PORES on the surface of fossilised leaves could hold clues to the rate at which mountains grew and how they changed the climate around them.
Mountains and plateaux present a physical barrier to air and wildlife, so discovering how fast they formed can be vital to understanding weather systems and the evolution of ecosystems millions of years ago. Till now, the only way to do this was to estimate the change in the elevation of particular rocks over time using, for example, the size and shape of fossilised leaves. This in turn relies on knowing the climate millions of years ago and the way temperature varied with altitude. Estimates of altitude obtained this way may be out by as much as 450 metres.
To improve on this, Jennifer McElwain of The Field Museum in Chicago, Illinois, has turned to carbon dioxide concentrations, which fall predictably with rising altitude. Plants compensate for falling CO2 levels by increasing the number of 鈥渟tomata鈥�, the pores on their leaves through which they exchange gas. McElwain studied historical and modern leaf collections from the California black oak, which grows at altitudes from 60 to 2400 metres. She found that the density of stomata can help determine the elevation the leaves came from to within 300 metres on average, and to within 100 metres in some cases (Geology, vol 32, p 1017).
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By comparing the density of stomata on fossilised leaves with densities on leaves from living trees of the same species at different elevations, McElwain hopes to get a better estimate of the growth rate of important mountainous regions, such as the Tibetan plateau. This could reveal the impact the mountains had on monsoons and the local vegetation. 鈥淚 can鈥檛 wait to apply [the technique] to real fossils,鈥� she says.
The idea has its drawbacks, however. CO2 is more concentrated low down in a canopy than near the top, so leaves from different parts of the canopy will give different results. And the technique can only yield useful information with a few kinds of leaf. 鈥淵ou need a modern plant with a broad altitude range and a fossil record,鈥� says Paul Kenrick of the Natural History Museum in London. 鈥淭hat limits its use.鈥