Savannas(tropical grasslands)

Distribution Climate Controlling factors: soils, fire,

grazing Savanna patchiness Climate change Desertification

African climate and savanna distribution

Savanna ecosystems in W Africa

250

500

1000

15002000

Low grass savanna

Tall grass savanna

Thornforest

Rain forest

Desertdesert

Sahel zone

Sudan zone

Guinea zone

savanna-forestrainforest

Precipitation Vegetation

Climatic control on savanna distribution and type in West Africa

Synoptic situation

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

25°

20°

15°

10°

5°

ITCZ = northern edge of rains

rainy season

Savanna ecosystems in South and East

Africa

Low grass savanna

Miombo woodland

Mopane woodland

S American climate and savannas Llanos

Cerrado

1500

1

2 3

4

5

6

7

1. Honduras2. Bolivar3. Llanos4. Rupununi5. Amazonas6. Cerrado7. S. Brazil

Shrubs(6 spp.)

1

2 3

4

5

6

7

0.5

1.00.9

0.9

0.75

0.75

1

2 3

4

5

6

7

0.67

1.00.83

0.830.83

0.5

Floristic similarity in S. American savannas

(with Rupununi)

Herbs(12 spp.)

Caribbean pine

savannas(Belize -

Nicaragua)

Are South American savannas primarily products of seasonal

drought?

Savanna Forest

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

Curatella americana Byrsonima crassifolia

Alternative (or supplementary) hypotheses

Savanna Forest

firesoil senility topography

seasonal droughtand inundation

Soil hydrologyVan 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

savanna

savanna

savanna

forest

forest

to Brazilian coast -->

Topographic control on savanna/forest distribution,

Rupununi

Savanna-forest boundary(Kakadu National Park, NT, Australia)

Frederick Hardy:

the senile soil

hypothesis

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

• 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

“..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)

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

Fire-maintained boundary:Rupununi savannas,

Guyana

Dry season fires in the forest-savanna zone, Africa, 1987

Fire temperatures in tall grass savanna

Low grass savannas with sub-shrubs,

Rupununi savannas; a

cold fire environment

Effects of fire protection on tree abundance, SW Nigeria

No.

of

trees

(0.2

ha p

lot)

0

100

200

300

400

500

Late burn Early burn Protected

Fire -tender

All species

Effects of fire protection on tree abundance in savanna, Zambia

No.

of

trees

(aft

er

14 y

rs)

0

100

200

300

400

500

Thornveld

Sandveld

Effects of fire protection on grass abundance in savanna,

Zambia

% c

han

ge (

194

9-1

963)

-40

-30

-20

-10

0

10

20

30

40

50

1-yr 2-yr 3-yr 5-yr Unburnt

Heteropogon contortus Themeda trianda

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.

“Oskar”:an

individual ofPalicourea rigida

subject to frequent, low-intensity grass

fires

[Rupununi savannas]

The “Oskar-Gulliver”

syndrome: field surveys of four

savanna areas in South Africa

subjected to fire exclusion treatments

for ~50 years

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

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

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 .

Savanna patch

dynamics

Acacia patches in savanna: Serengeti,

Tanzania

Herbivore defences in Acacia species

Herbivore thorns companion ants

Acacia patch characteristics

Termitaria in the Rupununi savannas, Guyana

Termitaria in burnt savanna,

Kakadu N.P. (NT, Australia)

Leaf litter decompositi

on by termites

Termite mounds:high nutrient patches

Grazing sequence: Serengeti

rainsrenewed grass growth

zebra …. wildebeest … gazelles

coarse ……. new ……….. forbsgrasses shoots

Serengeti food chain and interactions

Sere

ng

eti

d

yn

am

ics

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

Lines = simulation model; dots = observed data

rainfall(mm/month)

grass(‘00s tons)

adult wildebeest(‘000s)

lions(‘000s)

Grazing patchinesse.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

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

1980 1991

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

Long-term climate change:

Late Pleistocen

e lake levels in African

savannas

Evidence of climate change:

Holocene vegetation changes in the Lake Victoria region