There are four very distinct layers of trees in a tropical rain forest. These layers have been identified as the emergent, upper canopy, understory, and forest floor.
· Emergent trees are spaced wide apart, and are 100 to 240 feet tall with umbrella-shaped canopies that grow above the forest. Because emergent trees are exposed to drying winds, they tend to have small, pointed leaves. Some species lose their leaves during the brief dry season in monsoon rainforests. These giant trees have straight, smooth trunks with few branches. Their root system is very shallow, and to support their size they grow buttresses that can spread out to a distance of 30 feet.
· The upper canopy of 60 to 130 foot trees allows light to be easily available at the top of this layer, but greatly reduced any light below it. Most of the rainforest’s animals live in the upper canopy. There is so much food available at this level that some animals never go down to the forest floor. The leaves have “drip spouts” that allows rain to run off. This keeps them dry and prevents mold and mildew from forming in the humid environment.
· The understory, or lower canopy, consists of 60 foot trees. This layer is made up of the trunks of canopy trees, shrubs, plants and small trees. There is little air movement. As a result the humidity is constantly high. This level is in constant shade.
· The forest floor is usually completely shaded, except where a canopy tree has fallen and created an opening. Most areas of the forest floor receive so little light that few bushes or herbs can grow there. As a result, a person can easily walk through most parts of a tropical rain forest. Less than 1% of the light that strikes the top of the forest penetrates to the forest floor. The top soil is very thin and of poor quality. A lot of litter falls to the ground where it is quickly broken down by decomposers like termites, earthworms and fungi. The heat and humidity further help to break down the litter. This organic matter is then just as quickly absorbed by the trees’ shallow roots.
Urban Tree Decline
Importance. – Among the problems leading to urban tree decline are: air pollution, soil compaction, mower- and machinecaused injuries, poor pruning, heat reflected from streets and buildings, direct root damage from excavations and turf cultivation, paving (figure 16.1), improperly applied herbicides, potting above and below ground level, overplanting, and lack of understanding about tree growth and development. For these and other reasons, urban trees generally suffer a diseased existence and must be frequently replaced. Those that survive are often aesthetically unpleasing.
Identifying the Cause. – In addition to the above, a variety of fungi can attack trees that have been weakened. Most are heart, butt, and root rotting fungi that can affect trees structurally, making them unsafe. Others attack the roots, causing the tops to die back. Only rarely will all the causal agents in urban decline be identified.
Identifying the Injury. – Identifying the injury is usually easy. Affected trees show a dieback of the crown, beginning with the uppermost and outermost branches first. In the final stages, the trees may have only a few green sprouts and leaves attached to the main stem.
Biology. – Tree crowns most frequently begin to die back when the roots have been damaged or are diseased. This is due to the fact that plants grow with a carefully balanced root/ shoot ratio. When a portion of the roots ceases to function, a portion of the crown dies as well. Often, disease fungi enter the weakened portion of the tree and further damage it.
Control. – Protect, fertilize, and irrigate trees that are declining. Plant trees that are resistant to air pollution injury and drought, and provide trees with adequate root space and aeration. Remove dying trees to avoid danger to people and property.