Many plant species would not exist at all today if gardeners had not preserved them. The native American tree Franklinia alatamaha survives thanks to John and William Bartram, who collected seeds from the tree in Georgia in 1765 and began propagating it shortly before it disappeared completely from the wild around 1800. The spectacular Dove Tree, Davidia involucrata, exists in European and North America gardens through sheer luck; in 1900 plant collector E.H. Wilson went to China seeking the fabled tree only to discover that the one known specimen had been cut down for lumber. He searched far and wide in the woods to find a few more trees growing wild and thus managed to collect enough seed to introduce the tree to the nursery trade.
Botanical gardens have bred plants extinct in the wild to reintroduce them to their native ranges. For example, Sophora toromiro, a small tree from Easter Island, went extinct in its natural habitat but was preserved for reintroduction by the Bonn University Botanical Garden. (Maunder et al. 2000) The Royal Botanic Garden, Kew, has collaborated with the Seychelles Botanic Garden to cultivate Rothmannia annae, a plant native to the Seychelles that is nearly extinct in its home environment.
Kokia cookei, a tree endemic to Molokai, Hawaii, went extinct in the wild in 1918 but has been cultivated in botanical gardens since then; as of 2008, it was growing at Waimea Audubon, Lyon Arboretum, Volcano Rare Plant Facility, and the National Tropical Botanical Garden. The last known wild Cyanea pinnatifida, endemic to Oahu, died in 2001, leaving its cultivated progeny in the Lyon Arboretum and the National Tropical Botanical Garden. Lysimachia minoricensis, a plant of the Balearic Islands, went extinct in the first half of the 20th century, but botanists at the Botanical Garden of Barcelona cultivated plants from collected seed in 1926. The species is now found only in cultivation and seedbanks. Both of these plants have been the subject of experimental outplantings, in which propagated plants are reintroduced to the wild, though neither has produced self-sustaining wild colonies.
Some plants thought to be extinct have been rediscovered in the wild. The Hawaiian plant Cyanea truncata was thought to be extinct after the last known individual died in the 1980s. Subsequent surveys discovered a few more wild plants, three of which survived as of 2006, and which provided genetic material that botanists have used to propagate more plants and outplant them into a protected habitat. As of 2007 the state of Hawaii’s Genetic Safety New Program and the Lyon Arboretum were maintaining seeds and tissue samples that could be used to propagate more plants in the future.
When it was listed as an endangered species in 1996, Hibiscadelphus giffardianus was extinct in the wild. Only one tree has ever been known in the wild, from Kipuka Puaulu in Hawaii Volcanoes National Park. This tree died in 1930, but Giffard, its namesake, collected seed before the tree died. These seeds have been propagated, and have produced genetically unique offspring (cause they’re from seeds, which are the product of sexual reproduction.) Cuttings from these cultivated trees were planted back into the now fenced original habitat at Kipuka Puaulu. As of 2008, over 400 plants survived, many of them fruiting or flowering.
I know I keep going on about Hawaiian plants, which may seem to have little to do with Carolina botany. But they certainly have a problem with native species going extinct. Hawaii really is an astonishing hotbed of endangered species, most of which appear to have been destroyed by invasive introduced species – plants, pigs, goats, slugs. Ants – Argentine ants are a terrible problem, eating seeds and pollinators.
So without apology, I'll mention a couple more Hawaiian plants. Hibiscadelphus hualalaiensis was historically known from three populations, located in the Puu Waawaa region of Hualalai, on the island of Hawaii. The last known wild tree was in Puu Waawaa Plant Sanctuary, owned and managed by the Department of Land and Natural Resources, State of Hawaii. This tree died in 1992. BUT - as of 2008, there were about 100 reintroduced plants living. 50 were planted at Puuwaawaa Sanctuary cabin after 1997; they were fruiting and flowering and the oldest of those had produced seedlings by 2007. There were also ten trees below the highway at Puu Waawaa, seven at Kaloko-Honokohau National Historical Park planted in 2006, and unknown numbers of plants at Kaupulehu Forest Reserve and Makala-Ooma forest. These plants were all producing seed every year, and seeds were geminating well. The species is widely available in cultivation. So is it extinct?
Some stories are just depressing. Hibiscadelphus woodii was discovered in 1991 and first described as a species in 1995. When it was listed as an endangered species in 1996 it was known only from a single population of four individuals occurring at an elevation of 3,250-3,280 ft (991-1,000 m) at the site of its discovery in Kalalau Valley within the Na Pali Coast State Park on the island of Kauai. A falling boulder killed one plant and damaged two, so that by 2001 there were only two individuals left. By 2006 only one plant remained in the wild. There were no plants at all in cultivation.
All of this raises a question, of course: can a botanical garden, or even a network of botanical gardens, seedbanks, etc, really conserve anything? It a species has ten plants left in the wild (or did in 2002, but may have none today for all we know), can growing its progeny in a greenhouse or a living collection really make a difference? These reintroductions - will they amount to anything? And the business of scientific control of plant collection - some of these older plants exist because random gardeners collected them for the horticulture trade. It was dumb luck that they also happened to preserve endangered species.
What is certain is that without the work of the USFWS, the IUCN, and many random gardeners we would not know anything about the state of plant extinctions, and we probably wouldn't have the genetic material we have now. That's worth something.