The Rain Forest Biome: Is the
Climatic Devil in the Details?By now you ought to understand how much annual
air temperature and moisture patterns affect, indeed
determine, the actual nature of the world’s various
biomes. What then is a rain forest?
The physical structure of tropical rain forest will be
the subject of chapter 3. But let’s define tropical rain
forest here. It turns out that there are actually several
definitions of rain forest in use by ecologists, and while
generally similar, they are not precisely the same. You
will have to choose the one you like when you visit there.
The definition given by Holdridge in his life- zone
model represents extreme rain forest, one that receives
very high and continuous input of rainfall. Thus a
“real” rain forest, in its purest form, is essentially
a nonseasonal forest dominated by broad- leaved
evergreen trees, sometimes of great stature, where
rainfall is both abundant and relatively constant. Rain
forests are the very definition of lushness, with many
kinds of vines and epiphytes (air plants) draping the
statuesque trees. In general, a rain forest receives at
least 200 cm (just under 80 in) of rainfall annually,
though it can be much more), with this precipitation
spread relatively evenly from month to month.
Much of the tropics consists, however, of forests
where some seasonal variation in rainfall is both
typical and ecologically important. Tropical forest with
abundant but seasonal rainfall is often termed a moist
forest rather than a rain forest. That being said, a moist
forest is scarcely discernible from a rain forest. It is
an evergreen or partly evergreen (some trees may be
deciduous) forest receiving not less than 10 cm (nearly
4 in) precipitation in any month for two out of three
years, frost- free, and with an annual temperature of
at least 24° C (about 75° F) or more. Since the term
rain forest is in such widespread and common usage,
in this book I will continue to refer to lush, moist
tropical forests, seasonal or not, as rain forests. I’ve had
the pleasure of wandering in many moist forests from
Belize to Brazil, and believe me it rains a lot in those
moist forests, and gets pretty muddy too.
So what it comes down to is seasonality. How
extreme is dry season? That is really at the essence
of understanding the ecological dynamics of tropical
forests.The Importance of Seasonality
Precipitation, all of which falls as rain in the lowland
tropics, sometimes varies considerably from month
to month and from one location to another. Overall,
precipitation is highest in the central Amazon Basin
as well as on eastern Andean slopes and lowlands, and
is lower to the north and to the south, varying from
about 150 cm (59 in) in the extreme north or south
to about 600 cm (236 in) along some eastern Andean
slopes. Even within the central Amazon Basin, seasonal
rainfall is variable from place to place. For example,
Iquitos, Peru, along the Amazon River, receives an
average of 262 cm (103 in) of rainfall annually, while
Manaus, Brazil, also on the Amazon River, receives an
average of 177 cm (70 in) and experiences a distinct
dry season. Forest structure varies between Iquitos and
Manaus, mostly due to the difference in seasonal rainfall
pattern and overall amount. As a more extreme example,
Andagoya, in western Colombia, receives 709 cm (279 in)
annually. (The highest rainfall on Earth, according to
some authorities, occurs in Mawsynram, in Meghalaya,
India, which annually receives 1,187 cm/467 in.)
Where dry season is pronounced, many trees are
deciduous, shedding leaves during dry season, rather as
trees in the temperate zone do in the autumn months.
Such tropical dry forests are sometimes termed monsoon
forests, since they are in leaf only when the monsoon
rains are present. Dry season is defined as less than 10
cm (4 in) of rainfall per month, and rainy season features
up to 100 cm (39 in) of rainfall per month. A typical
lowland tropical forest receives a minimum of between
150 and 200 cm (approx. 60– 80 in) of rainfall annually.
The rainy season varies in time of onset, duration,
and severity from one area to another in the tropics.
For example, at Belém, Brazil, virtually on the equator,
dry season extends from August through November,
and the wettest months are January through April. In
Belize City, Belize, at 17° N, the rainy season begins
moderately in early June but in earnest in mid- July
and lasts through mid- December and sometimes into
January. The dry months are normally mid- February
through May. In general, when it is rainy season north
of the equator, it is dry season to the south. Because
the Amazon River flows on both sides of the equator
(which side depends on location), parts of the huge
river are experiencing wet season while other parts are
in dry season.chapter 2 why it is hot, humid, and rainy in the tropics 33