A study is reframing a fundamental question in plant evolution: What made trees possible? Researchers from Cal Poly Humboldt, Yale University, the University of Hohenheim in Germany and the Czech Academy of Sciences set out to understand how trees evolved and what allowed them to become some of the largest and longest-living organisms on Earth.

The Current Biology study suggests that surviving drought may have been a powerful force shaping the evolution of trees. It challenges conventional ideas about what defines a tree.

Rather than viewing trees simply as plants with trunks and wood, the researchers argue that a defining feature may be their ability to add and replace water transport tissues throughout their lives. Becoming tree-like and growing taller to compete for sunlight, they argue, required solving a critical challenge—moving water through a massive organism.

Water became the central challenge
To grow tall, trees must move water upward from their roots to their trunks, branches and leaves. But as they grew larger, keeping more and more water moving became increasingly difficult, especially when water became scarce.

Xylem, the microscopic tubes that carry water through plants, can become blocked by air bubbles called embolisms. If those blockages spread, they can cut off water to the rest of the plant and kill it. As plants evolved larger and more extensive water-transport systems, they became more vulnerable to embolisms and hydraulic failure.

But tree-like plants, the researchers propose, started to compartmentalize these systems to limit the spread of damage.

The strategy is similar to a power grid with circuit breakers or a ship with watertight compartments: If one section fails, the entire system doesn't go down with it.

That led the researchers to a new conclusion: The evolution of trees may have been driven by the need to move more water through their increasingly taller bodies and survive hydraulic failure...