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Savannas (tropical grasslands). Distribution Climate Controlling factors: soils, fire, grazing Savanna patchiness Climate change Desertification. African climate and savanna distribution. Desert. 250. Low grass savanna. 500. Tall grass savanna. 1000. Thorn forest. 1500. 2000.

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savannas tropical grasslands

Savannas(tropical grasslands)

Distribution

Climate

Controlling factors: soils, fire, grazing

Savanna patchiness

Climate change

Desertification

savanna ecosystems in w africa

Desert

250

Low grass savanna

500

Tall grass savanna

1000

Thorn

forest

1500

2000

Rain forest

Savanna ecosystems in W Africa

Vegetation

Precipitation

desert

Sahel zone

Sudan zone

Guinea zone

savanna-forest

rainforest

climatic control on savanna distribution and type in west africa
Climatic control on savanna distribution and type in West Africa

Synoptic situation

ITCZ = northern edge of rains

25°

20°

15°

rainy season

10°

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

slide8

2

3

1

4

5

1. Honduras

2. Bolivar

3. Llanos

4. Rupununi

5. Amazonas

6. Cerrado

7. S. Brazil

6

7

floristic similarity in s american savannas with rupununi
Floristic similarity in S. American savannas(with Rupununi)

1

1

0.5

0.67

2

2

3

3

0.9

0.83

1.0

1.0

4

0.83

4

0.75

0.9

0.83

5

5

0.75

6

6

0.5

Shrubs

(6 spp.)

Herbs

(12 spp.)

7

7

are south american savannas primarily products of seasonal drought
Are South American savannas primarily products of seasonal drought?

SavannaForest

500 1000 1500 2000 2500

“the vegetation is xerophytic in many places because of the dry season that lasts for months . . . But the xerophylly is also due to the the dry continental climate in general.”

E. Warming, 1909 (on the southern cerrados of Brazil)

the climate of the Venezuelan Llanos is “hostile to woodland”

Schimper, 1903

common sclerophyllous shrubs s american savannas
Common sclerophyllous shrubs, S. American savannas

Curatella americana Byrsonima crassifolia

alternative or supplementary hypotheses
Alternative (or supplementary) hypotheses

SavannaForest

fire

soil senility

topography

seasonal drought

and inundation

soil hydrology
Soil hydrology

Van Donselaar (1969), on the basis of work in Surinam, commented:

“Savanna communities are primarily correlated with the hydrology of the soil”

In the wet season the savannas of the Venezuelan Llanos (and other flat-lying savanna areas) may be inundated by flood waters for several weeks; in the dry season the water table may drop to depths of several metres. Such fluctuations may be too severe for rainforest trees.

topography monica cole s observations
Topography: Monica Cole’s observations

savanna

forest

savanna

forest

savanna

to Brazilian coast -->

characteristics of senile soils
Characteristics of senile soils
  • Low pH (4.4 - 5.2) Low cation exchange capacity(clay fraction dominated by kaolinite)
  • Very low base saturation
  • High soluble aluminium

Soil nutrients inadequate to support forest growth; only alumino-tolerant trees survive.

plinthite formation
Plinthite formation
  • Development of iron-rich horizon in zone of fluctuating seasonal water table.
  • Long-term lowering of water table causes irreversible induration of iron-rich horizon (plinthite / laterite / ferricrete).
  • Plinthite inhibits root penetration and causes perched water tables and seasonal inundation.
the role of fire

Fire as an agricultural tool in the Guinea zone, W. Africa

The role of fire

“..together with the degradation towards a poor savanna (following on the use of fire) many other changes occur; the soil definitely deteriorates and lateritic iron pans are formed.”

Budowski, 1959 (on savanna formation in Nicaragua)

the oskar gulliver concept
The “Oskar-Gulliver” concept

“repeated fires can keep (trees) small, but (they) rarely suffer mortality, and large (trees) are virtually immune from fire damage. This .. has been called the Oskar syndrome (after Günter Grass’ character Oskar Matzerath), which emphasizes the potentially advanced age of a small individual, or the Gulliver syndrome (after Jonathan swift’s character..), which emphasizes a tree’s potential to be a giant once it escapes fire. Simulation model(s) … have shown that the distribution of fire intensities at a site can shape the structure of the tree stratum”

Higgins et al., 2007 Ecology 88, 1119-1125.

slide30
“Oskar”:anindividual ofPalicourea rigida subject to frequent, low-intensity grass fires[Rupununi savannas]
slide31
The “Oskar-Gulliver” syndrome: field surveys of four savanna areas in South Africa subjected to fire exclusion treatments for ~50 years

Higgins et al., 2007 Ecology 88, 1119-1125.

fire use by hunter gatherers e g northern australia
Fire use by hunter-gatherers(e.g. northern Australia)

“The fine-scale mosaic of burnt and unburnt areas created by mid-dry season Aboriginal landscape burning has clear effects on the distribution of kangaroos. Kangaroos move into burnt moist habitats and away from burnt dry, rocky habitats. Isotopic analysis of scats suggests that the mechanism driving this effect is the increased abundance of nitrogen rich grasses in burnt moist habitats.”

Murphy, B.P. & Bowman, D.M.J.S. 2007. J. Biogeography,34, 237-250 .

herbivore defences in acacia species
Herbivore defences in Acacia species

Herbivore thorns companion ants

grazing sequence serengeti

renewed grass growth

zebra …. wildebeest … gazelles

rains

coarse ……. new ……….. forbs

grasses shoots

Grazing sequence: Serengeti
serengeti dynamics
Serengeti dynamics

rainfall

(mm/month)

grass

(‘00s tons)

adult wildebeest

(‘000s)

lions

(‘000s)

Wolanski, E., et al., 1999. American Scientist, 87, 526-531 (Fig. 9)

Lines = simulation model; dots = observed data

grazing patchiness
Grazing patchiness

e.g. interactions between zebra, Grant’s gazelle and wildebeest in the Serengeti: patchiness is a product of direct effects (grazing sequence), and indirect effects (nutrient cycle shunt = scat production)

What are the results of loss of herbivores?

(e.g. 70% of elephants in Serengeti poached in 1980s)

the last acacia land management issues in the serengeti
“The last acacia”:land management issues in the Serengeti

1890’s - outbreak of rinderpest in East Africa led to mega-death of cattle(and wildebeest) and starvation amongst pastoralists. Fewer people, therefore fewer fires to stimulate grass growth (and kill tree seedlings);

1930-40’s - reduction in fires led to expansion of acacia woodland;

1960’s - wet; cattle numbers increased, but wildebeest did not; fires in savanna were hot, killing tree seedlings;

1963 - rinderpest control program; ungulates recovered;

1970’s - Old acacias (which live to 60-70 years) were dying; few replacements; elephants blamed for destroying young trees; elephants culled;

1990’s - Numbers of buffalo and elephants are far lower due to heavy poaching (although elephants have been increasing since the 1990 ivory ban). The wildebeest population has soared to about 1 million; human-set fires are down to about a quarter of what they were--and the acacias have returned.

the last acacia views of the serengeti
The last acacia?Views of the Serengeti

1980 1991

Photos: Science 19 December 1997: 278. no. 5346, p. 2059

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