The Understory


July 31, 2012

Many of the seeds and fruits produced by canopy trees fall to the ground where they provide food for seed gatherers (rodents, birds, fish, etc) and create a natural seed bank in the leaf litter. There are two growth strategies once seeds reach the ground. One strategy is to produce large seeds with food reserves enabling the seedling to survive in the low-light conditions of the understory. The second method characteristic of many pioneer species is to produce huge numbers of small seeds which only germinate under certain conditions (usually light gaps). When a light gap opens, these seeds sprout and the seedlings rapidly grow to once again plug the hole in the canopy.

Since seeds are the agents for future growth, many have defenses against predators to ensure some will be left untouched to germinate. Plants reduce herbivory and seed eating with structural defenses like thorns, stinging hairs, or spines and the production of toxic compounds which interfere with digestion or are unpalatable. Often the seed is surrounded by an edible fleshy material to make it palatable to birds and mammals who can disperse it, but the seed is toxic or indigestible so it will be passed out or discarded. For example, the cashew has a sweet and juicy stem but a toxic seed, so monkeys eat the fleshy stem and discard the seed.

Rainforest fruit in Cambodia. Photo by Rhett A. Butler

Many rainforest trees, especially those of the forest interior, produce large cauliflorous fruits that grow directly out of the surface of the tree. This mechanism enables the tree to grow large, conspicuous fruits that attract larger animals. One of the largest cauliflorous fruits is the jackfruit of Southeast Asia which may be three feet long (1 m) and 110 pounds (50 kg). The jackfruit produces a strong odor that attracts nocturnal mammals like the flying fox.

A famous cauliflorous fruit of Southeast Asia is the durian, a thorn covered fruit that is said to "taste like heaven but smell like hell." The odor of the durian is so offensive that its presence has been banned in some public places. Nevertheless, the durian is an important export for Malaysia, Indonesia, and Thailand, which exported nearly 1 million metric tons in 1992. Both cocoa and coffee beans grow as cauliflorous fruit.

Dragon fruit. Click image for more pictures of seeds and fruit. Photo by Rhett A. Butler

Seeds and fruits provide an important food resource for forest-floor animals. To survive, a seed must elude these predators and many do so by being dispersed by specialized birds, mammals, and fish. These dispersers eat the fruit flesh without destroying the seed. Other seeds escape by being poisonous. Some of these toxins are useful to insects, which sequester them for their own purposes, and to humans. For example, many seed toxins have been chemically isolated and used to formulate drugs for humans, while umpteen insects naturally incorporate these compounds to render themselves toxic or use these toxins to produce scents to attract mates. For example, the male butterflies of the Danaiidae family convert alkaloids in the crotalaria plant into a scent to attract females.

Many large seeds have structured defenses against seed predators. In some cases seeds designed for gut passage by large mammals will have a sturdy seed coat, while bird-dispersed large-seeded fruit will often have seeds that are chemically defended. For example, the fruit of the nutmeg splits open to reveal a large seed covered with an orange-colored aril. The seed is highly aromatic, and the odor acts as a warning and toxic deterrent.

Cacao fruit in Peru. Photo by Rhett A. Butler

Review questions:

  • How do animals help the reproductive cycle of plants?
  • Why are many seeds poisonous?

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Continued / Next: Light Gaps, Seedlings, Shrubs