The New Neotropical Companion

“Hot, humid, and rainy” is the first observation

virtually everyone makes about the tropics, particularly

the lowland tropics. The aircraft door swings open,

and as you exit there is a rush of hot and humid air in

your face. The cool and dry air- conditioned cabin of

the aircraft is past tense as you enter a world where the

dew point is rarely out of the uncomfortable range and

often in the oppressive range. No, it is not just the heat;

it’s also the humidity. It takes some getting used to. In

this chapter I will explain why the tropical climate is so

hot and muggy and why that is really important with

regard to ecology, particularly of rain forests.

On Biomes and Life Zones: Just

Three Numbers

In 1947 a paper was published in the scientific

journal Science that bore the title “Determination of

World Plant Formations from Simple Climatic Data.”

The author was Leslie R. Holdridge (1907– 1999), a

specialist in tropical forest ecology and biogeography.

Holdridge was able, by taking three measures of

climate (and only three), to accurately determine just

what sort of terrestrial ecosystem would occur in any

area on Earth. Those three measurements are (1) mean

annual temperature, (2) total annual precipitation,

and (3) ratio of potential evapotranspiration (which

is a function of both moisture and temperature) to

mean annual precipitation. Holdridge published a now

famous triangular diagram in which each side of the

triangle represented one of the three climate variables

he had identified (fig. 2- 1). He was able to show that

each of the world’s major ecosystem types fell into a

climate- determined hexagonal spot on the triangle.

Holdridge referred to each of the hexagons as a life

zone. Rain forest was at the lower right of the triangle;

its climate was described as super humid and uniformly

warm, with high annual precipitation. Holdridge noted

that his life- zone diagram applied both latitudinally

and elevationally. The power of Holdridge’s approach

was that he showed unequivocally that climate and

climate alone is the principal determinant of the

structure of terrestrial ecosystems. All other variables

are secondary.

Though Holdridge’s triangle diagram is conceptually

simple, it may be further simplified by graphing mean

Chapter 2. Why It Is Hot, Humid, and Rainy in the Tropics

Latitudinal

regions

Potent

ial evapotranspiration ratio

Annual precipitation (mm)

Altitudinal

belts

Polar Alvar

Alpine

Subalpine

Montane

Lower montane

Premontane

Critical

temperature

line

248 C

128 C

68 C

38 C

1.5 8 C

Desert Desert

Dry

tundra

Moist

tundra

Wet

tundra

Rain

tundra

Rain

forest

Rain

forest

Rain

forest

Rain

forest

Wet

forest

Wet

forest

Wet

forest

Wet

forest

Moist

forest

Moist

forest

Moist

forest

Moist

forest

Dry

forest

Dry

forest

Thorn

steppe /

woodland

Thorn

woodland

Very dry

forest

Dry

scrub

Desert

scrub Steppe

Desert

scrub

Desert

scrub

Desert

Desert

Desert

Desert

Desert

Subpolar

0.25

0.12

(^5) 62.5
12
5
250
500
1000
2000
4000
8000
16000
0.5
1
2
4
8
16
32
Boreal
Cool temperate
Warm temperate
Subtropical
Tropical
Superarid Perarid Arid Semiarid Subhumid Humid PerhumidSuperhumid
Humidity provinces
Biotemperature
Figure 2– 1. Leslie R. Holdridge’s classic and innovative triangle diagram spoke clearly to the power of climate to determine
fundamental characteristics of terrestrial ecosystems. From Holeridge, L. R. 1947. Determination of world plant formations from
simple climatic data. Science 105: 357–368. Image adapted from Peter Halasz. Reprinted with permission from AAAS.
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