The Lāwa‘i Valley on Kaua‘i, Hawai‘i, is a tumble of green. Vines twine up tree trunks, lichens upholster stone, layers of vegetation leave the forest floor flecked with filtered light. And it’s all infused with water—humid, sultry air; the Lāwa‘i Stream snaking down the valley; and a constant drip, drip, drip of water falling from leaf to branch to soil. Once home to a sugarcane plantation and a private garden that was fueled by the bank account and plant collecting proclivities of its owners, the valley is now part of the National Tropical Botanical Garden (NTBG), an organization dedicated to the study and conservation of tropical plants. Safeguarding Hawai‘i’s native plants is a daunting task for an island state that’s often referred to as the endangered species capital of the world. Today, almost three-quarters of Hawai‘i’s 1,044 native vascular plant species are threatened.
On the western rim of the valley is the NTBG’s headquarters. Stepping through its doors is a transition from buzzy steam room to cool, clinical library hush. A sticker on the door to Dustin Wolkis’s lab makes the room’s purpose clear: Got Seed? Wolkis is the NTBG’s scientific curator of seed conservation, and in his small lab he and his colleagues care for over 18 million seeds from about 530 species, all chilling in the lab’s freezers.
Dustin Wolkis, scientific curator for seed conservation at the National Tropical Botanical Gardens (NTBG) on the Hawaiian island of Kaua‘i, assesses a tray of germinating seeds. Photograph by the National Tropical Botanical Garden
Seed banks have existed for over a century. The earliest were established to preserve the genetic diversity of agricultural crops. Seed banks dedicated to wild species started in the 1960s, and their numbers expanded slowly. The world’s largest is the Millennium Seed Bank in the United Kingdom, established in 2000, which holds about 2.4 billion seeds from 40,000 wild species. Today, growing interest in conserving wild plant biodiversity in the face of climate change and habitat loss has resulted in an estimated 350 botanical gardens in 74 countries maintaining seed collections of varying sizes. Since almost 40 percent of the world’s plant species are at risk of extinction, seed banks—and collaboration among their stewards—are critical players in international plant conservation. “Seed banking is the most efficient and lowest risk means of ex situ [outside of natural habitats] plant conservation,” says Wolkis. “Whether it’s time, dollars, human resources, electricity, storage space, or whatever, seed banking is the most efficient.”
While Wolkis is clearly on Team Seed, many of Hawai‘i’s seeds are not. In the seed banking pipeline, seeds need to be collected, dried, and stored, explains Wolkis. “With an exceptional species, there’s a problem with at least one of those steps.” They might not survive desiccation or cold storage, for instance, or may not make seeds in the wild because they’ve lost their native pollinator. (“Exceptional species” is the operative term used by botanists, as opposed to “orthodox species,” whose seeds can be banked using conventional protocols for drying and freezing.) Wolkis has a lot of exceptional species under his watch—up to one-third of Hawai‘i’s threatened and endangered plant species may defy conventional seed banking for one or multiple reasons. For these plants, keeping them alive in the wild or in living collections is currently the go-to conservation strategy. But Wolkis, as well as several of his colleagues from the NTBG and around the world dealing with similarly reluctant species, are experimenting with another tool: banking pollen.
Tucked into a freezer is the NTBG’s nascent sperm bank—several triple-layered, hermetically sealed foil pouches containing the dust-like pollen of a fan palm called Pritchardia minor. Pollen is the male reproductive cells of flowering plants, and each grain holds two sperm cells. When pollen germinates, it grows a pollen tube that delivers sperm to the eggs tucked in the plant’s ovary at the base of the flower. This pollen came from flowers in the NTBG’s living collection in the Lāwa‘i Valley, where Wolkis’s colleagues care for at least 25 species of Hawaiian Pritchardia, or loulu. Gathering pollen from the NTBG’s living collection is both a practicality and a reality for Wolkis and his team—almost all of Hawai‘i’s Pritchardia are listed as endangered or critically endangered by the International Union for Conservation of Nature, and they’re challenging to access in the wild. The palms evolved on the islands over millions of years, but they’ve almost disappeared in a blink of geological time.
