Field Testing Satellite-Derived Vegetation Health Indices for a Koala Habitat Managers Toolkit
- To identify the most appropriate remote sensing tools for the landscape-scale assessment of koala habitat conditions and changes.
- To develop a rapid koala habitat health check to enable a qualitative assessment of local habitat condition as a trigger for the quantitative evaluation and potential management intervention; and
- To compile a suitable koala habitat management toolbox.
1.1. Objective 1—Investigate Remotely Sensed Habitat Condition Indices
1.2. Objective 2—Koala Habitat Health Check
1.3. Objective 3—Habitat Management Toolbox
1.4. Image Acquisition and Processing Considerations
2. Materials and Methods
2.1. Biophysical Parameters Selection
- Normalised Difference Vegetation Index (NDVI)—a numerical dimensionless indicator of plant ‘greenness’ or photosynthetic activity that uses the visible and near-infrared bands of the electromagnetic spectrum .
- Normalised Difference Red-Edge Index (NDRE)—similar to NDVI but suitable for detecting plant foliar stress earlier than NDVI and a consequence of satellite radiometric design incorporating a ‘red-edge spectral capability .
- Equivalent Water Thickness (EWT) is a leaf moisture proxy using a mid-infrared band available on some imagers for characterising moisture absorption .
- NDRE—satellite band ratio of NIR reflectance and another NIR reflectance where the band radiometric wavelength is close to the R band (known as a ‘red-edge’ or RE band) reflectance :NDRE = (NIR842 − RE703.8)/(NIR842 + RE703.8)
- EWT—this index is constructed from satellite short-wave infra-red (SWIR) reflectance located at electromagnetic energy wavelengths known to be absorbed by water (~1.6 µm) :EWT = SWIR1600/NIR820
2.2. Satellite Imager Selection
2.3. Image Processing Software Selection
2.4. Methods—Vegetation Health Indices Field Measurements
- July 2018—St Bees Island (off the coast near Mackay CQ).
- August 2018—Minerva Hills National Park (near Emerald CQ).
- August 2018—Daisy Hill Conservation Park (south of the City of Brisbane).
- August 2018—private property ‘Hollow Log’ in the vicinity of Mt Byron and D’Aguilar National Park (northwest of the City of Brisbane).
2.5. Satellite Image Processing
2.6. Field and Map Correlation
3. Results—Vegetation Health Indices
3.1. RapidEye Compared to Sentinel-2
3.2. Field-measured and Satelite Image-Derived Parameter Correlation
- The four scatter plots at Figure A3 compare field-measured LAI with Sentinel-2 derived LAI maps for the four fieldwork site locations.
- The four scatter plots at Figure A4 compare field-measured LAI with Sentinel-2 derived NDVI maps for the four fieldwork site locations.
- The four scatter plots at Figure A5 compare field-measured LAI with Sentinel-2 derived NDRE maps for the four fieldwork site locations.
3.3. Satellite-Derived Vegetation Health Proxy Maps
- below 0.3 as rock, sand or built environment;
- from 0.3 to around 0.6, grassland and some open woodland; and
- 0.6 to 0.8 rainforest land cover.
3.4. Minerva Hills NP
- NDVI (photosynthetic health);
- declined in ground cover made up predominately of grasses,
- declined in patches in a riparian zone—some areas of the stream bed allow for more resilient vegetation (narrow-leafed ironbark),
- appeared to provide more biota resilient conditions (for the period of image difference) on low ridgelines adjacent to the alluvial plain, and
- is uneven across the habitat range (on a tree species basis).
- LAI (leaf area with respect to the ground area);
- in areas of grass exhibited neutral changes (as expected for LAI),
- declined in patches in a riparian zone, and
- iron-barks adjacent to the stream suffered the least decline.
- EWT (a proxy for leaf water content);
- strongly declined in areas of bare ground (as expected for EWT),
- declined in patches in a riparian zone, generally more neutral declines than NDVI or LAI,
- indicated grassland drying, and
- iron-barks adjacent to the stream suffered the least decline suggesting species resilience.
