2011 International Year of Forests – ‘undergrowth’ and ‘canopy’

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Bromeliad species in the Christoffel Park. Curaçao is also home to epiphytes. However, there is no real 'canopy', because of the already low character of the 'mondi'. Photo: Michelle da Costa Gomez

When you travel to Latin America or the Caribbean as a tourist, almost everywhere you’ll get offers for a tour with the charming title “Canopy Tour”. Costa Rica as well Suriname, Puerto Rico, and a whole set of other islands in the Caribbean are known to offer this lucrative activity. When you decide to participate you will be strapped into a kind of harness and hooked up to a so called “zipline” you’ll then ‘fly’ through the treetops of forests, a monumental adrenaline rush.

“Zipping” for science

This activity was not invented for recreational purposes at first, but served as a practical application for scientists who were involved in the investigation of forests, and rainforests in particular. Through data collecting aimed at describing the species diversity of mainly rainforest, it soon became clear that the bulk of the flora and fauna was to be found in the treetops. And since those forests typically consist of very large trees whose tops are almost invisible from ground level, it became necessary to develop technical solutions for the exploration of the “canopy”. Using cables and platforms at different heights in the forest, scientists could map the hitherto unknown world of the “canopy”, which resulted in a wealth of discoveries.

Stratification of forests – the ‘undergrowth’

Forests are not only made up of (large) trees, they are usually divided into three segments: the lower section or ‘undergrowth’ (0.3-3 meters), the middle layer and the top layer or ‘canopy’. All three segments have their own characteristics and properties. The “undergrowth” is where the seeds of plants and trees fall and begin to germinate. The young plants will usually not get very far in their development because the foliage of large trees around them will block most sunlight, which is a necessity for growth. Many die, the strong will hang on while awaiting the day that one of the large trees will fall over, allowing a portion of light to penetrate that will promote growth. Apart from young trees the undergrowth is also home to a lot of plants that thrive in dark, damp places. These herbaceous plants need a little sunlight, but only indirect and in low quantities. The soil of the forest will stay pretty damp, because the low levels of sunlight do not cause much evaporation. This moist environment promotes decomposition of fallen branches, leaves and complete trees.

Mushrooms ensure the processing of organic material into useful vitamins and minerals for plant growth. Photo: Michelle da Costa Gomez

This process is assisted by a wealth of species of fungi (mushrooms) and insects, which are responsible for supplying essential vitamins and minerals to the soil. Without these processes, the big trees will not survive. The decomposition of organic matter and uptake of vitamins and minerals goes very quickly, contrary to what many people think, and the resulting layer of humus on the floor of tropical rainforests is usually very thin and will disappear in no time, if allowed. It is very fertile, but only present in very low quantities. These biological processes attract other wildlife to the ‘undergrowth’, as such adding to an impressive species diversity.

The middle layer and the ‘canopy’ of a forest

The middle layer consists of the stems of young trees and the few young plants that developed further than usual. The “canopy” is the real treasure chest of the rainforest. The tops of the trees are covered with so called epiphytes. These are plants that grow on other plants, but do not harm, unlike parasites. Usually these plants have tiny roots that will extract minerals from thin layers of organic material that are deposited on the trees. A layer of rotting leaves on a branch is the ideal place for a spore or an epiphyte seed to germinate. Within the crown of large trees there are numerous places where small amounts of organic material will accumulate, and where epiphytes grow in large quantities. The ‘canopy’ is usually loaded with ferns, orchids, bromeliads and thousands of other plants. Various animals such as monkeys, insects and frogs will get their supply of moisture through the water collection strategies of these plants, such as from the cup shaped reservoirs of the bromeliads. The epiphytes are also providing shaded, cooler places for animals to hide from the fierce sun. The plants serve as food as well.

"Canopy" bridge in St. Elena, Costa Rica. These suspension bridges are also very useful for long term studies of the upper layers of forest. Photo: Dirk van der Made (Wikipedia)

Canopy – treasure trove of new species

Most new animal or plant species discovered in the Amazon forest during the last few years, were found in the upper layer of the forest. Every time a scientist navigates through the tree tops with his or her tools there is a high probability of discovery of new species of flora or fauna. What might be even more important in the study of the forests and its stratification, are the complex relationships that exist between plant and animal species and the various segments of the forest. If no one would have looked at those relationships they never would have discovered that certain frog species, that lay eggs in small puddles that remain on and in between dead leaves on the ground, totally depend on the water reservoirs of bromeliads that grow 20-30 meters above the ground. After the tadpoles hatch and grow to a few days old, the male frog will attach them one by one on his back in a drop of water, and will jump up to 20-30 meters height to deposit them into the water-filled cup of a bromeliad. Here, the tadpoles will feed themselves with mosquito larvae, hatched from eggs that were laid in these pools. Also, one would never have discovered that certain species of monkeys are responsible for the fertilization of certain tree species. This process of fruit production is essential to the survival of birds and other mammals during the difficult dry season.

Don’t skip a visit to the forest!

If you get a chance to walk through a tropical forest during a future vacation, make sure to jump to the occasion. Especially if it can be combined with a “canopy” tour. But also focus your attention on your surroundings when you zip along the lines and zoom through the treetops, who knows what you’ll discover!

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