Origins and Evolution of Tropical Rain Forests
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24/02/2015 Origins and Evolution of Tropical Rain Forests Jaboury Ghazoul Chapter 6: From the beginning: origins and transformation 1 24/02/2015 The Growth and Spread of Trees Gilboa: the earliest tree Archaeopteris: the earliest branched tree 2 24/02/2015 The First Trees Devonian (416‐359 Ma) Two growth forms: Single‐stemmed ‘Gilboa’, perhaps the world’s oldest tree Archaeopteris, a multi‐ stemmed pro‐gymnosperm Archaeopteris Gilboa The First Forests 3 24/02/2015 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 (360‐300 Ma) Carboniferous coal deposits due to: Lepidodendron Sigillaria • Appearance of lignin‐rich woody trees • Lower sea levels –created extensive lowland swamps and forests • Few animals and bacteria could digest the new lignin 50 m40 m Accumulating woody debris eventually formed coal deposits 4 24/02/2015 Lepidodendron Scale trees of the Late Carboniferous Cone The Rise of Land Plants and Animals 5 24/02/2015 Carboniferous Arthropods High oxygen levels (up to 35%) resulted in frequent fires and giant arthropods Encounters with Arthropleura in the Carboniferous forests The Permian (299 – 251 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 24/02/2015 Eduard Suess (1831 –1914) Proposed, in 1885, the existence of Gondwana, based on Glossopteris fossil distribution Terra Nova Expedition, 1911‐1912 Edward Wilson 7 24/02/2015 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 (250–145 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. Dicroidium, a tree‐like From the Jurassic (210 Ma) seed fern Petrified Araucarioxylon trunk from Arizona 8 24/02/2015 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 24/02/2015 Insect Diversification 110‐100 Ma Gondwana was breaking‐up and warm wet climates were extensive Start of Tropical angiosperm families diverged Angiosperm radiation Ants, bees and other pollinating insects diversified alongside angiosperms … … but evidence for coevolution is limited 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 24/02/2015 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 10‐15 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 24/02/2015 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 24/02/2015 Regional differences: Southeast Asia The Dipterocarpaceae: Gondwana origin, India rafting, and Asian radiation 13 24/02/2015 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 24/02/2015 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 Agathis microstachya (Araucariaceae) Gondwanan forests • Many angiosperm families also occur elsewhere because of the ancient Gondwana link 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.