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 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

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J F M A M J J A S O N D

Leaf flush Flowering Fruiting Ripe fruit

% o

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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

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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

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Open pasture Branch perch

Cecropia sp.

Dendropnax sp.

Ficus spp.

Inga sp.

Ocotea sp.

Mean

no.

seed

s /

m2

*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

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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

: N

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s (1

99

9)

J. B

iogeogr a

phy, 2

6, 4

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-48

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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 evidence

low intermediate and high stands

Holocene LGM

Lake Pata pollen record

Gra

sses

Podo

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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.