3.5. St Bees Island
- declined little in the perennial and littoral rainforest and mangrove regions,
- declined somewhat for poplar gums (but less than for Minerva Hills NP) and the QLD forest red gum, and
- declined for grass-cover.
- provided more variability than the NDVI change detection indicating patchy leaf area decline but continuing photosynthetic health generally,
- for the rainforest (subject to further inspection) may indicate vine thicket phenological cycle, and
- poplar gums exhibit decline, and QLD forest red gum marked decline.
- some widespread leaf water content decline in poplar gum areas (not substantial),
- a stronger decline for QLD forest red gum—for this habitat, a significant koala food tree, and
- slight difference in rainforest assemblages.
3.6. Daisy Hill CP
- indicated taller and more comprehensive tree crown environment than the other three sites,
- and a neutral change, and
- the per-pixel values of the index would be less influenced by understory and groundcover species.
- indicated more vegetation stage change than NDVI—a mosaic mottling of decreasing LAI—the extent to which this map indicates issues for koala food tree species in this location should be examined, and
- the March image has a cloud shadow contamination—an area evident in the east side of the image, mid-way between north and south.
- Indicated neutral leaf water loss across the seasons for this particular time series, and
- the small areas that appeared to retain higher water content into the dry season may be related to landscape location (this could be further examined).
3.7. Mt Byron
- indicated a strong seasonal phenological stage signal—declining grass photosynthetic health is most apparent, and
- while considering the image processing impact of terrain slope, north-facing vegetation has suffered the most decline.
- LAI—as for NDVI;
- EWT—areas in the landscape that show small retention of leaf water content need to be further examined to understand the mapping result.
Conflicts of Interest
Appendix A. Satellite Imager Specifications
|SPOT-1 to 4||1||10/20||120||PGRNS||-||1986–2012|
|Number of Satellites||5|
|Orbit Altitude||630 km in Sun-synchronous orbit|
|Equator Crossing Time||11:00 am local time (approximately)|
|Sensor Type||Multi-spectral push broom imager|
|Spectral Bands||Blue||440–510 nm|
|Pixel size (orthorectified)||5 m|
|Swath Width||77 km|
|Revisit time||5 days (at nadir)|
|Dynamic Range||12 bit|
|Central Wavelength (nm)||Bandwidth (nm)||Central Wavelength (nm)||Bandwidth (nm)|
Appendix B. Spatial Resolution Sentinel-2 and RapidEye
Appendix C. Comparing Fieldwork and Satellite Vegetation Indexes
Appendix D. Vegetation Index Maps
Appendix E. Rainfall Data for Selected Locations in the Study Area of Interest
|2017 Months||St Bees Island||Minerva Hills||Daisy Hill||Mt Byron||2018 Months||St Bees Island||Minerva Hills||Daisy Hill||Mt Byron|
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|R2 for satellite NDVI vs. measured LAI.||0.4703||0.5004|
|R2 for satellite NDRE vs. measured LAI.||0.4688||0.6079|
|R2 for satellite LAI vs. measured LAI.||0.4458||0.4765|
|Field Site||Image Date during a Rain Period||Image Date during Dry Period|
|Minerva Hills||17 March 2018||30 July 2018|
|St Bees Island||1 April 2017||28 October 2017|
|Daisy Hill||16 March 2018||23 August 2018|
|Mt. Byron||10 April 2018||2 September 2018|
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Hewson, M.; Santamaria, F.; Melzer, A. Field Testing Satellite-Derived Vegetation Health Indices for a Koala Habitat Managers Toolkit. Remote Sens. 2022, 14, 2119. https://doi.org/10.3390/rs14092119
Hewson M, Santamaria F, Melzer A. Field Testing Satellite-Derived Vegetation Health Indices for a Koala Habitat Managers Toolkit. Remote Sensing. 2022; 14(9):2119. https://doi.org/10.3390/rs14092119Chicago/Turabian Style
Hewson, Michael, Flavia Santamaria, and Alistair Melzer. 2022. "Field Testing Satellite-Derived Vegetation Health Indices for a Koala Habitat Managers Toolkit" Remote Sensing 14, no. 9: 2119. https://doi.org/10.3390/rs14092119