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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
Tropical forest:
regional climate
Tropical forests:
productivity and diversity
• Primary productivity (forests) [g m-2 yr-1]:
Tropical:
Temperate:
Boreal:
1500
1000
500
Diversity Malaysia
[1800]
[1300]
[800]
2000
1500
1000
Amazonas
Africa
• Plants:
60 000
50 000
30 000
• Birds:
127
(3 km2)
81
270
(3 km2)
98
150
(50 km2)
115
• Bats:
Canopy
stratification:
(how many strata?)
multiple strata
facilitate high
productivity and
diversity
Density variations in
rainforest stands
High stem density
Diversity:
majority of
trees are
rare densities
<1/ha.
Characters:
• lots of small
poles
• ‘drip-tip’
leaves
• thin bark
Leaf shape:
acute (‘drip-tip’), entire margin
‘scratch and sniff’
taxonomy
lichen growth on palm leaf
Treefalls
Tree
stability on
wet, clayrich tropical
soils
Buttresses
Plexus
Stilts
Cauliflory
Lianas and
vines
Epiphytes:
bromeliads and
orchids
Phenology: Malaysian rainforest
% of trees
40
30
20
10
0
J
F
M
A
Leaf flush
M
J
J
Flowering
A
S
Fruiting
O
N
D
Ripe fruit
Triggers: degree of water stress and photoperiod. Daylength variations
of 15 minutes can trigger flowering in some tropical tree species.
Biomass variations in
rainforest stands
Necromass variations in
rainforest stands
Nutrient
storage:
nitrogen
Nutrient storage: phosphorus
Nutrient storage: potassium
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 vesiculararbuscular).
Nutrient shunts:
leaf-cutter ants and termites
Herbivore
and
insectivore
mammals
Seed/fruit eaters
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
Wapishan
woman
with
cassava
press,
Guyana
Regeneration and the maintenance
of diversity
Regeneration into gaps:
intense competition for light
Gap microclimates
Antropogenic gaps and
succession
“milpas” Belize and Guyana
Nutrient loss
from shifting
cultivation plot
results from
severance of
direct nutrient
cycle and
changes in soil
microclimate
and hydrology
Forest clearance:
Rondonia, Brazil
1975
100 km2
1992
Forest clearance for pasture, Guatemala
[compare with size of milpa clearing]
“Pasturization”:
log, burn, seed in Amazonas
Succession on abandoned
pastures, Amazonia
 60,000 km2 land in pasture (mid-1980’s)
 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
Pasture use history
Biomass
and
necromass
“From
green hell
to red
desert”?
Abandoned
pastures nutrient
stocks
(NB: top 0.5m
of soil only;
N values / 5)
Rates of species replacement in
rainforest succession
Biodiversity on abandoned pastures
undergoing succession
Heavy
Recovery of tropical forests
following disturbance
1.
2.
3.
4.
5.
Karen Holl (UC Santa Cruz) working on abandoned cattle
pasture in Costa Rica has identified the following obstacles to
TRF recovery:
Tree seeds have short viability
Tree seed dispersal is generally short (large seeds;
commonly animal-dispersed) seedfall in pasture is only
1/10th that in the forest.
Heavy predation of seeds in pasture
Low survivorship of germinating seeds (severe microclimate,
low mycorrhizal infection and high herbivory)
Competition from non-native pasture grasses (e.g.
Imperata cylindrica)
Seed dispersal into abandoned
pasture, Costa Rica
30
25
Cecropia sp.
20
Dendropnax sp.
15
Ficus spp.
10
Inga sp.
5
0
Ocotea sp.
Open pasture
Branch perch*
*dispersal more effective when tree branches placed
in pasture as perches for forest birds
Rainforest fragments:
Thomas Lovejoy’s experiments
Forest species:
survival?
recruitment?
dispersal?
Patch:
minimum size?
Were tropical rain forests restricted to small refuges at LGM?
The rise
of refuge
theory*:
endemism
in the
Neotropical
forest
avifauna
* Haffer (1969)
Science, 165, 131-137.
from: Prance and Lovejoy (1985) Amazonia, Oxford U.P.
Caryocar
ranges
Ranges of related forest
bird species and subspecies
Trumpeters
(Psophia)
Jacamars
(Galbula)
Ranges of related forest
bird species and subspecies
Aracaris
(Pteroglossus)
Toucans
(Rhamphastos)
Species
and
subspecies
ranges:
Heliconius
butterflies
Inferred LGM
forest refuges
based on:
1. birds
2. lizards
3. butterflies
4. four tree
families
5. scorpions
Lake Pata
TRF refuges: a
minimalist
reconstruction
forest
desert
from: Tallis (1991) Plant Community History,
Chapman and Hall
Late Quaternary climate change in
intertropical Africa: the lake-level
evidence
low intermediate and high stands
Holocene
LGM
LGM Holocene
Lake Pata pollen record
Colinvaux et al., 1996, Science, 247, 85-88
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.