Plants use unique strategies to budget water in the wild

It’s common knowledge that plants need water to survive, but what’s less obvious is how differently they manage it. From forests to grasslands, plants have developed remarkably varied strategies to cope with water stress.

Some plants burn through water like there’s no tomorrow. Others ration it carefully. The kapok trees of the Amazon have different tactics than the tough grasses on the American plains.

But until now, scientists had no easy way to compare how these strategies play out across different ecosystems and how they change when the environment shifts.

Water shortages and plant behavior

Scientists at UC Santa Barbara and San Diego State University have found a way to make sense of this. They built a model that uses soil moisture data to detect how plants behave when it comes to water.

The team applied this model to a global dataset, covering ecosystems all over the planet. The results show that two key factors drive plant behavior: how dry a place is, and how dense the vegetation is.

“When plants face water shortages, grasslands act like spendthrifts (using water aggressively until it’s gone). In contrast, forests act more like careful budgeters (cutting back water use early to avoid disaster),” said senior author Kelly Caylor, a professor at UCSB’s Bren School of Environmental Science & Management.

The study also points out a flaw in existing models, noting that they likely overestimate how quickly ecosystems lose water during droughts. That has big implications for climate models, agriculture, and water planning.

A smarter way to read soil

Plants respond to water stress in a wide variety of ways. That’s part of what makes them so good at surviving. But it’s also made it hard for scientists to find broad patterns in their behavior.

Lead author Bryn Morgan noticed something others had missed: the way soil moisture changes over time might hold the key.

Until now, models of soil drying were focused on being easy to calculate, not on accurately reflecting how plants actually use water. Most treated the relationship between plants and soil as linear – simple input, simple output.

“But there is nonlinearity in the data,” said co-author Ryoko Araki, a joint doctoral student at UCSB and SDSU.

To address this, Araki led the development of a new, nonlinear model. It captures how soil dries after rain – not just as a mathematical curve, but in a way that reflects how plants take risks (or don’t) when water is scarce.

How plants spend their water budget

With this model in hand, the team looked at soil moisture readings from a global network of sensors. They found the parts of each record where soil was drying and fit the model to each episode.

By studying the shape and slope of each drydown curve, they could tell how aggressive or conservative the plants were.

Grasslands and dry regions tended to show more aggressive water use. Forests and wetter regions leaned more conservatively.

“If water is the currency of vegetated ecosystems, grasses are out here like, ‘YOLO,’ while trees are investing for retirement. They’re playing the long game,” said Morgan.

Risky behavior vs. cautious planning

It’s not just about water – it’s about strategy. Grasses often live fast and die young. They might only have one shot at life per year. Trees are in it for the long haul. They can’t afford to damage their water-transport systems.

“Grasslands are like sprinters who give everything they have until they hit the wall,” Caylor explained. “Forests are like marathon runners who pace themselves and slow down strategically to finish a longer race.”

“This is the overall behavior, but, of course, we see a range of aggressive to conservative strategies within a given ecosystem,” said Morgan.

Adaptive behavior of plants

Once the researchers had a sense of how ecosystems differ from each other, they looked at how behaviors shift within a single ecosystem.

The team found that as things get drier, or as water demand rises, plants change their strategy. They become more aggressive. For example, in a place like the Mojave Desert, where rainfall is highly seasonal, plant behavior can swing wildly across the year.

Until now, scientists mostly studied water use either up-close in the lab or from afar using satellites. This model bridges that gap.

“Bryn’s approach is clever in using in-situ measurements at a global scale,” Araki said. That middle-ground perspective helped them see patterns that had been hidden.

Building better climate models

Soil moisture plays a role in global carbon and climate models, but these models rarely include the subtleties of plant behavior.

“The uncertainty in estimates of the global carbon cycle is really sensitive to how vegetation responds to soil moisture limitation,” Morgan said.

Better models mean better predictions. Morgan has submitted a proposal to NASA to explore how this new nonlinear parameter could be folded into existing earth system models. She’s also interested in how these plant strategies feed back into the system over time.

Capturing soil moisture dynamics

Araki, on the other hand, is focused on scaling. Soil moisture isn’t uniform. It changes a lot, even over short distances. That makes it hard to build accurate large-scale models.

“In reality there is topography, vegetation, soil,” said Araki. “So scientists can’t apply simple statistics to the system.”

“Understanding how soil moisture values themselves scale is something hydrologists have been interested in for decades,” noted Morgan.

The challenge now is figuring out how to capture those dynamics – from a single measurement site all the way up to an entire watershed.

The full study was published in the journal Nature Ecology & Evolution.

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