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
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
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
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!
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!)
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%)