In a new study, researchers analyzed a distinct type of DNA marker, which shows there are likely more scrub mint species waiting to be scientifically described. And at least one species has been left without federal protection because of a technicality.

“The Titusville balm is currently considered to be a recent hybrid,” said lead author Andre Naranjo, who conducted the study while completing a Ph.D. with the Florida Museum of Natural History. “When you describe something as a hybrid, that implies that it’s not a true species, and it can’t be protected under the Endangered Species Act.”

Naranjo found no evidence for recent hybridization in Titusville balms (Dicerandra thinicola), and his results suggest that a group called the calamints may contain cryptic diversity that requires further study.

Scrub mints evolved during a period of rapid climate change

Naranjo and his colleagues conducted the study to learn about the evolutionary history of scrub mints. The group is native to the southeastern United States and originated during a turbulent time in Earth’s past.

Three million years ago, during a period called the Pliocene, temperatures were 2-3 degrees C warmer than they are today, and sea levels were up to 30 meters higher. At the time, the central and southern half of Florida was an archipelago. But as temperatures cooled over the next several million years, the waters receded, and the Florida Peninsula took on its modern-day dimensions.

Much of this newly surfaced habitat wasn’t exactly prime real estate. Where soils did exist, they were primarily composed of sand, and the cooler temperatures resulted in less rainfall. This was particularly true of the elevated areas that had been islands before sea levels fell.

What’s left of these ancient shorelines is now located near the center of the Florida Peninsula and is often referred to as sand pine scrub. Plants and animals that moved into these vacant spaces had to contend with little water, few nutrients and rampant wildfires.

The species that managed to survive tended to do well within this narrow framework of harsh conditions but lost the ability to live just about anywhere else. Today, 40-60% species that live in these areas are endemic, meaning they can be found only in southeastern scrub habitats.

Scrub mints are among the few plants that staked a claim in the new Florida frontier. Originating in the panhandle, the ancestor of modern scrub mints dispersed south as soon as there was land to grow on.

At the height of the ice ages during the Pleistocene, when much of the planet’s reserve of water was locked away in massive glaciers, Florida was up to twice the size it is today, and scrub mints flourished.

“These plants had a much wider range in the past and were readily sharing DNA with one another,” Naranjo said.

But their habitat soon shrank. There were at least 17 ice ages during the Pleistocene, when scrub mints were evolving, and each cold period was separated by warm intervals in which much of Florida was swallowed by the sea.

Widespread scrub environments were repeatedly reduced to islands, severing the connection between mint populations. They began to grow apart, and soon each scrub island contained its own unique mint species. During the cold periods, when sea levels fell, scrub mint populations again overlapped, and these unique species hybridized with each other.

This ancient intermingling created the scrub mints as they’re known today.

The storied history of scrub mints cut short by development

Naranjo sequenced nuclear DNA from scrub mints for the study. Unlike the plastid DNA often used to study plants, which is produced by structures called chloroplasts, the DNA from plant nuclei is especially useful for scientists trying to tease apart historical interactions between species.

According to his results, annual scrub mints in the genus Dicerandra — which grow north into South Carolina and die back during the winter — originated from a back-to-back hybridization event between the ancestors of perennial scrub mints, which have a distribution further south and grow year-round.

Hybridization is a common form of diversification in plants, so much so that nearly every group of plants you might come across has had a hybridization event occur at some point in its evolutionary history.

Crucially, Naranjo’s findings indicate the scrub mints that currently exist have been on separate evolutionary trajectories for hundreds of thousands of years. When modern humans diverged from Neanderthals around 500,000 years ago, scrub mints were already well on their way to becoming separate species.

The study also suggests that calamints are genetically diverse, so much so that new species designations are likely warranted. This is especially true for those with large ranges in the southeastern U.S., including the scarlet calamint (Clinopodium coccineum) and Georgia calamint (Clinopodium georgianum), neither of which is considered to be endangered.

Even if additional species are afforded protection, Naranjo fears it may not be enough to stave off declines and eventual extinction. Lakela’s mint (Dicerandra immaculata), for example, is listed as critically endangered and only grows along a three-mile stretch of scrub, almost all of which is privately owned.

Conditions that once allowed these plants to thrive, such as periodic wildfires, are now impractical, due to nearby urban areas that would be negatively affected. And invasive species are encroaching on what little pristine scrub is left. Work to remove invasives is often done by volunteers, if they’re removed at all.

“If we continue with business as usual, this entire group of plants could go extinct within the next 100 years. And we won’t just lose these species. We’ll lose the scrub, one of the most truly authentic and formerly ubiquitous Florida habitats will just go away,” Naranjo said.

The study was published in the journal Molecular Phylogenetics and Evolution.

Christine Edwards of the Missouri Botanical Garden, Matthew Gitzendanner of the University of Florida, and Pamela and Douglas Soltis of the Florida Museum of Natural History are also co-authors on the study.

The researchers’ bleak findings are before factoring in human impact on the climate since the Industrial Revolution. The ANU-led team also found that 20th century droughts in southwestern and eastern Australia, including the Murray-Darling Basin, were longer on average compared to pre-industrial times.

According to the scientists, the findings paint a worrying picture of future droughts in Australia that are far worse than anything in recent experience.

Megadroughts are exceptionally severe, long-lasting and widespread. They can last multiple decades or even centuries. An example of this is the megadrought in the United States’ southwestern region that started in the year 2000 and has continued for more than two decades.

Co-lead author Dr Georgy Falster, from the ANU Research School of Earth Sciences, said that if a megadrought occurred in Australia today, the consequences would be made even worse because of climate change, as any drought would occur against a backdrop of hotter weather.

