HOW A RAINFOREST FUNCTIONS

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HOW A RAINFOREST FUNCTIONS

• Dawn R. Black

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Questions

• What factors influence productivity?
• How does primary productivity in tropical
  rainforests compare to other biomes?
  
• Where are most of the rapidly recycling minerals
  in tropical rainforests found?
  
• What are the three general types of soils found
  in the tropics?

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Questions

• What are the nutrient retention adaptations found
  in oligotrophic soils?
  
• How do rainforest plants receive nitrogen?

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GPP NPP/Biomass

• Gross Primary Productivity (GPP)
• The total amount of photosynthesis accomplished
• Net Primary Productivity (NPP)
• Amount of carbon added to the plant for growth
  and reproduction
  
• Biomass Detritus Soil Organic Matter
• Biomass
• Total storage of organic carbon in plant tissues

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Factors Influencing Productivity

• Adequate light (low light intensity limits
  understory species)
  
• Moisture
• CO2 levels
• Soil minerals/nutrients (many soils old and
  mineral poor)

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Tropical vs. Other Ecosystems

• GPP vastly higher in rainforests than in any
  other ecosystem
  
• High rates of respiration (temperature stress)
• 50-60 of GPP spent on maintenance
• NPP higher than any terrestrial ecosystem

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Comparisons of NPP
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Global Distribution of Carbon in Plant Biomass
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Productivity by Biome
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Tropical vs. Temperate PP

• Huston (1994)
• Productivity per unit time no greater in the
  tropics than in temperate zone (high PP due to
  length of growing season)
  
• Kricher (1997)
• Maybe plant tissue grows faster in tropics
• Tropical species grew by an order of magnitude
  more than temperate species (red oak, red maple)
  when length of growing season was corrected for
  
• Suggests that per tree productivity is
  considerably enhanced in the tropics

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Nutrient Cycling

• Decomposition and subsequent recycling is the
  process by which materials move between the
  living and nonliving components of an ecosystem

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Decomposition

• Fungi bacteria convert dead
• organic tissue back into simple
• inorganic compounds reavailable
• to plant root systems
• Fungi immensely abundant in tropics
• Mycelial mesh covers parts of some
• tropical forest floors

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Supporting Decomposers

• Slime molds
• Actinomycetes
• Algae
• Animals (vultures, arthropods, earthworms,
  invertebrates)
  
• Protozoans

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Leaching of Nutrients

• Leaching washing of essential minerals and
  other chemicals from leaves and soils by water
  

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Leaf adaptations

• Drip tips (speed water runoff)
• Protective cuticle with lipid-soluble secondary
  compounds that retard water loss discourage
  herbivores and fungi

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Leaching of Soil

• Rainfall increases H ions in soil (lowers pH),
  which bind to (-)-charged humus clay
  
• ()-charged minerals (Ca, K) washed to deeper
  part of soil
  
• Acidity of soil increased

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Rapid Recycling of Nutrients

• Most of rapidly recycling minerals are in the
  biomass in the tropics
  
• Decomposition recycling of fallen parts occur
  with much greater speed in rainforests than in
  temperate forests (thin litter layer)
  
• 80 of total leaf matter in Amazon rainforest
  annually returned to soil (Klinge et al. 1975).

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Role of Mycorrhizae

• Substitute for poorly developed root hairs
• Mostly vesicular-arbuscular (VAM)
• Aid in uptake of phosphorous
• Some ectomycorrhizae, especially in poor soils
• Aid in uptake of both minerals and water
• VAM status of Dicorynia guianensis seedlings is
  critical factor controlling regeneration in
  primary tropical forest of French Guiana (Bereau
  et al., 1997)

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Soil Characteristics

• Determined by several factors (Jenny 1941)
• Climate
• Vegetation
• Topographic position
• Parent material
• Soil age

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Rainforest Soil Types

• Three general classifications of soils throughout
  humid tropics
  
• Ultisols
• Oxisols
• Alfisols
• Comprise 71 of land surface in humid tropics
  worldwide
  
• Only 15 of moist tropical forests moderately
  fertile (in young soils of recent origin)

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Ultisols

• Well-weathered
• Minerals leached from upper parts of soils

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Oxisols

• Deeply weathered
• Old
• Acidic
• Found on well-drained soils of humid regions
• Also found on Guianan Shield (common throughout
  global tropics)
  
• Reddish color due to iron aluminum oxides

1 M
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Alfisols

• Closer to neutral pH (still acidic)
• Less overall leaching
• Common in subhumid semiarid tropics

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Mineral Cycling on Oligotrophic Soils

• Up to 26 of roots on the surface
• Root mats several cm thick can develop
• Root mat mycorrhizae directly absorb available
  minerals
  
• 99.9 of Ca P absorbed into root mat in Amazon
• Presence of buttresses may allow roots to spread
  widely at surface, where they reclaim minerals

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Nutrient Retention Adaptations

• Surface roots/mats
• Apogeotropic roots roots grow upward from soil
  onto stems of neighboring trees, absorb nutrients
  leached from trees from throughfall
  
• Arrested litter epiphytes understory plants
  catch litter from canopy
  
• Canopy leaves algae lichens on leaves absorb
  nutrients from rainfall and trap on leaf

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Nitrogen Fixation

• Legumes Rhizobium abundant in biomass
  biodiversity in tropics, take up gaseous N from
  atmosphere convert to nitrate
  
• Certain epiphytic lichens fix nitrogen
• Leaf-surface microbes liverworts may facilitate
  uptake of gaseous nitrogen
  
