Climate & Landscapes — AID Forests

Tropical Dry Forests: Climate & Landscapes

Tropical dry forests are dominated by deciduous vegetation and experience highly seasonal rainfall.

What is a tropical dry forest?

The answer to this question is not actually as simple as you might think. There is not a standard definition that is agreed upon by scientists, nor a standard name! Even though there is no clear consensus for the definition, tropical dry forests are generally considered to be dominated by deciduous vegetation and experience highly seasonal rainfall. The debate mainly centers around the percentage of vegetation that must be deciduous, the number of months without rainfall, and the amount of vegetation that must be woody plants. Here is a table summarizing the various definitions.

One consequence of this lack of a standard definition, however, is that it complicates the program of accurately assessing the health and conservation status of tropical dry forests around the world. Regardless of the exact definition used, however, tropical dry forests around the world are in trouble.

Inside a Tropical Dry Forest

Most people imagine lush jungles when they think about tropical forests. However, tropical environments are comprised of more than evergreen rain forests. One of the striking features of tropical dry forests is how much they change over the course of the year. As Janzen describes, “Its 4- to 7-month rain-free dry season is sufficiently harsh that many species of trees, vines, and herbs are deciduous for 2 to 6 months. Its rainy season, during which 1 to 3 meters of rain can fall, is as wet, if not wetter, than that of a rain forest. In the dry season, the sun penetrates to the forest floor, the leaf litter becomes very dry (and virtually ceases to decompose), watercourses dry up or greatly diminish in flow, and daytime relative humidity ranges from 20 to 60%.”

The experience of walking through a tropical dry forest really depends on when you are there. During the rainy season, the forest is a raucous cacophony of insects, birds, and other animals, as well as dripping or running water, while in the dry season the forest can often be eerily silent except for the wind. It can look fairly dead. Many plants lose their leaves and can become dormant. Animals may migrate or hibernate. And yet, some plants will flower or fruit during this time, somehow finding enough water for these water intensive phases of reproduction despite months without rain! Then, when the rains finally return, the forest reawakens. Little green freckles of tiny new leaves appear throughout the forest and then quickly develop into full-grown, almost neon-green leaves. The transformation back into a lush forest can happen within just a few weeks.

In Madagascar, for example, Axel found that 6% of the primary tropical dry forest in the Kirindy Mitea National Park burned within just 3 months in 2013 due to fires that were started outside of the park to improve grazing conditions for cattle but then escaped control.

Agricultural expansion is often associated with the clearing of tropical dry forests.

Tropical dry forests are disappearing due to charcoal production in areas where people have little or no access to electricity or gas for cooking and boiling water so that it is safe to drink. The timber is needed for building houses, furniture, and in southwestern Madagascar, the small and large boats that are critically important for fishing, transportation, and cultural identity.