(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?

Why Preserve 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

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?

The Tropical Rain Forest

• 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?

The tropical rain forest is the most productive and species-rich terrestrial ecosystem on earth

What we wish to learn:

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) 25,000 ± 800Latin America (Brazil) 25,000 ± 14,000Africa 800 ± 300 Global 58,000 ± 14,000

Deforestation of Tropical Rainforests is proceeding rapidly! (see them while you can…)

Photo gmagazine.com

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 Soil Fertility & Nutrient CyclingCycling

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 soilsBy comparison, in temperate-zone forests there can be 10x as much N and P in the soils as in the canopy.

(kg/m2)

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

Data on Tropical Forests

* No relationship between aboveground biomass and soil nitrogen

* Inverse relationship between root biomass and soil nitrogen

* Rich soils have faster turnover time of leaves

From the Millennium Ecosystem Assessment, 2005, World Resources Institute

BIODIVERSITY (of vertebrates BIODIVERSITY (of vertebrates only)only)

Example of increasing biodiversity toward the

tropics –# of bird species

BIODIVERSITYBIODIVERSITY

Why are tropical forests so Why are tropical forests so diverse?diverse?

(1) Ecological specialization

Multi-layered forests

(2) Evolutionary history

Geographic isolation

Episodes of climate change

(3) Other factors

High year-round productivity

Tropical forests contain ~50% of the 5-30 million species on earth.

Up to 5Layers of vegetation exist in the tropical forest, compared to only 2-3 layers in a temperate-zone 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, mmArea 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

SummarySummary• 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…”