BIL 336 SAVANNAS  R. Hofstetter (Cox rm 259)

WHAT IS A SAVANNA?

The term “savanna” is used for those tropical and subtropical terrestrial and fresh-water communities that are dominated by a continuous cover of herbaceous vegetation, especially graminoids (grass-like plants). Trees (and shrubs) may be present, but without a closed canopy.

The term “savanna” may also be applied to ecosystem and landscape where savanna communities dominate.

WHERE DO SAVANNAS OCCUR?

Savannas occur between the Tropics of Capricorn and Cancer, (23.5^o North and South latitude respectively), i.e., the tropics and subtropics, where there is a pronounced annual dry season (monsoonal climate), especially in the interior or in rain-shadow regions of large land masses. They occur between lowland rain forests and either deserts or seasonally-dry deciduous forests. For total area occupied within the tropics and subtropics, savannas rank second to lowland tropical rain forests. Large areas of savannas occur in Africa (65% of area), Australia (60% of area), South America, Malesia, and India.

WHAT CAUSES SAVANNAS?

Savannas may result where and when the controlling forces favor development and persistence of herbaceous vegetation over woody vegetation. Three general classes -- “Natural”, “Anthropogenically Derived” and “Other” -- of savannas have been identified, according to the prevailing forces.

1. NATURAL SAVANNAS

Natural savannas are those savannas whose existence is due primarily to forces other than those that are human-related. Four basic types of naturally derived savannas have been identified. These are:

1A. CLIMATIC savannas are the “typical” savannas with a total annual rainfall often comparable to that of lowland rain forests, but with an annual prolonged, almost completely rainless, dry season in which few tree species can survive.

There are several different types of natural savannas whose presence can not be explained by climate alone. Combinations of several other natural forces inhibit tree development. These types are:

1B. EDAPHIC savannas develop and persist primarily because soil (edaphic) conditions inhibit the development of trees. These sites may have:

1Ba)impeded drainage, where the soils are waterlogged for too long for survival of most trees species (this type includes those WETLANDS in which trees are not dominant); OR

1Bb)excessive drainage, where too little water is available in the soil year round to support development of forests; OR

1Bc)insufficient nutrients to support forest development.

1C. FIRE savannas exist primarily because natural fires occur too frequently for trees to develop.

1D. ALPINE savannas exist at elevations above the tree line.

2. ANTHROPOGENICALLY DERIVED (SECONDARY) SAVANNAS exist primarily because humans have prevented forests (or some other community type) from developing. Such human activities include:

1)cutting of trees (land clearing and fire wood) of a former  forest,

2)excessive (frequency and/or intensity) burning of the site,

3)certain agricultural techniques that affect edaphic conditions, 4)increasing density of certain native animals (browsers), and

5)raising domesticated animals that are browsers.

Humans have existed for tens of thousands of years in some tropical regions where climates have been changing and their relative influence is not fully known. Some people consider humans to be a natural entity and force in ecosystems, and so do not differentiate between anthropogenically derived savannas and natural savannas.

3. OTHER (types of) SAVANNAS are those savannas that can not be explained by any of the above forces or processes. Some of these may be “relic” communities of earlier natural or anthropogenic conditions, others may be successionally too young for forests to have developed.

Any reasonable combination of the above (or any other) conditions that favors herbaceous vegetation over woody vegetation may result in the development of savannas.

WHAT CHARACTERIZES SAVANNAS?

Because savannas differ in their development and controlling forces, there is no single typical savanna. Savannas, however, do share combinations of the following characteristics:

1)marked seasonality in development and activity of individuals related to conditions that prevail in the wet and dry seasons, such as, for plants the time of onset of growth and dormancy, time of flowering and fruiting; and for animals the time of birthing, growth, migration/nomadism and aestivation;

2)except in certain wetlands, savannas are moderately rich in plant species diversity, but have few dominant species, rapid population turnover and marked fluctuations in populations of dominant species; the flora consists of perennial plants including bunch grasses, succulents and deciduous or evergreen sclerophyllous woody plants; tree species, which are unique to savannas, have umbrella-like crowns or are coarsely branched;

3)net primary production and standing crop is seasonally variable, being much higher in amount and in quality in the wet season than in the dry season;

4)especially in herbs, more biomass is below ground (in roots and underground stems) than above ground in shoots;

5)savanna grasses have maximum rates of photosynthesis at high temperatures and at high light levels, and have low rates of photorespiration, compared to temperate-zone grasses; wetland-type savannas have many C3 plant species, while the other types of savannas have many C4 and CAM plant species;

6)savanna plants have great drought tolerance through combinations of biochemical/physiological, anatomical, morphological, and behavioral adaptations;

7)the content of secondary (chemical) compounds in savanna grasses is low, compared to temperate zone grasses; and

8)savanna grasses are low in crude proteins, high in fiber and so are low in nutrition to grazers than are temperate zone grasses.

ADAPTATIONS OF PLANTS TO SAVANNA CONDITIONS:

Because savannas differ in the sets of forces that control their development and character, each savanna has plants with different sets of adaptations. Adaptations in plants of wetland savannas will be discussed in the second lecture. The primary environmental forces that effect selection of flora in most non-wetland savannas are regular drought, fire and herbivory. Following are some adaptations to these forces.

Anatomical/Morphological Adaptations:

1) leaves (“sclerophylls”) with reduced evaporation and  transpiration (small, revolute leaves with sunken stomata,  thick cuticle, hairs);

2) deciduous leaves and shoots;

3) extensive root systems (more biomass is in roots than in  shoots, especially in graminoids);

4) water- and food-storing roots and underground stems (rhizomes)  in many species, thick water- and food-storing shoots in  “succulents” (thick, fleshy plants);

5) ability to develop new growth loci along stems (coppice).

