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O Christmas Tree! How Evolutionarily Constrained Are Your Branches?

A study shows that most of the variation in plants and trees comes down to just two factors: overall size and the "leaf economics spectrum."
The Christmas tree at Rockefeller Center in New York City.

The Christmas tree at Rockefeller Center in New York City.

It's the holiday season, and that means lots of plants and trees: douglas firs, white pines, Colorado spruces, not to mention all the mistletoe, holly, and poinsettias. So much variety! Well, sort of. When you step back, it turns out most plants differ on just two general traits—basically, how big they are, and how they manage the costs and benefits of producing large leaves—according to a paper published today in Nature.

There's a tremendous variety of plants and trees out in the world, a result of a Darwinian evolution operating in a wide range of environments, writes a team headed by Sandra Díaz, a professor of community and ecosystems ecology at the National University of Cordoba's Multidisciplinary Institute of Plant Biology. "Yet it has long been thought that there is a pattern to be found in this remarkable evolutionary radiation—that some trait constellations are viable and successful whereas others are not."

In other words, when you think about it, doesn't it seem like most plants have a lot in common despite all that variety?

To find out, Díaz and 34 of her colleagues turned to a database of vascular plants—those that have sophisticated systems for transporting water and nutrients, or, more simply, most any plant or tree apart from mosses and algae. That database, known as TRY, includes all sorts of traits for 46,085 vascular plants from 423 biological families, though the biologists focused on six particularly important features: plant height, stem density, the ratio of leaf mass to its area, nitrogen density in leaves, and the mass of the plant's seeds or spores. Each of those traits reflects some kind of tradeoff. For example, it takes more resources and time to build a tall tree, but tall trees can also gather more light and spread their seeds over a larger area. Similarly, leaf mass per area reflects a tradeoff between the ability to gather light and the durability—and thus longevity—of a leaf.

With even six variables, there's a lot of room to move around, so to speak, but it turns out plants and trees are relatively tightly constrained. In fact, the team found, about three-quarters of the all the differences between different species come down to just two variables. One variable describes overall size, ranging from short plants with small seeds to tall species with larger seeds. The second variable, as the team describes it, runs from species with "cheaply constructed" leaves to those with more robust leaves that last longer but take more resources to build.

This observation is, of course, largely of interest to scientists, such as those interested in "theories that have focused ... on different aspects of the Darwinian struggle for existence," the team writes. If that's not you, well, now you have some good banter should you find yourself sipping egg nog next to a Christmas tree with nothing to talk about.