Reflection and Absorption

The analysis of reflection, absorption and transmission of radiation by individual leaves, plants or by the vegetation canopy has become an important method in ecophysiology. The principle relations are shown in Fig. 2.3. The range of photo-synthetically active radiation (PAR: 400 - 700 mm) is largely identical to that of the visible light. Here radiation absorption is dominating. Chlorophyll has an absorption minimum in the green range of the spectrum (550 nm), and this is identified by reduced absorption, and increased reflection and transmission (Fig. 2.3). In the infrared range of the spectrum (above 800 nm) radiation reflection and transmission are dominating. At very high wavelengths absorption increases again, although, this is not so relevant as solar emission contains little radiation above 2,000 nm.

The contrast between absorption of the radiation in the visible range and reflection in the infrared range of the spectrum by green plants has been used to develop a dimensionless vegetation index Q related to reflection between 580 and 680 nm (^680) and between 725 and 1,100 nm (R1)™), respectively, as follows:

It results from this equation that at very low reflection between 580 and 680 nm and very high reflection between 725 and 1,100nm, vegetation is dense and Q tends towards +1. In contrast, at very high reflection between 580 and 680 nm and very

Fig. 2.3 Relationship between reflection, absorption and transmission of radiation by a green leaf at varied wavelengths. UV Ultraviolet; PAR photosynthetically active radiation; IR infrared. (After Gates 1965; from Nobel 1983: Biophysical Plant Physiology and Ecology, Copyright 1983 by W.H. Freeman and Company; used with permission)

Fig. 2.3 Relationship between reflection, absorption and transmission of radiation by a green leaf at varied wavelengths. UV Ultraviolet; PAR photosynthetically active radiation; IR infrared. (After Gates 1965; from Nobel 1983: Biophysical Plant Physiology and Ecology, Copyright 1983 by W.H. Freeman and Company; used with permission)

low reflection between 725 and 1,100nm Q tends towards -1, indicting sparse vegetation (Running 1990). The two values of R, R^so and R ^o, can be measured from aeroplanes or meteorological satellites equipped with two sensors for the respective range of wavelengths. The results can be depicted on maps using false colours, which provide informative images at the global level. Formations with particularly dense vegetation (e.g. the tropical forests and the extended forest regions of the northern hemisphere) are readily distinguished from poorer areas like deserts, steppes and savannas (Malingreau and Tucker 1987).

Thus, analysis of reflection and absorption with the rough vegetation index obtained by comparing reflectance at two rather broad bands of wavelengths provides information on a given state of vegetation and, if followed in time, also about its dynamics. Although this type of analysis has been used successfully to predict harvests, it does not provide a real picture of the physiological state and vitality of vegetation. This can be improved by high-altitude aircraft or space based imaging spectroscopy with much higher spectral resolution of the solar radiation reflected from the Earth's surface in contiguous narrow bands. A National Aeronautics and Space Administration (USA) Earth Observing (EO) device allows measurement of reflected radiance in 242 spectral bands from 0.4 to 2.5 |m at a spatial resolution of 30 m (Asner et al. 2004). The sampling of narrow bands of the optical spectrum allows deduction of more specific biochemical properties of canopies, e.g. by giving a photochemical reflectance index (PRI), an anthocyanin reflectance index, a spectroscopic water absorption index (Asner et al. 2004) and an index of nitrogen concentration (Asner and Vitousek 2005) in canopies. With spectrometers of particularly high resolution gross biochemical composition of vegetation utilizing the light absorption of quantitatively dominant organic compounds (e.g. sugars, cellulose, starch, lignin, protein) can also be measured (Wessman 1990).

The PRI uses the wavelength of 570 nm, which is affected only by chlorophyll absorbance, and the wavelength of 531 nm which is affected by both chlorophyll and carotenoid absorbance (Nichol et al. 2006). Carotenoid absorbance is modulated by the epoxidation-state of components of the xanthophyll cycle which is related to the dissipation of surplus photosynthetic excitation energy as will be explained below in the context of light in tropical forests (Sect. 4.1.4). PRI is calculated in different ways in the literature, i.e. as

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