Savannas and Dry Forests : Savannas and Dry Forests Harvey E. Ballard, Jr.
Department of Environmental
and Plant Biology
Ohio University
Savannas and Dry Forests : 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 : Savannas and Dry Forests Geologic History
Ecological patterns
Diversity and endemism
Characteristic plants and animals
Conservation
Answers to initial questions
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 : More Questions Are Neotropical savannas species-rich or species-poor communities?
Which groups of organisms are well represented in savannas?
Brief Introduction : Brief Introduction “Rainforest” covers broad range of communities
Includes savannas and dry forests
These range from S Mexico to SE Brazil
Brief Introduction : 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 : Brief Introduction Dominated by xeromorphic plants
“Treed grassland” or “treed desert”
Soils typically nutrient-poor
Fire important in some regions
Brief Introduction : 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 : Definitions and Classifications Vegetation Formation Classes (Dansereau 1957)
Class Stratification Cover
woodland woody pls>8 m 25-60%
savanna woody pls=2-10 m 10-25%
herbs=0-2 m 25-100
steppe woody pls=0.1-2 m 0-25%
herbs=0-2 m 10-50%
Definitions and Classifications : 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 : Definitions and Classifications Proposed Savanna Terminology (cont.)
Low tree and shrub savanna--widely spaced low-growing 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 : 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 : Types of Savanna and Dry Forest “Dry” Communities
Dry pine savanna (Belize)—Caribbean pine- dominated 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 : Determining Factors Climate
Soil characteristics
Fire
Human influences
Determining Factors : Determining Factors Climate
Determining Factors : Determining Factors Soil
characteristics
Determining Factors : 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 : Determining Factors No one natural factor explains distribution of the dry forest/ savanna biome
Determining Factors : 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 : 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 : 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 : 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 : 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 : Floristic Composition-- An Example
Trees/shrubs: 1000-2000 species
Herbs/subshrubs: 2000-5250 species
Total: 3000-7000+ species
Cerrado vegetation much more species-rich than previously believed
Floristic Composition-- An Example : Floristic Composition-- An Example How does this compare to other regions?
Region Families Genera Spp.
Cerrados: 88 363 3000- 7000+
N. America 210 ?? 15000
Ecuador* 254 2110 15306
*includes angiosperms + gymnosperms
Geologic History : 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 populationsarid species in mesic matrix
Well developed and extensive dry system by Mid-Pliocene (4 mya)
Geologic History : Geologic History Reciprocal invasions of genera between continents only since late Pliocene (2 mya)
Geologic History : Geologic History Repeated glacial periodsdrier climate, expansion of savanna
Increased speciation?
Ecological Patterns : 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 : 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 : 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 : Ecological Patterns Drought tolerance and responses--stems
Diversity and Endemism : Diversity and Endemism Species diversity influenced by a variety of factors, including latitude
Diversity and Endemism : Diversity and Endemism Life form diversity changes along aridity gradient
Diversity and Endemism : 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 : 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 : Diversity and Endemism Endemism not previously evaluated at species level!
Each different dry forest/savanna type needs focus of conservation efforts
Characteristic Plants : Characteristic Plants Legumes (Fabaceae) are commonest woody plants [and in Tropics generally!]
Characteristic Plants : Characteristic Plants Many succulents (e.g., Agavaceae, Cactaceae)
Characteristic Plants : Characteristic Plants Grasses (Poaceae) are dominant
Characteristic Animals : 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 : 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 : 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 : 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) human-induced
Human influences--cutting, agriculture
Answers to Initial Questions : 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 : 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 : 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 : 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 : 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 : 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.