The Carolina hemlock (Tsuga caroliniana) is a rare, but beautiful sight in this region, native only to a small sliver of the southern Appalachian mountains from southern Virginia to northern Georgia, with a majority of its range occurring in western North Carolina. It is one of only ten distinct species of hemlock across the world today. Of these ten, only four exist in North America, and only Carolina and eastern hemlock are native to eastern North America.
Differences between Carolina and eastern hemlock
Although Carolina and eastern hemlock overlap in their ranges, these two species have many distinct differences. Unlike Carolina hemlock, eastern hemlock can be found across a large swath of eastern North America. Differences even extend to the genetic level — Carolina hemlock is not as closely related to eastern hemlock as it is to other Asian species of hemlock, despite their locational contiguity. It is smaller than the eastern hemlock, typically growing to between 40 and 70 feet tall with a slender trunk and narrow crown of slightly droopy branches. Its needles are longer than eastern hemlock and spread from the twig at all angles, giving their stems a whorled appearance when looking directly down upon them. The Carolina hemlock is also likely to be found in more rugged habitat than the eastern hemlock — dry and rocky slopes, ridges, and gorge walls are where this species thrives. The eastern hemlock prefers shadier, moist habitats meaning that the two species rarely stand side by side despite existing within the same regions.
Differing root structures enable these two species to occupy different habitats. Carolina hemlocks have a taproot, which is a large, central, and dominant root that grows directly downward and from which smaller roots branch off. The taproot allows Carolina hemlocks to exist in the more extreme conditions it inhabits, anchoring the tree in relatively less stable soil and allowing for a deep reach into the ground for acquiring water during times of drought. The eastern hemlock has a fibrous root system, which is a shallow, mat-like structure of roots that branch out horizontally in the soil allowing for uptake of water and minerals across a greater surface area, and providing the ecological benefit of reducing soil erosion.
Carolina hemlock: a unique tree
The shaggy green branches of the Carolina hemlock are a sparse but beloved highlight of the southern Appalachian landscape, providing aesthetic beauty to the craggy and rugged outcroppings of the mountains. Beyond beauty, the Carolina hemlock offers numerous ecological benefits. Many birds and mammals feed on its seeds and rabbits are known to eat its bark. Birds and other creatures use hemlocks as shelter and a place to retreat from the elements. White-tailed deer are especially reliant on hemlock, using its foliage as an occasional food source and relying on the tree for essential shelter and bedding in the winter. Hemlocks are also thought to be a species relied on by the endangered Carolina northern flying squirrel at lower elevations within its range, helping maintain the limited populations of this species in the southern Appalachians. The rarity of the Carolina hemlock also means that it is an important contributor to biodiversity, which increases ecosystem productivity and supports overall ecosystem health.
Carolina hemlocks are uniquely found in tiny, isolated populations that have high levels of genetic differentiation among populations, a highly unusual trait for a coniferous tree. This is likely due to the structure of Carolina hemlock seeds. Though they are winged and twirl and float through the air like helicopter wings, they are the largest of any North American hemlock, which means they often fail to cover the long distances in between individual Carolina hemlock populations. This means that a hemlock stand in one area may be very different, genetically speaking, from another stand found only a few miles away. In contrast, within the same populations, Carolina hemlocks are more genetically similar than most other conifers. This lesser genetic diversity means that the individual populations are likely more vulnerable to environmental stress and climate change. The interesting genetics of Carolina hemlock heightens the importance of protecting these rare trees from hemlock woolly adelgid.
Carolina Hemlock Forest Natural Communities
Though you can find Carolina hemlock scattered across its range, large stands of Carolina hemlock are considered rare and recognized as three distinct natural communities by the NC Natural Heritage Program (NCNHP) based on their unique presence on the landscape. A natural community is a distinct and reoccurring assemblage of populations of plants, animals, bacteria, and fungi naturally associated with each other and their physical environment. The NCNHP describes three distinct subtypes of Carolina Hemlock Forest: Typic, Pine, and Mesic subtypes and are distinguished by associated vegetative species and position on the landscape. Carolina Hemlock Forests are small patch communities and range from one to twenty acres in size. From a natural history perspective, the small patch community distribution and lack of genetic exchange between populations implies long term stability and persistence on the landscape over time, due to its inability to recolonize, making the communities rare and patchy. A small Carolina Hemlock Forest can be seen as an ancient relic of Pleistocene glaciations that have survived for centuries if not longer.
