Rainforests

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Rainforests

• The rainforest is incredibly complex
• The rainforest is incredibly productive
• The rainforest cycles nutrients incredibly
  quickly
• The rainforest soils are incredibly fragile

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Rainforests

• Productivity the amount of solar radiation,
  sunlight, converted by plants into complex
  molecules such as sugars
• This is accomplished through photosynthesis
• CO2 H2O sunlight sugar O2

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Rainforests
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Rainforests

• Oxygen production very important both in
  ecological and evolutionary time
• E.g. oxygen-free to 21 O2
• E.g. rainforest
• (2x) temperate forest
• (4-5x) grassland

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Rainforests

• NPP vs. GPP
• GPP very hard to estimate, typically NPP
• Despite high rates of respiration (50-60 in
  maintenance), highest NPP of all terrestrial
  systems
• So what

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Rainforests

• Conversion to less productive systems
• As rainforests are replaced by other systems
  (predominately agriculture), all are much less
  productive!
• E.g. 40 of worlds NPP co-opted

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Rainforests

• Leaf-area Index (LAI) is the leaf area above a
  square meter of forest floor.
• In NH, LAI is almost 6, BCI almost 8
• In Caatinga, 5.1, and in rainforest, a range of
  10.6-22.4 (soil conditions)
• The relationship between LAI and understory cover
  is inversely related

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Rainforests

• Tropical growing season is year-round compared to
  temperate areas
• Easy to suggest comparable on a per unit time
  metric
• Answer is unclear
• But, some studies suggest tropical growth is
  significantly greater than temperate growth (an
  order or magnitude greater)

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Rainforests

• Growth what influences it??
• Water
• Light
• CO2
• Minerals
• Tropics do well in first 3, and plants have done
  relatively well in adapting to poor soil
• Why??

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Nutrient Cycling

• Earth is a closed system
• Consequently, atoms present in dead tissue must
  be reacquired, recycled back into the living
  world (or tissues)
• The recycling is in the form of decomposition

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Nutrient Cycling

• Consider a unit of rainforest unit of energy
  fixed during net primary productivity can move in
  one of two major directions either it can be
  consumed as part of living tissue (caterpillar),
  or it can remain as part of the leaf until it
  drops and enters the decomposer food web

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Nutrient Cycling

• Have you been to the tropics?
• Which route do you think the majority of energy
  takes? Why?

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Nutrient Cycling

• Who are the players in the decomposer food web?
• Fungi and bacteria
• But also slime molds, actinomycetes, algae,
  arthropods, earthworms, protozoans
• Having said that, our understanding of how the
  decomposer (microbial) food web works is
  relatively poor

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Nutrient Cycling

• It may be the biodiversity in soil community
  rivals that of the canopy, but there are
  relatively few studies on the pathways and extent
  of this community

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Nutrient Cycling

• Organisms facilitate a process called
  humification, in which complex soil organic
  matter is maintained at the interface between the
  tree roots and soil.
• Humus is important in forming colloids that
  cement soil particles, in helping aerate the
  soil, in possessing a negative charge, and
  important aid in retaining critical minerals, and
  in overall conservation of nutrients

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Nutrient Cycling

• The soil is an important storage bank for
  essential minerals
• E.g. nitrogen, calcium, magnesium, phosphorus,
  and potassium
• Nutrient cycling is often termed biogeochemical
  cycling because of the process moving things in
  between the living (bios) and non-living (geos)

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Nutrient Cycling

• The rate of biogeochemical cycling is strongly
  influenced by temperature and rainfall
• Since minerals are always taken up through roots
  via water, the uptake of water is essential the
  uptake of minerals as well. But evaporation can
  be a mixed blessing. Plants can lose too much
  water when subjected to constant high temperature.

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Nutrient Cycling

• Plants can lose too much water when subjected to
  constant high temperature. Many tropical plants
  retard evaporative water loss both by closing
  their stomata and by producing waxy leaves.

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Leaching

• Water can wash essential minerals and other
  chemicals from leaves, a process called leaching.
  Leaching can be especially sever in areas subject
  o frequent heavy rains.
• A defense against this is the thick waxy coating
  that aid in retarding water loss as well as
  deterring herbivores and fungi

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Leaching

• Rainfall also leaches minerals from the soil,
  washing them down into the deeper soil layers.
• Clay particles and humus have negative
  electrostatic charges tat attract mineral with
  positive charges such as calcium and potassium.
• Because H2O adds a positive H to the soil, these
  elements get washed deep into the soil or may
  wash out of the soil completely

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Leaching

• Consequently in the tropics, the combination of
  high temperature and heavy rainfall can often
  result in much leaching and strongly acidic
  soils.
• Typical Amazon soils (up to 75) are frequently
  mineral-poor, high in clay, acidic, and low in
  available phosphorus

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Leaching

• As a result, in tropical forests most of the
  rapidly cycling minerals are in the living
  plants, the biomass.
• In the temperate zone, minerals are more equally
  distributed between the vegetation and soil bank.

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Mycorrhizae

• Most plants in the world exhibit an intimate,
  mutualistic association between tree roots and a
  diverse group of fungi collectively termed
  mycorrhizae
• Although located in tree roots and appearing
  parasitic, they are essential in mineral uptake
  from the forest litter.

