Savannas (tropical grasslands)

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Savannas(tropical grasslands)

• Distribution
• Climate
• Controlling factors soils, fire, grazing
• Savanna patchiness
• Climate change
• Desertification

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African climate and savanna distribution
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Savanna ecosystems in W Africa
Vegetation
Precipitation
desert Sahel zone Sudan zone Guinea
zone savanna-forest rainforest
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Climatic control on savanna distribution and type
in West Africa
Synoptic situation
ITCZ northern edge of rains
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20
15
rainy season
10
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J F M A M J J A S O N D
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Savanna ecosystems in South and East Africa
Miombo woodland
Low grass savanna
Mopane woodland
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S American climate and savannas
Llanos
1500
Cerrado
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2
3
1
4
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1. Honduras 2. Bolivar 3. Llanos 4. Rupununi 5.
Amazonas 6. Cerrado 7. S. Brazil
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Floristic similarity in S. American
savannas(with Rupununi)
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1
0.5
0.67
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2
3
3
0.9
0.83
1.0
1.0
4
4
0.83
0.75
0.9
0.83
5
5
6
0.75
6
0.5
Shrubs (6 spp.)
Herbs (12 spp.)
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7
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Caribbean pine savannas(Belize - Nicaragua)
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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
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Common sclerophyllous shrubs, S. American savannas
Curatella americana
Byrsonima crassifolia
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Alternative (or supplementary) hypotheses
Savanna Forest
fire soil senility topography seasonal
drought and inundation
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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.
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Topography Monica Coles observations
savanna
forest
savanna
forest
savanna
to Brazilian coast --gt
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Topographic control on savanna/forest
distribution, Rupununi
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Savanna-forest boundary(Kakadu National Park,
NT, Australia)
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Frederick Hardy the senile soil hypothesis
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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.
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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.

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The role of fire
Fire as an agricultural tool in the Guinea zone,
W. Africa
..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)
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Fire-maintained boundaryRupununi savannas,
Guyana
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Dry season fires in the forest-savanna zone,
Africa, 1987
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Fire temperatures in tall grass savanna
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Low grass savannas with sub-shrubs, Rupununi
savannas a cold fire environment
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Effects of fire protection on tree abundance, SW
Nigeria
No. of trees (0.2ha plot)
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Effects of fire protection on tree abundance in
savanna, Zambia
No. of trees (after 14 yrs)
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Effects of fire protection on grass abundance in
savanna, Zambia
change (1949-1963)
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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 swifts character..), which emphasizes a
trees 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.
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Oskaranindividual ofPalicourea rigida
subject to frequent, low-intensity grass
firesRupununi savannas
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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.
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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 .
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Savanna patch dynamics
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Acacia patches in savanna Serengeti, Tanzania
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Herbivore defences in Acacia species
Herbivore thorns
companion ants
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Acacia patch characteristics
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Termitaria in the Rupununi savannas, Guyana
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Termitaria in burnt savanna, Kakadu N.P. (NT,
Australia)
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Leaf litter decomposition by termites
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Termite moundshigh nutrient patches
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Grazing sequence Serengeti
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Serengeti food chain and interactions
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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
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Grazing patchiness
e.g. interactions between zebra, Grants 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)
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The last acacialand management issues in the
Serengeti
1890s - 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) 1930-40s - reduction
in fires led to expansion of acacia
woodland 1960s - wet cattle numbers increased,
but wildebeest did not fires in savanna were
hot, killing tree seedlings 1963 - rinderpest
control program ungulates recovered 1970s -
Old acacias (which live to 60-70 years) were
dying few replacements elephants blamed for
destroying young trees elephants culled 1990s
- 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.
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The last acacia?Views of the Serengeti
1980 1991
Photos Science 19 December 1997 278. no. 5346,
p. 2059
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Long-term climate changeLate Pleistocene lake
levels in African savannas
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Evidence of climate change Holocene vegetation
changes in the Lake Victoria region