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, wherethermal 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.
Forests cover almost a third of the earth’s land surface, some 3.9 billion hectares (9.6 billion acres). These are vast and often foreboding regions often inaccessible to human entry. Forest researchers therefore take advantage of remote-sensing satellites to monitor forest wellbeing, obtaining detailed information about tree growth and decline that would otherwise be difficult and costly to obtain.
In Israel, researchers of the Remote Sensing Laboratory (RSL) and BGU Department of Geography and Environmental Development are applying Landsat satellite images to explore – for the first time – how drought effects forest health in a desert fringe area. Their studies deal with the Yatir Forest, some 23-km (14 miles) northeast of Beer Sheva, which was originally planted in the late 60s and early 70s by the Keren Kayemeth LeIsrael – Jewish National Fund.
A Landsat -TM image of central Israel. Note the location of the Yatir forest on the the desert fringe, visible as sharp contrast between bright tones (semi-arid zone) and dark tones (sub-humid zone).
The 3000 ha (6500 acre) forest serves to reduce soil erosion, and rehabilitate the dryland. The scientists, in a seven year study between 1994 and 2001, measured rainfall and calculated Normalized Difference Vegetation Index (NDVI) values (Vegetation Health Technology)throughout the Yatir Forest, from Landsat data. The results are presented as false-color maps of the forest, indicating well-growing areas in various shades of green and less satisfactory growth presented in yellow-green, yellow, and brown. Areas appearing in red, indicate trees experiencing severe water stress.
NDVI in similiar seasons (winter and spring) but for a wet year (1995) and for a drought year (2000).
NDVI vegetation health-index maps taken at intervals throughout the year enable comparison of forest growth in rainy and dry months. Moreover, maps produced over a period of high, average, and low preciptation can evalutae how different parts of the forest withstand drought and subsequently recover when rainfall returns to normal.
The Yatir study clearly demonstrates the high photosynthetic activity – and health –of the trees in the winter rainy months when water availability is at a maximum and evaporation is reduced due to cooler temperatures.
Annual dynamics of the photosynthetic fluxes of the Yatir forest for the year 2000/2001 (mmol m-2s-1) in comparison to the annual dynamics of NDVI for the year 1994/1995.
However, many other important facts emerged as wells.
- The major forest tree in Yatir, the Aleppo pine (Pinus halepensis), clearly demonstrates its ability to adjust to sharp changes in environmental conditions. When growing in Europe, its high NDVI and photosynthetic activity occurs in rainy, warm summers, while in the Negev its growth takes place primarily in the winter, as evidenced by the NDVI maps.
- Large downward changes in NDVI values were observed during the poor rainfall winters of 1996, 1997, 1999, and 2000, as compared with their higher maximums in years of greater precipitation, namely 1995, 1998, and 2001. Thus, the forest’s physiological reaction to drought stress was clearly demonstrated and evaluated. The results indicated that the forest is resilient and did not suffer irreversible damage in years of poor rainfall.
- Planting of the Yatir Forest took place over some 30 years, with each section of the forest having trees of different ages. The NDVI images clearly show that the trees planted in the 1990s were less likely to be affected by drought and water shortage than the older trees. In fact, the most significant factor effecting on plant health and resistance to drought across the whole forest was tree age and soil type. Other environmental variables, such as slope, aspect, soil type, and the underlying geological formations, had only weak effects on the NDVI, when the tree age was taken into account.
Thus remote sensing was able to determine various environmental factors effecting the health of a model forest operating on the desert edge, as well as its response to drought and satisfactory rainfall. This approach could well provide vital information for planning and monitoring forests planted by countries around the world seeking to halt desertification.
