The intensity of the incident solar radiation at the earth’s surface is one of the key factors in photosynthesis, as shown in Chapter 3. Except in a few rare cases, nat­ural biomass growth will not occur without solar energy. Isolation varies with ge­ographic location, time of day, and season of the year, and as is well known, it is high in the tropics and near the equator. The approximate changes of insolation with latitude are illustrated in Table 4.1 (Brinkworth, 1973), and Fig. 4.1 shows how the mean annual insolation varies at the earth’s surface with geographic location (Crutchfield, 1974). A more quantitative summary of average total daily insolation values over the continental United States is shown in Table 4.2 (U. S. Dept, of Commerce, 1970). At a given latitude, the incident radiation is not constant and often exhibits large changes over relatively short distances. Although several environmental factors influence biomass productivity, there is usually a relatively good correlation between the annual yields of dry biomass per unit area and the average insolation value (see Table 3.1). All other factors being equal, it is generally true that the higher the insolation, the higher the annual yield of a particular energy crop provided it is adapted to the local environment. C4 biomass species often exhibit higher productivities in terms

of growth rates and annual yields because of their capability to utilize incident solar radiation at higher efficiencies for photosynthesis.