086_sm.jpg
Ecosystems such as the grassland and shrubland on the sevilleta
show lots of heterogeneity on the ground, are the satellite
measurement’s averages missing something?
The “Medium” Picture
Marcy Litvak’s lab uses flux towers set up
at 2 locations on the Sevilleta measuring
Net Ecosystem Exchange of Carbon
(NEE), meteorological variables, and
optical vegetation indexes of NDVI, and
PRI over a scale of < 0.5km2
The “Little” Picture
- This summer I have measured NDVI,
PRI (Photochemical Radiation Index)
and LAI on small scales (<1m2 plots)
measuring ~60cm above ground
- I used a portable sensor developed by
Lee Vierling at the University of Idaho
with a Campbell Scientific datalogger
(CR23X)
Research Questions
1) Is there a relationship on the ground between
NDVI , LAI, and PRI in grassland and
shrubland biomes? How do they correspond
with precipitation?
2) How well do optical sensing methods
compare to traditional biomass harvesting
methods for LAI measurement?
3) How well do plot scale measurements
compare to full ecosystem carbon uptake?
How does this correspond with precipitation?
Grassland and Shrubland ecosystems on the
Sevilleta
Plant species that dominate the
ecosystems:
• Creosote in shrubland: Larrea
tridentata
• Blue Grama (Bouteloua
gracilis),and Black Grama
(Bouteloua eriopoda) in
grassland, which use C4
photosynthesis and are
perennial.
•Larrea tridentata, C3 photosynthesis
Locations of CO2 flux towers and my transects
Light green = grassland site
Dark green= shrubland site
Transects
•
•
•
•
•
•
Set up in each biome
Three 100m transects, one 50 m transect for biomass harvesting/allometry.
Little squares represent a plot every 10 m.
Laid out in cover of ecosystem tower
At each plot I measured NDVI and PRI for 5 minutes
Quantified % covers with quadrat for each (understanding of the make-up of each plot, soil,
grass, shrub, rock)
100 m
N
50 m
LAI Methods
• Measurements below and above canopy at each plot (<1m2)
along transects at Grassland and Shrubland CO2 Flux tower
locations
• using a Decagon PAR/LAI ceptometer
• Ratio of how much photosynthetically active radiation (PAR)
gets through the canopy
Taking LAI measurements with ceptometer
Photo credit: Jennifer Johnson
NDVI/PRI methods
•The specific wavelengths
correspond to pigments of plant
tissue responsible for photosynthesis
•
=PRI
•
=NDVI, corresponds to
absorption of chlorophyll a (also a
band at 800nm, not on graph)
•PRI corresponds to photosynthetic
light use efficiency, equivalent to the
rate CO2 uptake by foliage per unit
energy absorbed and carotenoid
pigments
NDVI= (p800-p675)/ (p800+p675)
PRI= (p531-p570)/(p531+p570)
P800= Near Infrared Radiation (NIR) spectrum
P675=Red spectrum
Net Ecosystem Exchange of Carbon
•NEE: Net Ecosystem Exchange of CO2 – direct measurement of exchange of
CO2 between atmosphere and ecosystem
•NEE=GPP– (Rplant+Rhetertrophic). When NEE is positive, more carbon is released
into atmosphere (carbon source). When NEE is negative, more carbon is
sequestred by ecosystem (carbon sink)
Eddy covariance instruments
on towers measure the
integrated fluxes of all of the
biota in the tower’s range.
Flux tower instruments over a forest
1) Is there a relationship on the ground between
NDVI and LAI in these two biomes?
LAIvNDVI combined Shrub and Grass June 15th-18th
y = -0.0609x - 0.151
R2 = 0.0554
0.1000
These trends are describing both
biomes together
0.0500
0.0000
0
0.2
0.4
0.6
0.8
1
1.2
NDVI
-0.0500
-0.1000
-0.1500
•Barely negative, very weak
correlation in mid june. NDVI and LAI
not strongly related to one another.
-0.2000
•Positive slightly stronger correlation in
mid July. As LAI increases, NDVI
increases.
-0.2500
-0.3000
LAI (m 2 leaf per m 2 ground)
LAIvNDVI combined Shrub and Grass July 18th-21st
y = 0.1142x - 0.1929
R2 = 0.3613
0.15
0.1
0.05
•Relationship becomes apparent after
monsoon season has started (july)
0
NDVI
0
0.2
0.4
0.6
0.8
1
1.2
-0.05
-0.1
-0.15
-0.2
-0.25
-0.3
LAI (m 2 leaf per m 2 ground)
1.4
1.6
1.8
•As “greenness” increases,
relationship strength increases.
