Npq

In many of these examples analysis of temporal variations is essential, integrated by repeated observation. Another important aspect is the need for "calibration", which may allow a more detailed interpretation of the radiation signals obtained in remote sensing. Due to the complex structure and dynamics of ecosystems, significant efforts are required for calibration, involving measurements on a hierarchy of levels with different resolution of area, e.g. on the ground, on measuring towers, in aeroplanes and helicopters and in satellites (Sellers et al. 1990). With better calibration, interpretation of finer detail and more sophisticated resolution will be possible from remote-sensing data. However, calibration or "ground truthing" itself is a great problem, especially in inaccessible tropical areas.

2.3.2 Fluorescence

A way different from PRI (Sect. 2.3.1) for obtaining information on the photosyn-thetic activity of vegetation is the analysis of fluorescence. The fluorescence from chlorophyll is not only directly related to the concentration of chlorophyll but is also inversely related to the efficiency of photosynthesis. This will be explained in more detail later in relation to photosynthetic light use in tropical environments (Sect. 4.1.7).

Remote fluorescence excitation is possible using powerful lasers. A laser-induced fluorescence spectrum is given in Fig. 2.5. The fluorescence maxima at 690 nm and 740 nm are particularly variable in response to stress as shown in Fig. 2.6 for some conditions studied in the laboratory. As plants senesce fluorescence decreases due to the degradation of chlorophyll (Fig. 2.6A). Conversely, when the photosyn-thetic process is impaired, e.g. by K+ deficiency, drought stress or herbicide action (Fig. 2.6B,C), fluorescence increases (Chapelle et al. 1984a). For a more sensitive analysis it has also been suggested that the ratio of fluorescence at the peak of 690 nm and in the far-red region at 730 or 740 nm should be used (Hak et al. 1990; Lichtenthaler et al. 1990).

Fig. 2.5 Laser-induced fluorescence spectrum of maize leaves (Chapelle et al. 1984b)

Fig. 2.6A-D Changes of laser-induced fluorescence due to various kinds of stress. A Senescence. B Potassium deficiency. C Drought. D Action of the herbicide DCMU (dichlorophenyl-dimethyl-urea) inhibiting photosynthesis. A, C, D soybean; B maize. (Chapelle et al. 1984a)

Fig. 2.7 Profile of laser-induced fluorescence emission (wavelength of fluorescence 685 nm) along a flight path of 6 km above forests and fields. The bright profile gives fluorescence. The dark profile indicates terrain elevation and at the same time the different parts of the landscape, i.e. green fields, brown and freshly ploughed fields and trees. (After Hoge et al. 1983)

Flight path (km)

Fig. 2.7 Profile of laser-induced fluorescence emission (wavelength of fluorescence 685 nm) along a flight path of 6 km above forests and fields. The bright profile gives fluorescence. The dark profile indicates terrain elevation and at the same time the different parts of the landscape, i.e. green fields, brown and freshly ploughed fields and trees. (After Hoge et al. 1983)

Thus, when used in remote sensing, fluorescence analysis allows large-scale diagnosis of stress effects by abiotic factors such as the availability of growth resources (e.g. water, mineral nutrients, photosynthetically active radiation etc.) or environmental pollutants and biotic factors, such as pests and pathogens. The resolution of analyses of laser-induced fluorescence of 685 nm during flights in meteorological aeroplanes flying at a height of 150 m and a nominal flight-speed of 100 m s-1 is between 10 and 80 m. An example is given in Fig. 2.7 for a flight path of 6 km, resolving green and brown fields and forests.

2.4 Gas Analysis

Another means of analyzing the effects of plant life across a range of scales is that of infrared gas-analysis (IRGA). It is of great importance because the method can measure the gas exchange of plants, particularly respiratory and photosynthetic CO2 exchange (but not O2!) and also transpirational loss of water vapour. Many molecules which play a role as environmental pollutants, such as sulfur dioxide, nitrogen oxides, ammonia and carbon monoxide can also be analyzed. Therefore, the IRGA is an important technique in ecophysiological studies encompassing photosynthesis as well as environmental control.

Box 2.2 Infrared active and non-active gases

Was this article helpful?

0 0
Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

Get My Free Ebook


Post a comment