Tropical%20forests

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Tropical forests

• Climate and distribution
• Forest characteristics and phenology
• Direct nutrient cycling
• Regeneration and gap dynamics
• Anthropogenic disturbance - shifting cultivation
  and pastures
• Forest fragmentation and conservation
• Late Quaternary climate change and conservation

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Tropical forest regional climate
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Tropical forestsproductivity and diversity

• Primary productivity (forests) g m-2
  yr-1 Tropical 1500 1800 2000 Temperate 100
  0 1300 1500 Boreal 500 800 1000

• Diversity Malaysia Amazonas Africa
• Plants 60 000 50 000 30 000
• Birds 127 270 150 (3 km2) (3 km2) (50
  km2)
• Bats 81 98 115

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Canopy stratification(how many
strata?)multiple strata facilitate high
productivity and diversity
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Density variations in rainforest stands
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High stem density
Diversity majority of trees are rare - densities
lt1/ha.

• Characters
• lots of small poles
• drip-tipleaves
• thin bark

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Leaf shape acute (drip-tip), entire margin
scratch and sniff taxonomy
lichen growth on palm leaf
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Treefalls
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Tree stability on wet, clay-rich tropical soils
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Buttresses
Stilts
Plexus
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Cauliflory
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Lianas and vines
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Epiphytesbromeliads and orchids
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Phenology Malaysian rainforest
of trees
Triggers degree of water stress and photoperiod.
Daylength variations of 15 minutes can trigger
flowering in some tropical tree species.
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Biomass variations in rainforest stands
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Necromass variations in rainforest stands
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Nutrient storage nitrogen
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Nutrient storage phosphorus
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Nutrient storage potassium
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Root distribution and the direct nutrient cycle

• Dense root mats in surface soil exploit nutrients
  released by rapidly decaying organic matter on
  the forest floor.
• Nutrient capture by tree roots facilitated by
  mycorrhizal associations (predominantly
  endomycorrhizal and vesicular-arbuscular).

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Nutrient shunts leaf-cutter ants and termites
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Herbivore and insectivore mammals
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Seed/fruit eaters
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Herbivore resistance

• mechanical spines e.g. on climbing palms
• lactiferous rubber (Hevea sp.) or
• chemical secondary chemicals in roots, stems,
  leaves or seed coats to dissuade herbivores from
  attacking tissue (see next slide).The tropical
  forest as a pharmaceutical factory.
• biological companion ants on Acacia shrubs in
  Central America

?
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Wapishan woman with cassava press, Guyana
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Regeneration and the maintenance of diversity
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Regeneration into gaps intense competition for
light
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Gap microclimates
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Antropogenic gaps and succession
milpas Belize and Guyana
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Nutrient loss from shifting cultivation plot
results from severance of direct nutrient cycle
and changes in soil microclimate and hydrology
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Forest clearance Rondonia, Brazil
1975
1992
100 km2
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Forest clearance for pasture, Guatemala compare
with size of milpa clearing
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Pasturizationlog, burn, seed in Amazonas
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Succession on abandoned pastures, Amazonia

• 60,000 km2 land in pasture (mid-1980s)
• Generally abandoned after 4-8 years
• Pasture disturbances larger, more prolonged and
  more intense than slash and burn agriculture

abandonment as a result of soil infertility
(especially phosphorus deficiency), insect
attack, and weed competition
Uhl et al., 1988. J. Ecology
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Pasture use history
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Biomass andnecromass
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From green hell to red desert?
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Abandoned pastures - nutrient stocks(NB top
0.5m of soil onlyN values / 5)
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Rates of species replacement in rainforest
succession
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Biodiversity on abandoned pastures undergoing
succession
Heavy
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Recovery of tropical forests following disturbance
Karen Holl (UC Santa Cruz) working on abandoned
cattle pasture in Costa Rica has identified the
following obstacles to TRF recovery 1. Tree
seeds have short viability 2. Tree seed dispersal
is generally short (large seeds commonly
animal-dispersed) seedfall in pasture is only
1/10th that in the forest. 3. Heavy predation of
seeds in pasture 4. Low survivorship of
germinating seeds (severe microclimate, low
mycorrhizal infection and high herbivory) 5. Compe
tition from non-native pasture grasses (e.g.
Imperata cylindrica)
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Seed dispersal into abandoned pasture, Costa Rica
Mean no. seeds / m2

dispersal more effective when tree branches
placed in pasture as perches for forest birds
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Rainforest fragmentsThomas Lovejoys experiments
Forest species survival? recruitment? dispersal?
Patch minimum size?
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LGM in the humid tropics plant and animal
responses
Were tropical rain forests restricted to small
refuges at LGM?
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The rise of refuge theoryendemism in the
Neo-tropical forest avifauna
Haffer (1969) Science, 165, 131-137.
from Prance and Lovejoy (1985) Amazonia, Oxford
U.P.
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Caryocar ranges
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Ranges of related forest bird species and
subspecies
Trumpeters (Psophia)
Jacamars (Galbula)
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Ranges of related forest bird species and
subspecies
Aracaris (Pteroglossus)
Toucans (Rhamphastos)
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Species and subspecies rangesHeliconius
butterflies
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Inferred LGM forest refuges based on 1.
birds2. lizards3. butterflies4. four tree
families5. scorpions
From Nores (1999) J. Biogeography, 26, 475-485
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TRF refuges a minimalist reconstruction
Lake Pata
forest desert
from Tallis (1991) Plant Community History,
Chapman and Hall
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Late Quaternary climate change in intertropical
Africa the lake-level evidence
low intermediate and high stands
Holocene LGM
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Lake Pata pollen record
Podocarps
Grasses
LGM Holocene
Colinvaux et al., 1996, Science, 247, 85-88
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Refugia a failed hypothesis?
we conclude that the Amazon was not arid at any
time in the Pleistocene, that the lowlands were
in the main always forested, that forest biota
were never fragmented into isolates called
refugia, and that the critical global changes in
Amazon history were the warmings of interglacials
that intermittently perturbed the great and
persistent ice-age forests. Much or all of this
needs testing with more data. Colinvaux et al.,
2000. Quat. Sci. Rev. 19, 141-169.