The New Neotropical Companion

annual temperature against mean annual precipitation
(fig. 2- 2). Tropical rain forests will fall into the area on
the graph where mean annual temperature is 20– 30° C
(68– 86° F) and annual precipitation is 250– 450 cm
(98– 177 in). Tropical seasonal forests (often referred to
as tropical moist forests) are in exactly the same range
of annual temperature range, but precipitation is lower,
between 150 and 250 cm (59– 98 in), and often uneven
throughout the year, so there are distinct rainy and dry
seasons. Note that for most visitors to the tropics, what
passes for tropical “rain forest” is actually within the
definition of tropical moist forest. Nonetheless, these
forests are extremely wet and lush, and they look and
“feel” 100% tropical. In areas with even less annual
precipitation the tropics support dry deciduous forest or
perhaps savanna, primarily a form of grassland, usually
with scattered trees of varying density (chapter 14).
The power of Holdridge’s analysis is in how it shows
the pervasive influence of climate in determining the
structure of terrestrial ecosystems. Other variables
also come into play but to lesser degrees. Geology as it
affects soil characteristics (what ecologists call edaphic
factors) is very influential on ecosystem characteristics,
as are various biological factors. Nonetheless, rain

forests and other ecosystems are products primarily

of climate. The meaning of that statement will become

increasingly clear as we look at rain forest plant

characteristics and plant adaptations (chapter 3).

How’s the Weather Today? The Feel

of the Tropics

Should you decide to move to Manaus, Brazil, or

perhaps to Iquitos, Peru, both well within the Amazon

Basin, you should expect at least 130 days of rain per

year and in some places up to 250 days with at least some

rain. So, bring an umbrella. But wear light clothing

because the air temperature will be consistently warm,

indeed, often rather hot, with daytime high of about

31° C (88° F) and a nighttime low of about 22° C

(72° F). The relative humidity is rarely less than 80%,

so the combination of heat and humidity means the

“feel” of the weather can be much more stifling than

temperature alone would indicate. Though it can rain

on any given day, rainfall, in most places, is relatively

seasonal (fig. 2- 3). In the Amazon Basin, the very heart

of the lowland Neotropics, the climate is permanently

30

50

Mean annual precipitation (cm)

100 150 200 250 300 350 400 450

25

20

Desert

15

10

5

0

 5

 10

 15

Mean annual temperature (

C)

W

oo

dla

nd

W

oo

dl

an

d

Cold

6 22 temperate

14

15,16

19

21

Tundra, etc.

Taiga 7

Warm

temperate

Temperate

rain forest

Temperate

forest

Shrubland

Thorn scrub

Semi-desert(Grassland)(13)

18 11 10

20

17

(Savanna)

(12)

9 8

4, 5 3

Tropical Tropical

rain forest

1

Tropical

seasonal

forest

2

Arctic

alpine

Figure 2– 2. By comparing mean annual precipitation and mean annual temperature it is easy to see that the area we call the

humid tropics falls into the maximum category for both climatic variables. Reprinted with permission from Kricher, John.

Tropical Ecology. Princeton, NJ: Princeton University Press, 2011.

30 chapter 2 why it is hot, humid, and rainy in the tropics