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Rapid Recycling

• Walk through a rainforest and youll be surprised
  how little litter there is (think deciduous or
  more, coniferous)
• Rainforest have the same processes, they just
  occur much more rapidly
• E.g. One study estimates 80 of total leaf matter
  in an Amazon rainforest is annually returned to
  the soil.

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Rainforest Soils

• Despite previous statements, generalizations of
  tropical soils should be made cautiously
• Eastern and central Amazon Basin, soils are very
  old and mineral-poor (oligotrophic), while in
  other regions, such as volcanic areas of Costa
  Rica or much of the Andes, soils are young and
  mineral-rich (eutrophic)

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Rainforest Soils

• Soil characteristics vary regionally (and quite a
  bit) because soil is the product of several
  factors climate, vegetation, topographic
  position, parent material, and soil age
• Much of the humid tropics falls into one of three
  categories ultiols, oxisols, or alfisols, all of
  which are reddish to yellowish in color

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Rainforest Soils

• Ultisols well-weathered (leached)
• Oxisols deeply weathered, old, acidic, and found
  on well-drained soils (generally occurring on old
  geologic formations). Widespread and world wide
• Alfisols common in the subhumid and semiarid
  tropics and are closer to a neutral pH (though
  still acidic), which less overall leaching than
  oxisols

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Rainforest Soils

• However, not all tropical soils are old or
  heavily weathered or infertile
• Perhaps 15 of moist tropical soils are situated
  on soils of at least moderate fertility. Soils
  generated from deposits during the flood cycle or
  from recent volcanic activity typify these
  categories

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Rainforest Soils

• A general pattern in the tropics is that heat and
  heavy moisture input cause the formation of
  oxides of iron and aluminum (neither of which is
  useful to plants), yielding a reddish color
• Clay content is high

be leery of wet trails and mountain roads
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Rainforest Soils
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Rainforest Soils
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Rainforest Soils

• At the extreme, one can encounter laterization,
  the combined effects of intensive erosion and
  heat acting on soil
• If vegetation is removed and bare soil is exposed
  to extreme downpours and heat, it can bake into a
  bricklike substance, ruining it for future
  productivity (also utilized for bricks)
• In Amazonia, only 4 are at risk

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Rainforest Soils

• However, even without this extreme, the attempt
  to farm the Amazon and other tropical forests
  have failed due to the quick loss of soil
  fertility
• Limited agriculture could be supported (soil
  similar to SE US)

more on this later
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Rainforest Soilswhite soils

• In some parts of the Amazon Basin, white and
  sandy soils predominate, from the Brazilian and
  Guianan Shields
• These are extremely old (100s of millions of
  years) and have lost their fertility
• However, relatively lush broadleaf rainforests
  grow on these soils

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Rainforest Soils

• Oligotrophic soils support less lush and smaller
  (in height) forests
• Up to 26 of roots can be on the surface and root
  mats can be very thick
• Certain minerals (e.g. calcium and phosphorus)
  are extremely efficiently taken up by root mat
  and mycorrhizae (99.9)

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Rainforest Soils

• How might buttresses impact the nutrient cycling?
  Roots can spread widely at the surface
• Do you understand the value and need for a thin
  layer of forest humus with its mycorrhizal fungi
• Generalizations should be avoided as there are
  many variants as to what is truly productive
  and what is tight

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Nutrient Adaptations

• Some tropical plants have root systems that grow
  vertically upward, from the soil onto the stems
  of neighboring trees. These apogeotropic roots
  grow as fast as (5.6cm) in 72 hrs).
• Why?

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Nutrient Adaptations

• Arrested Litter many plants (i.e. palms,
  epiphytes) catch litter before it reaches the
  forest floor. This is one way in which plants in
  the upper canopy can obtain the entire suite of
  nutrients needed to grown and reproduce

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Nitrogen Fixation

• Some plants, such as legumes, can take up gaseous
  nitrogen directly from the atmosphere and convert
  it to nitrate, a chemical form in which it can be
  used by the plant.
• How effective? One estimate 20kg of nitrogen
  fixed per ha per yr in the Amazon Basin (gt3x from
  precip)

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Tropical Rivers

• White sandy soils are usually drained by
  blackwater rivers
• Color derived by tannins, phenolics, and related
  compounds (humic matter)
• Not confined to tropics (e.g. bogs)
• Poor soils contribute to such water
• Leaf production expensive, hence concentrate
  defenses

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Rainforest Gaps
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Tropical Rivers

• In contrast to white, sandy soils, soils in
  places such as Puerto Rico, much of Costa Rica,
  and much of the Andes Mountains are not
  mineral-poor but mineral-rich. They are much
  younger, mostly volcanic in origin, high rainfall
  and temperatures.
• Much sediment leaches into the river, they are
  cloudy and termed whitewater

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Tropical Rivers
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Tropical Rivers

• A dramatic confluence of such water is the
  wedding of the waters between the Rio Negro and
  Rio Brancho
• It lasts from 15-25km until the mixing is complete

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Tropical Rivers
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Tropical Rivers