Phenological Adaptations:

Phenology is the study the relationships of biological phenomena to meteorological and climatological conditions. Many different phenologies have evolved in the savanna flora to adapt to the different sets of stresses and opportunities. Most perennial plants grow and flower in the wet season, a few flower in the dry season, but vegetative reproduction appears to be triggered mostly by rain or fire, not temperature. The diversity of phenological patterns in the following groups is considerable.

A. Those with continuous carbon assimilation (evergreen)may show

a) continuous growth, with

 1. precocious flowering: vigorous growth and flowering at onset of wet season, seeds produced within a few weeks; this is common in the dominant tussock-forming grasses in neotropics;

 or

 2. delayed flowering: growth begin with rains, reaches maximum after several months; sexual reproduction occurs in middle to late wet season; many dominant neotropical graminoids are in this group;

 or

 3. tardy flowering: flowering in dry season, or once each season; this pattern is uncommon, but includes some trees;

 or

 4. everflowering: producing flowers throughout the year; perennial sub-shrubs & some rhizomatous herbs with new shoots replacing dying ones;

 or

 5. opportunistic flowering: flowering after a fire in any season;

OR

b) seasonal growth, with

 6. precocious flowering: growth & flowering triggered by rainy season or fire; little growth in dry season

 or

 7. delayed flowering: flowering occurring 4-5 mon. after start of rainy season;

 or

 8. tardy flowering: flowering in the dry season (common in cerrados & llanos of South America)

B. Those plants with seasonal carbon assimilation may be:

 1. perennials with precocious flowering: plants complete growth and flowering before the end of rainy season, followed by a long dormancy with perennating organs deep in the soil;

 or

 2. annuals with precocious flowering: plants appear and flower only in first few months of the wet season;

 or

 3. perennials with delayed flowering: these species have a definite dry-season resting stage and flower several months after the onset of the growth phase;

 or

 4. annuals with delayed flowering: plants whose life cycle is completed within 7 months of the onset of the wet season;

 or

 5. perennials with tardy flowering: deciduous woody plants that flower in the dry season;

 or

 6. annuals with tardy flowering: taxa restricted to seasonally-waterlogged soils where they germinate, grow and flower after the water drains off;

 or

 7. opportunistic annuals: plants whose life cycle is completed in a few weeks at any time of year (uncommon).

FIRE AS AN ENVIRONMENTAL FORCE:

Worldwide, fire is the prevalent force in maintaining early and intermediate successional stages in essentially all of the communities dominated by graminoids - grasslands and marshes - as well as most of the communities dominated by coniferous trees.

Fires provide many benefits to fire-adapted plants, including:

1) controlling invasion of successional woody species, by pruning;

2) release nutrients from litter;

3) promoting germination of seeds and new vegetative growth (due to  fertilizer effect, and increased exposure);

4) promoting flowering growth (due to fertilizer effect, and  increased exposure);

5) reducing the incidence of diseases and pests;

Fires also

6) improve the nutritional quality of the plants for vertebrate  herbivores

Natural fires that commonly occur from late dry season to early wet season do not drastically alter the normal course of phenological events as much as do fires at other times.

Even in fire-adapted plants, there are phenological stages,(e.g., while they are growing rapidly or flowering) when they can be detrimentally affected by fire.

REFERENCES:

For those who wish to learn more about savannas and fire, any of the following references may be borrowed from the instructor, except for the bolded references, which are available at the Reserve Desk, second floor, Richter Library.

Batchelder, R.B. 1967. Spatial and temporal patterns of fire in  the tropical world. Pp. 171-206 in: “Proceeding of the Sixth  Annual Tall Timber Fire Ecology Conference”, Tallahassee, FL.

Bourliere, F.C., & M. Hadley. 1983. Present-day savannas: an  overview. Pp. 1-17 in: F. Bourliere (ed.) "Tropical  Savannas", Ecosystems of the World, Vol. 13. Elsevier Pub.  Co., N.Y.

Gillon, D. 1983. The fire problem in tropical savannas. Pp. 617- 641 in: F. Bourliere (ed.) "Tropical Savannas", Ecosystems of  the World, Vol. 13. Elsevier Pub. Co., N.Y.

Lamotte, M. 1975. The structure and function of a tropical  savannah ecosystem. Chapter 15, pp. 179-222 in: Golley, F.B.,  & Medino, E. (eds.) “Tropical Ecological Systems: Trends in  Terrestrial and Aquatic Research”, Vol. II.

Menaut, J.-C. 1983. The vegetation of African savannas. Pp. 109- 149 in: F. Bourliere (ed.) "Tropical Savannas", Ecosystems of  the World, Vol. 13. Elsevier Pub. Co., N.Y.

Nix, H.A. 1983. The climate of tropical savannas. Pp. 37-62 in: F.  Bourliere (ed.) "Tropical Savannas", Ecosystems of the World,  Vol. 13. Elsevier Pub. Co., N.Y.

Phillips, J. 1974. Effects of fire in forests and savanna  ecosystems of Sub-Saharan Africa. Chapter 13, pp. 435-481 in:  Kozlowski, T.T., & Ahlgren, C.E. (eds.) “Fire and  Ecosystems”. Academic Press, New York.

Sarmiento, G., & M. Monasterio. 1983. Life forms and phenology.  Pp. 79-108 in: F. Bourliere (ed.) "Tropical Savannas",  Ecosystems of the World, Vol. 13. Elsevier Pub. Co., N.Y.