MIAMI — About half of Miami’s signature native trees — including the live oaks that line the streets of Coral Way and towering sabal palms that are the state’s official tree — might not be able to handle the rising temperatures caused by climate change.

Researchers from the University of Miami looked at hundreds of South Florida’s diverse plant species and tried to answer how, or if, the trees will survive the heat expected in the coming decades.

Kenneth Feeley, a UM biology professor and director of the university’s Gifford Arboretum, believes the findings will shape the look of the South Florida landscape over the next century — and perhaps should be guiding some tree planting decisions being made today.

“Unfortunately many trees will be lost and that’s a consequence of modern climate change,” Feeley said. “Should we continue planting with some of these same native species or do we need to diversify our urban forest to incorporate more species for hotter places?”

In the next 50 or 60 years, he imagines the trees we see at our parks, or on the way to the grocery store might shift toward more exotic species that flourish in the greater Caribbean. That also might create more habitat for exotic animals as well, like the populations of tropical parrots that already thrive in South Florida.

Some natives are clearly more vulnerable than others. Take the live oak. Miami and the Florida Keys already one of the hottest place in the world that it grows. Feeley expects the oaks, and many other natives trees, like the bald cypress, slash pine and red maple, will show signs of stress as the average temperatures continue to rise and be at greater risk of die off.

Feeley and his doctoral graduate student, Alyssa Kullberg, were able to estimate up to 41 percent of Miami’s native trees will experience heat stress by looking at global data bases of where the trees occur around the planet and cross referencing that with the Miami’s current and predicted maximum temperatures.

For many South Florida natives, the mid-80s is the best temperature for photosynthesis, the process where plants use light, carbon and water to grow and make oxygen. With extreme temperatures, Feeley said plants and trees will struggle and, for humans, provide less shade and cooling.

“They’ll be taking up less carbon. Their leaves might not be as lush so they won’t provide as much canopy cover and as they get hotter, they also become more susceptible to disease and pests,” he said.

UM’s work on trees isn’t just academic curiosity. Jane Gilbert, Miami-Dade County’s chief heat officer, told the Herald that the county met with the researchers and will factor the findings into decisions on what to plant along streets, in public places and what to recommend that residents plant.

That’s not as simple at it may sound. There are lots of considerations city planners think about. Can trees withstand hurricane-force winds? Are they invasive species, likely to spread and crowd out natives? Feeley also wants to add heat tolerance to the criteria list.

But some trees may be better than others — depending on where you put them. For instance, there are many palms that would do well in hotter temperatures, but they might not be the best for providing cooling shade, which is important in a place like Miami where trees are “a natural air-conditioner.”

For shade along suburban streets, more heat resilient native species like mahogany and gumbo limbo might be the best choice. Sabal and cabbage palms might survive longer in areas where they are protected from intense sun like forests or parks. But variety is also critical, he said, to protect against the risk of a sudden die-off. That’s what happened to commercial avocado trees in South Dade, which have suffered a dramatic decline due to a fungus spread by a beetle.

“You might say 50 to 60 years that’s too far into the future but remember we’re talking about trees that can live hundreds of years and so we need to be thinking long term,” Feeley said.

While Feeley believes more tropical exotics are inevitable in South Florida, others are cautious about moving too soon.

The Florida Native Plant Society, which focuses on promoting native plants in their natural habitats, urges against the introduction of non-native species in anticipation of climate change. There are many examples of exotics going wrong, said Steve Woodmansee, the society’s Miami-Dade chapter president. Tens of millions of dollars have been spent on removing Brazilian pepper and Austrailian melaleuca that over-ran thousands of acres of Everglades wetlands.

“We’d cross that bridge when we come to it, right now native plants are doing very well,” said Woodmansee. “It’s very complicated issue for us to think that we know exactly what’s going to happen in every detail so that we can start preemptively mitigating for that.”

But the change is coming for sure and not just in South Florida.

More than a third of the world’s tree species are threatened by extinction, according to a report released at the end of Oct. by the International Union for Conservation of Nature Red List, a global environmental union. Climate change is an emerging threat, the researchers told the New York Times, although there are still open questions on how warming will affect the majority of trees.

Trees adapt slowly

There are a few reasons why trees in particular are vulnerable to climate change. Unlike animals, they are slower to adapt and while they can expand to new areas, that process is also slow.

As an example, Feeley pointed to the floss silk tree during a class tour of the arboretum at the University of Miami. It’s an unusual species, with a trunk that has a swollen hump to store water in its mushy, spongy wood like a camel. Most notably, it’s also covered in hard-spiked knobs.

Those spikes were adapted ago to protect the trees from pre-historic relatives of elephants that roamed Central and South America 10,000 years ago. That threat is long gone but the trees remain the same.

“There’s an important lesson there. These trees respond really slow to their environment. Adaptation, evolution, is very slow,” Feeley said on the tour. “So when we say these trees might adapt to climate change or rising temperature we’re not being realistic. Give it 50 years, 100 years to respond to rising temperatures — it can’t do it.”

One part of the ongoing research is assessing how tree respond to heat. UM researchers use special sensors to see how much the tree grows or shrinks. As the tree grows it pushes against a spring-loaded pin, and the sensor measures pressure changes every 15 minutes.

“If this thing grows as much as a hair’s width, we know it,” Feeley said.

It will take a few years to get conclusive data from the sensors on UM’s trees, but the data already shows that the trees shrink during the day as they use water and grow overnight. A few more decades of rising temperatures and humidity, especially hotter nights when the trees typically grow, could hurt the trees, he said.

Feeley says some answers about South Florida’s landscape may emerge from research he is also doing far south in the steamy hot zone of the Boiling River in Peru.

Feeley said based off of his decades of work studying forests, his back of the envelope calculation is that for every degree Celsius the planet warms, there will be a loss of up to 10 percent of tree species.

“A lot of us think of trees as immortal,” he said. “But trees are being stressed all the time and a lot of them are dying. We need to be thinking long term.”

Ashley Miznazi is a climate change reporter for the Miami Herald funded by the Lynn and Louis Wolfson II Family Foundation in partnership with Journalism Funding Partners.

A heat sensor on the silk floss tree at Gifford Arboretum on University of Miami’s campus. The tree has dull spikes on it to ward off elephants that used to roam the area more than 10,000 years ago. It’s an example of how trees adapt and evolve slow and its wrong to expect them to adapt to rising temperatures in the next 50 to 60 years. (Ashley Miznazi/Miami Herald/TNS)

Kenneth Feeley gave a tour of the Gifford Arboretum to English major students at the University of Miami on Nov. 8, 2024. (Ashley Miznazi/Miami Herald/TNS)