Shrubland v Grassland
Shrubland LAI v NDVI total points
Top graph shows shrubland relationship
Between LAI and NDVI
• Points more spread out than grass,
• Positive trend apparent
y = 0.1171x - 0.1952
R 2 = 0.3072
0.15
0.1
0.05
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
NDVI
-0.05
• Shrubland shows a slightly stronger
relationship over the whole summer
Between LAI and NDVI.
Possibly due to wider range of LAI
(shrubs taller and larger than grasses)
-0.1
-0.15
-0.2
-0.25
-0.3
LAI (m2 leaf per m2 ground)
0.15
0.10
0.05
0.00
0
0.2
0.4
0.6
0.8
1
1.2
-0.05
-0.10
•
-0.15
Points with low NDVI but LAI of around
.8 are result of patchy areas where a
plot has one shrub/group of grass but
mostly bare ground
y = 0.0807x - 0.1744
R 2 = 0.138
Grassland LAI v NDVI total points
NDVI
Bottom graph shows grassland relationship
Between LAI and NDVI
• Points more clumped
• Positive trend apparent
-0.20
-0.25
-0.30
LAI (m2 leaf per m2 ground)
1.4
1.6
1.8
2
2) How well do sensing methods compare to traditional
biomass harvesting methods for LAI measurement?
y = 0.0041x + 0.2524
R2 = 0.9132
Shrub Biomass V LAI
0.9
•Strong positive relationship
0.8
0.7
LAI (ceptometer)
Shrubland Biomass (allometry) v LAI
(ceptometer)
0.6
•Possible saturation or leveling out
0.5
0.4
•Not sufficient data points to draw
strong conclusions
0.3
0.2
0.1
0
0
50
100
Biomass (allometry of life leaf in grams)
150
Grassland LAI and Biomass
Grassland Biomass Harvest v LAI (ceptometer)
1
Grassland Live biomass v NDVI
y = -0.0005x + 0.341
R2 = 0.0108
0.0000
0
0.9
0.8
10
20
30
40
50
y = 0.0003x - 0.1701
R2 = 0.0512
-0.0500
LAI (ceptometer)
0.7
LAI
0.6
0.5
0.4
0.3
-0.1000
-0.1500
0.2
-0.2000
0.1
0
0
50
100
Biom ass in gram s
150
200
60
-0.2500
Biomass in grams
We didn’t find correlation between harvested biomass in the grassland and
LAI measured by the ceptometer. (on left, r2 value of .01)
We also compared live biomass harvested in the grassland to NDVI, also
very little correlation (r2=.05).
3) How well do plot scale measurements compare to full
ecosystem Carbon uptake? How does this correspond with
precipitation?
Shrubland
Grassland
1.2
1.2
1
1
0.8
0.8
NEE (gC m-2 d-1)
NEE (gC m-2 d-1)
0.6
0.6
0.4
0.2
0
0.4
0.2
0
-0.2
-0.4
-0.2
-0.6
-0.4
-1
6/1/09
7/6/09 7/11/09 7/16/09 7/21/09
6/6/09
6/11/09 6/16/09 6/21/09
Date
7/1/09
7/6/09
7/11/09 7/16/09
Date
25
60
50
Daily preecipitation (mm)
20
15
10
40
30
20
10
5
Date
7/
11
/2
00
9
6/
26
/2
00
9
6/
21
/2
00
9
6/
16
/2
00
9
6/
11
/2
00
9
6/
6/
20
09
7/
11
/2
00
9
7/
16
/2
00
9
7/
6/
20
09
6/
11
/2
00
9
6/
16
/2
00
9
6/
21
/2
00
9
6/
26
/2
00
9
7/
1/
20
09
6/
6/
20
09
6/
1/
20
09
0
0
6/
1/
20
09
Daily preecipitation (mm)
6/26/09
7/
6/
20
09
6/6/09 6/11/09 6/16/09 6/21/09 6/26/09 7/1/09
7/
1/
20
09
-0.8
-0.6
6/1/09
Shrubland responses
NEE of C and LAI
Shrubland PRI and NEE
1.2
1.2
0.6
-0.1
0.8
NEE (gC m-2 d-1)
0.6
-0.21
0.4
-0.215
0.2
0
0.4
0.4
0.2
0.3
0
-0.22
0.2
-0.2
-0.2
0.1
-0.4
-0.6
-0.23
6/1/09 6/6/09 6/11/09 6/16/09 6/21/09 6/26/09 7/1/09 7/6/09 7/11/09 7/16/09 7/21/09
-0.6
6/1/09
Date
0.6
-0.12
0.4
-0.14
0.2
0
-0.16
-0.2
-0.18
-0.225
-0.4
NEE (gC m-2 d-1)
0.5
0.6
LAI (m 2 leaf m -2 ground)
-0.205
0.8
-0.08
1
0.8
1
NEE (gC m-2 d-1)
1.2
0.7
1
-0.2
NEE of C and NDVI
NDVI
NEE of C and PRI
6/11/09
6/21/09
7/1/09
7/11/09
-0.4
-0.6
-0.2
6/1/09 6/6/09 6/11/09 6/16/09 6/21/09 6/26/09 7/1/09 7/6/09 7/11/09 7/16/09 7/21/09
0
7/21/09
Date
Date
Grassland responses
0.45
1
-0.185
1
-0.195
0.4
0.2
-0.2
0
-0.2
-0.205
-0.4
-0.6
-0.21
-0.8
-1
6/1/09
NEE (gC m-2 d-1)
