**1. Introduction**

220 Solar Radiation

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Neotropical grasslands or savannas cover almost half the surface of Africa and large areas of Australia, South America, India and Southeast Asia (FAO, 2010). In South America vegetation like savanna covers over 2.1 million km2 mainly in central Brazil (also called Cerrado), Colombia and Venezuela. Tropical savannas are currently undergoing rapid and radical transformation due to human interventions on land-use patterns (Lehmann et al., 2009). In the last 30 years about 54 million hectares of native Cerrado vegetation have been replaced by cultivated pasture, mainly African grasses of the genus *Brachiaria* spp. *(*Boddey et al., 2004, Meirelles et al., 2011). At the same time, the majority of deforested land in the Amazon Basin has become cattle pasture, making forest-to-pasture conversion an important contributor to the carbon and climate dynamics of the region (Asner et al., 2004).The terminology here follows the International Forage and Grazing Terminology Committee (Allen et al., 2011):"the term grassland bridges pastureland and rangeland and may be either a natural or an imposed ecosystem. Grassland has evolved to imply broad interpretation for lands committed to a forage use".

At present it is unclear whether neotropical grasslands are governed by the same factors in Australia, Africa and South America, the three major continental regions of this biome (Lehmann et al., 2009). What is clear is that significant portions of native or cultivated grasslands on every continent have been degraded due to human activities (FAO, 2010). In the tropical regions of Brazil where it is estimated at least half of the cultivated pastures are degraded, the two main drivers of degradation processes are low soil fertility (especially low soil N) and excessively high animal stocking rates (Boddey et al., 2004).On a global scale, overgrazing alone can account for about 7.5 percent of grassland degradation (FAO, 2010).

There is consensus that the sustainability of pastoral ecosystems demands more appropriate livestock management practices. Besides, for the ecological management of these agrosystems, it is necessary to increase the level of understanding of the interactions between its biotic and abiotic components. Central to this understanding, the diverse facets of the interaction between solar radiation and grassland vegetation is of theoretical and practical importance. They are the subject of this chapter, and range from forage grass as monocultures to the complex interrelationships that exists among trees and grasses in silvipastoral agro-systems. In this chapter we will explore a few selected subjects within this broad chain of processes. Recently, focusing on the role of theory in plant science, Woodward (2011) noted that "the development of plant science is based on observations, the development of theories to explain these observations and the testing of these theories."Besides the need of theory to overcome empirical approaches, the theoretical basis is also functional for a better understanding of possibilities and limitations of the new available instrumentation from advances in remote sensing and other technologies for grassland monitoring and assessment. We give emphasis to the concept of sward canopy structure, discussing the central role of Leaf Area Index in the pasture trophic program via light interception. We also give some theoretical and practical emphasis on methodological aspects and procedures for measurements of canopy structure and radiation interception by vegetation. And finally we considered the efficiency use of radiation.
