Savannas
and Dry Forests
Harvey E. Ballard, Jr.
Department of Environmental
and Plant Biology
Ohio University
Savannas and Dry Forests
 Questions
 Brief
introduction
 Definitions and classifications
 Types of savanna and dry forest
 Determining factors
 Floristic composition--an example
Savannas and Dry Forests
 Geologic
History
 Ecological patterns
 Diversity and endemism
 Characteristic plants and animals
 Conservation
 Answers to initial questions
Questions
What type of savanna vegetation is the
most widely distributed?
What factors determine the development of
savanna vegetation?
What are some ecological patterns
characteristic of savannas and dry
forests?
More Questions
Are Neotropical savannas species-rich or
species-poor communities?
Which groups of organisms are well
represented in savannas?
Brief Introduction
• “Rainforest” covers broad range of
communities
• Includes
savannas and
dry forests
• These range
from S Mexico
to SE Brazil
Brief Introduction
• Vegetation intermingles with wet
rainforest
• Savanna may be uniformly wet, dry,
or seasonal
• “Savanna” and “dry forest” differ in
tree cover (intergrade)
Brief Introduction
• Dominated by xeromorphic plants
• “Treed grassland” or “treed desert”
• Soils typically
nutrient-poor
• Fire important
in some regions
Brief Introduction
• Negative human influences from cutting,
agriculture
• May be much
more imperiled
than previously
thought--even
worse off than
wet rainforest!
Definitions and Classifications
Vegetation Formation Classes (Dansereau
1957)
Class
woodland
savanna
steppe
Stratification
woody pls>8 m
woody pls=2-10 m
herbs=0-2 m
woody pls=0.1-2 m
herbs=0-2 m
Cover
25-60%
10-25%
25-100
0-25%
10-50%
Definitions and Classifications
Proposed Savanna Terminology (Cole 1963)
Savanna woodland--deciduous/semi-deciduous
woodland of tall trees>8 m high, and tall
grasses>80 cm high; spacing of trees>canopy
diameter
Savanna parkland--tall grasses 40-80 cm high with
scattered deciduous trees<8 m high
Savanna grassland--tall tropical grassland, no
woody plants
Definitions and Classifications
Proposed Savanna Terminology (cont.)
Low tree and shrub savanna--widely spaced lowgrowing perennial grasses<80 cm high) with
abundant annuals, widely spaced, low-growing
trees and shrubs often<2 m high
Thicket and scrub--trees and shrubs without
stratification
THOUGHT BY SOME TO BE MOST FLEXIBLE
Types of Savanna
and Dry Forest
“Wet” communities
• Llanos (Venezuela)—seasonally wet nearly
treeless grassland of Orinoco floodplain
• Pantanal (W Brazil, E Bolivia)—vast wet
grassland in interior basin
“Mesic” Communities
• Chiquitanos (SE Bolivia)—dry to moist treed
savanna/forest, newly discovered
Types of Savanna
and Dry Forest
“Dry” Communities
• Dry pine savanna (Belize)—Caribbean pinedominated savanna E of Maya Mountains
• Dry forest (Costa Rica)—NW quarter of
country, well studied by Dan Janzen et al.
• Campos cerrados (central Brazil)—largest
savanna area in Latin America, on deep sands;
large pocket across central Guianas region
• Caatinga (Brazil)—highly seasonal deciduous
forest w/thorny woodies
Determining Factors
•
•
•
•
Climate
Soil characteristics
Fire
Human influences
Determining Factors
Climate
Determining Factors
Soil
characteristics
Determining Factors
Fire
• Most types, including Campos Cerrados,
show evidence of frequent fire
• Morphological “adaptations” to
withstand it
• Extent of its role still debated—
determines local savanna/forest borders?
• [One source also suggested herbivory was
important]
Determining Factors
No one
natural
factor
explains
distribution
of the dry
forest/
savanna
biome
Determining Factors
Human influences
• Conversion of savanna/forest to
agriculture, grazing land; charcoal
extraction
• Higher ground-level temperatures
(“albedo”) by loss of vegetation cover
• Increased erosion, soil compaction
• Reduced soil nutrients
Floristic Composition-An Example
Meta-analysis of
floristic richness
in Brazilian
cerrados (Castro
et al. 1999)
• Compiled 145
lists for 78
repeatedly
visited localities
Floristic Composition-An Example
•
•
•
•
973 species
363 genera
88 families
Many other
unknown woody
plants in all 3
categories
• more research!
Floristic Composition-An Example
• 387 species
(39%) grew at
only one site
• Over 50% of
species grew in
1-2 sites!
• Lower, upper
limits inferred
Floristic Composition-An Example
• Lower limit--assumes all unknown
already recorded in “known” species
• Upper limit--takes data as is, all
unknowns = new species
• Other considerations, e. g., most
sampling methods miss 5-20% of species
present in sampled area
Floristic Composition-An Example
Trees/shrubs:
Herbs/subshrubs:
Total:
1000-2000 species
2000-5250 species
3000-7000+ species
Cerrado vegetation much more
species-rich than previously believed
Floristic Composition-An Example
How does this compare to other regions?
Region
Families Genera Spp.
Cerrados:
88
363
30007000+
N. America
210
??
15000
Ecuador*
254
2110
15306
*includes angiosperms + gymnosperms
Geologic History
• No extensive macrofossil floras available,
only microfossils (e.g., pollen, fibers)
• Oldest savanna/dry forest fossils show up
in Mid- to Late Eocene (12 mya)
• Fossils only as isolated species or small
populationsarid species in mesic matrix
• Well developed and extensive dry system
by Mid-Pliocene (4 mya)
Geologic History
Reciprocal
invasions of
genera
between
continents
only since
late
Pliocene (2
mya)
Geologic History
• Repeated
glacial
periodsdrier
climate,
expansion of
savanna
• Increased
speciation?
