Transcript forests
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]: Tropical: Temperate: Boreal: 1500 1000 500 Diversity Malaysia [1800] [1300] [800] 2000 1500 1000 Amazonas Africa • Plants: 60 000 50 000 30 000 • Birds: 127 (3 km2) 81 270 (3 km2) 98 150 (50 km2) 115 • Bats: Canopy stratification: (how many strata?) multiple strata facilitate high productivity and diversity Density variations in rainforest stands High stem density Diversity: majority of trees are rare densities <1/ha. Characters: • lots of small poles • ‘drip-tip’ leaves • thin bark Leaf shape: acute (‘drip-tip’), entire margin ‘scratch and sniff’ taxonomy lichen growth on palm leaf Treefalls Tree stability on wet, clayrich tropical soils Buttresses Plexus Stilts Cauliflory Lianas and vines Epiphytes: bromeliads and orchids Phenology: Malaysian rainforest % of trees 40 30 20 10 0 J F M A Leaf flush M J J Flowering A S Fruiting O N D Ripe fruit 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 vesiculararbuscular). Nutrient shunts: leaf-cutter ants and termites Herbivore and insectivore mammals Seed/fruit eaters 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 Gap microclimates 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 100 km2 1992 Forest clearance for pasture, Guatemala [compare with size of milpa clearing] “Pasturization”: log, burn, seed in Amazonas Succession on abandoned pastures, Amazonia 60,000 km2 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 Uhl et al., 1988. J. Ecology 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 1. 2. 3. 4. 5. Karen Holl (UC Santa Cruz) working on abandoned cattle pasture in Costa Rica has identified the following obstacles to TRF recovery: Tree seeds have short viability Tree seed dispersal is generally short (large seeds; commonly animal-dispersed) seedfall in pasture is only 1/10th that in the forest. Heavy predation of seeds in pasture Low survivorship of germinating seeds (severe microclimate, low mycorrhizal infection and high herbivory) Competition from non-native pasture grasses (e.g. Imperata cylindrica) Seed dispersal into abandoned pasture, Costa Rica 30 25 Cecropia sp. 20 Dendropnax sp. 15 Ficus spp. 10 Inga sp. 5 0 Ocotea sp. Open pasture Branch perch* *dispersal more effective when tree branches placed in pasture as perches for forest birds Rainforest fragments: Thomas Lovejoy’s experiments Forest species: survival? recruitment? dispersal? Patch: minimum size? Were tropical rain forests restricted to small refuges at LGM? The rise of refuge theory*: endemism in the Neotropical forest avifauna * Haffer (1969) Science, 165, 131-137. from: Prance and Lovejoy (1985) Amazonia, Oxford U.P. Caryocar ranges 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 Lake Pata TRF refuges: a minimalist reconstruction 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 LGM Holocene Lake Pata pollen record Colinvaux et al., 1996, Science, 247, 85-88 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.