shkrobius (shkrobius) wrote,

An essay on falling leaves

Быть может, прежде губ уже родился шепот
И в бездревесности кружилися листы...
It's possible the whisper was born before the lips
And leaves were spiraling in treelessness...

With all respect to Mandelstam's poetry, he was gravely mistaken about the leaves. They would not be in the cards without the trees: the leaves make trees possible and the trees make leaves possible. For 50 Myr after the colonization of the land there were some shrubby plants but almost no leafy ones. Then, in the late Devonian, the concentration of atmospheric CO2 plummeted and there were leafy plants everywhere. It is an interesting (but disturbing) story:

... Plants take in carbon dioxide through channels on their surface called stomata. Living plants can adjust the number of stomata that grow on their leaves. If you raise them in a greenhouse flooded with carbon dioxide, they will develop significantly fewer stomata. That's because the plants can gather the same amount of carbon dioxide they need to grow while allowing less water to evaporate out of their stomata.

Geological evidence shows that 400 Mya, the atmosphere was loaded with carbon dioxide--about 10X the level before humans began to drive it up in the 1800s. A big leaf sitting in the sun risks overheating. The only things that can cool a leaf down are--once again--stomata. As water evaporates out of these channels, it cools the leaf: unable to cool, early Devonian leaves would have been a burden to plants, not a boon.

About 380 Mya, however, carbon dioxide levels began to drop. Over the next 40 Myr they crashed 90%, almost down to today's levels. The decline in carbon dioxide brought with it a drop in temperature: the planet cooled enough to allow glaciers to emerge at the poles. Two different groups of plants--ferns and seed plants--began to sprout leaves. As years passed, the leaves became longer and wider. And at the same time, the leaves became increasinly packed with stomata. From 380 to 340 Mya they became eight times denser. The drop in carbon dioxide and temperature turned leaves from burden to boon.

It's possible that plants themselves may have ultimately been responsible for the emergence of leaves. Before leaves evolved, roots appeared on plants. Unlike moss and liverworts, which can only soak up the water on the ground, plants with roots can seek out water, along with other nutrients. Their probing eroded rocks and built up soil. The fresh rock that the plants exposed each year could react with carbon dioxide dissolved in rainwater. Some of this carbon was carried down rivers to the ocean floor and could no longer rise back up into the atmosphere. In other words, roots pulled carbon dioxide out of the atmosphere and made it possible for leaves to evolve. The evolution of leaves in turn led to the rise of big trees, which could trap even more carbon, cooling the climate even more.

...As atmospheric CO2 levels declined, maximum leaf width in several independent groups increased by a factor of up to 25. Larger leaves signaled the parallel evolution of trees, whose maximum height quickly increased, leading to the formation of stratified forest. By the end of the Devonian, when leaf size had reached 80% of its maximal enlargement, stem diameter increased logarithmically from 3 mm to 1.5 m, implying a rise in height from a just a few centimeters to ≈30 m. Trees entrained the evolution of more complex rooting systems to provide anchorage and exploit larger volumes of soil for water and nutrients. This increased nutrient removal and the surface area of the soil–root interface, both features enhancing rates of chemical weathering of silicates and increasing the rate of removal of CO2 from the atmosphere. The expanding terrestrial biomass further promoted CO2 removal because of an enormous increase in organic burial of plant-derived organic matter (mostly on land but also in the sea), which tracked the rise in leaf and plant size and was evidenced most importantly in the formation of vast coal deposits.

The leaves are the living remnant of the global climate catastrophe caused by the evolution of roots in land plants. Before the latter evolved, there was no need for leaves whatsoever. As the roots started to erode the soil, the silicates began to weather and bind CO2 from the atmosphere, cooling the planet. Soon it became more difficult to find CO2 to grow, and the leaf was invented to solve this problem. But that only made the erosion worse, and so there was runaway depletion of CO2 to its modern level. The leafy plants would gladly deplete it more, but when they do that, the glaciations begin, the forests die, and the story begins all over again.

The root of all our problems is, literally, the root, and the fate of the planet has been sealed before the leaves started to fall on the cold ground.

И те, кому мы посвящаем опыт,
До опыта приобрели черты.
And those to whom we dedicate our trials
Acquired their features before we tried.

Tags: evolution

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