The earliest fossil angiosperms (plants with enclosed seeds) date from the early Cretaceous, c. 120 Ma. They evolved rapidly and the world population is now believed to be around 400 000 species, many of them trees or shrubs. The beech family (Fagaceae) seem to have arisen in the montane tropics; its members migrated and diverged into the current living genera by the late Cretaceous some 60 Ma. Rapid speciation of the oaks began in the Eocene (40-60 Ma) in response to the expansion of drier and colder climates, and has continued in response to various other changes ever since. Though the gymnosperms (plants with naked seeds - the conifers and their relatives) arose as long ago as the Middle Devonian (365 Ma) there have never been more than a few thousand species. The expansion of the angiosperms and the concurrent decline of the gymnosperms has indeed been referred to as one of the most important phytogeographic processes in the history of the Earth (Richardson and Rundel,
1998). In the early Cretaceous 113 Ma, the angiosperms formed just 1% of the pollen records of the Atlantic coast of North America but within around 15 million years this had risen to 40%. Since the early angiosperms were almost all insect-pollinated (and so comparatively little of the pollen is carried by the wind to land on the ground) these figures do not show the true impact of angiosperms. When pollen reached 40% angiosperm, the vast majority of the leaf fossils preserved were angiosperm, showing how rapidly and completely they took over from gymnosperms (Ingrouille, 1992 gives a fuller description). This competition has resulted in the restriction of most gymnosperms, including pines and other conifers, to the harsher environments of the globe. Nevertheless, certain gymnosperms, especially the pines, are still notably successful and the tallest existing trees are all gymnosperms (though eucalypts appear to have a greater potential).
Competition between gymnosperm and angiosperm trees, which occurs in very many of the world's forests, is discussed by Bond (1989) who points out that by accumulating several cohorts of leaves, trees of gymnosperms often achieve higher productivity than those of angiosperms, as explained in Section 3.6. In contrast, solute flow (the mixture of water and nutrients moving up through the wood or xylem) is more efficient in angiosperms, and their seedlings are markedly more efficient than those of gymnosperms, which at this stage are dependent on a single cohort of relatively stereotyped and not very efficient leaves. Angiosperms tend to dominate favourable habitats (warm, high light, high nutrients), where their greater plasticity and initial rapid growth, frequently accompanied by use of insect pollination and seed dispersal by animals, confers marked advantages.
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