TROPICAL DRY FORESTS AND SAVANNAS |
I. Factorsinfluencing vegetation types in other tropical forest types
A number of ecological factors dictate the type of tropical and subtropical forest type. We have already seen how elevation can produce an incredible gradient of vegetation types in tropical montane settings - for example, cloud forests. We will first consider the influence of salt and soil type on vegetation along tropical and subtropical coasts - peat swamps, mangroves, and beach communities. We will then look at the importance of seasonality, wind patterns, and rainshadows as they influence the formation of monsoon forests, dry tropical forests, and various thorny forests or scrub vegetation. We will finally look at the close interplay of fire and soil in the production of various kinds of savanna vegetation.
II. TropicalCoastal Communities
A. Peat Swamp Forests
Peat swamp forests typically form near drainages of large tropical rivers (e.g., Mekong Delta) or other inland sites where peat accumulation can occur to deep depths. Water level fluctuations, low pH values, acid stained black water, and anaerobic conditions are extreme and select for specialized species. The peat swamp forests often integrade into the next type of forest - mangrove forest.
B. Mangrove Forests
These forests are found throughout all the tropical and subtropical regions of the world and are confined to the saltwater of tidal regions. Mangroves are delimited in distribution by a water temperature that must exceed 75°F or 24°C in the warmest month.
Adaptations to the halophytic lifestyle ("salt lover" literally) include stilt roots - support in semiliquid substrate or shifting sand, pneumatophores - erect roots going above water level for oxygen exchange, salt glands in some for salt excretion, and viviparous seedlings - seeds germinate on mother plant with long radicle or root to allow for quick seedling establishment.
80 species are considered mangroves; these are placed in 30 genera of 20 families. 60 species are found in the richer paleotropics, and only 20 in species poor neotropics.
a. Rhizophora (Rhizophoraceae): red :outermost
b. Avicennia (Verbenaceae): black :intermediate
c. Laguncularia (Combretaceae): white :innermostd. Sonneratia (Lythraceae): common in old world
C. Tropical Beach Communities
1. grades with mangroves
Hibiscus tiliaceus (swamp hibiscus, Malvaceae) : one of few mangrove plants in both hemispheres; also common on beaches
2. trees
a. Terminalia catappa (Combretaceae)
b. Cocos nucifera (Palmae)
c. Hippomane manicella (Euphorbiaceae)
d. Coccoloba uvifera (Polygonaceae)
3. runners
a. Ipomoea pes-caprae (Convolvulaceae)
b. Scaevola (Goodeniaceae)
III.Dry Tropical Forests [Rain Green vegetation]
·India
·Indochina
·W. Africa & W. Madagascar
·Southern Africa
·S. Amer. N & S of Amazon, C. Amer. & W. Indies
·N. Australia
- climate
· wet-dry tropical climate due to seasonal alternation between dominance by moist air of intertropical convergence zone during summer solstice &
dry air masses of subtropical-high belt during winter solstice
· note: ITC migrates to most northly position at summer solstice
· a large soil-water shortage occurs, especially severe during the hottest time prior to onset of rains
Monsoon climate
W. India
Burma
Bangladesh
C. & W. Africa (e.g., Sierra Leone)
S. BrazilÞ hi and low pressure systems on continental interior
Þ extreme peak of rainfall during high-sun period (June/July - N)
Þ well developed dry seasons at low-sun (Dec-Mar - N)
A. Tropical Deciduous Forest
1. location ; hot lowlands outside equatorial zone
2. nature of vegetation
· canopies lower and more open
- Panama Canal Zone
- Santa Rosa, Costa Rica in "summer green condition"
- Jalisco, Mexico (very dry type)
- Enterlobium (legumes important)· understory vegetation more developed - better light
· green stems (Hildegardia, Sterculiaceae in Africa)
· trees and understory often leafless (deciduous) during long dry season
· flowering and fruit maturation synchronized to dry season (maximum temps induce flowering)
- Cochlospermum
- Vochysia
- Tabebuia· few lianas, epiphytes but parasites common (Psitticanthis, Loranthaceae)
· spines common
- Ceiba (kapok tree)
- Astrocaryum (palm)
- Deckenia (palm)· most expansive forests south of equator in Africa
- Adansonia digitata (Bombacaceae) "baoab" - bottle tree
120,000 liter of water, 20m dbh, 1,000 years old
- bottle trees convergent with other Bombacaceae and Sterculiaceae in South America (Cola, Ceiba) and Queensland (Brachychiton)
· this forest type is cut over extensively! (Jalisco, Mexico)
IV.Thornforest - Thorn Scrub
· gradation of dry forest types in Venezuela (neat place)
- coast - thorn forest (Cactaceae & Agave) - deciduous tropical forest - montane forest
· tropical and subtropical thorn forests are low tree vegetation types that grow in hot, somewhat dry to semiarid lowlands adjacent to tropical deciduous forests
· lack leaves in the dry season and forms dense herbaceous layer during wet season
· dominant plants are small, spiny or thorny shrubs or trees
Acacia and other legumes - pantropical!
[convergence!]
Cactaceae: neotropic
Euphorbia: Africa, Madagascar
Senecio: Canary Islands, Africa
· thorn forest in Madagascar
Didieriaceae (related to Cactaceae - replacement?)
Euphorbiaceae (striking adaptive radiation in life form!)
· grades into thorn scrub (where less than 30 cm rainfall) and finally into deserts
e.g. Mexico [note convergence in small leafless, spiny treelets]
Opuntia (Cactaceae)
Zygophyllaceae
Oleaceae
Rutaceae
Koeberlinia (Koeberliniaceae)
V.Tropical Savanna Woodland
A. vegetation type and adaptations
· tall grasslands with widely scattered trees and shrubs
· low to intermediate elevations where seasonal drought and fire combine to favor perennial grasses and limit tree growth
· fire and termites seem to go together as seen in Venezuela llanos and edge of northern Australia's monsoon forest
· trees of medium height, crowns flat or umbrella-like
· trunks thick and rough
· leaves xeromorphic or are shed in dry season
B. Llanos on the Orinoco
· Curatella (Dilleniaceae)
· Byrsonima (Malphighiaceae)
C. Amazonian sand savannas
· Brocchinia (Bromeliaceae)
· Ruizterania (Vochysiaceae)
· Rapateaceae & Xyridaceae
· Utricularia (Lentibulariaceae) carnivores!
D. Amazonian "lajas" - edaphic type
· Vellozia (Velloziaceae), Pseudobombax (Bombacaceae), Bromeliaceae, Melocactus (Cactaceae)
E. Brazilian "cerrados"
· subterranean xylopodia - drought resistance
· Velloziaceae
· Paepalanthus (Eriocaulaceae)
