Tropical Rain Forests - Physical and Biotic Characteristics - Posted 1/4/01
- I. Defining a rain forest
- A. Strict definition
- 1. receive >1500 mm of rainfall each year
- 2. rainfall relatively evenly distributed throughout the year, >100 mm/month
- 3. evergreen trees
- 4. relatively little change in temperature throughout year
- 5. high humidity
- B. Other terms, characteristics that do not always apply
- 1. moist forests
- 2. high diversity of plants
- 3. lowland
- 4. acidic soils
- 5. broad-leaved trees
- C. Social constructions of rain forests also important
- II. Rainfall
- A. Reasons for high rainfall
- 1. global circulation model
- a. solar radiation is strongest near the equator
- b. the equator is mostly covered by water so a large amount of water
evaporates
- c. as air heats it become less dense and rises
- d. as air rises it cools and water condenses to form rain which is why the
tropics are wet
- e. air flows from high pressure to low pressure areas
- f. Hadley cell - system of air and water vapor circulation near the equator
- g. Intertropical convergence zone (ITCZ) - convergence of northern and
southern Hadley cells, moves from 23N to 23S with the change in season
- h. Rain forests fall within boundaries of ITCZ
- i. Spinning of the earth (Coriolis effect) means that Hadley cells are
actually curved
- 2. Orographic uplift - in areas where there are mountains near the coast and
prevailing winds are towards the continent, water evaporates from the ocean, as
it rises over the mountains it cools and water condenses to form rain - this is the
reason for most temperate rain forests
- B. Adaptations to high rainfall
- 1. drip tips
- 2. buttresses
- 3. no seed dormancy
- III. Temperature/elevation
- A. Temperature in most tropical forests usually changes more diurnally than seasonally
- B. Temperature is largely a factor of elevation
- IV. Soils
- A. Tropical soils are varied
- B. Factors affecting soil type
- 1. parent material
- 2. climate
- 3. topography
- 4. age
- 5. vegetation
- C. Typical characteristics
- 1. old - tropical soils (100,000 to 1 million years) vs. temperate rain forest soils
(10,000-40,000)
- 2. usually nutrient poor, especially with respect to phosphorus
- 3. most of nutrients are tied up in biomass
- 4. rapid nutrient cycling
- 5. high in aluminum
- D. Adaptations to soils
- 1. mycorrhizae - fungi that form associations with plants, they help with the
uptake of phosphorus in return for carbohydrates from plant
- 2. aluminum tolerance
- 3. shallow root systems
- V. Microclimate
- A. Wind speed, humidity, temperature, light all vary with level in the forest
- B. High humidity
- C. Light levels in tropical forests are commonly very low
- D. Adaptations to high light levels
- 1. Plants use short periods of light (sunflecks)
- 2. Plant remain as seedlings for many years until trees fall
- VII. Why are rain forests so diverse?
- A. Age hypothesis
- B. Climatic stability hypothesis
- C. Environmental heterogeneity hypothesis
- D. Productivity hypothesis
- E. Habitat diversity hypothesis
- F. Bottom line - we just don't know!
- VIII. Are rain forests "stable" ecosystems
- A. Tree fall gaps
- B. Flooding
- C. Volcanoes
- D. Human disturbance
- IX. Rain forest regeneration - treefall gaps
- A. pioneer vs. mature species
- B. seedlings vs. seed bank
- C. resprouting
- X. Rain forest plants
- A. Forest layers
- B. Epiphytes - plants that live on other plants, nutrients through rainfall and litterfall
- C. Lianas - woody vines
- D. Few herbs
- E. Predominantly animal-dispersed seeds
- F. Flower throughout the year
- G. Aerial roots and buttresses
- XI. Characteristics of rain forest
- A. coevolution - when species A evolves a response to a trait of species B, and species
B in turn evolves a response to the trait in species A
- B. mutualism - an interaction between two organisms where both benefits
- C. herbivory and anti-herbivory mechanisms
- D. predators and anti-predator mechanisms
- E. Trophic cascades
- F. high animal diversity
- G. know very little about rain forest biology, most ecological dogma based on studies
in the temperate zone
XII. Comparisons with temperate rain forests