• Termites N-fixation due to activities
• of microbes in termite guts

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Rainforest Gaps

• Microclimates dependent on gap size
• Affects light, moisture, wind conditions
• Treefalls are normal part of rainforest function,
  peak in rainy season
  
• Creates heterogeneous forest

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Gap-Dependent Pioneer Species

• Produce an abundance of small seeds dispersed by
  bats or birds
  
• Seeds capable of long dormancy periods
• Different growth patterns among pioneers may
  explain coexistence of so many different species
  in rainforest ecosystems

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Forest Demographics

• Forest turnover varies with species region
• La Selva, Costa Rica 118 years
• Cocha Cashu, Peru 63 years
• Manaus, Brazil 82-89 years

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Disturbance Ecological Succession

• Jungle early succession in tropics
• High species richness
• Highly variable from site to site
• Early succession Colonizers
• Small in stature, grow fast, produce many-seeded
  fruits
  
• Late succession Equilibrium species
• Larger, grow more slowly, fewer seeds per fruit,
  persist in closed canopy
  
• Can take 500 years to reach equilibrium

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Answers

• What factors influence productivity?
• Light levels, moisture, CO2 levels, soil
  minerals/nutrients
  
• 2. How does primary productivity in tropical
  rainforests compare to other biomes?
  
• Both GPP NPP are higher than other biomes

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Answers (cont.)

• Where are most of the rapidly recycling minerals
  in tropical rainforests found?
  
• In the plant biomass
• 4. What are the three general types of soils
  found in the tropics?
  
• Ultisols, Oxisols, Alfisols

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Answers (cont.)

• 5. What are the nutrient retention adaptations
  found in oligotrophic soils?
  
• Surface roots/mats, apogeotropic roots, arrested
  litter, algae/lichens on leaves
  
• 6. How do rainforest plants receive nitrogen?
• Legumes Rhizobium, epiphytic lichens,
  leaf-surface microbes/liverworts, termites

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Roggy et al. (1999)

• Study of plant N nutrition in legumes pioneer
  species at Piste de St Elie in the ECEREX
  research area, French Guiana
  
• Used d15N method to estimate nitrogen input by
  N2-fixing legumes to natural rainforest

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Roggy et al. (1999)

• Results
• N2-fixing legumes contributed 136 t ha-1 to total
  above-ground plant biomass
  
• N2-fixation estimated to be 7 kg ha-1 y -1
• d15N of non- N2-fixing plants could be related to
  soil nitrogen availability
  
• Could be used as indicator of nitrogen-cycling
  efficiency in rain forests
  

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Chave et al. (2001)

• Biomass study
• 2 study sites
• Nouragues Research Station (100 km inland)
• Piste de Saint-Elie Research Station (coastal
  rain forest
  

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Chave et al. (2001)

• Results
• Significant spatial variability of biomass at
  fine-scale resolution
  
• Illustration of disturbance-driven, mosaic-like
  pattern in old-growth forest
  
• Biomass accumulation of 3.2 Mg ha-1 y -1
• 2.8 Mg ha-1 y -1, which agrees with literature
  NPP of 2-4 Mg ha-1 y 1 (Phillips et al., 1998)
  
• -Variability of biomass correlated with canopy
  gap openings

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Granier et al. (1996)

• Transpiration of natural rainforest its
  dependence on climatic factors
  
• Objectives
• Analyze transpiration at tree level through sap
  flow measurements performed on several major
  species growing in their natural environment
  
• At stand level, analyze dependence of
  transpiration to climatic factors, by scaling up
  allowing calculation of stand.

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Granier et al. (1996)

• Dependent Factors
• Late stage species (high flow rates)
• Pioneer species (low flow rates)
• Crown Status
• Codominant trees exhibited lower flow rates than
  dominant trees of same species
  
• Sap flow showed remarkable concordance with
  variations of air vapor pressure deficit

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Literature Cited

• Bazzaz, F.A. 1984. Dynamics of wet tropical
  forests and their species strategies. In E.
  Medina, H.A. Mooney, and C. Vazquez-Yanes (eds).
  Physiological ecology of plants of the wet
  tropics. Junk, Dordrecht, pp. 233-243.
• Bereau, M., E. Louisanna, and J. Garbaye. 1997.
  Effect of endomycorrhizas and nematodes on the
  growth of seedlings of Dicoryniaguianensis
  Amshoff, a tree species of the tropical rain
  forest in French Guiana. Annales des Sciences
  Forestieres 54 271-277.
• Chave, J., B. Riéra, M-A. Dubois. 2001.
  Estimation of biomass in a neotropical forest of
  French Guiana spatial and temporal variability.
  Journal of Tropical Ecology 17 79-96.
• Granier, A., R. Huc, S.T. Barigah. 1996.
  Transpiration of natural rain forest and its
  dependence on climatic factors. Agricultural and
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• Huston, M.A. 1994. Biological diversity the
  coexistence of species on changing landscapes.
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  Press. 
• Kricher, J. 1997. A Neotropical Companion An
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  of the New World Tropics. 2nd ed. Princeton, New
  Jersey Princeton University Press.  
• Lescure, J-P. and R. Boulet. 1985. Relationship
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  forest in French Guiana.
  
• Biotropica 17 155-164.
• Phillips, O. L., Y. Malhi, N. Higuchi, W.F.
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• Roggy, J.C., M.F. Prévost, F. Gourbiere, H.
  Casabianca, J. Garbaye, and A.M. Domenach. 1999.
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  concentration as potential indicators of plant N
  nutrition in legumes and pioneer species in a
  rain forest of French Guiana. Oecologia 120
  171-182. 
• Turnbull, M.H., S. Schmidt, P.D. Erskine, S.
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