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Tree Saplings in the Understory

by:Annika Keeley.

Jungles are green down low, green in the middle, and green up high. And dark. Therefore the plants that grow underneath the canopy of a tropical rain forest must be adapted to grow in the shade. Even trees that help form the canopy when they are large, have to start small down low to the ground. (Strangler figs are an exception: they start as epiphytes in the tree canopy and grow roots down to the ground, eventually enveloping the host tree) . Therefore, shade-tolerance is one of the most important factors characterizing tree species in the tropical rainforest. Trees are often classified as shade-tolerant, intermediately tolerant, or intolerant. Researchers measure growth patterns, and ecophysiological characteristics of representative tree species to determine shade tolerance.

At Tirimbina, N. Vera, A. Newton, and Bryan Finegan measured photosynthetic characteristics of tree saplings that belonged to species representative of one of the shade-tolerance groups . They studied tree saplings (0.5-1.8 m / 1.5-6 ft tall) because this stage is critical in the life histories of trees due to slow growth and high risk of mortality. They used saplings in two plots that differed in the amount of sunlight reaching the ground. You would expect the energy production to be high in shade tolerant species, even when it is shady. Vice versa, in the shade you would expect the energy production to be low for shade-intolerant species.

The researchers found no clear relationship between shade tolerance groups and their energy production. They did find though that the energy production in all saplings was higher in the plot with more light compared to the shadier plot. So, even saplings of the species classified as intermediate or shade-tolerant had a higher energy production rate when more light was available. This means that the description is just right: the saplings tolerate the shade rather than require it. During the study the researchers concluded that the behavior and chemistry of a tree may change throughout its life; saplings even of light-demanding adult trees may be adapted to tolerate the shady conditions of the understory. Other adaptations to life in the understory of tropical rainforest (that were not part of this study), such as the efficient use of short-term light available in sun flecks and the prevention of damage by insects, may play a more important role in the survival of tree saplings than the maximum energy production.

tree saplings in the understory 1tree saplings in the understory 2
tree saplings in the understory 3

Just for Fun: 2 Tree Species One tree that is considered shade-tolerant is the oil tree (Pentaclethra macroloba, gavilán in Spanish), the most common tree in this area. Like most of the plants in the legume family (which also contains beans), they can fix nitrogen from the air. This makes them a great pioneer in the regeneration of disturbed areas. As a member of the subfamily Mimosoideae, oil trees can ‘sleep’ by closing their leaves at night. Thus forests dominated by oil trees are dark inside during the day, but light on a moon-lit night. The seeds are very oily (hence the name oil tree), and are used to make soaps and hair conditioners . They are poisonous though and are avoided by birds and most mammals.

The botarama tree (Vochysia ferruginea) is a shade-intolerant tree. It is fast growing, long-lived, and common in secondary forests. It only gets established in canopy gaps where lots of light reaches the ground. The Spanish name ‘botarrama’ means ‘throwing-branch’, describing the characteristic of self-pruning. A tree will shed dead lower branches which didn’t receive enough sunlight to survive. In April -May, and again in September – October, the botarama puts on a show with its bright yellow flowers. It is then easy to see how common this species is in the Sarapiquí region. It is now grown in plantations because it is fast growing and has beautiful wood. Ecophysiology: the study of the interrelationship between an organism's physical functioning and its environment. Vera, N.E., B. Finegan, and A.C. Newton. 1999. The photosynthetic characteristics of saplings of eight canopy tree species in a disturbed neotropical rain forest. Photosynthetica 36: 407-422.;

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