Tropical Forests. II. Ecophysiological Responses to Light

Abstract

Although gradients of several different environmental factors are noticeable in tropical forests (Sect. 3.4), intensity of irradiance is most highly variable and appears to play the most prominent role in determining the ecophysiological comportment of forest plants. At the top of the canopy and in larger clearings in full sun-light intensity of photosynthetically active radiation (PAR) at 400 – 700 nm wavelength may range from well above 1,000 up to 2,500 μmolm−2 s−1 photons. On the forest floor there may be less 5 μmolm−2 s−1 photons (see Figs. 3.26 and 3.28). Thus, light can become a stress factor from both too much (when it causes overenergization of the photosynthetic apparatus and hence photoinhibition or even photodestruction), or too little (when it becomes limiting as an energy source of photosynthesis). Focussing of light by leaf epidermal cells may increase irradiance intensity in the mesophyll of under story plants of tropical forests (Vogelmann et al. 1996). However, the major responses are those of the photosynthetic machinery.