Light Environment of Tropical Forests
Light is the aboveground environmental factor that affects plants the most, and it is also the most variable. For example, on a sunny day, instantaneous measurements of photosynthetically active radiation range over three orders of magnitude, from less than 10 ^mol m-2 s-1 in closed-canopy understory of mature forest to well over 1,000 ^mol m-2 s-1 in exposed micro-sites of gaps and large clearings, or at the top of the forest canopy (Chazdon and Fetcher 1984). The light environment is important for understanding natural forest regeneration and designing techniques for natural forest management. Pertinent questions are: what are the light requirements of the preferred species? How much should the canopy of a forest be opened to allow for increased light in the understory to favor growth of desired species? Knowledge of the light environment is also important for the proper design and management of agroforestry systems and plantations. What species should be in the upper, mid, or lower layers of a multi-strata agroforestry system based on their light requirements? One can learn how to manage the light environment depending on species requirements, or alternatively, how to design a system with species that match the light availability offered by the different microenvironments that are already present in the system.
Attention to the light requirements of species is basic for designing sustainable forest management techniques. For example, it is well known that mahogany is a light-demanding pioneer species that regenerates after catastrophic disturbances such as fires, floods, and landslides that open up large areas of the forest. Most other tree species associated with mahogany have little or no commercial value; therefore, mahogany logging is highly selective and opens up very little of the forest canopy (Lugo et al. 2002). The result is insufficient light levels at the forest floor, a condition inhospitable to regeneration of mahogany. Enrichment planting of mahogany in the understory of natural forests (described in Chap. 6) has had poor results because mahogany seedlings require full light to grow rapidly, a condition found only in larger canopy openings. In most cases, mahogany has responded favorably to silvi-cultural treatments intended to release mahogany seedlings from competition for light by cutting climbers, girdling surrounding trees, thinning saplings, and pruning.
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