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Light gap in the Brazilian Amazon. (Photo by R. Butler)
LIGHT GAPS
The majority of the world's remaining forests are not the classical rainforest with towering trees, an open interior, and virtually no ground growth. Instead, most rainforests have been impacted in their recent history by storms, fires, logging, and landslides and subsequently have scattered areas in various stages of regrowth.
A common event in the tropical rainforest is the fall of an emergent tree, usually during a tropical thunderstorm. In fact, it is estimated that tree turnover rates in some rainforests are every 80-135 years. When one of these giants—laden with lianas connected to neighboring trees—falls, it takes out a sizeable portion of the canopy. This hole in the canopy is known as a "light gap" because direct sunlight reaches the floor in contrast to the usual 1-5 percent under full canopy conditions. The opening of a light gap brings many changes to the section of rainforest.
The light gap is rapidly colonized by the same pioneer species that colonize clearings including trees like cecropia, balsa, macaranga, musanga, and bamboo, and shrubby plants like gingers, bananas, nightshades, climbing lianas, and rattan palms. These species are well-adapted for rapid growth, but not for long-term existence in the forest. Their often white wood and leaves with poor chemical protection are subject to infection and infestation by insects. Generally, these pioneers flower rapidly and produce numerous fruits, but are soon overtaken by the hardier, better adapted hardwood trees which fill in the gap in the canopy. Many forest tree species are dependent on light gaps to complete their life cycle.
Undisturbed primary rainforest, Brazil 1999
Nearby light gap, Brazil 1999
As a result of the increased light and abundance of fruits produced by gap colonists, light gaps are areas of increased animal activity. Carnivorous animals follow the herbivorous animals that are attracted to the fruiting plants.
SEEDLINGS
The fall of a tree gives new opportunity to the hundreds of hardwood seedlings that have been waiting years in a state of dormancy for just such an occasion of increased sunlight. The saplings are able to persist in such an extended state of dormancy thanks to several adaptations that enable them to cope with the poor light conditions: the 1-5 percent of sunlight that reaches the forest floor is not typical sunlight, but has high proportions of long-wavelength red and infrared light. Many species have colored pigments better adjusted to harvesting energy from the altered light, while others have red undersides thought to improve light exposure by reflecting light back through the leaf so the tissue gets a double dosage of light. Interestingly, the leaves of saplings on the forest floor differ substantially from those of the canopy, even within the same tree. Canopy leaves are rarely equipped with dark undersides and are usually small and waxy to conserve moisture and protect leaf tissues from the strong sun. Conversely, the leaves of saplings are often large, soft, and non-moisture conserving. In addition, saplings have low rates of growth to conserve the few precious sugars they are able to manufacture. Over the course of a decade, a sapling in the full shade of the canopy may show virtually no noticeable growth. For this reason, forest dwellers cut saplings to make semi-permanent trails since they will not need to cut them again for some time. These trails are suitable to the rainforest because they inflict little environmental harm and are undetectable to most non-forest peoples who may wander into the rainforest. New leaves of both floor and canopy height trees are often bright red to discourage predators by warning them of the leaves' bitter taste or toxic qualities.
As previously mentioned, there are hundreds of tree saplings waiting for the opportunity to take the place of the fallen tree. When the saplings are touched by direct sunlight, they are stimulated to grow rapidly. Each sapling seeks to become the first to reach the canopy, since those that do not slowly perish under the shade of the newly formed canopy.
RAINFOREST SHRUBS
Like hardwood tree saplings, ground-dwelling rainforest plants and shrubs have adapted to life in the low light levels of the forest floor. They often have structures very similar to those of saplings: dark undersides, deeper colored leaves, and large leaves. Even so, because of the shaded conditions, a true shrub story is unusual in forests and only scattered vines and shrubs are present. Many of the herbaceous plants of the rainforest are epiphytes, including ferns, bromeliads, orchids, and arums, although there are also exclusively terrestrial plants like some of those of the Zingiberales Order including heliconia, gingers, bananas, and birds of paradise. These plants are known for their colorful bracts and are also found in light gaps and under secondary growth conditions.
The world's largest flower, from the rafflesia epiphyte, is found on the rainforest floor. The largest recorded specimen had a diameter of 45 inches (1.14 m), but the average diameter is around 30 inches (75 cm). The plant exists most of its life as vine root, and only becomes visible when it emerges as cabbage-like structure. From this ugly structure comes the maroon and yellow flower which blooms for 3-4 days before dying. The flower releases a putrid smell which attracts flies for pollination. Rafflesia seeds are thought to be dispersed when large mammals like pigs tread on them and carry the seeds away, stuck to their feet. The loss of large mammals may be hindering rafflesia populations.
Another noteworthy group of forest-floor plants from Southeast Asia are the monkey cups. These plants, like the pitcher plants of other parts of the world, have modified leaves for catching insects, which serve as a source of supplementary nutrients like nitrogen and carbon. Interestingly, pitcher plants raised with good soil conditions produce few, if any, pitchers.
Near light gaps and forest edges where sunlight is stronger, big-leaved plants like heliconia (bananas) are more common. Heliconia are famous in the New World for their large colorful flower-like bracts. These bright red, orange, and yellow structures house flowers that produce copious amounts of nectar which attract hummingbirds and insects. Hummingbirds like to visit more than one species of heliconia, creating a risk of hybridization, so different species of heliconia have flowering parts of different lengths allowing the heliconia to deposit its pollen on a specific part of the bird. When the hummingbird visits another heliconia of the same species, the flower has receptor parts that catch the pollen off the particular part of the bird.
Review questions:
What is a light gap and why is important to forest regeneration?
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