Ecological Patterns
Drought tolerance and responses--roots
• root:shoot biomass ratio ca. 2 times
higher (0.42-0.50) than that in wet forests
(0.23)
• root production higher
• legume trees (Fabaceae) dominant,
developing significant mycorrhizal
assocations for N2 fixation
Ecological Patterns
Drought tolerance and responses--leaves
• not well studied (e.g., desert plants)
• trees and shrubs have sclerophyllous
leaves, with thick cuticle and leathery
texture
• energy “cost” of producing these leaves
much higher than in deciduous species
• may also help against herbivory, etc.
Ecological Patterns
Drought tolerance and responses--stems
• canopy trees ca. 1/2 as tall as those in wet
forests
• avg. canopy height in lowland tropical
forest inverse to # months of <200 mm
precipitation
• root:shoot ratio differences of dry and
wet forests may be partly due to stature
Ecological Patterns
Drought tolerance and responses--stems
Diversity and Endemism
Species
diversity
influenced
by a
variety of
factors,
including
latitude
Diversity and Endemism
Life form
diversity
changes
along
aridity
gradient
Diversity and Endemism
• Climatic regime “sifts out” non-drought
resistant herbs and woodies
• Many epiphytes, lianas and creepers resistant to
environmental extremes become more dominant
• CAM and C4 plants become important in
hotter, drier sites
• Succulents increase in dominance
Diversity and Endemism
• For conservation, areas of endemism are more
important than areas of high diversity
• Mexico’s dry forests have higher endemism
than other communities; deserve attention
• But endemism at generic level is lower over
most savanna communities
Diversity and Endemism
• Endemism not previously evaluated at
species level!
• Each different dry forest/savanna type
needs focus of conservation efforts
Characteristic Plants
• Legumes (Fabaceae) are commonest
woody plants [and in Tropics generally!]
Characteristic Plants
• Many succulents (e.g., Agavaceae,
Cactaceae)
Characteristic Plants
• Grasses (Poaceae) are dominant
Characteristic Animals
• Some birds “spill over” from nearby wet
forests or wet grassland areas; some more
restricted to dry forest/savanna
• Reptiles and amphibians frequent
• Substantial beetle and other insect
populations, especially Carabids, Scarabs
Conservation
• Significant tracts in Mexico, southern
Belize, NW Costa Rica, Bolivia,
Venezuela, central Brazil
• Some protection afforded to Guanacaste
region of Costa Rica, little elsewhere
• May be too late in some peripheral areas
(Ecuador, Colombia, much of Central
America)
• Inaccessible tracts (Guianas)?
Answers to Initial Questions
What type of savanna vegetation is most widely
distributed?
The “campos cerrados”, a unique natural
mosaic of grassland and open woodland on
acidic, deep sandy soils, that dominates much
of central Brazil but has many outliers
The “caatinga”, highly seasonal dry
forest/scrub, is also widely distributed
Answers to Initial Questions
What factors determine the development of
savanna vegetation?
• Climate--including seasonality (if exists)
• Soils—nutrient-poor, but either well or poorly
drained
• Fire—natural or (more commonly) humaninduced
• Human influences--cutting, agriculture
Answers to Initial Questions
What are some ecological patterns characteristic
of savannas and dry forests?
• High root:shoot biomass—partly due to short
stature?
• Substantial soil mycorrhizal relationships—
principally in dominant Fabaceae
• Xeromorphic leaf structure—thick cuticle,
leathery leaves
• Deciduous leaves—leaf fall during droughty
periods
Answers to Initial Questions
Are Neotropical savannas species-rich or
species-poor communities?
Study of Brazilian cerrados suggests that
this region is unexpectedly species-rich,
(3,000-7,000 species)
Answers to Initial Questions
Which groups of organisms are especially
well represented in savannas?
• Angiosperms (Fabaceae, Agavaceae,
Cactaceae, Poaceae)
• Birds
• Reptiles and amphibians
• Beetles
Bibliography
Bullock, S. H., H. A. Mooney, and E. Medina (eds.). 1995.
Seasonally dry tropical forests. Cambridge University
Press, Cambridge, England.
Castro, A. A. J. F., F. R. Martins, J. Y. Tamashiro, and G.
J. Shepherd. 1999. How rich is the flora of Brazilian
cerrados? Ann. Mo. Bot. Garden 86:192-224.
Cole, M. M. 1986. The savannas-Biogeography and
geobotany. Academic Press, London, England.
Goodland, R. J. A. 1966. On the savanna vegetation of
Calabozo, Venezuela and Rupununi, British Guiana.
Bol. Soc. Venezolana Cienc. Nat. 26:341-359.
Bibliography
Goodland, R. 1970. The savanna controversy:
Background information on the Brasilian cerrado
vegetation. McGill Univ. Savanna Res. Ser. 15.,
Montreal, Quebec.
Hills, T. L. The savanna landscapes of the Amazon Basin.
McGill Univ. Savanna Res. Ser. 14., Montreal, Quebec.
Kricher, J. 1997. A Neotropical companion. Princeton
Univ. Press, Princeton, NJ.
Proctor, J. (ed.). 1989. Mineral nutrients in tropical forest
and savanna ecosystems. Blackwell Scientific, Oxford,
England.
Bibliography
Sarmiento, G. 1984. The ecology of Neotropical savannas.
Harvard Univ. Press, Cambridge, MA.
Solbrig, O. T., E. Medina, and J. F. Silva (eds.). 1996.
Biodiversity and savanna ecosystem processes-A global
perspective. Springer-Verlag, Berlin, Germany.