The Tropical Rain Forest

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Why Preserve Tropical Forests?
(1) Tropical forests produce a lot of oxygen by
photosynthesis, and if we cut down all tropical
forests, we would lose that oxygen production and
we (as oxygen consuming organisms) would be in
trouble. (2) If we burn all the tropical
forests, we not only lose the oxygen they produce
each year, but we consume lots of oxygen from the
atmosphere in the burning process, really putting
us in trouble.
Homework Questions - Can you defend the above
statements using principles of residence
time and mass balance? - What is the residence
time of O2 in the atmosphere, and how much
would be consumed by burning tropical forests?
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Homework Problem Answer these Questions - Can
you defend these statements using principles of
residence time and mass balance? - What is the
residence time of O2 in the atmosphere, and how
much would be consumed by burning tropical
forests?

• DATA
• Total mass of oxygen in the atmosphere 1.2 x
  1021 g O2
• Total net ecosystem production from global
  land photosynthesis
• 1.1 x 1015 g O2/yr (tropical forests
  are 20 of this total).
• Amount of C in all tropical forests 206 x
  1015 g C
• Amount of C in all land plants 610 x 1015 g C

• Because of molecular weight differences for C
  and O2, you need to know that 2.67 g of O2 are
  used to respire or burn 1 g of C.
• The highest permanent human settlements are
  found in mountains where the oxygen is only 45
  of that at sea level

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The Tropical Rain Forest
The tropical rain forest is the most productive
and species-rich terrestrial ecosystem on earth
What we wish to learn

• How can such productive forests grow on soils of
  extremely low fertility?
• Why is biological diversity so high in the
  tropics?
• What are the consequences of the expected,
  almost-total loss of humid tropical forests?

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Several different types of forests exist in the
tropics
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Timber production is a major cause of
deforestation
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Tropical Land Use Change in 1990-1997 Annual
Deforested Area (km2)
Southeast Asia (Indonesia) 25,000 800 Latin
America (Brazil) 25,000 14,000 Africa
800 300 Global 58,000 14,000
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Photo gmagazine.com
Deforestation of Tropical Rainforests is
proceeding rapidly! (see them while you can)
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Tropical forests are highly productive, and so it
is natural to assume that the soils are fertile
(many nutrients). But, that would be wrong
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Soil Fertility Nutrient Cycling

• 1. Rapid weathering and time have led to
  depleted soil nutrients
• 2. Waters draining tropical landscapes are
  nutrient poor
• 3. Nutrients are found mainly in the plant
  biomass
• 4. Experiments show the effectiveness of roots
  at capturing nutrients
• 5. Comparisons of 7 tropical forests and their
  biomass and nutrient stocks

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Compare the amounts of Phosphorus in different
locations most P is in the canopy, not the soils
By comparison, in temperate-zone forests there
can be 10x as much N and P in the soils as in the
canopy.
(kg/m2)
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Data on Tropical Forests
Amazon, Venezuela Oxisol forest, Venezuela Montane rainforest, Puerto Rico Evergreen forest, Ivory Coast Dipterocarp forest, Malaysia Lowland rainforest, Costa Rica Moist forest, Panama
Aboveground Biomass (tons/ha) 268 264 228 513 475 382 326
Root Biomass (tons/ha) 132 56 72 49 20 14 11
Total Soil Nitrogen (kg/ha) 785 1697 - 6500 6752 20,000 -
Total Soil Phosphorus (kg/ha) 36 243 - 600 44 7000 23
Turnover time of leaves (years) 2.2 1.7 2.0 - 1.3 - 0.9
Modified from J. Terborgh, 1992, Scientific
American Library
No relationship between aboveground biomass and
soil nitrogen Inverse relationship between root
biomass and soil nitrogen Rich soils have
faster turnover time of leaves
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BIODIVERSITY (of vertebrates only)
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Example of increasing biodiversity toward the
tropics of bird species
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BIODIVERSITY
Tropical forests contain 50 of the 5-30
million species on earth.
Why are tropical forests so diverse? (1)
Ecological specialization Multi-layered forests
(2) Evolutionary history Geographic
isolation Episodes of climate change (3)
Other factors High year-round productivity
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Up to 5 Layers of vegetation exist in the
tropical forest, compared to only 2-3 layers in a
temperate-zone forest.
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Guilds
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There are more bird guilds in the tropics
(light bars) than in the temperate zone (dark
bars)
of species
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Species have specific ranges, and geographic
isolation can lead to the evolution of new
species.
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Repeated climate change in the tropics may have
led to frequent geographic isolation, and thus
more species.
Annual precipitation, mm
Area receiving lt1500 mm (tan)
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Species loss is studied in deliberately created
forest fragments, here shown in Brazil
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Deforestation is due both to slash and burn for
agriculture and to cutting for timber and
firewood How does this affect the nutrients?
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Crop yield drops quickly after slash and burn
agricultural
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Deforestation causes massive erosion and disrupts
the water cycle, which may alter the regional
climate
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Summary

• High productivity, but low nutrients in soils
• Most nutrients in biomass, Efficient nutrient
  cycling
• 50 of worlds species - why?
• Specialization in a complex, stable environment
• Evolutionary history - climate change and forest
  fragmentation
• Human impacts - loss of area (20 originally to
  7 now, to lt1 when? -- soon)
• Interplay of people, ecosystem function
  (agriculture, logging, ranching, mining), and
  politics influence the rainforest

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Take Home Message
Sometimes knowing the science just isnt
enough