When water runs out, some trees refuse to stop growing

When drought hits the African savanna, most trees shut down, hold back, and wait for rain. But one small, tough tree doesn’t follow the usual rules.

The umbrella acacia, known for its flat-topped shape and resilience, does something unexpected when water dries up. Instead of slowing down, it ramps up.

The tree keeps growing, photosynthesizing, and using every drop of water it can reach – pushing forward when others pull back.

This isn’t how most trees respond to stress. But it might explain why the umbrella acacia has survived for centuries in one of the world’s harshest environments.

How umbrella acacias defy drought

Most plants under drought stress shut down their systems to conserve water. The umbrella acacia goes the opposite way. As soon as water becomes scarce, it puts its growth into overdrive.

“You would expect most plants, if they’re being water stressed, will shut down, but at the early stage of drought stress, umbrella acacias ramp up – they go for broke,” said James Pease, an evolutionary biologist at the Ohio State University who helped lead the study.

The researchers compared the umbrella acacia (Vachellia tortilis) with its cousin, the splendid thorn acacia (Vachellia robusta), a species more common in wetter areas. The contrast was clear.

“The splendid thorn acacia tends to be more of a water saver – holding on to water, not growing a lot. Umbrella acacia does the opposite – it tries to grow more and do more photosynthesis and capture more carbon that it’s going to stockpile,” Pease said.

“Once water’s not going to come for a while, it lets the above-ground biomass die and waits for water to try again the next season.”

Living against the odds

Umbrella acacias have figured out how to survive on their own terms. They have to grow in hyper-arid conditions that are really difficult for a large woody plant to grow in, noted Pease.

“They’re being eaten by giraffes, they’re being knocked over by elephants. They have to compete with the grasses. The grasses catch fire. So there’s this whole set of pressures on them.”

According to Pease, the early seedling establishment phase is when a lot of them either make it or don’t based on their habits of how well they can acquire energy and water.

That’s where the umbrella acacia pulls ahead. Even as it burns through its water, it’s investing in deep roots – long before it looks like a full-grown tree on the surface.

“If you dig up a little acacia seedling, it has a tree’s worth of roots. And once it gets the right combination of water and nutrients, it has the rootstock to support a full tree and it will transition to that,” said Pease.

Similar tools, different timing

Scientists tracked the genetic response of the two tree species to drought by sequencing their transcriptomes – essentially, a snapshot of which genes switch on at different stages of stress.

Both trees used similar systems to deal with drought, including ways to manage photosynthesis and cellular stability. But they triggered these systems in different ways.

The splendid thorn played defense – slow, careful, and conservative. The umbrella acacia played offense – fast, aggressive, and all-in.

The differences didn’t come from genetic mutations, which are permanent changes in DNA. Instead, they came from when and how the genes were activated.

“You don’t necessarily see gene sequences and gene expression changing together,” said Ellen Weinheimer, the study’s lead author.

“The genes that are differentially expressed in response to drought don’t necessarily have sequence changes, which shows that those two mechanisms are largely independent of each other.”

Lessons beyond the savanna

The research helps explain how certain trees survive in extreme environments. It also adds to the small but growing field of genetic studies on tropical trees.

“Drought stress and climate habitat shifts are not a unique problem to African acacias. But there are very few genomic studies of tropical trees and how water stress impacts them,” said Pease.

These insights don’t just apply to Africa. As the climate shifts worldwide, plants everywhere are under new kinds of pressure.

Understanding how some trees adapt could help scientists develop better conservation plans – or even design more drought-resilient crops. Pease’s lab continues to study how gene expression changes across species and over time.

“We’re layering how gene expression levels are changing among different species,” he said.

“And over evolutionary time, we’re finding expression as important as the mutations, in that a mutation in one gene could affect the expression of another gene. We’re learning very different things than we would if we just looked at the mutations.”

The full study was published in the journal The Plant Journal.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–