tropics

Tropics

Regions between 23.5 ° N and S (tropics of Cancer and Capricorn). These are areas where the sun is directly overhead at some point during the year.

This area includes 40-50% of the globe’s surface area (including both land and oceans).

Some defining characteristics of most tropical areas

Warm temperatures

Biological complexity (many species and thus many species interactions)

Many soils with low levels of mineral nutrients

In temperate areas (like Michigan), low temperatures and low soil moisture in winter constrain biological processes, like the growth and reproduction of individuals

In tropical areas, other factors like biological interactions and low soil nutrients may take over this constraining role

Terrestrial tropical ecosystems vary greatly

Rain forests (these get most of the attention)Deciduous forestsScrub forestsSavannaMangroves

Holdridge life zones (1947)

Based classification of life zones on

1. Biotemperature (based on temperatures starting at 0° C--below 0° C plants are dormant)

2. Precipitation3. Ratio of mean annual potential

evapotranspiration to mean total annual precipitation.

Evapotranspiration is evaporation from ground surface plus the water released from plant leaves (transpiration)

When ratio of annual evapotranspiration to annual precipitation is more than 1, what can we say about the climate?

http://www.city-data.com/states/Michigan-Climate.html

Detroit’s annual rainfall is about 32.9 inches or 836 mm per year

Detroit’s daily average temperature of 49 degrees F or 9 degrees C

A strong influence on tropical climates is the Intertropical Convergence Zone (ITCZ)

Easterly winds from north and south converge near the equator to produce a low pressure area

This band of low pressure shifts north and south over the year according to where the earth is in relation to the sun (to changes in solar angles and radiation)

ITCZ does not cover the same area (or latitude range) in different areas of the globe—because of features like mountains

The shifting location of the ITCZ over the year leads to varying patterns of rainfall

When ITCZ is present, rainfall is heavy

Rainfall diminishes as ITCZ leaves a region

Since the ITCZ is in the vicinity of the equatorial regions (O° latitude) yearround, these regions tend to be wet year round, often with two “rainy” seasons

Further from the equator (20° latitude), ITCZ is not in the vicinity as much of the year. These regions often are somewhat drier and have one “rainy” season

Image source: Pearson Education, Inc.Troipical rain forests.

Tropical Rain Forests

Image source: Pearson Education, Inc.Tropical savanna.

Tropical Savanna

Conclusions from two previous slides

1. Tropical rain forests and savannas are not very seasonal with regard to temperature.

2. Tropical rain forests and savannas both show seasonal precipitation patterns

3. Seasonal precipitation patterns are more pronounced in tropical savannas.

There is much variability in rainfall, even within an ecosystem type (like savannas), because of features like mountains

El Niño Southern Oscillation Cycle (ENSO) influences tropical climates

Multi-year cycle of surface air pressure changes in the tropical Pacific Ocean, resulting in changes in ocean temperature

El Niño events happen every 2-8 years and vary greatly in strength

In a normal year, pressure systems are as above. Trade winds over the Pacific Ocean move from east to west. The easterly trade winds bring warm surface waters westward, bringing convective storms to Indonesia and coastal Australia. Along the coast of Peru, cold bottom water wells up to the surface to replace the warm water that is pulled to the west.

http://www.physicalgeography.net/fundamentals/7z.html

In an El Niño year, air pressure drops over large areas of the central Pacific and along the coast of South America. The normal low pressure system is replaced by a weak high in the western Pacific (the southern oscillation). This change in pressure pattern causes reduced trade winds. This reduction means warm ocean water along the coastlines of Peru and Ecuador. This accumulation of warm water causes the thermocline to drop in the eastern part of Pacific Ocean which cuts off the upwelling of cold deep ocean water along the coast of Peru.

http://www.physicalgeography.net/fundamentals/7z.html

Three phases in ENSO cycle

El Niño phase—warm ocean surface temperatures

La Niña phase—cool ocean surface temperatures

Normal phase—normal surface temperatures

Strong El Niño years generally lead to

Dry periods in rain forests, which influence biological systems:

These dry periods can lead to mass flowering in Indonesian and Malaysian forests, then fruiting, then the appearance of many animals following the fruiting cycle

In other tropical areas, these dry periods can lead to tree mortality

Predicted effects of El Niño

http://www.eoearth.org/images/b/b2/El_nino_global_map.gif

Throughout the course, keep in mind that the “tropics” are not the same from place to place

Much variability, so generalizations are often difficult

Neotropics Africa MadagascarSoutheast Asia New Guinea

Main Geographical Feature

Amazon River basin, Andes

Congo River basin

Forests in east

Peninsula and islands on Sunda Shelf

Large, mountainous island

Distinctivebiological features

Bromeliad ephiphytes, high bird diversity, small primates

Low plant richness, forest elephants, many forest browsers

Lemurs, low fruit abundance

Dipterocarp tree family, mast fruiting of trees, large primates

Marsupial mammals, birds of paradise

Annual rainfall (mm) 2000-3000 1500-2000 2000-3000

2000-3000, often > 3000

2000-3000, often > 3000

Largest Country Brazil

Democratic Republic of Congo

Malagasy Republic Indonesia

Papua New Guinea

From the Times Atlas of the World 1994

Major rain forest regions




Congo River basin

Forests in east










2000-3000, often > 3000

2000-3000, often > 3000




Papua New Guinea

From The Times Atlas of the World 1994


(Wettest areas of Michigan receive 1000 mm)




Congo River basin

Forests in east










2000-3000, often > 3000

2000-3000, often > 3000




Papua New Guinea

From The Times Atlas of the World 1994


Over 30 species of gliding species in Borneo

None in the Amazon

A few in Africa

Why?

It could be that the dipterocarp trees that dominate Bornean forests produce fruit at unpredictable intervals, making food harder to find in these forests than in other tropical forests. Therefore, many species have evolved gliding to be able to range more widely in search of food.

tropics

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