A Carolina Hemlock Forest has a canopy dominated or co-dominated by Carolina hemlock with lesser components of pine, oak, and other hardwood tree species. The Typic and Pine subtypes occur on steep rocky upper slopes, rocky bluffs, and along ridges, usually with a north-northwest aspect in dry habitats with acidic soils and low nutrients. The Mesic subtype is rarer, occurring on topographically sheltered or valley bottom environments in moister, more sheltered sites. The Pine subtype occurs with pines species and usually adjacent to Pine Oak Heath. The Typic subtype occurs with chestnut oak along with other oak species and deciduous trees. The Mesic subtype occurs with eastern hemlock and along with other mesic species typical growing with eastern hemlock.
The NCNHP maps and ranks each Carolina Hemlock Forest in the state of North Carolina based on its condition and size. The tracking of Carolina Hemlock Forest in North Carolina highlights its biological significance globally and all three communities are ranked as globally rare by NatureServe. Even though the three community types are tracked by the NCNHP, the individual trees are not, based on their relative local abundance. They are however on the state’s Watchlist with a “W5” designation, due to severe decline in populations or habitat caused by HWA. (W5 species are species that have declined sharply in N.C. but do not appear to warrant site-specific monitoring.) That said, if the decline continues, many W5 species including Carolina hemlock will be threatened with extirpation in all or a major part of their ranges in N.C. This potential risk and the fact that the vast majority and best examples of Carolina Hemlock Forest occur in North Carolina together underline the importance of protecting the species.
Protecting Carolina hemlock
A common misconception is that Carolina hemlock is less susceptible to hemlock woolly adelgid than its eastern counterpart, but this is not true. Carolina hemlocks are more often found on the edge of landscapes where they receive increased sunlight which hinders propagation of hemlock woolly adelgid. They are also more drought and stress tolerant than eastern hemlock, which may allow it to better withstand the occasional double dose of stress caused by less ideal environmental conditions and hemlock woolly adelgid. However, the innate susceptibility to hemlock woolly adelgid, excluding other factors, is essentially the same for both species.
Protection of Carolina hemlock presents several unique challenges. The hemlock woolly adelgid is established in 100% of Carolina hemlock’s natural range, making extinction of the tree a real possibility. Due to their preference for ridges and exposed, rocky outcrops, Carolina hemlock stands are often hard to access. Their relative rarity makes certain conservation efforts such as seed collection a more onerous task. Populations of Carolina hemlock are also small and fairly spread out meaning that treatment of these trees requires substantial time and coordination. Finally, the lack of Carolina hemlock genetic diversity creates a lesser basis of adaptation and resilience to environmental stress and change. Luckily, HRI and its partners, along with other entities, are working hard to preserve the species.
Camcore, an imperiled tree conservation program affiliated with NC State University, is working to conserve the genetic diversity of Carolina hemlocks by collecting seed and planting trees in conservation nurseries. The Forest Restoration Alliance, another non-profit program affiliated with NC State, is conducting trials of crossbreeding Carolina hemlock with other, adelgid-resistant species of hemlocks with the hope to eventually introduce these populations back into its native region where they will be able to reproduce and thrive. The North Carolina Forest Service is working to perfect protocols to grow Carolina hemlock en masse for use in future restoration projects and produces seedlings that are made available for research and public purchase. Such protocols will be extremely useful once a hemlock woolly adelgid resistant hemlock is available for reforestation efforts. Finally, chemical and biological control methods are being used by HRI and others to preserve the Carolina hemlocks that are still standing.
The unique nature of the Carolina hemlock, its many ecological benefits, and its special connection to the southern Appalachian Mountains make its preservation of utmost importance. Protecting the rare tree is no easy task, but HRI and others are rising to the challenge to maintain the presence of this iconic conifer on the southern Appalachian landscape. When traversing the ridges and cliffs of the southern Appalachian Mountains, be sure to keep an eye out for a shaggy, green tree standing tall and proud against the backdrop. It might just be a Carolina hemlock, and it might just be there for many years to come.