When Polynesians first landed on the shores of Hawai‘i about a thousand years ago, pollinators across the islands busily moved buttercup-yellow pollen between P. minor’s pendulous clusters of flowers, resulting in shiny green seeds the size of olives. With thick leaves that spiraled out like massive folding hand fans, the palm proved indispensable. The people transformed the leaves into roof thatching, mats, fans, lei, and hats; the fruit was sweet and reminiscent of young coconut.
Like so many of Hawai‘i’s flora, 90 percent of which are endemic, loulu have faced a litany of assaults since that first landing: introduced rats with a penchant for palm seeds; feral pigs, goats, and deer that nibble seedlings; strawberry guava and other rambunctious invasives that smother plants with thug-like abandon; and a human population that has razed forests for everything from sugarcane to condos. A more recent threat is the arrival of the coconut rhinoceros beetle (Oryctes rhinoceros). It bores into the center of palms to feast on the sap, severely injuring the plants and sometimes killing them outright.
Invasive coconut rhinoceros beetles threaten endangered native palm trees in Hawai‘i. Photograph by Jou_1/Shutterstock (Used with permission.)
Trouble on these multiple fronts has increasingly isolated Pritchardia, mostly to remote, rugged, or high-elevation sites, making successful sex problematic. A potential pollinator, or even another plant of the same species, might not be nearby to help get the job done. Without pollination there is no fruit and no seeds, which is why NTBG staff have stepped in—monitoring what few Pritchardia remain in the wild, caring for trees in living collections, and figuring out how to successfully collect and store their pollen.
Researchers in agriculture, forestry, and horticulture bank pollen for some species, but the strategy has been largely overlooked for wild plants. Wolkis sees it as an exciting possibility for conserving the genetic biodiversity of exceptional species with little need for extra infrastructure investment. He says that if a facility has a conventional seed bank they essentially have all they need for a pollen bank, too.
Several of his colleagues working in conservation botany around the world agree that pollen is an additional tool in plant conservation, and they recently published a correspondence in Nature Plants, making a case for increasing pollen banking efforts. Pollen banking, they suggest, is a more feasible way to conserve genetic material long term than relying solely on plants, either in the wild or in living collections. Like the gametes preserved in a human sperm bank, stored pollen takes up little space today but offers enormous potential for the future. It safeguards genetic diversity as fall outs from climate change, including the decline of pollinators, accumulate.
As a process, pollen banking is similar to seed banking: determine best practices for collection, drying, and storage. As Wolkis explains it, he asks a series of questions, “from pretty boring to more interesting.” For him, the most boring and the most labor-intensive, but also the most critical, is figuring out what pollen needs to germinate in the lab. Germination is one way for researchers to test for viability—signs of life—as they finesse experimental treatments, such as responses to the standard storage temperatures of -18 degrees Celsius (-0.4 degrees Fahrenheit) or -80 degrees Celsius (-112 degrees Fahrenheit). (Cryopreservation is another storage option, but accessing a steady supply of liquid nitrogen is not practical for all banks.) Stains are also used to test for viability, but, so far, Wolkis finds germination the most promising.
Wolkis and his colleagues are experimenting with how best to collect, dry, and store pollen of the fan palm, Pritchardia minor, one of 25 species of Pritchardia, most of which are endangered. Photograph by the National Tropical Botanical Garden
Wolkis and his team tested several growth media with eight different concentrations of sucrose—an energy source for the pollen—before landing on recipes that worked for their trial species. Wolkis is jazzed by how fast the science can move now that he’s nailed this crucial step. “Depending on what question you’re trying to answer, you could go from pollen on a plant in the field to analyzed data in one day,” he says, explaining the revelation could be something like the temperature at which fresh pollen germinates. That’s a quick reward; seeds can take months to germinate, trying his patience. He gives seeds a year to sprout. If there’s no sign of life, he’ll terminate the test.
In this snippet from the last half of a 4-hour time-lapse recording, Wolkis caught the growth of Pritchardia minor’s pollen tubes. Video by Dustin Wolkis
Wolkis’s lab has been trialing pollen banking strategies on a few Hawaiian plants, but has had the most success with P. minor, which is endemic to Kaua‘i. It once thrived in the island’s wetter valleys and on exposed slopes, but now an estimated 200 plants live in 13 pockets. While P. minor is the only species of loulu he’s tested, it’s a reasonable assumption that what works for one species of Pritchardia would likely work for others in the genus. At least it’s a place to start.
