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Tropical forests Climate and distribution Forest characteristics and phenology Direct nutrient cycling Regeneration and gap dynamics Anthropogenic disturbance.

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Presentation on theme: "Tropical forests Climate and distribution Forest characteristics and phenology Direct nutrient cycling Regeneration and gap dynamics Anthropogenic disturbance."— Presentation transcript:

1 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

2 Tropical forest: regional climate

3 Tropical forests: productivity and diversity Primary productivity (forests) [g m -2 yr -1 ]:Primary productivity (forests) [g m -2 yr -1 ]: Tropical:1500[1800]2000 Temperate:1000[1300]1500 Boreal:500[800]1000 DiversityMalaysiaAmazonasAfrica PlantsPlants : 60 00050 00030 000 Birds:Birds: 127 270 150 (3 km 2 ) (3 km 2 ) (50 km 2 ) Bats:Bats:8198115

4 Canopy stratification: (how many strata?) multiple strata facilitate high productivity and diversity

5 Density variations in rainforest stands

6 High stem density Characters: lots of small poles ‘drip-tip’ leaves thin bark Diversity: majority of trees are rare - densities <1/ha.

7 Leaf shape: acute (‘drip-tip’), entire margin lichen growth on palm leaf ‘scratch and sniff’ taxonomy

8 Treefalls

9 Tree stability on wet, clay- rich tropical soils

10 Buttresses Plexus Stilts

11 Cauliflory

12 Lianas and vines

13 Epiphytes: bromeliads and orchids

14 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.

15 Biomass variations in rainforest stands

16 Necromass variations in rainforest stands

17 Nutrient storage: nitrogen

18 Nutrient storage: phosphorus

19 Nutrient storage: potassium

20 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).

21 Nutrient shunts: leaf-cutter ants and termites

22 Herbivore and insectivore mammals

23 Seed/fruit eaters

24 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 ?

25 Wapishan woman with cassava press, Guyana

26 Regeneration and the maintenance of diversity

27 Regeneration into gaps: intense competition for light

28 Gap microclimates

29 Antropogenic gaps and succession “milpas” Belize and Guyana

30 Nutrient loss from shifting cultivation plot results from severance of direct nutrient cycle and changes in soil microclimate and hydrology

31 Forest clearance: Rondonia, Brazil 1975 1992 100 km 2

32 Forest clearance for pasture, Guatemala [compare with size of milpa clearing]

33 “Pasturization”: log, burn, seed in Amazonas

34 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

35 Pasture use history

36 Biomass and necromass

37 “From green hell to red desert”?

38 Abandoned pastures - nutrient stocks (NB: top 0.5m of soil only; N values / 5)

39 Rates of species replacement in rainforest succession

40 Biodiversity on abandoned pastures undergoing succession Heavy

41 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)

42 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 *

43 Rainforest fragments: Thomas Lovejoy’s experiments Forest species: survival? recruitment? dispersal? Patch: minimum size?

44 LGM in the humid tropics: plant and animal responses Were tropical rain forests restricted to small refuges at LGM?

45 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.

46 Caryocar ranges

47 Ranges of related forest bird species and subspecies Trumpeters (Psophia) Jacamars (Galbula)

48 Ranges of related forest bird species and subspecies Aracaris (Pteroglossus) Toucans (Rhamphastos)

49 Species and subspecies ranges: Heliconius butterflies

50 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

51 TRF refuges: a minimalist reconstruction Lake Pata forest desert from: Tallis (1991) Plant Community History, Chapman and Hall

52 Late Quaternary climate change in intertropical Africa: the lake-level evidence low intermediate and high stands Holocene LGM

53 Lake Pata pollen record Grasses Podocarps Colinvaux et al., 1996, Science, 247, 85-88 LGM Holocene

54 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.

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