Presentation on theme: "Tropical forests Climate and distribution Forest characteristics and phenology Direct nutrient cycling Regeneration and gap dynamics Anthropogenic disturbance."— Presentation transcript:
Tropical forests Climate and distribution Forest characteristics and phenology Direct nutrient cycling Regeneration and gap dynamics Anthropogenic disturbance - shifting cultivation and pastures Forest fragmentation and conservation Late Quaternary climate change and conservation
Tropical forest: regional climate
Tropical forests: productivity and diversity Primary productivity (forests) [g m -2 yr -1 ]:Primary productivity (forests) [g m -2 yr -1 ]: Tropical:15002000 Temperate:10001500 Boreal:5001000 DiversityMalaysiaAmazonasAfrica PlantsPlants : 60 00050 00030 000 Birds:Birds: 127 270 150 (3 km 2 ) (3 km 2 ) (50 km 2 ) Bats:Bats:8198115
Canopy stratification: (how many strata?) multiple strata facilitate high productivity and diversity
Density variations in rainforest stands
High stem density Characters: lots of small poles ‘drip-tip’ leaves thin bark Diversity: majority of trees are rare - densities <1/ha.
Leaf shape: acute (‘drip-tip’), entire margin lichen growth on palm leaf ‘scratch and sniff’ taxonomy
Tree stability on wet, clay- rich tropical soils
Buttresses Plexus Stilts
Lianas and vines
Epiphytes: bromeliads and orchids
Phenology: Malaysian rainforest % of trees Triggers: degree of water stress and photoperiod. Daylength variations of 15 minutes can trigger flowering in some tropical tree species.
Biomass variations in rainforest stands
Necromass variations in rainforest stands
Nutrient storage: nitrogen
Nutrient storage: phosphorus
Nutrient storage: potassium
Root distribution and the “direct nutrient cycle” Dense root mats in surface soil exploit nutrients released by rapidly decaying organic matter on the forest floor. Nutrient capture by tree roots facilitated by mycorrhizal associations (predominantly endomycorrhizal and vesicular- arbuscular).
Nutrient shunts: leaf-cutter ants and termites
Herbivore and insectivore mammals
Herbivore resistance mechanical: spines e.g. on climbing palms; lactiferous: rubber (Hevea sp.) or chemical: secondary chemicals in roots, stems, leaves or seed coats to dissuade herbivores from attacking tissue (see next slide). The tropical forest as a “pharmaceutical factory”. biological: companion ants on Acacia shrubs in Central America ?
Wapishan woman with cassava press, Guyana
Regeneration and the maintenance of diversity
Regeneration into gaps: intense competition for light
Antropogenic gaps and succession “milpas” Belize and Guyana
Nutrient loss from shifting cultivation plot results from severance of direct nutrient cycle and changes in soil microclimate and hydrology
Forest clearance: Rondonia, Brazil 1975 1992 100 km 2
Forest clearance for pasture, Guatemala [compare with size of milpa clearing]
“Pasturization”: log, burn, seed in Amazonas
Succession on abandoned pastures, Amazonia Uhl et al., 1988. J. Ecology 60,000 km 2 land in pasture (mid-1980’s) Generally abandoned after 4-8 years* Pasture disturbances larger, more prolonged and more intense than slash and burn agriculture * abandonment as a result of soil infertility (especially phosphorus deficiency), insect attack, and weed competition
Pasture use history
Biomass and necromass
“From green hell to red desert”?
Abandoned pastures - nutrient stocks (NB: top 0.5m of soil only; N values / 5)
Rates of species replacement in rainforest succession
Biodiversity on abandoned pastures undergoing succession Heavy
Recovery of tropical forests following disturbance Karen Holl (UC Santa Cruz) working on abandoned cattle pasture in Costa Rica has identified the following obstacles to TRF recovery: 1.Tree seeds have short viability 2.Tree seed dispersal is generally short (large seeds; commonly animal-dispersed) seedfall in pasture is only 1/10th that in the forest. 3. Heavy predation of seeds in pasture 4.Low survivorship of germinating seeds (severe microclimate, low mycorrhizal infection and high herbivory) 5.Competition from non-native pasture grasses (e.g. Imperata cylindrica)
Seed dispersal into abandoned pasture, Costa Rica Mean no. seeds / m 2 * dispersal more effective when tree branches placed in pasture as perches for forest birds *
LGM in the humid tropics: plant and animal responses Were tropical rain forests restricted to small refuges at LGM?
The rise of refuge theory*: endemism in the Neo- tropical forest avifauna from: Prance and Lovejoy (1985) Amazonia, Oxford U.P. * Haffer (1969) Science, 165, 131-137.
Ranges of related forest bird species and subspecies Trumpeters (Psophia) Jacamars (Galbula)
Ranges of related forest bird species and subspecies Aracaris (Pteroglossus) Toucans (Rhamphastos)
Species and subspecies ranges: Heliconius butterflies
Inferred LGM forest refuges based on: 1. birds 2. lizards 3. butterflies 4. four tree families 5. scorpions From: Nores (1999) J. Biogeography, 26, 475-485
TRF refuges: a minimalist reconstruction Lake Pata forest desert from: Tallis (1991) Plant Community History, Chapman and Hall
Late Quaternary climate change in intertropical Africa: the lake-level evidence low intermediate and high stands Holocene LGM
Lake Pata pollen record Grasses Podocarps Colinvaux et al., 1996, Science, 247, 85-88 LGM Holocene
Refugia: a failed hypothesis? “…we conclude that the Amazon was not arid at any time in the Pleistocene, that the lowlands were in the main always forested, that forest biota were never fragmented into isolates called refugia, and that the critical global changes in Amazon history were the warmings of interglacials that intermittently perturbed the great and persistent ice-age forests. Much or all of this needs testing with more data.” Colinvaux et al., 2000. Quat. Sci. Rev. 19, 141-169.