Quantifying Biome specific relationships and Monsoon Event responses

Maxine PaulMarcy Litvak, Mentor

Using LAI , NDVI, PRI, and net ecosystem exchange in desert grassland and shrubland on the Sevilleta

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)

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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

Photo credit: Jennifer Johnson

Taking LAI measurements with ceptometer

•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/PRI methods

NDVI= (p800-p675)/ (p800+p675)

PRI= (p531-p570)/(p531+p570)

P800= Near Infrared Radiation (NIR) spectrumP675=Red spectrum

Flux tower instruments over a forest

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.

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

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LAI (m2 leaf per m2 ground)

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LAIvNDVI combined Shrub and Grass July 18th-21st

y = 0.1142x - 0.1929

R2 = 0.3613

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LAI (m2 leaf per m2 ground)

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These trends are describing both biomes together

•Barely negative, very weak correlation in mid june. NDVI and LAI not strongly related to one another.

•Positive slightly stronger correlation in mid July. As LAI increases, NDVI increases.

•Relationship becomes apparent after monsoon season has started (july)

•As “greenness” increases, relationship strength increases.

Shrubland LAI v NDVI total points y = 0.1171x - 0.1952

R 2 = 0.3072

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LAI (m2 leaf per m2 ground)

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Grassland LAI v NDVI total points y = 0.0807x - 0.1744

R 2 = 0.138

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LAI (m2 leaf per m2 ground)

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Top graph shows shrubland relationship Between LAI and NDVI• Points more spread out than grass, • Positive trend apparent

Bottom graph shows grassland relationshipBetween LAI and NDVI• Points more clumped• Positive trend apparent

• 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)

• 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

Shrubland v Grassland

2) How well do sensing methods compare to traditional biomass harvesting methods for LAI measurement?

Shrub Biomass V LAI y = 0.0041x + 0.2524R2 = 0.9132

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Biomass (allometry of life leaf in grams)

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Shrubland Biomass (allometry) v LAI (ceptometer)

•Strong positive relationship

•Possible saturation or leveling out

•Not sufficient data points to draw strong conclusions

Grassland LAI and BiomassGrassland Biomass Harvest v LAI (ceptometer) y = -0.0005x + 0.341

R2 = 0.0108

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Biomass in grams

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IGrassland Live biomass v NDVI

y = 0.0003x - 0.1701

R2 = 0.0512

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Biomass in grams

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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?

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Shrubland Grassland

Grassland responses

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Grassland PRI and NEE

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NEE of C and LAI

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Shrubland responses

Shrubland PRI and NEE

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NEE of C and PRI

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 mid-summer 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

Conclusions

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?Questions?