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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
Trop
ical f
ores
t: re
gion
al c
limat
e
Tropical forests:productivity and diversity
• Primary productivity (forests) [g mPrimary productivity (forests) [g m-2-2 yr yr-1-1]:]:Tropical:1500[1800]2000
Temperate:1000[1300]1500Boreal:500[800] 1000
DiversityDiversity MalaysiaMalaysia AmazonasAmazonas AfricaAfrica• PlantsPlants: 60 000 50 000 30 000• Birds: Birds: 127 270 150
(3 km2) (3 km2) (50 km2) • Bats:Bats: 81 98 115
Canopy stratification
:(how many
strata?)
multiple strata facilitate high
productivity and diversity
Density variations in rainforest stands
High stem density
Characters:• lots of small
poles• ‘drip-tip’
leaves• thin bark
Diversity:majority of trees are rare - densities <1/ha.
Leaf shape: acute (‘drip-tip’), entire margin
lichen growth on palm leaf‘scratch and sniff’taxonomy
Treefalls
Tree stability on wet, clay-
rich tropical
soils
Buttresses
PlexusStilts
Cauliflory
Lianas and vines
Epiphytes:bromeliads and orchids
Phenology: Malaysian rainforest
0
10
20
30
40
J F M A M J J A S O N D
Leaf flush Flowering Fruiting Ripe fruit
% o
f tre
es
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 vesicular-arbuscular).
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
micr
oclim
ates
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 1992
100 km2
Forest clearance for pasture, Guatemala
[compare with size of milpa clearing]
“Pasturization”:log, burn, seed in Amazonas
Succession on abandoned pastures, Amazonia
Uhl et al., 1988. J. Ecology
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
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
disturbanceKaren 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 viability2. 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 pasture4. Low survivorship of germinating seeds (severe
microclimate, low mycorrhizal infection and high herbivory)
5. Competition from non-native pasture grasses (e.g. Imperata cylindrica)
Seed dispersal into abandoned pasture, Costa
Rica
05
1015202530
Open pasture Branch perch
Cecropia sp.Dendropnax sp.Ficus spp.Inga sp.Ocotea sp.M
ean
no. s
eeds
/ m
2
*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?
LGM
in th
e hu
mid
trop
ics:
plan
t and
ani
mal
resp
onse
s
Were tropical rain forests restricted to small refuges at LGM?
The rise of refuge theory*:endemism in the
Neo-tropical forest
avifauna
from: Prance and Lovejoy (1985) Amazonia, Oxford U.P. * Haffer (1969) Science, 165, 131-137.
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. lizards3. butterflies4. four tree
families5. scorpions
From
: Nor
e s ( 1
999 )
J. B
ioge
o gr a
phy,
26 ,
475
-485
TRF refuges: a minimalist
reconstruction
Lake Pata
forest desert
from: Tallis (1991) Plant Community History, Chapman and Hall
Late Quaternary climate change in intertropical Africa: the lake-
level evidencelow intermediate and high stands
Holocene LGM
Lake Pata pollen record
Gras
ses
Podo
carp
s
Colinvaux et al., 1996, Science, 247, 85-88
LGM
Hol
ocen
e
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.