A never-before-documented flower blooms on one of world’s rarest trees

Only about two dozen trees from the Karomia gigas tree exist in its wild Tanzanian habitat. Its new flower is a hopeful sign for its survival.

As far as the plant scientists at the Missouri Botanical Garden in St. Louis know, the tiny purple-and-white flower that recently grew in their greenhouse has never before been seen, at least by experts like them.

On May 3, Justin Lee, a senior horticulturist at the garden, was checking on a group of Karomia gigas tree saplings in a greenhouse when he spotted the flower. The tree, related to mint and originally from Africa, is one of the world’s most critically endangered tree species.

The one-inch-long flower had a halo of light purple petals that sloped downward while a cluster of four white, pollen-bearing stamens poke out.

“It’s a bit odd for a mint flower. It looks flipped inside out,” says Lee. The mint family, Lamiaceae, more commonly puts out tube-like flowers. The tree’s caretakers think it’s likely the flowers attract pollinating bees, butterflies, and moths, but it’s also possible that the tree is capable of self pollinating.

In the coming weeks, more Karomia gigas flowers are expected to grow at the greenhouse where, instead of attracting insects, they’ll attract the human hands that are working to keep this species from going extinct. By successfully getting multiple flowers to grow, the trees can be cross-pollinated and have a better chance at survival.

Only about two dozen are known to grow in the wilds of Tanzania. Roy Gereau, the Tanzania program director at the Missouri Botanical Garden, wasn’t too surprised the flower hasn’t yet been spotted. Karomia gigas is a tall, straight tree that can reach 80 feet whose branches don’t appear until about 35 to 40 feet off the ground, making the flowers hard to spot.

The tree is so rare, it doesn’t have a known nickname in English, Swahili, or the local languages around the forest reserves it's found in. Of the more than 60,000 tree species known to exist, the Karomia gigas is among the closest to extinction and one of the most endangered in Africa.

“To the best of our knowledge, there certainly is no record of the flowers in the scientific literature,'' says Gereau.

And now that the tree has produced a flower, its conservators are confident they can keep it from disappearing.

“From a standpoint of actual extinction, it’s looking really good,” says Andrew Wyatt, the vice president of horticulture at the Missouri Botanical Garden. “We can make sure the species doesn’t go extinct.”

Growing the trees abroad 

Growing the plant has been a challenge. In the wild, it’s highly susceptible to a fungus that may be spread by insects.

In September 2018, thousands of seeds were collected from field expeditions in Tanzania and shipped to St. Louis, but only about 100 seemed viable. To complicate matters, Karomia gigas' growing conditions in its Missouri greenhouse had to replicate the soil, water intake, and sunlight of the East African climate it evolved in.

The horticulturalists were eventually able to grow saplings by first germinating the seeds on wet paper towels to reduce the likelihood of an infection, and then planting them in peat. There are now 30 young trees grown from seeds and one propagated from a cutting.

“We were debating whether it would flower in our careers,” Wyatt says.

With so few trees from this species left in the world, trying to save them and seeing them grow successfully is emotional.

“You celebrate every stage. They become like your children. You’re a steward to these species,” Wyatt says. “You have a scientific connection, and you also have an emotional connection to the species.”

Lee agrees: “They’re kinda my babies.”

The flower helps scientists better understand the tree, confirming it’s in the right genus and, based on the shape of its petals and stamen, likely pollinated by insects. It’s unclear if flowers on adult trees will grow singularly or in clusters. 

“The single flower ... may not be the normal arrangement,” says Gereau. “It’s a juvenile putting out a flower for the first time.”

Importantly, it also helps secure the tree’s survival. Horticulturalists can propagate the plant with cuttings, but those trees are clones with the same DNA. Having genetic diversity in a species helps ensure it can resist deadly conditions like pests.

“Without flowering in our collections, we have to rely on the wild plants to produce seeds and the viability is very low,” says Wyatt. While some species can pollinate themselves, it’s unclear if Karomia gigas can. Lee hand pollinated the species before the flower wilted but says having more flowers from more trees will help produce more genetically resilient plants.

“Towards the end of the day I spread some pollen around. It's kind of a question mark how readily some plants self-pollinate. Sometimes it’s like a sliding scale. This one was not successful,” says Lee. “Ideally since we have so many [saplings], we will be able to get that flowering consecutively and cross pollinate, and that's better for genetic diversity.”

“Having flowering plants is a great start in efforts to recover the species,” says Emily Beech, an expert on endangered trees at Botanic Gardens Conservation International. Though not involved with propagating the St. Louis trees, in 2016 Beech joined Gereau and others from the Tanzanian Forest Service on an expedition to document the conservation status of known trees.

“There were no visible seedlings in the forest when we visited but the fact that there are now flowering plants provides a real hope for the species in the future,” she says.

A step toward regrowth 

The tree was first discovered in 1977 in Kenya, but when only two known Kenyan trees were cut down, it was thought extinct. It was rediscovered in Tanzania in 1993, and Gereau and Tanzanian botanists have been slowly finding more in the wild since 2011.

According to Fandey Mashimba, the manager of seed biology at the Tanzanian Forest Service, there are likely more than two dozen trees in the wild. Known populations are all in the country’s Mitundumbea Forest Reserve and Litipo Forest Reserve. Those forests are home to a type of woody ecosystem called Miombo woodlands that span central and southern Africa. Mashimba says it’s common to see animals there like baboons, wild pigs, buffalo, and a small type of antelope called dik-dik.

The woodlands were once the ocean floor, and so have a unique substrate. “The soil is essentially termite poop and remnants of coral beaches,” Lee says.

While the tree has been studied in its native habitat and as a specimen growing in St. Louis, its flowers had remained a mystery.

“We have a guy from the nearest village interested in the conservation of the forest area who keeps an eye on them,” Gereau says of the wild trees. “He has alerted us when he thought they were starting to flower."

But by the time someone makes the long drive to the forest reserve, no flowers have been found.

“They are in forest reserves protected by the government, but people go there for logging,” says Mashimba. Karomia gigas wood has been compared to teak, a very highly sought wood, making it a valuable source of timber.

“As far as survival, we’ve got this one,” says Wyatt, referring to the Karomia gigas species. “We can actually make sure it does not go extinct. The idea of actually preserving the species is entirely possible. It’s protected in Tanzania. We have collections in the botanical garden. Once we’ve got enough seed, we hope we can store it [in a freezer] and create a buffer between loss.”

Gereau says he’s reluctant to repatriate the trees, concerned they’re too fragile to make the trip between continents, but he says the group will actively share their knowledge with the Tanzanian government and botanists at the University of Dar es Salaam, where research on the tree has been conducted.

For now the single flower is a hopeful sign of things to come. To the surprise of the botanical garden staff, the flower in St. Louis came and went quickly, dropping off the tree in less than 24 hours.

“It basically shriveled it up,” Wyatt says. “And I picked it up off the floor and composted it.”

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