“The combination of climate change on top of naturally occurring megadroughts that could last for 20 years means that in the future Australia could see droughts that are worse than anything in recent historical experience,” Dr Falster said.

“We must consider, and prepare for, the possibility that one of these multi-decade megadroughts could occur in the near future.

“One of the problems with understanding protracted droughts in Australia is that our climate observations since the 1900s give us only a handful of examples to work with. This isn’t representative of the worst-case scenarios that are possible just through natural climate variations.

“Thinking about when we might expect to see a 20-year-long drought in the Murray-Darling Basin in southeastern Australia, this varies a lot. We could see a megadrought occur every 150 years or 1,000 years.

“In this study, we paid particular attention to the Murray-Darling Basin. As the largest agricultural region of Australia, it’s important to know how bad droughts in this region could be.”

The ANU-led team looked at the full spectrum of droughts Australia could experience, including length and intensity, even without the effects of climate change. They also wanted to find out how human-caused climate change is now altering the characteristics of Australian droughts.

The researchers used multiple climate models to simulate droughts that occurred during the past millennium — from the year 850 to 2000 — to determine how they might change in the future.

This includes predicting how long Australian droughts could last for, and how dry they could be.

“One of the confronting findings of our work is that it is possible for droughts in Australia to be much longer than any of the droughts that we’ve experienced in recent times. Droughts that continue for 20 years or more are something that we should expect to happen,” Dr Falster said.

“Megadroughts are part of the natural variations in Australia’s climate. But worryingly we are now also adding human-caused climate change into the mix, and that is probably increasing the chances of the next megadrought here.

“We compared simulated droughts in the 20th century, from the year 1900 to 2000, with those from the pre-industrial period, before the year 1850, to see if human-caused climate change has impacted how Australians experience droughts today.”

Co-author Professor Nerilie Abram, also from ANU, said human-caused climate change is contributing to longer droughts in southwestern and eastern Australia, including the Murray-Darling Basin.

She said these are also the regions where we can expect future rainfall declines due to climate change, thereby increasing the risk of droughts.

“It is likely that changes to drought intensity could still arise as climate change continues to worsen,” Professor Abram said.

“One example of this is the 21st century ‘Tinderbox Drought’, which was only three years long but was exceptionally intense and set the conditions for the Black Summer bushfires. The Tinderbox Drought was likely made more severe by climate change.

“The only thing we can do to lessen the potential severity and length of future droughts is to rapidly reduce greenhouse gas emissions. For example, by rapidly transitioning to renewable energy sources.

“We can also reduce the impacts of future droughts by being prepared with water storage and management plans, and community support networks.”

The research is published in a special edition of the journal Hydrology and Earth System Sciences. This work was co-led by ANU and The University of Sydney in collaboration with the University of New South Wales (UNSW), the University of Wollongong and the University of Monash.

Professor Junsuk Rho from the Departments of Mechanical Engineering, Chemical Engineering, and Electrical Engineering and PhD/MS student Geon Lee from the Department of Mechanical Engineering at Pohang University of Science and Technology (POSTECH) have joined Professor Miso Kim from the School of Advanced Materials Science and Engineering at Sungkyunkwan University (SKKU) to collaborate on a research project. Together, they developed a multifunctional elastic metasurface that can be freely configured by attaching and detaching components for practical applications. This research was published in one of the international journals in materials science, Advanced Science.

Metamaterials are artificially designed structures that exploit the relationships among wavelengths to manipulate wave energy such as light, vibration, and sound. Harnessing this capability in energy harvesting allows for the gathering of elastic waves in piezoelectric components, thereby increasing the efficiency of electricity production. However, limitations in the theoretical analysis of the beams constituting metamaterials confine their operation to a single frequency and restrict their utility to specific purposes, posing challenges for their practical application in real structures.

The research team overcame these limitations by employing the Timoshenko-Ehrenfest beam theory instead of the conventional Euler-Bernoulli beam theory. What distinguishes the former is its consideration of the fundamental characteristics of elasticity, including shear deformation and rotational inertia effects of the beam. This study marks the first application of this theory to elastic metamaterial research.

The researchers succeeded in interpreting and modeling elastic metamaterials for phase modulation of elastic waves using the Timoshenko-Ehrenfest beam theory. Furthermore, they fabricated a new type of Timoshenko-Ehrenfest beam-based reconfigurable elastic metasurface (TREM) capable of attaching and detaching multiple structures. The TREM can reconstruct its surface depending on its application, enabling control over various wave phenomena such as anomalous wave refraction, wave focusing, self-accelerated wave propagation, and total wave reflection across a wide frequency range.

Notably, the team’s TREM demonstrated outstanding effectiveness in harvesting elastic wave energy, enhancing the electrical output power of piezoelectric components by up to eight times. This highlights its value as a piezoelectric energy harvesting system.

Professor Junsuk Rho, the lead researcher, stated: “I believe that our newly developed metasurface, designed to operate across multifunctional and wide-frequency ranges, will prove invaluable in energy harvesting, most notably in the eco-friendly utilization of ambient energy. This technology, along with its applications in structural health monitoring, wireless sensing, and the Internet of Things, holds great potential for significant contributions across diverse fields.”

This work was supported by the N.EX.T. Impact Project of POSCO Holdings, as well as by funding from various programs including the Pioneer Research Center Program, the Regional Leading Research Center (RLRC) Program, and the Laboratory for Future Technology Program, all administered by the National Research Foundation of Korea and funded by the Ministry of Science and ICT of the Korean government.