It is uncertain how far the C02 level has fluctuated on longer geological timescales, over tens and hundreds of millions of years. Some calculations based on balancing the rate at which volcanoes push up C02, and the rate at which C02 is taken up into weathering reactions with rocks, suggest that there must have been some rather large fluctuations in the C02 level of the atmosphere over geological time, perhaps by a factor of 20 or more (Chapter 7). Another way to estimate past C02 levels comes from looking at the stomatal indices of leaves that lived during each phase of geological history. By looking at the surface of very well-preserved fossil leaves under a microscope, one can compare the abundance of stomata with similar leaves in the modern-day world, and perhaps deduce the concentration of C02 in the ancient atmosphere. It is important to compare the stomatal index of groups of fossil plants that have close present-day relatives for comparison, because the average stomatal index varies quite a lot between different groups of plants. Good candidates include "living fossils'' like the maidenhair tree Ginkgo, which has existed nearly unchanged for around 150 million years, and the "tulip tree'' Liriodendron which has existed for 70 million years. When their fossil leaves are very well preserved, it is possible to use a microscope to see the stomata on the leaf surfaces, and to compare their frequency to the abundance of normal epidermal cells in order to work out the stomatal index. By comparing that with the stomatal index of modern specimens grown experimentally under a range of different C02 concentrations, it is possible to estimate what the atmospheric C02 level was back then, so many millions of years ago. The general picture from stomatal densities seems to agree fairly well with that from balancing volcanoes and rock reactions, supporting the idea that there have been several tenfold fluctuations in the C02 concentration over the last few hundred million years. Some other independent indicators of C02 levels in the chemistry of rocks seem to corroborate these estimates, while others disagree, so the picture is perhaps not totally clear overall. However, most geologists who have studied the evidence seem to be convinced that there really were major fluctuations in atmospheric C02 levels over geological time.
It is unclear what these fluctuations in C02 would have been capable of doing to the ecology of plants. Presumably, the higher C02 levels in the distant geological past made plants less drought-susceptible, because they would not have needed to open their stomata quite as much to get the carbon they needed. So, they would be better able to eke out whatever supply of water they had around their roots. It has been suggested that the rise to dominance of the flowering plants (angiosperms) between 120 and 60 million years ago was caused by a large decrease in atmospheric C02 levels during the same period. Various features that are common in flowering plants, but rare in other types of plants, seem to be favorable for getting water up from the roots quickly in an environment where leaves are often short of water. For example, the elaborate branching networks of veins in the leaves of flowering plants, and the long open vessels that conduct water up through their stems, may allow better movement of water. It has been suggested, then, that the low C02 world exposed leaves to greater drought stress as they had to keep their stomata open for longer to bring in enough C02. The flowering plants, having the correct features for keeping leaves supplied with water, were able to flourish under these conditions while other older groups of plants were pushed out. The trouble with such assertions is that the rise of the flowering plants was a one-off event that we cannot re-run under different circumstances: there could in fact be many other reasons why the angiosperms did so well after they first appeared in the lower Cretaceous.
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