As you move about 10 to 25 degrees north and south of the equator, the unchanging wetness of the rain forest begins to dissipate, giving way to a more seasonal climate. The temperatures are still warm throughout the year, but precipitation in tropical dry forests comes in bursts of only five or six months. The rest of the year is dry and relatively bare; in some areas during the dry season, these forests may resemble savannas or even deserts.
The soils are often relics of the ancient continent Gondwana, especially in directly detached, isolated systems like New Caledonia. The soils are more fertile than that of the rain forests, but erosion is higher, especially in deforested areas.
The dichotomous nature of the climate in the dry forest drives life to cope with the extreme seasonality of precipitation in different ways. The driest areas are dominated by deciduous trees that drop their moisture-exuding leaves once the rains stop, allowing sunlight to pass through the once-thick canopy, to reach the lower levels of the forest. Wetter areas, like Southeastern Indochina, are home to large forests of dipterocarps (pdf) and other evergreens, able to utilize the blessings of the monsoon rain season to keep growing throughout the year. And, unlike the plants of the rain forests, that rely almost entirely on animals to disperse seeds, seeds in the dry forest are swept through open areas by strong winds.
Wildlife is diverse in the dry forests, but necessarily migratory in some areas due to the extreme seasons. During the dry season, many animals will travel to a certain area in the neighboring rain forest to subsist, waiting for the rains to return to their home. This extra level of complexity has given ecologists a new challenge: Tracing the migratory paths of animals reliant on both tropical dry and wet forests. How much do these species rely on each biome for subsistence?
Dry forest systems are not quite as biologically diverse as rain forests, but they are home to many rare species found nowhere else: tigers, leopards and jaguars, sloth bears, a myriad of kingfishers and flycatchers, the Komodo dragon, the maned wolf, the elusive Javan rhino and unique plants like Gouania and Cycas beddomei. The Madagascar dry forests are home to seven species of baobab tree. The whole of Africa can claim only one.
While I'm generally working under the assumption that the reader knows the dangers faced by each biome, the dry forest has been massively altered by human influences. There is a huge difference in human population between tropical wet and dry forests. Murphy and Lugo provided an estimate in 1986: In Central America, only 7 percent of the people lived in wet forest areas, while 79 percent live in dry forest areas. The relatively fertile soil drives deforestation for agricultural purposes, and has whittled dry forests down to just about 2 percent of their former area in places like Central America.
Guanacaste National Park in Costa Rica is usually hailed as the prevailing example of how dry forests can be saved and care of them placed in capable, educated hands of native peoples. Ecologist Dan Janzen began with a simple problem - why does the guanacaste tree produce so much fruit if it just lays around - found a simple answer - it had evolved to rely on herbivores like camel and ground sloths to disperse its seeds; unfortunately, those animals were hunted to extinction 10,000 years ago - and then used this knowledge to set up a system to save the tree - by introducing domesticated herbivores to the park - and encouraging the Costa Rican people to preserve their lands. Janzen called it a "biocultural restoration."
Next in the order of increasing latitude is the savanna, tropical grasslands that actually cross the boundaries with dry forests. More on that later.