Transcript Rainforest_lecture_2..

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?
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
Photosynthesis (Reduction) 
CO2 + H2O  CH2O + O2
 Respiration (Oxidation, Combustion)
• 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
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?
Several different types of forests exist in the tropics
Timber production is a major cause of deforestation
Tropical Land Use Change in 1990-1997
Annual Deforested Area (km2)
Southeast Asia (Indonesia)
Latin America (Brazil)
Africa
Global
25,000
25,000
800
58,000
± 800
± 14,000
± 300
± 14,000
Photo gmagazine.com
Deforestation of
Tropical
Rainforests is
proceeding rapidly!
(see them while
you can…)
Tropical forests
are highly
productive, and so
it is natural to
assume that the
soils are fertile
(many nutrients).
But, that would
be wrong…
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
Compare the
amounts of
Phosphorus
in different
locations –
most P is in
the canopy,
not the soils
By comparison,
in temperatezone forests
there can be
10x as much N
and P in the
soils as in the
canopy.
(kg/m2)
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
BIODIVERSITY (of vertebrates only)
From the Millennium Ecosystem Assessment, 2005, World Resources Institute
Example of increasing
biodiversity toward the tropics –
# of bird species
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
Up to 5
Layers of
vegetation
exist in the
tropical
forest,
compared to
only 2-3
layers in a
temperatezone forest.
“Guilds”
There are more
bird “guilds” in
the tropics (light
bars) than in
the temperate
zone (dark bars)
# of species
Species have specific “ranges”, and geographic
isolation can lead to the evolution of new species.
Repeated climate change in the tropics may have led to
frequent geographic isolation, and thus more species.
Annual precipitation, mm
Area receiving <1500 mm (tan)
Species loss is studied in deliberately created forest
fragments, here shown in Brazil
Deforestation is due both to slash and burn for
agriculture and to cutting for timber and firewood
– How does this affect the nutrients?
Crop yield drops quickly after slash and burn agricultural
Deforestation
causes massive
erosion and
disrupts the
water cycle,
which may alter
the regional
climate
Summary
• High productivity, but low nutrients in soils
• Most nutrients in biomass, Efficient nutrient cycling
• 50% of world’s 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 <1% when? -- soon…)
• Interplay of people, ecosystem function (agriculture,
logging, ranching, mining), and politics influence the
rainforest
Take Home Message
“ Sometimes knowing the science
just isn’t enough…”