Flowers & Rainfall

Flowers & Rainfall Photo Credit: Clipart.com

Flowering plants keep the world cooler and wetter than it would be otherwise.


Flower-powered rainfall. I'm Bob Hirshon and this is Science Update.

Without flowers, the world would be a little more drab. It would also be a lot drier and hotter, according to
University of Chicago paleontologist Kevin Boyce. He explains that the leaves of flowering plants have a much higher vein density than any other plants, past or present.

And that matters because in order to take in carbon dioxide, they have to be able to lose the water. And the more veins they have, the more water they're capable of losing.

That water comes out through the leaves, evaporates, and eventually returns as rain. Using climate models, Boyce and his colleague Jung-Eun Lee showed that just replacing flowering plants with non-flowering types would dramatically decrease rainfall around the world. It's a step toward learning how flowering plants shaped today's climate. I'm Bob Hirshon, for AAAS, the science society.

Making Sense of the Research

Even little kids understand that rain helps flowers grow. But this study suggests that the reverse is also true: that flowering plants increase rainfall throughout the world.

Flowering plants (a division of the plant kingdom known as angiosperms) dominate most of the world's ecosystems today, but this wasn't always the case. In fact, flowering plants first appeared on the Earth about 120 million years ago, well into the reign of the dinosaurs. No other type of plant in Earth's history had anywhere near the vein density of these new flowering species. Within about 20 million years, flowering plants had become the world's most prevalent and diverse plant type.  

Flowering plants had some key advantages over their competition. The flowers themselves, which are the plant's reproductive organs, allowed them to disperse themselves much more efficiently. Their high vein density also allowed them to make their own food, through photosynthesis, very efficiently. That's because photosynthesis requires the loss of water, which happens through transpiration—the release of water vapor through small pores in their leaves. (Think of it as plant sweat.) The more veins a plant has, the more water it can lose. 

As flowering plants thrived and spread, they pumped more and more water vapor into the atmosphere through transpiration. This affected the climate in far-reaching and complex ways, but the most basic result was that the world became wetter. The researchers here used a sophisticated climate model to compare what the world would be like today if only one variable were changed: that all the flowering plants were replaced, one-for-one, with non-flowering plants. Among the findings: tropical rainforests would be 80 percent smaller than they are today, and the American Northeast would get 40 percent less rainfall. The effects of flowering plants were strongest in the tropics, but their influence could be seen just about everywhere.

The findings suggest that our climate depends on flowering plants to function. Not only that, flowering plants may have helped themselves out by increasing rainfall, which helps still more flowering plants to grow. The diversity of the world's plant and animal species also owes a lot to flowering plants, since they made the climate more hospitable for a greater variety of living creatures. Unfortunately, we may be turning back the clock: recent research suggests that deforestation has reduced rainfall in many parts of the world, and that clear-cutting one area can alter the climate thousands of miles away.

Now try and answer these questions:

  1. What's the key difference between flowering plants and non-flowering plants that's relevant to this research?
  2. How does that difference affect rainfall?
  3. When humans clear-cut wild forests, we sometimes do so to create farmland. Farm crops generally consist of flowering plants. Yet, the farm crops are not as diverse, or densely populated, as the plants in the wild forest. Would this also affect rainfall, based on the study's findings?
  4. In what ways might flowering plants have helped other species emerge and thrive?

Going Further

For Educators

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