Responses of Plants to Light

The response of plants to light, i.e., their capture of light energy for photosynthesis, is influenced by several leaf characteristics, including (1) sclero-phyllous character (sclerophyllous leaves are more common in forests of nutrient-poor soils; because of their thickness, there is less light absorbed per unit of chlorophyll); (2) predominant leaf size (larger, horizontal leaves are more effective in capturing light, while smaller, vertical leaves on the tops of many tropical trees are more efficient in catching the light from the rising and setting sun); (3) presence of epiphylls (mosses, lichens, and algae) which can interfere in the capture of light; and (4) presence of a layer of anthocya-nins (red pigments) which enhance the capture of PAR (Mooney et al. 1984).

Leaf life span is often correlated with overall plant productivity. Long-lived leaves often occur on plants in soils low in moisture or nutrients. In such environments, it is energetically expensive to synthesize new leaves. In lowland tropical forests, leaves often live 3-13 months, while in upper montane forests the leaf life span can be 14-18 months, possibly because in montane forests, limited light or nutrients may inhibit leaf synthesis (Mooney et al. 1984; Jordan 1985). Reich et al. (2004) found that variation in light availability had consistent effects on leaf life span for all species studied in the rain forest at San Carlos de Rio Negro in the Venezuelan Amazon: species native to tierra firme forest in deeply shaded understory microsites, in small gaps, and in sunlit mature tree canopies had average leaf life spans of 3.2, 1.9, and 1.6 years, respectively. Species native to caatinga forest (poorer soils) had average leaf life spans of 4.2, 3.4, and 2.5 years, respectively, in these same micro-site types. Two species common in gaps and in disturbed sites had much longer leaf life span in shaded understory locations than in open, disturbed micro-sites.

The R/FR ratio found in the forest floor is key in determining many important biological processes, as, for example, breaking of dormancy of certain seeds and provoking their germination. When a canopy gap opens due to forest disturbance, the changes in the R/FR ratio may provoke germination of seeds of pioneer species such as Cecropia that are in the seed bank in the forest floor (Vazquez-Yanes and Orozco-Segovia 1990).

Understory plants have low photosynthetic capacity and low saturation levels, while canopy trees generally are not saturated, and often have higher photosynthetic capacity. The shaded understory of a tropical forest presents a particular challenge for photosynthetic acquisition of sufficient energy and carbon to support growth and survival (Chazdon et al. 1996). Light in the shaded understory is depleted of PAR and has a low R/FR ratio (i.e., there is lots of FR, because leaves absorb the red and do not absorb the FR). Leaves in the understory are adapted to low flux densities and also to responding very fast to changes in light availability, especially to sunflecks. The sun-flecks, or light that passes through holes in the canopy, have similar spectral quality as direct sunlight plus some additional wavelengths from light reflected by leaves. The light intensity and quality of sunflecks thus vary with time of day, angle of the sunfleck, colors of leaves and other plant parts. Sun-flecks are less intense than direct light, but between two and five times more intense than full shade. Individual sunflecks usually last less than 2 min.

Carbon assimilation in the understory is strongly dependent on sunflecks, which can provide 30-40% of the daily PPFD (Chazdon et al. 1996). In microsites where sunflecks are more abundant, they could be expected to account for an even larger fraction of the daily carbon gain of understory plants. Shade-adapted plant species have photosynthetic capabilities such as induction that allow them to respond very quickly to sunflecks. Induction means that if a leaf has been exposed to light it will respond faster to the next coming sunfleck (Chazdon and Pearcy 1991).

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