Published on November 26, 2007
Savannasand 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 Savannaand 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 Savannaand 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.