The two warmest time periods on Earth in the past 140 million years drove both mass extinctions and explosions in plant diversity, according to researchers in Australia and Austria who say this could explain how flowering plants became dominant among land plants. The study, published in Nature Plants, examined fossilised pollen and spores to show that the mid-Cretaceous and early Cenozoic periods of extreme global warmth were central to plant evolution. This applies not only to flowering plants, but also to the other main land plant groups.
Land plants are critical for life on land. The energy they capture from the sun underpins the existence of almost all terrestrial animals, fungi and bacteria. Lead author Kale Sniderman, Honorary Research Fellow at the University of Melbourne, said that flowering plants are the most productive and diverse groups of land plants with approximately 400,000 species globally.
"They dominate most ecosystems on land. They are the most important terrestrial players in capturing carbon dioxide from the air. They have evolved complex relationships with animals, fungi, and bacteria," Dr Sniderman said.
"This created vast numbers of species, each specialising in eating, pollinating, dispersing seed and decomposing flowering plants of different species or in different ways."
It's not clear how flowering plants came to rule the world. In the middle of the Cretaceous Period, during 'peak dinosaur' time (~100 million years ago), flowering plants began to displace older plant groups that had dominated the land for 200 million years before. Climate may have been important in driving this transition, but until now a clear link between climate and plant diversity has been elusive due to bias in fossil records.
"The number of available fossils varies over geological time and between regions. Because of this, simple counts of fossilised species can give a misleading sense of diversity over evolutionary time," Dr Sniderman said.
The researchers compiled data of over 60,000 individual fossil occurrences, and applied methods that overcome the major biases in the fossil record. They then generated revised estimates of fossil pollen and spore diversity for successive time periods throughout the past 130 million years.
"Our research shows that both extinction and the emergence of new but evolutionarily related land plants occurred mostly during two intervals of globally extremely warm climate," said Greg Jordan, Professor of Botany at the University of Tasmania and a co-author of the study.
"Each of these extremely warm 'turnover intervals' lasted a few million years. Thus, the warmest greenhouse climates simultaneously destroyed and promoted plant diversity, but only when considered over million-year timescales."
"In contrast, cooler climates were marked by little change. Other potential causes of plant diversity trends, like interactions between plant groups, or geographic isolation (e.g., when Australia began moving away from the rest of the southern supercontinent, Gondwana) did not explain the plant diversity trends as much as the climate."
Dr Chris Mays, Curator of Palaeontology at the Natural History Museum of Vienna (Austria) and co-author of the study, says that current, human-driven warming is occurring much faster than the mid-Cretaceous and early Cenozoic time intervals.
"Human-driven warming is likely to cause plant extinctions much faster than those during the extreme warming events of the past, Dr Mays warns. "While extinct species will eventually be replaced by new species, not many of us will be satisfied having to wait the few million years for them to evolve, especially since we have the power to prevent the loss of species today, if we stop emitting CO₂ from burning fossil fuels."