NEE (gC m-2 d-1)
0.6
0.35
0.6
-0.19
0.8
0.4
0.8
0.3
0.4
0.25
0.2
0
0.2
-0.2
0.15
-0.4
-0.6
6/6/09 6/11/09 6/16/09 6/21/09 6/26/09 7/1/09
Date
-1
6/1/09
-0.02
0.8
-0.04
-0.06
0.4
-0.08
0.2
-0.10
0
-0.12
-0.2
-0.14
-0.4
0.1
0.05
-0.8
-0.215
7/6/09 7/11/09 7/16/09
0.00
1
0.6
NEE (gC m-2 d-1)
1.2
1.2
0
6/6/09 6/11/09 6/16/09 6/21/09 6/26/09 7/1/09 7/6/09 7/11/09 7/16/09
Date
-0.6
-0.16
-0.8
-0.18
-1
6/1/09
6/6/09 6/11/09 6/16/09 6/21/09 6/26/09 7/1/09
Date
-0.20
7/6/09 7/11/09 7/16/09
NDVI
1.2
LAI (m 2 leaf m -2 ground)
Grassland PRI and NEE
Conclusions
1)
Is there a relationship on the ground between NDVI and LAI in these two
biomes?
• In both biomes, after the monsoon season has started, there is a
positive correlation between NDVI and LAI. As biomass increases,
Normalized difference vegetation index becomes more positive.
• In the shrubland the relationship between NDVI and LAI is slightly
stronger. One possibility is that it is due to larger range of LAI for
creosote than for gramas.
My results suggest there are stronger relationships in midsummer in both biomes, compared to early summer. This
correlates with an increase in rainfall and available water for
photosynthesis.
Conclusions
2) How well do optical sensing methods compare to traditional biomass harvesting
methods for LAI measurement?
•
For shrubland, allometric biomass was positively related to LAI, and had
strong correlation. In this study ceptometer use is similar to allometry of
Creosote.
•
For grassland, no strong correlation between biomass harvesting and LAI by
ceptometer. Results suggest that optical measurements are better for
understanding relationship between LAI and NDVI in grassland.
Conclusions
3) How well do plot scale measurements compare to full ecosystems Carbon
uptake? How does this correspond with precipitation?
•
Because grass seems to respond quicker to rainfall events, the
grassland LAI, NDVI show a stronger correlation to carbon uptake
•
In the shrubland, there is an increase in LAI most likely in response
to precipitation, however the relationship to C flux is less clear. With
partitioning out GPP and respiration, we expect a stronger
relationship.
•
PRI, NDVI, LAI in both biomes show a seasonal progression
corresponding to precipitation
Future Work,
Connections
-
Southwest US undergoing climate
change – semi-arid ecosystems are
particularly sensitive to changes in
climate
-
Over the year I will continue to take
NDVI, LAI, and analyze PRI
measurements and compare the
“little” to “medium” to “big”
pictures
-NEE chamber added into thesis, more
PRI analysis
-Look for me at the Earth and
Environmental Science Senior
Thesis presentation at Columbia
University in the spring in NYC
References
-L. Fan, Y. Gao, H. Bruck, Ch. Bernhofer, Investigating the relationship between NDVI
and LAI in semi-arid grassland in inner Mongolia using in-situ measurements.
Theoretical and Applied Climatology. March, 2007
-M. Van Wijk, M. Williams, Opitical Instruments for Measuring Leaf Area Index in Low
Vegetation Application in Artic Ecosystems, Ecological Applications, 2005.
- National Aeronautic and Space Administration, Goddard Space Flight Center, One
Year Vegetation Average- NDVI measures plant growth
http://svs.gsfc.nasa.gov/stories/drought/
Acknowledgements
-Marcy Litvak
-Andrew Fox
-Litvak Lab
-Jennifer Johnson
-Cesar Coronado, Frankie Reyes, Giomara LaQuay,
Diana Guzman, Laura Sanchez Jardon, Ernest
Herrera, all interns and REUs
-Sevilleta LTER
-National Science Foundation
-UNM Biology
Questions?