Great Smoky Mountains National Park, Acid Cove-Hemlock Forest

Forest Summary

  • Found on acidic sites.
  • Found on creek flats and protected (often steep) slopes in the low (1500-2500′) and middle (2500-4500′) elevation zones.

  • Hemlock dominates the canopy along with three or fewer associates.
  • Acid-loving species like Heath family members common.
  • Rosebay rhododendron frequently dominates the understory.
  • At successional maturity, hemlock dominates exclusively.
  • The hemlock wooly adelgid is presently decimating this forest type.

Detailed Description

Acid cove forests are distinguished from hemlock forests in the All Taxa Biodiversity Inventory that describes the Park’s forests in detail. Acid cove forests are abundant in the Park, occurring along many streams as they tumble down the mountain through rocks like those of the Anakeesta Formation where acidic compounds leach into the surrounding soil. Hemlock forests also grow on moist acidic sites, but hemlock dominates the canopy exclusively, refusing to share with the hardwoods.

In reality, the difference between these two forest types may be a matter of successional stage, with acid cove forests representing the intermediate stage of forest succession in what will eventually become a mature hemlock forest. Most of the Park’s designated hemlock forests are very old forests (either virgin forests or disturbed old-growth forests) located on the same kinds of sites that acid cove forests occupy in more recently disturbed (younger) sections of the Park.

Cataloochee provides a good example. Its middle elevation streams support the largest chunk of hemlock forest in the Park. These are virgin forests, and they grow on sites where acid cove forests occur elsewhere. This book combines the two forest types into one; the acid cove-hemlock forest to indicate their successional relationship, and to anticipate the reduced presence of hemlock in the future.

In the low elevation zone (1500-2500′), acid cove-hemlock forests grow on acidic sites along streams, especially creek flats. In the middle elevation zone they are found on creek flats and protected slopes, especially steep protected slopes. During early stages of succession, hemlock shares the dominant canopy with poplar, black birch and red maple on more moist sites, and with white pine and oaks on dryer sites. American sycamore and American hornbeam, trees diagnostic of the river cove forest are usually absent from the acid cove-hemlock forest.

The subcanopy of acid cove-hemlock forests is usually dominated by acid-loving plants like sourwood and American holly. Rhododendron is almost always present in the understory, and often dominant there. Mountain laurel and doghobble characteristically share the understory with rhododendron as well as its acid-loving tendency. Unlike classic cove hardwood forests, acid cove-hemlock forests usually lack a rich herbaceous layer.

As the acid cove-hemlock forest matures, hemlock assumes exclusive canopy dominance simply by virtue of its great size and age. It just outlasts everything else. At this late stage of succession, its dispersed canopy cohorts are likely to be other shade tolerant trees like buckeye, basswood and beech along with just a few huge poplars and white pines, both long-lived trees from earlier successional stages.

It is not uncommon to find examples of both river cove forest and acid cove-hemlock forest growing in the low elevation zone along the same stream as conditions of soil acidity vary with location. In fact, nature regularly refuses to be pinned down to a strict regimen of classification, changing along with the environmental conditions that sustain her. But she does exhibit elevational continuity.

As we follow a mountain river upstream we expect first to encounter river cove forest where sycamore and hornbeam are characteristic along with a variety of lowland and mountain trees. With increasing elevation, the river becomes a prong loosing much of its bottomland, becoming narrower, steeper and more musical. We walk through acid pockets lacking sycamore and hornbeam where hemlock is prominent in the canopy with much rhododendron and mountain laurel below.

As we ascend into the middle elevation zone at around 2500′, the dominant canopy trees of the acid cove-hemlock forest change. Yellow poplar and white pine drop out. Mountain silverbell becomes more common along with sugar maple in places. The acid-loving rhododendron remains as a prominent understory component.

On less acidic stream sites, the classic cove hardwood forest appears above 2500 feet elevation. Basswood, white ash and sugar maple become more abundant along with yellow poplar and some hemlock. Still higher up, we realize yellow poplar, cucumber tree and others have dropped out of the classic cove hardwood mix. Basswood remains and buckeye and yellow birch are now more abundant as we transition to the cool cove forest, especially on the south-facing North Carolina side of the main divide. By now our prong is a creek and is growing smaller. Above around 4500′ elevation the creek may become a waterless ravine. Only a few hardwood trees remain. Yellow birch, yellow buckeye and American beech dominate, often with red spruce, especially on the north-facing Tennessee side of the main divide. We have entered the northern hardwood forest.

Yellow poplar is the prominent pioneer tree in low elevation acid cove-hemlock forests on moist acidic sites. It would dominate the site during the pioneer stage of forest succession (50-100 years) as forest succession advanced on abandoned fields or cut-over land. At age 80 years (time elapsed since the Park’s establishment) we would expect dominant canopy poplars to be in the 19-20 inch diameter range. We would look for intermediates like red maple, blackgum and black birch in the dispersed canopy and subcanopy ready to steal dominance from the poplars in a few years. Hemlocks, the future inheritors of the site would fill the subcanopy. By the mature stage of succession (200-250 years) they will dominate the canopy as the forest approaches climax.

Eastern hemlock is disappearing from southern Appalachian forests along with other beautiful, valuable, seemingly indispensable tree species. The potential for more species devastation is firmly on the horizon as the world continues to share pathogens at an alarming rate. Mass extinctions have been common in earth’s history, from the trilobite die-offs of the late Cambrian Period to the more recent disappearance of Pleistocene mega-fauna. It is hoped that there will be people around in the future to read about this potential latest episode!

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About Dan Williams

Forest manager & environmental educator with the Warnell School of Forestry and Natural Resources. I have spent 26 years interpreting forest research for non-scientists interested in learning more about the forests of the Southeastern United States.
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