Introduced trees are becoming more common in the eastern United States, while native diversity declines

The researchers included more than 5 million measurements from individual trees across much of eastern North America and showed the rate at which introduced species are spreading has increased over the last two decades. Additionally, native tree diversity is on the decline in areas where exotic species originally introduced by humans have encroached.

This might seem like a no-brainer. If you increase the number of non-native species, it makes sense that the number of natives would go down. But no one's actually demonstrated that this happens at a large scale before.

"There's this assumption that introduced species are not a good thing, but we don't always know what that means," said study co-author Doug Soltis, a distinguished professor at the Florida Museum. "People have tried to get at their impact using fine-scale studies. What this paper does is take a more macro-level approach."

There are numerous studies that catalogue the negative effects plants can have when they're introduced to a novel ecosystem. Without the pressures of pathogens and herbivores in the region where they evolved, they can quickly outcompete native species, change the pH of the soil, alter the growth and behavior of animals, disrupt the flow of nutrients from tree to tree through underground fungal networks, and make it easier for some species to move into an area while pushing others out by altering the environment.

What we don't know is how all of these changes affect native plant diversity as a whole.

To find out, the study authors left no stone unturned. Like a physician subjecting a patient to a barrage of clinical tests, they looked at changes in biological diversity over time in response to the arrival and establishment of non-native species.

Using data collected as far back as 1995, the authors found two conspicuous patterns. First, introduced species are picking up speed rather than losing steam. Second, in areas with introduced species, the number of native species is decreasing over time.

The diversity of introduced species in many areas is also increasing. This could be because certain areas are more amenable to the growth of non-natives; they could be close to metropolitan or suburban areas, for example, where exotic trees are grown as ornamentals. Or it could mean the arrival of one introduced species somehow makes way for others.

"We don't actually know the mechanism behind the increase," said lead author Yunpeng Liu, a postdoctoral research associate at the University of Florida's Invasion Science Institute.

The answer to that riddle will have to await further research. Despite a solid understanding of how introduced trees affect natives, the authors say they aren't any closer to finding a solution for the problem. The study of introduced species is primarily focused on mitigation and prevention rather than the full-scale restoration of ecosystems. Once Pandora's box has been opened, it's practically impossible to corral the furies.

"We can't eradicate species once they've become widespread," Liu said.

But studies like these are valuable nonetheless for the simple reason that a problem can't be solved unless it's first identified. Large-scale and long-term datasets like the Forest Inventory and Analysis Program of the USDA provides directions for the unknown road ahead. People doing the hard work of removing introduced species and planting natives -- as co-authors Pam and Doug Soltis are doing with a tract of forest at the University of Florida that has been granted a temporary stay from development -- need to know where to look and focus their efforts.

"What we can offer is a risk map," Yunpeng said. "We can tell people which region or ecosystem they should pay more attention to in the future."

Case study: Tallow tree, "a most useful plant."

Tallow trees (Triadica sebifera) have spade-shaped leaves with much of the same visual appeal as a gaudy flower. In spring, the new leaves grow out crimson and give way to kelly green as they mature. Near the end of the growing season, the leaves are flushed with anthocyanins that turn them ruby red with bronze veins and a halo of dappled sunset yellows.

They have actual flowers, too, which hang down in long racemes that look like elephant trunks attached to the tip of each branch. When the fruit is ready for dispersal, an outer woody layer splits and falls away, revealing three seeds coated in a thick, waxy layer of tallow that can be harvested to make candles and soap.

The seeds also contain a type of oil that hardens when exposed to air. Unlike water, oils don't evaporate but instead go through a process called polymerization, a chemical transformation that results in a solid. Tallow tree oils polymerize quickly, making them useful for oil-based paints and varnishes.

The tree, which can grow up to 50 feet tall, also has an elegant profile, with a wide, flowing canopy that looks like it was draped in a sheet of leaves rather than growing them itself.

Tallow trees are native to Asia, where they've been cultivated for hundreds of years. With all of its pleasing and useful attributes, it's little wonder the species was imported to North America. In fact, the first tallow tree seeds were shipped over by none other than Benjamin Franklin. In a letter dated 1772, he wrote: "I send also a few seeds of the Chinese tallow tree, which will I believe grow and thrive with you. 'Tis a most useful plant."

Franklin shipped the seeds to a man in Georgia. By 1784, they were spotted growing near Charleston, South Carolina. Young tallow trees can grow as much as 13 feet a year, allowing them to outcompete slower-growing native flora for light and space. They can also grow in a variety of environments, including flood plains and dry uplands, deep shade and full sun, giving them the potential to spread widely.

Tallow trees are now considered one of the most invasive plant species in the United States, with a primarily coastal distribution from North Carolina through Texas, as well as naturalized populations in California. But their current pervasiveness isn't all Franklin's fault. In the early 1900s, the U.S. Department of Agriculture incentivized growing tallow trees in the Southeast with the goal of creating a soap industry in the region. Even when the incentive was gone, local nurseries continued to grow and sell tallow trees as ornamentals. As many as 300,000 were grown in Houston during the 1980s. The species was also introduced to California as a shade tree.

Many birds eat tallow tree fruit and disperse the seeds, making it easy for the plant to escape cultivation and infiltrate natural areas, where it can radically and rapidly alter ecosystems.

A stark example of this comes from Southeast Texas, to which tallow trees were first introduced in the early 19th century. Initially, they spread primarily through riparian landscapes, following the course of rivers and streams. Then, beginning in the 1970s, they moved onto higher ground.

Before Houston, Galveston and the surrounding areas were developed or converted to farmland, they were covered with vast prairies. The area received sufficient rainfall to support forests, but the soil is composed primarily of thick, cloying clay that makes it difficult for plants to absorb water and nutrients. Grasses like little bluestem were among the only plants that grew there, and wildfires were common. Thus, any trees that could withstand the infertile soil were periodically cleared away by conflagrations.

But this didn't stop tallow trees. After building up a robust seed bank in the nearby riparian zones, they quickly and efficiently went about converting prairies into woodlands made primarily of tallow trees. They're also good at taking over dormant pastures, and their spread was likely facilitated by abandoned farm and ranch lands, which lay fallow as agriculture declined in the region and properties were earmarked for development.

Once invasive species become established, they also make subtle changes to the environment that scientists are only now beginning to understand.

Tallow leaves are relatively thin compared with some native trees in temperate North American forests. If they happen to fall in a body of water, such as a pond or a lake, bacteria rapidly begin breaking down the leaves, digesting their sugars and tannins, and using up dissolved oxygen in the water during the process.

This has been shown to reduce the hatching success and tadpole survival of southern leopard frogs in laboratory settings. Gray tree frogs also seem to avoid making mating calls in tallow trees, preferring the natives instead, which would become problematic in places where tallows are the main type of woody vegetation.

Replacing an entire ecosystem, whether through development or the introduction of non-native species, also displaces organisms that cannot withstand the transition. This is the case for Attwater's prairie-chicken, which is listed as federally endangered and lives exclusively in the disappearing coastal plains of Texas.

The study was published in the Proceedings of the National Academy of Sciences.

Samuel Scheiner, Aaron Hogan of the USDA Forest Service, Matthew Thomas and Jeremy Lichstein of the University of Florida and Robert Guralnick of the Florida Museum were also co-authors of the study.

Funding for the study was provided in part by the National Science Foundation, the USDA Forest Service and the University of Florida.