The portion of radiant energy important for plants is called photosyntheti-cally active radiation (PAR): 400-700 nm (nanometers), which is short-wave, energy-rich radiation. The amount of PAR that reaches a certain area where plants grow is measured in irradiance units, or photosynthetic photon flux density (PPFD), and is expressed in micromoles per square meter per second (Longman and Jenik 1987). Light availability can be expensive to measure in a direct manner, because it requires equipment such as light sensors. Indirect measurements of light availability or total illuminance can be more practical. Indirect measurements of light availability in a forest can include indices of canopy closure based on direct observation of canopy characteristics, or on the use of spherical densiometers consisting of a concave or convex mirror that reflects the canopy of trees and allows estimations of area covered. Hemispheric photography uses fish-eye lenses to obtain pictures of the canopy, and can be complemented with computer software that can calculate the size of the open and canopy areas in a more precise manner. Simple light sensors can also be used for instantaneous measurements, but their reliability is low due to the high variations that can be found in the light environment according to time of day and weather conditions.
Solar radiation is spectrally altered by passage through the forest canopy, affecting both the quantity and the quality of PAR in the understory (Chaz-don et al. 1996). Chlorophyll in green plants absorbs some light energy in the blue (400-500 nm) but mainly in the red (600-700 nm) portions of the light spectrum. Leaves (chlorophyll) reflect light around 550 nm (the green band). Leaves are very transparent to far-red (FR, beyond 700 nm) wavelengths, i.e., they reflect and transmit most of the FR radiation that reaches them; thus a high proportion of FR radiation penetrates all the way down to the forest floor. The greatest spectral change after canopy filtering, therefore, is the reduction in R/FR ratio (Chazdon et al. 1996).
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