Wolkis adds pollen banking research to his lab’s conservation efforts when time, funding, and opportunity permit, but Jeremy Foster, Wolkis’s colleague and coauthor on the Nature Plants note, is fortunate to focus on pollen full time as pollen bank manager at the Chicago Botanic Garden (CBG). Foster currently concentrates on native oaks and orchids, primarily from the Great Lakes region, but also on other rare and endangered plants from around the world. The oak pollen he’s stored in small vials feeds into an international project in the small, but enthusiastic, world of pollen bankers. Based at the University of Valencia in Spain, Daniel Ballesteros leads the Global Oak Pollen Bank, started in 2023. The initiative aims to preserve the genetic diversity of all of the world’s 425 oak species. That includes two endangered California oaks that Foster is working on at the CBG—the coastal sage scrub oak (Quercus dumosa) and the Engelmann oak (Quercus engelmannii).
Jeremy Foster is the pollen bank manager at the Chicago Botanic Garden. Presently, his work focuses on the conservation of native oaks and orchids. Here he collects pollen from alula (Brighamia insignis), a planet endemic to Kaua‘i that no longer exists in the wild. To help preserve what little genetic diversity is left in this species, its pollen can be shared with other botanic gardens housing live alula. Photograph by Chicago Botanic Garden
Oak seeds do not tolerate the drying required for long-term storage, so, other than trees living in the wild and in arboretums, nothing backs up the genetic diversity, explains Ballesteros. If fire or disease ravages groves of rare oak in Spain, for instance, there is no reserve. Storing pollen fills that gap.
The oak pollen project gives Ballesteros a platform for pollen evangelism. Through seminars and trainings, he’s encouraging organizations—including Spain’s national seed bank—to expand their collections to include pollen.
The Global Oak Pollen Bank is an initiative originally funded by the International Oak Society that currently includes partners from eight institutions in four countries: the United States, Spain, the United Kingdom, and China. Although Ballesteros is the lead, the bank itself is decentralized, with each institution storing its own pollen. A final goal is to create a master oak database, which will track all the oak holdings—primarily, live trees and pollen. This meta-collection tracking is critical for making “genetically informed crosses” when the time comes to share the pollen between institutions and humans take on the role of pollinators.
In conservation biology, tracking parentage is critical to preventing inbreeding and to preserving the broadest possible genetic diversity. A group of scientists, including those from the CBG and the NTBG, developed a plant “stud book” approach—similar to how pedigrees are tracked in captive-bred animals—as a way to manage the genetics of living collections of endangered plants. Foster followed this protocol when he was doing his masters on alula (Brighamia insignis), a Kaua‘i endemic that is extinct in the wild. Genetic diversity in live alula collections is dangerously low, so tracking relatedness of plants held in various collections ensures the best possible crosses when it comes time to share pollen. As Wolkis says, “with these rare species, we can’t add genetic diversity, but pollen banking combined with pedigree management allows us to maintain what’s left.”
Endemic to Kaua‘i, the alula only exists in collections. Photograph by Steve Pridgeon/Alamy Stock Photo (Used with permission.)
With the world’s wild plant heritage in such a dire state, healthy cross-pollination between plants—and also between researchers and botanical institutions—is critical. As champions of pollen banking, Wolkis, Ballesteros, Foster and their colleagues are energized by the challenge of tackling overlooked species.
“There is a whole group of [exceptional plants] that people are not banking by seed because it’s too tricky,” says Ballesteros.
“This is the frontier of ex situ conservation biology,” says Wolkis. “It offers such exciting information and the ability to save species, yet so little is known.”
It’s an exciting challenge for any botanist, but daunting for one working in a state that’s lush with life, yet home to so many critically endangered species being squeezed of real estate. Most visitors to Hawai‘i are unaware that invasive species are creating places that, Wolkis says, “are like an old folks home. You go into the forest and there’s a nice mature tree, and that’s it. There’s no recruitment, no seedlings, nothing.”
In the face of it all, banking on pollen is a hopeful act, a way to act today to bolster the biodiversity of tomorrow.
Photograph by the National Tropical Botanical Garden