Lecture Schedule back


 I.Introduction to tropical rainforest (evergreen tropical forest)

Associated with warm, moist tropical lowlands; equatorial rainbelt; short dry season sometimes but not intense

Most complex structurally and diverse

Multi-layered, evergreen canopy provides a mosaic of ncihes which support great diversity of biota

Recurrent structural and physiognomic adaptations to similar environmental conditions, however, these convergent traits have arisen in very different floras

 II.Location of tropical rainforest biome

Equator to 10° or 25° North and South and between 0 - 1000 m in elevation in the Americas, Africa, SE Asia

Along coasts windward to the trades in E. Brazil, Madagascar, NE Australia

East coasts with orographic precipitation in E. Panama and Costa Rica, E. Puerto Rico

3 floristically diverse formations

1. American: largest; 50% of biome
2. African - Congo Basin of N. Zaire and coastal zone from Nigeria to Guinea: 20% of biome
3. SE Asian - Indomalaysia; Sumatra east to island of W. Pacific; 30% of biome

 III.Relationships to other forest types (see Holdridge system)

Grades into montane and cloud forest in elevational gradient

Grades into monsoon forest, subtropical evergreen, and tropical and subtropical deciduous tropical forests along water gradient ( & latitude)

Alluvial forest

Mangrove forest

Swamp forest: seasonally flooded

Várzea: flooded by muddy water tributaries of Amazon
Igapó: flooded by nutrient poor waters of sandy soils

 IV.Rainforest Climate

Diurnal patterns! (not seasonal); up to 2.8 C daily temperature range (sometimes greater than seasonal change!)

25 C mean annual temperature; average of 5 C difference between warmest and coldest months; 18 C exceeded in all months

Large annual rainfall; continual day time heating produces convectional uplift and results in predictable daily cycle of cloud build up and rain. 2000 - 4000 mm rain (2-4 meters!) or greater (especially in orographic areas). Therefore large annual water surplus and high soil-water storage throughout the year

 V. Rainforest Soils

Warm soil and water surplus combines to promote decomposition of rock to great depths

Laterite soil; red, little litter, low nutrients; silica leached out; Al and Fe left behind; iron oxides give red color, pH 4.5-5.5

Temperature and rainfall allow rapid decompostion of litter; therefore no organics; soil incapable of holding nutrient base cations; therefore infertile and nutrient limiting; nutrients all tied up in biomass

Severe nutrient limitation in other areas; white sand areas

Plants respond in a number of ways as we will see to deal with this nutrient deficiency

 VI. Rainforest Vegetation

A. General overview

Warm and uniform temperature and wet climate allows for broadleaf evergreen forest to dominate.

Net productivity is highest of any terrestrial biome

Diversity is high. 100,000 species or roughly 40% of worlds angiosperm flora. High divresity possibly due to (1) stable ecologically, (2) climatic change and allopatric speciation, and (3) coevolution

Large number of species in tree stratum; 40-100 species per ha; 3,000 species trees in just few km2

Amazon                  Wisconsin
60,000 spp                2,000 spp
6,000 tree spp             50 tree spp

As many tree species in 2 or 3 hectares of Amazonas as in all of Wisconsin !

Exceptions do occur: Mora excelsa (Leguminosae) dominates in Trinidad

Floristic dissimilarity of the three regions making up the biome

Palms basically lacking in Africa (but not Madagascar!)

Dipterocarpaceae diverse in SE Asia; making up 80% of canopy and 40% of understory; in monsoon regions (related biome) of India, one species (Shorea robusta, sal) can dominate

We will discuss this later in the course, but does suggest a couple of things; antiquity of the rainforests and early separation of the tropical forest if ever connected

Al Gentry's family lists of 3 rainforest regions

B. Trees

Tall and closely set trees forms continuous canopy cover; therefore dense shade below

Trees smooth barked and unbranched in lower 2/3

Characteristic and important classficiation of tree architecture

3 strata generally (is it sampling artifact?)

emergent crowns 40 m (130 ft); discontinuous; with buttress or plank roots for support (shallow rooted)

15-30 m (50 - 100 ft); continuous canopy

5-15 m (15-50 ft) lower zone; small, slender, narrow crowns, stilt roots for support; often palms dominate here

shallow feeder roots, often with mycorrhizal relationship (like saprotrophs), takes up nutrients quickly; therefore efficient recycling of nutrients

C. Leaves

Leaf size and evergreeness increases with temperature and precipitation

Canopy leaves exposed to recurrent dry periods have thick cuticle and leathery but not xeromoprhic

36% of spp in Ecuador lowland forest have compound leaves vs. 9% in adjacent montane forest (usually feature of drier forests). Perhaps provides competitive edge, rapid deployment (Givnish, 1978) or adaptation for heat dissipation (Gates et al., 1968)

New leaves often lax and reddish or white; anthocyanins to prevent photoxidation until cuticle gets fully develped

Interior forest more stable (very dark and very humid) produce drip tips. Rapid rate of growth produces nodding foliage or drip tips

Ghana undergrowth study showed 90% of spp have drip tips and drip tips dry w/in 20 min vs. still wet after 90 min

Increase photosynthesis and prevent accumulation of epiphylls?

