Forest Niche and Life Cycle

In the true rainforest the tabonuco tree is queen. True rainforest is the "tallest, most luxuriant, and most complex type of vegetation in the American tropics." It is where the forest "gives the impression of the vault of cathedral aisles." (Mom found those quotes for me.) This is the mature forest, the old-growth forest, virgin forest, climax forest. (Scientists use a lot of different names.) Mature forest species are those that live in a forest that has been undisturbed for decades, or even centuries. Here you won’t find the tree I’m not allowed to mention (as my brother so dumbly put it). You will find the greatest variety of trees. There are more than 150 species. You’ll also find the tallest, straightest trees.

Trees actually grow to several heights here. Some are short (10 to 30 feet or 3 to 9 meters). Some are mid-size (60 to 70 feet or 18 to 21 meters). Some, like tabonuco, are tall (100 to 120 feet or 30 to 36 meters). Of course, there are more than trees in this forest. There are ferns, orchids, birds, mushrooms, lizards, even yucky things (to me) like slime molds. They interact with each other. This is called symbiosis. For example, you find a lot of epiphytes or air plants. They live on tabonuco trees. The plant has a home on the tree, and the tabonuco is neither harmed nor helped by the air plant. Dad calls this commensalism. 

Sometimes the interaction within the forest community isn’t so pleasant. There is competition when different species need the same food or same resources, like water or sunlight.

Some local scientists call the old-growth forest the tabonuco forest type, in honor of my tree. Other scientists call it the subtropical wet forest life zone. It is only found where growing conditions are ideal. The land, below 2,000 feet (600 meters), is well drained. The soil is deep and porous (so the rain can pass through it). Temperatures are around 73 degrees F (28 degrees C). Rainfall is between 90 and 140 inches (225 to 350 cm). There is shelter from strong winds.

In El Yunque about 70 percent of its forest is considered "true" rainforest. Here, the tabonuco tree dominates. Its crowns (leaves and branches) make up one-third of the mature forest canopy. At the peak of the time when its leaves fall (in July), the tabonuco tree contributes more than half of the forest’s total amount of leaf fall. It is a very important tree. 

I'll go on to the life history. I must admit my brother surprised me in the good job he did with the life history of the yagrumo (Dad must have helped him a lot). Like all (well, most) trees, the tabonuco starts out as a seed. Of course, to form the seed, the tree's flowers must be fertilized. My brother didn’t go into that, so I will. 

Like the yagrumo, the tabonuco tree is dioecious. It has male and female flowers on separate trees. Adult trees produce flowers. The flowers are small and green in color. The male flowers have tiny stalks and pouches that contain pollen. The pollen contain sperm cells. The female flowers have a different kind of stalk. It is sticky on top and swollen on the bottom. The swollen part is called the ovary. It contains eggs. Okay, so far so good, I hope. Certain insects in the forest recognize the tabonuco flowers as a source of food. When they eat from the male flowers, they get covered with pollen. When they eat from the female flowers, some of this pollen comes off on the sticky tip of the stalk. The pollen reaches the ovary through a special tube. Fertilization occurs. The fertilized egg develops into a seed. This process is called pollination. It is an example of mutualism because both the insects and the trees benefit.    

The tabonuco seeds are big. They are carried into the forest by large birds (no creepy bats) such as the Puerto Rican parrot. The birds eat the seeds, and they also help spread the seeds to other parts of the forest. This is another example of mutualism.

Sometimes the seeds are damaged by parasites. Parasites live on, feed off, and often harm the seeds (this is not mutualism). Unlike the yagrumo, the tabonuco seeds sprout quickly. Then they sit around on the forest floor. You can say they form a seedling bank in the forest.

These seedlings can sit in the shade for a long time before they grow. They do need light to grow, but not the strong direct light needed by pioneer species. They produce less sugars during photosynthesis and have less nitrogen in their leaves than the pioneer species. So they grow very slowly. It takes several decades for tabonucos to become mature and produce seeds. Openings in the forest may come and go several times before the tabonuco tree actually reaches the top of the forest. Because of its slow growth, the tabonuco wood is very dense and strong.

As the tabonuco trees grow, the forest becomes shady. Eventually the pioneer species die. Remember, they need sunlight. The old-growth trees become dominant. Scientists call this succession. 

I feel a little sorry for the short life of my brother’s tree. I’ll tell you something not so good about the tabonuco. Once upon a time it was the most common and one of the most valuable of the tall trees growing in the lower slopes of Puerto Rico’s mountain rainforests. However, people came into the forest and cut the tabonucos down. Then they planted crops. In the 1930s the Forest Service replanted these lower slopes. This produced what foresters call secondary forest. They discovered that the tabonuco does poorly in growing back in secondary forest. 

It cannot grow back in open areas. Mom explained to me that foresters have tried to grow it in nurseries, but most of the seedlings don’t survive the shock of being transplanted. Also, the few that do survive grow very slowly. So tabonuco remains common in the old-growth forests but uncommon in the new forests. 

And that is the life history of the tabonuco tree.