Origins and Evolution of Tropical Rain Forests

Transcription

1 Origins and Evolution of Tropical Rain Forests Jaboury Ghazoul Chapter 6: From the beginning: origins and transformation 1

2 The Growth and Spread of Trees Gilboa: the earliest tree Archaeopteris: the earliest branched tree 2

3 The First Trees Devonian ( Ma) Two growth forms: Single stemmed Gilboa, perhaps the world s oldest tree Archaeopteris, a multistemmed pro gymnosperm Archaeopteris Gilboa The First Forests 3

4 Pro gymnosperms: the first forest trees Archaeopteris, extinct relative of seed plants: the first large and widespread tree Appeared in the Middle Devonian (375 Ma) and became extinct in the Early Carboniferous (345 Ma) The Carboniferous Coal Forests ( Ma) Carboniferous coal deposits due to: Appearance of lignin rich woody trees Lower sea levels created extensive lowland swamps and forests Few animals and bacteria could digest the new lignin Accumulating woody debris eventually formed coal deposits Lepidodendron 50 m 40 m Sigillaria 4

5 Lepidodendron Scale trees of the Late Carboniferous Cone The Rise of Land Plants and Animals 5

6 Carboniferous Arthropods High oxygen levels (up to 35%) resulted in frequent fires and giant arthropods Encounters with Arthropleura in the Carboniferous forests The Permian ( Ma) Extensive rain forests of the Carboniferous disappeared during the hot and dry Permian Smaller areas of wet tropical vegetation persisted in coastal areas (in what is now China) and comprised ferns, pteridosperms (seed ferns), and the first Ginkgoales Gingko trees diverse and widespread in Permian tropical forests Leaf of a fern and Sphenopteris (a seed fern) 6

7 Eduard Suess ( ) Proposed, in 1885, the existence of Gondwana, based on Glossopteris fossil distribution Terra Nova Expedition, Edward Wilson 7

8 Permian Temperate Forests The fossil distribution of the Glossopteridales provided evidence for continental drift theory Glossopteris woody, seed bearing plant reaching 30 m Distribution of Glossopteris across southern Pangea in the Permian Fossil leaves of Glossopteris Triassic and Jurassic ( Ma) Rain forests on coasts, with Araucariaceae trees reaching 60 m, understorey of ferns, cycads and giant horsetails. Large herbivorous dinosaurs were common. Late Jurassic (152 Ma) initial break up of Gondwana Petrified cone of Araucaria sp. From the Jurassic (210 Ma) Dicroidium, a tree like seed fern Petrified Araucarioxylon trunk from Arizona 8

9 Cretaceous: the first flowering plants Isolation of rain forests on separate continental fragments lays the foundation for distinctive modern biotas. Late Cretaceous (80 65 Ma) extensive wet forests of angiosperms (flowering plants), and first modern rain forests Leefructus mirus, 125 Ma Angiosperm origins and diversification 9

10 Insect Diversification Ma Gondwana was breaking up and warm wet climates were extensive Tropical angiosperm families diverged Ants, bees and other pollinating insects diversified alongside angiosperms but evidence for coevolution is limited Start of Angiosperm radiation Birds, mammals, lizards, freshwater fish, and other groups also underwent radiations Insect familial diversity from Triassic to present Angiosperm diversity in the Cenozoic Fossil pollen reveals long term diversity changes in South America a gradual rise then fall in plant diversity from 65 to 20 million years ago 10

11 The Cenozoic (65 Ma present) Extensive forests largely disappeared at the end Cretaceous extinction (65 Ma) Rain forests replaced by ferns, but recovered within 1.4 million years Early Tertiary climate favoured rain forests Angiosperms, as well as mammals and birds, diversified once more After 35 Ma the world began to cool and forests retreated The last million years has seen C4 grasslands emerge as a major tropical biome The evolution of grazing species along with fire restricted the distribution of forests Regional differences: Neotropics 11

12 Forests Through Glacial Cycles Under glacial conditions wet tropical climates cooled and dried, montane vegetation descended and sea levels dropped Cooling lowered altitudinal vegetation bands by as much as 1500 m and temperate species (e.g. Quercus, Alnus and gymnosperms) migrated into the tropics Regional differences: Madagascar 12

13 Regional differences: Southeast Asia The Dipterocarpaceae: Gondwana origin, India rafting, and Asian radiation 13

14 Regional differences: New Guinea Coastal extension and deep sea division Land bridges during lower sea levels permitted faunal exchange across Southeast Asia Deep water channels limited exchange across the Wallace Line 17,000 yrs BP 9,500 yrs BP 14

15 Regional differences: Australasia Australasia separated from Gondwana at about the same time as South America Evolved its own distinctive flora Rainforest flora comprises ancient conifer groups (Agathis) that dominated Gondwanan forests Many angiosperm families also occur elsewhere because of the ancient Gondwana link Agathis microstachya (Araucariaceae) Bowenia spectabilis (Stangeriaceae) Take home messages Tropical forest with complex structures have arisen repeatedly over geological time. Tropical rain forests acted as a cradle of angiosperm and animal diversification. Tectonic and biogeographical processes have interacted to shape tropical rain forest distributions and compositions 15