Not all plants handle water the same - here’s why it matters

If you’ve ever forgotten to water your plants for a couple of days, you probably noticed something interesting. The flowers droop quickly. The grass gets dry. But the trees? They’re fine.

That’s because not all plants handle water the same way. Some use water freely, while others hold back, adjusting to dry conditions in very different ways.

Yet for years, many climate models haven’t taken those differences into account. They’ve relied on simple assumptions that treat every plant alike. That may soon change, thanks to new research that puts plant behavior into the equation.

Climate model gets plant upgrade

Scientists from UC Santa Barbara and San Diego State University have built a better way to look at how soil dries out after rain.

Their model shows that plants don’t follow a one-size-fits-all response to water stress. Instead, they behave in complex, adaptive ways – something the researchers say could help sharpen predictions about climate, weather, and water use.

Kelly Caylor, a professor at UCSB’s Bren School of Environmental Science & Management, is the senior author of the study.

“We found that plants don’t respond to water stress in a simple, straight-line way,” she said. “Instead, they have dynamic response patterns that reveal whether they’re ‘water spenders’ or ‘water savers.’”

Where rainwater really goes

After it rains, water goes in a few directions. Some of it runs off into rivers. Some seeps down into the ground.

The rest goes back up into the air – either by evaporating or by moving through plants, which release water through their leaves in a process called transpiration. Together, these are known as evapotranspiration.

How quickly soil dries out affects everything from crop growth to wildfire risk to weather forecasts. But until recently, scientists didn’t have good large-scale data on how this drying process worked. Instead, they built models based mostly on rainfall and streamflow.

“Ironically, most models do not use soil moisture data, although the soil moisture is a central component to hydrological behavior,” said lead author Ryoko Araki, a joint doctoral student at UCSB and SDSU.

Plants respond in unique ways

Until now, even the more advanced models made a big assumption: that all plants slow down their water use in the same way, at the same rate, under the same conditions.

“All plants – no matter young or old, summer or winter, tree or grass, small or large,” Araki said. But that assumption skips over a critical part of the picture. Different plants behave differently under stress.

By ignoring that, the models miss out on useful clues. So Araki and her colleagues wondered – could a nonlinear model, one that accounts for more complex plant responses, offer better insights?

A smarter way to model moisture

The team began with a standard, linear model based on time and moisture levels. Then they added a nonlinear factor that could reflect how plants adjust their water use as conditions change.

One thing that set this project apart was the data. The scientists used information from NASA’s SMAP satellite, which tracks soil moisture across the globe using microwave sensors. That gave them a way to test their model against real-world conditions.

And it worked. The nonlinear model outperformed the two most widely used linear models. The older models tended to overestimate how fast soil dries out. That can lead to bad predictions—especially in regions where water supply is already tight.

Still, not everyone in the field is on board. Some researchers say the added complexity isn’t worth the trouble.

“Even though the nonlinear model is more complicated, it fits the data better, and it captures more of the system’s behaviors,” Araki said. “It provides a way to investigate plant adaptations.”

Watching plants work, from space

There’s no easy way to track how individual plants manage water stress. You can’t just walk into a forest and see whether a tree is slowing down its water use or pushing forward through dry soil.

“Do they keep growing as much as they can while they still have some amount of water, or do they just completely stop transpiring to prevent tissue damage?” said co-author Bryn Morgan, a former UCSB doctoral student who is now a postdoctoral fellow at MIT.

That’s why this model matters. It gives scientists a window into plant behavior on a massive scale. It also shows that there’s more going on beneath the surface – literally – than many models have accounted for.

For gardeners, farmers, and anyone keeping an eye on climate trends, this kind of research could lead to better tools and smarter predictions. Because as it turns out, understanding how a tree survives a dry spell might help all of us prepare for what’s ahead.

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