North America has a rich history of forest change due to climate change following the last Ice Age, and due to more current changes following European settlement. All this change has resulted in pockets of very interesting, often half-forgotten trees smattered throughout the continent. I wanted to give them a little recognition.
But I had a problem: How do you define rare? You could call a species rare if it has a very small range size. But with that criterion, you would miss some species that are rare, because some trees can be widespread but extremely rare within their range. Is it the smallest number of individuals? There are so many species for which this number is unknown. Does rare implicate the trees that have experienced the most decline? Again, this may or may not have been measured. I decided to use the IUCN Red List as a guide, because range, population size, and decline are taken into account cohesively to qualify for listing. Botanists often use the term “rare” to denote an uncommon species that is not necessarily declining or at major risk of decline. In this case, I have chosen trees that are at major risk of decline, either because their numbers are so few that if some die off randomly, the entire population’s ability to reproduce will be affected, or because of some external factor exacerbating decline, or both.
#5. Maple-Leaf Oak (Quercus acerifolia)
This small, shrubby deciduous tree (3-9 meters tall) with palmate leaves that resemble a maple’s grows on steep, rocky terrain on four of the Ouachita Mountains in four Arkansas counties. Populations have avoided logging because of the unfavorable terrain and growing conditions. Six populations of a few hundred individuals each have been identified. While some botanists debated whether this tree was a unique species or a variant of the Shumard oak (Q. shumardii), a consensus was reached that these populations constitute a separate species. This oak is considered endangered by the IUCN because its small population size means any large-impact disturbance could put this species below a viable threshold.
#4. Four-Petal Pawpaw (Asimina tetramera)
This small tree (1-3 meters) is shaded out by oaks and pines in the absence of fire and other disturbances. According to the IUCN Red List, a population of about 500 grows in Jonathan Dickinson State Park in Palm Beach County, Florida. Although trees are scattered outside the park, the populations are fragmented, and commercial and real estate development are encroaching upon their habitat. The flowers have a fetid scent, attracting no one except beetle and fly pollinators. The fruit smells like bananas and is more widely popular––even gopher tortoises like them. The IUCN lists this species as critically endangered.
#3. Boynton’s Sand Post Oak (Quercus boyntonii)
This shrubby tree (1-6 meters) was originally identified in Angelina County, Texas but was last seen there in 1953. Much of that original Texan habitat has been altered for pine plantations or pasture. The species was suspected to be extinct until a population was discovered in St. Clair County, Alabama in 1994. Very little information is known about this tree, where it is, and how many are left. Five to twelve populations are known to be left in Alabama sandstone glades in mixed oak-pine-hickory forest, according to Al Schotz of the Alabama Natural Heritage Program in 2010. Other sources (e.g., the IUCN) note that the original populations are extinct, but other populations may exist in Louisiana and Mississippi. The IUCN cites the need for updated information and lists the tree as endangered.
#2. Florida Yew (Taxus floridana)
This small tree (up to 6 meters tall) only occurs in a 100 km² (24,700 acres) area on bluffs and ravines along the Apalachicola River in (you guessed it) Florida. The number of adult trees dying each year outnumbers the number of seedlings growing up, resulting in a net decline for two decades. Why the tree fails to regenerate from seed is not understood. Some populations are protected in the Torreya State Park and the Apalachicola Bluffs and Ravines Preserve; however, U.S. and Florida policies do not require protection of endangered species on private lands, so some populations on private lands are in danger of destruction. This tree’s genus has bark that contains taxol, a chemical used in cancer treatment, although Florida yew is not usually cultivated for this purpose, as its congener Pacific yew (T. brevifolia) is. This critically endangered tree is often used as an example of why conservation is important for future unknown uses, since taxol’s use as a cancer treatment was discovered in 1971.
#1. Florida Torreya (Torreya taxifolia)
Growing in the same small area as Florida yew above, the Florida Torreya myteriously declined for a half century, until a fungal disease was identified as the cause of decline. A recent UnderStory blog post covered that discovery. The gist is this: The tree experienced catastrophic decline beginning in the 1950’s, and within 10 years virtually no adult trees remained in the already limited native range. This tree used to be big, but is now restricted to a small size because its aboveground parts die off due to disease. Its roots re-sprout, and each tree gets stuck in this purgatory-like adolescence because of recurring infection. They rarely, if ever, get to a size where they can make reproductive parts. The tree has declined 98% over three decades. According to the IUCN, population viability analyses indicate that extinction is inevitable. Why is this species so rare? Ecologists think it is a relict species left in the wake of the last glaciation. In other words, the last time the glaciers grew southward between 20,000 and 30,000 years ago, the cold climate pushed species that were common in the north southward. Florida torreya was likely much more common and widespread then, but as the climate warmed, it got stuck in disjunct pockets too far apart to interbreed until there was only one pocket left. Very little fossil evidence for this idea exists, because it’s impossible to distinguish many Torreya plant parts like pollen from other trees in the family Taxaceae. I found some mentions of macrofossils in GA and NC. If this relict species idea is true, Florida torreya would be better off in colder climates, which has spawned a lively debate about whether populations should be established outside of the original range.
Honorable (Vulnerable) Mentions
I conducted as exhaustive a search as time permitted to compile this list of rarest trees. I noticed immediately that the list is exclusive to trees in the south. I don’t know whether this is because more research on tree conservation has occurred in the southern U.S., whether more conservation issues arise there, or something else. Some trees I considered that are listed as Vulnerable on the IUCN Red List, but are not as rare as the trees enumerated above, were: Fraser fir, Port Orford cedar, Redwood, Giant Sequoia, Florida willow, Longleaf pine, and Santa Rosa Island pine. I also considered Virginia round-leafed birch (Betula uber). However, there appears to be some debate as to whether it is a distinct species from Betula lenta, and this tree is not listed on the IUCN Red List. I also only included trees that still grow in the wild, thus excluding one of North America’s rarest trees, Franklinia alatamaha, which is extinct in the wild.
Please comment on rare trees you’d like to add to the list or that have been important to you.