Leaf shedding not synchronized; some species have their own timing perhaps cueing on slight changes in light period (march of seasons is slight but present)

D. Herbs

70% of all species are phanerophytes or trees

Low light levels of forest floor discourage herbs: certain families are specialists - Gesneriaceae, Melastomataceae, Commelinaceae and other slow growing perennials. These often have velvety or variegated leaves; white or red patches; metallic shimmer.

Riparian (along streams) or gaps also common - especially the order Zingiberales and other large coarse monocots

Saprophytes are common in these conditions: low nutrients (mychorrhizal) and low light (nonphotosynthetic) - especially families Gentianaceae, Burmanniaceae

E. Lianas

One cost effective method in struggle for light

Exploit tree as support for rapidly growing flexible stem which can be rope like, up to 20 cm or 8 in diameter; can form old! secondary thickenings but pliable to follow growth of tree (eg. Bauhinia)

Forms profusely branched crowns in canopy (eg. Combretum)

90% of all lianas confined to tropics, most of them in rainforests because of difficulties of water transport in long stems confines them to wet tropics

Important families include Bignoniaceae, Asclepiadaceae, Apocynaceae, Leguminosae, Passifloraceae, Cucurbitaceae

F. Epiphytes

One cost effective method in struggle for light

Germination in top most branches of trees; host solely as means of physical support

Important examples: Orchidaceae, Bromeliaceae, Araceae, Cactaceae, ferns, mosses, liverworts, lichens, epiphylls: microscopic, algae, mosses

Adaptations to problems (similar to living on rocks)

1. take up water only when raining or fog; frequency of uptake greater importance than absolute quantity of rain. Thus epiphytes are greatest in windward sides, mountain slopes for example with ascending air and orographic rain; leaves constantly dripping

2. resist dessication (in dry intervals)

many ferns can tolerate dessication
store water (succulence as in Cactaceae)
succulent leaves: orchids, bromeliads, peperomiads
leaf tubers as water reservoirs (Orchidaceae)
velamen of aerial roots ensure rapid water uptake (Orchidaceae)
water absorbing scales in leaves forming funnel (Bromeliaceae)
other ferns more sensitive: produce "trash baskets" soil from litter with feeding roots going in (eg. Asplenium nidus)
CAM in many epiphytes in Brazil

G. Stranglers

Start as epiphytes and grow roots down host tree; when root reaches ground, shoot elongates and roots thicken and coalesce; strangulation of host via "root" stem; palms without secondary growth can not be strangled but die if canopy is too covered with strangler

Ficus (Moraceae), Clusia (Clusiaceae), Metrosideros (Myrtaceae)

H. Hemi-epiphytes

Germinate on ground, grow up as lianas (root climbers); bottom dies, becomes epiphytes

Araceae : known to "walk" or "snake" through the forest looking for light; as juveniles they search for dark places first (i.e., tree trunks where most shade is) as that will ensure a path to light!

 VII. Cloud Forests

A. Elevation and humidity related

NE trades in Caribbean brings rain when wind is confronted by obstacles such as mountains (see handouts of Venezuela). From coast to coastal mountains, there is considerable variation in climate and vegetation.

Cloud forests form at cloud level, maximum humidity; not definite altitude, but correlated with humidity at the foot of the mountain; greater the humidity, the lower the cloud forest (e.g., Panama is lower than Andes)

B. Vegetation

Epiphytes most abundant here: (Ericaceae); trees smaller; lianas rare

Tree ferns, ferns - eskpecially Hymenophyllaceae (filmy ferns), lycopods, Selaginella

Gunnera (Gunneraceae), Rubiaceae

C. Elfin forest

(stunted and mossy!)

 VIII.Reproductive strategies in tropical wet forests [plant-animalinteractions]

A. Pollination

Outcrossing mechanisms well developed (e.g. dioecy or dioecious)

20%+ of tall trees in Costa Rica are dioecious vs. 12% for small trees and shrubs (Tom Croat, 1979))

26% in Sarawak (Asian forests; Peter Ashton, 1969)

40% in Nigeria (Jones, 1955)

Wind pollination rare in mature rain forests (common in seral stages but dropped from 38% to 8% in two years of large light gap in Costa Rica)

Animal pollination

bats : cauliflory

hummingbirds (Neotropics); sunbirds and honeyeaters elsewhere

bees (small and large), butterflies and moths, beetles [some very specific or coevolution]

some mammal

B. Fruit/seed dispersal

Fleshy fruits the rule: correlation with dioecy

bats: green and yellow fruits
frugivorous birds: arillate seeds
larger mammals (monkeys)

Wind dispersal (5-10%)

Water dispersal (1-2%)

 IX. Other animals