When interpreting remotely determined vegetation health indexes, regional ecology may have entirely unanticipated effects on the significance of measured index levels. Scientists at The Blaustein Institutes for Desert Research Remote Sensing Laboratory collaborating with theRemote Sensing Center of the Mongolian Ministry of Nature and Environment, carried out index studies in Mongolia, a territory of some 1.5 million sq km (0.6 million sq miles) with six different ecological settings.
Ecosystem map of Mongolia. |
Their studies involved comparing vegetation health as determined by Normalized Difference Vegetation Index (NDVI) values and that as affected by land surface temperature (LST), as determined from satellite-measured “brightness temperatures.”
Data was obtained from the Mongolian southern ecosystems – comprised of the desert, desert steppe, and steppe – and the northern ecosystems, which include the forest steppe, high mountains, and taiga. The changes in NDVI and LST were compared and statistically evaluated over a period of 17 years (1982 – 1999). The results clearly showed that in the cold desert increases in NDVI were associated with decreasing LST values, whereas in the taiga, increases in NDVI were associated with increases in LST. The relationships also changed from negative to positive in a gradual way when moving north through the various ecologic patterns where in the forest steppe, the NDVI was not related in any significant manner to the LST values. Therefore when attempting to use both indexes to characterize vegetative health, previous long-term knowledge of their behavior with respect to the ecosystem is required.
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Scattereplot of LST against the NDVI. Note an overall significant negative relation between the two variables. When examining each ecosystem separarely, the northern ecosystems are characterized by a positive trend.
The researchers note that in the desert, where water is the limiting factor of plant growth, increasing temperature increases transpiration and evaporation, reducing the amount of water available to the plant and vegetative growth. Thus NDVI decreases as temperature increases. However, in the extremely cold, higher latitudes, where thermal energy is the major limiting factor for growth, higher external temperatures, which increase the plant's thermal energy, is conductive to growth. Therefore, when attempting to combine the effects of temperature and NDVI (chlorophyll and biomass) values into a single, convenient vegetation health index or VH, the ecological background situation must be taken into consideration.
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Time series of NDVI and LST values for the (a) Desert Steppe ecosystem, and (b) Taiga ecosystem. Note the out-of-phase and in-phase relationship in (a) and (b), respectively.
