Extinction is forever

A news article about the rarest tree on Earth caught my eye about a month ago, and since it is still living, it is worthy of some attention. Life is a process that generates genetic and biological diversity, and also destroys it. The ultimate fate of all species is extinction, but some species have a longer run than others and the Phactor has commented about the reigning king of longevity, the cinnamon fern, before. Many species are close to extinction, and human activities are way too commonly the cause.
A couple of rare trees provide some examples of how close to extinction you can get. Pennantia baylisiana is a species of tree that has consisted of a single individual at least for the past 65 years, which grows on one of the Three Kings Islands off the coast of New Zealand. Introduced herbivores in the form of goats may be the reason only one big tree remains, but it has a sex problem. Like many other plants, it cannot pollinate itself, so while you can make cuttings and root them, they are all genetically the same individual. But there may be an out; sex determination in plants is complicated, and occasionally a female produces some male flowers or flower parts and hand pollinations have yielded some seeds (and surely you understand that designations of male and female are totally inaccurate because the flowering plants with which we are familiar are the asexual spore-producing generation). With a bit of human assistance seedlings may be produced to help repopulate the island, however, they would all be siblings, so genetic diversity would be reduced, and this is a problem for any rare species.
Another contestant for the down-but-not-out extinction sweepstakes is the King’s Lomatia a tree that grows on the southern end of Tasmannia. About 500 trees exist, but this is a clone and again one genetic individual that has survived by asexual reproducing itself for 50,000 to 150,000 years. And because it has 3 sets of chromosomes, it may be the result of a hybridization that produced a sterile “mule” that has persisted by growing as a clone. Not much hope this species will “recover” unless it undergoes a spontaneous chromosome doubling to restore fertility. Although you may be unfamiliar with it, hybridization followed by polyploidy may be one of the most common types of speciation in plants.
Clearly both trees are in the running as the rarest organisms on Earth, each reduced to a single genetic individual.