White Oak Sinks, GSMNP, TN   Jess Riddle
  Apr 17, 2007 18:55 PDT 


Metamorphosed sedimentary rocks, most commonly sandstone, underlie the
vast majority of the Great Smoky Mountains, and they tend to weather
into acidic soils. Hence, the scattered small areas in the range
underlain by limestone, with their circumneutral to basic calcium rich
soils and associated assemblages of calcium loving plants, have
attracted the attention of naturalists studying the mountains. White
Oak Sinks, the largest of those areas now in the national park, sits
just inside the park's northern border northeast of Cades Cove. The
area's name comes from all the streams flowing into the basin flowing
underground once they reach the limestone and the half dozen or so
sinkholes in the area. All of the sinkholes occur on the relatively
flat floor of the closed basin that drains an area of about 900 acres.
To the southwest, steep slopes rising over 800' above the floor of
White Oak Sinks make the area feel more like a bowl, and help to
generate cold air that pools in the bowl's bottom (assuming the
topography functions the same way cold sinks do in the Rockies). As a
consequence of that cold air pooling, tuliptrees in the sinks remain
dormant when other tuliptrees 1000' higher in elevation and on more
acidic substrates, which tend to leaf out later, have already broken

The forests in the basin surrounding White Oak Sinks do not hint at
the unusual area's existence. Scarlet oaks, white oaks, white pines,
and hemlocks cover the gentle slopes along some streams over a patchy
understory of rhododendron, mountain laurel, huckleberry and open
areas with galax beneath. The steeper northeast facing slopes also
suggest only quick draining, acidic conditions since white oaks,
chestnut oaks, and tuliptrees dominate. While the limestone generally
correlates to the flat land in the bottom of the basin, even where the
rock extends up the north side of the basin the canopy belies the
bedrock. The white oak, chestnut oak, black oak mix on that south
facing slope does not seem unusual for the topography, although the
lack of a heath understory might not be expected.

However, settlers found the area and recognized in it an excellent
farm site. Consequently, determining what species originally
dominated the floor is difficult, and well drained acidic forests of
white oak with some pines and a dense midstory of hemlock now cover
much of the bottom. The limestone under those forests may have little
influence on the vegetation due to the depth of the colluvium covering
the bedrock, but the rest of the floor, especially around sinkholes
and outcrops, still features species mixes unusual for the region.
Black walnut with some slippery elm dominates the youngest forests on
floor. More mature black walnuts grow amongst cliffs and rock
outcrops on the edge of the bottom in association with white oak,
chinquapin oak, and some shagbark hickory. Those sites appear the
least disturbed, though probably not most representative, in the area,
so black walnut appears to be native to the site and not introduced by
farmers. Tuliptrees now dominate on the lowest and moistest areas of
the bottom, and hemlock also forms a nearly pure stand on one slope
with multiple limestone outcrops.

Shrubs may be even patchier in distribution on the limestone.
Spicebush occurs frequently in the shrub layer and American hazelnut
also forms thickets in multiple areas. American plum likewise grows
in multiple areas, but forms much smaller colonies, sometimes where
calcareous forests interface with acidic forests. Multiple sinkholes
supported alternate leaf dogwood on their slopes, and one sinkhole
hosts American bladdernut, a rare species in the Smokies.

The herbaceous layer similarly contains a mix of common rich site
species and species rare or absent elsewhere in the park. The
combined spring floral display has attracted enough visitors to
establish through use an extensive and well defined trail network in
and around White Oak Sinks without any maintenance or publication by
the Park Service. More common species contributing to the display
include may apples, bishops cap, and bluntleaf waterleaf. A phlox has
especially high coverage and, since the air does flow out, strikingly
perfumes the forest over multiple acres. Amongst the phlox, grow the
rare shooting star and largeleaf waterleaf. Barren strawberry grows
under the hemlocks on an adjacent slope, one outcrop harbors dwarf
larkspur and green violet, purple cliffbrake clings to other outcrops,
and Virginia bluebells cloaks one sinkhole; all rare in the park.

Only four trees were measured during our visit; past farming and
generally dry site conditions precluded most species from reaching
exceptional sizes. On the floor of the basin, slippery elm reaches
6'0" cbh. Along end of the bottom, but still influenced by limestone,
mockernut hickory reaches 6'3"cbh x 121.2' tall and chinquapin oak
7'2"cbh x 105.1', a new park height record. The gentle sections of
the surrounding basin support many impressively tall scarlet oaks
including a 7'4" x 121.1'+ individual.

Jess Riddle & Josh Kelly