The 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.
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.
"The same strategy appears again and again throughout the history of trees," explains Alexandru Tomescu, a Cal Poly Humboldt Botany professor and paleobotanist who co-authored the study.
Tomescu and his colleagues observed similar patterns across a wide range of living and fossil plants, even though they evolved independently over millions of years. In each case, their water-transport systems became more compartmentalized over time.
That ability allows trees to withstand and recover from damage, adapt to changing environmental conditions, and survive for decades, centuries, or even millennia, explains Tomescu.
"The pressures that shaped the first forests are still affecting forests today," he adds.
"It has been very rewarding to team up across disciplinary boundaries to connect recent discoveries about fossil and living plants,” says Martin Bouda, study co-author and professor at the University of Hohenheim and researcher at the Czech Academy of Sciences. “Together, we can both identify this convergent pattern and propose a drought-driven explanation. The next step will be most exciting: testing these ideas directly against fossil evidence."
"Understanding how trees evolved to manage water stress may help us better understand how forests will respond to a changing climate," Tomescu adds.