A Floristic Summary of a Post Glacial Aquiclude in the Winkler Botanical Preserve

Keith P. Tomlinson, Botanical Manager
Winkler Botanical Preserve
Jeffersonia, The Newsletter of Virginia Botany, Spring 1989

ABSTRACT

The Winkler Botanical Preserve is located on forty five acres in Alexandria, Virginia. The forest cover is a xerophytic Oak-Hickory woodland. The shrub layer is comprised of several species dominated by Ericads. In addition, a varied herbaceous community is present. The preserve’s landforms are composed largely of an alluvial outwash derived from the paleo-Potomac River, and secondarily, from Coastal Plain sediments. The composition and distribution of these soil profiles give rise to a unique hydric plant community located in an otherwise xeric woodland.

GEOMORPHOLOGY

Consequences of Pleistocene glaciation are clearly evident throughout the Potomac River Valley. Although no glacial ice entered the Potomac drainage, the river’s basic morphology did undergo substantial change. The drop in sea level during Pleistocene glaciation caused the river to deepen its channel due to increased water volume and velocity. Mather Gorge, immediately below Great Falls, clearly represents a dramatic change in the river’s pre-glacial and post-glacial form (Reed, Sigafoos, Fisher 1980).

Below the fall line the paleo-Potomac discharged vast areas of gravel, sands, and cobbles onto the Coastal Plain (Johnston 1964; Wentworth 1930). These deposits consist primarily of former gravel bars (Malcom 1986) located above the fall line between the Coastal Plain and Pedimont. Ultimately several large lobes of this river outwash were deposited on Cretaceous soils of the Coastal Plain. These older soil profiles are assigned to the upper and lower Potomac soil groups (Woodward, Clyde 1987).

The retreat of the Wisconsin Glacier and subsequent rise in sea level over the past 16,000 years has allowed the Potomac Watershed to stabilize. Presently, the Pleistocene outwash forms a large alluvial fan that underlies much of Alexandria and adjacent Fairfax County. This underlying strata is referred to as the River-Terrace Gravel formation (Johnston 1964). Eight kilometers to the east this formation grades into Holocene alluvium forming the present Potomac River shoreline. Two kilometers to the west the Sykesville formation is encountered. This boundary marks the fall line between the Coastal Plain and Pedimont (Darton 1947).

The topography of the region is marked by rolling hills with intermittent ravines forming small creek beds. Additionally, linear ridges are present, often with a north-south declination. Within this area evidence of River-Terrace Gravel is commonly seen as rounded cobble stones protruding through upper soil horizons. In effect, the River-Terrace Gravels form a veneer of alluvium over the older Potomac soil groups (Dietrich 1970). The junction of these two substraights creates a surface aquiclude *(Allaby 1989). The aquiclude facilitates a consistent low level flow of water to a specific area in an otherwise xeric woodland.

SOIL COMPOSITION

Soil conditions on higher elevations of the River-Terrace Gravel display on a very short hydroperiod. Similarly, relatively little organic matter is present in most areas. Once precipitation percolates through the gravel cap its vertical migration slows at the older Potomac soils. These subsoils evolved in a deltic environment some 140 million years ago. They are relatively impermeable, high in clay, and micaceous in some locations (Woodward, Clyde 1987). Upon reaching this substraight most ground water is channeled laterally to the seam between the River-Terrace Gravels and the Potomac soils, resulting in the surface aquiclude. This drainage system is recharged by rainfall and preexisting ground water.

The aquiclude seam is approximately 350 meters long and 5-20 meters wide. The average elevation is 62 meters above sea level (Sutton-Kennerly 1987). It is located at the base of a northwest facing slope. Soil conditions adjacent to the aquiclude differ substantially from the surrounding substraight. A moist leaf litter averaging 15 cm. in depth is present. Below the leaf litter a shallow, fibrous rootmat is apparent. The rootmat is subtended by sand and gravel 2-5 dm. deep, derived from the post-glacial outwash. Immediately below the outwash, upper Potomac soil horizons are encountered. The overall soil composition is strongly acidic with an average pH of 5.0-5.5. Moisture levels are consistently high in horizons above the underlining Potomac soils.

FLORISTIC SUMMARY

Floristic components of the aquiclude seepage seam are unique in contrast to the surrounding woodland. Several genera of mesic and hydric habitats are present. Many genera of cosmopolitan habitat are also noted. Herbaceous vegetation is dominated by various Pteridophytes. Lycopodium obscurum forms a large colony in the northern portion of the aquiclude. Osmunda cinnamomea constitutes the largest single species distribution in the community with several solid stands illustrating areas of constant seepage. Osmunda regalis is present in less abundance. Pteridium aquilinum is interspersed throughout the drainage.

Various herbaceous angiosperms are present among the dominant ferns. Medeola virginiana is present in several prolific colonies. Monotropa hypopithys, M. uniflora and Symplocarpus foetidus are found in limited numbers. Demonstrating bimodal moisture tolerance, Cypripedium acaule is present in the seepage area and adjacent dry woods. At the extreme southern end of the seepage a colony of Isotria verticillata is found. In addition, Smilax glauca and S. rotundifolia are prominent in a few locations. Two Carex spp. are growing near the trail. Where the seepage grades rapidly into a dry hill side, Epigaea repens and Chimaphila maculata are established.

The shrub layer is dominated by Ericads. In the wettest areas of the seepage Lyonia ligustrina, Vaccinium corymbosum, Leucothoe racemosa, and Rhododendron viscosum are common. Also growing here is Aronia arbutifolia (Rosaceae). Kalmia latifolia is a principle component of the shrub layer both in and around the seepage. Grading into the slightly dryer ecotone between the seepage and the surrounding woodland Rhododendron nudiflorum, Gaylussacia frondosa, and G. baccata are found. In the adjacent woodland Lyonia mariana, Vaccinium staminium, and V. pallidum dominate. Notable among the species mentioned here are several specimens of Vaccinium corymbosum, that attain nearly 5 meters in height.

Arboreal species in the seepage illustrate a relatively diverse composition. Among the lower canopy trees Amelanchier arborea, Hamamelis virginiana, Chionanthus virginica, Cornus florida, Alnus serrulata and Sassafras albidum are found. Upper canopy species in the wetter areas include Acer rubrum, Quercus rubra, Liriodendron tulipifera, Magnolia virginiana and several large specimens of Pinus virginiana. Grading into the dryer areas Carya tomentosa, Prunus serotina, Quercus alba, Q. velutina and Q. prinus are dominant. Of cosmopolitan habitat, Nyssa sylvatica occurs in abundance throughout the area. In general, composition and distribution of arboreal growth seems less affected by aquiclude soils than herbaceous growth. However, the distribution of Magnolia virginiana and Liriodendron tulipifera clearly reflects a substraight of increased moisture and organic matter.

CHECKLIST OF VASCULAR FLORA

The checklist follows Taxonomic arrangement used in The Flora of West Virginia, 1977 by Strausbaugh and Core. New county records for Alexandria are indicated by an asterisk (*), where relevant. This information is based solely on maps of plant distribution found in The Atlas of the Virginia Flora by Harvill and Associates. In addition, each species is assigned a number code to illustrate habitat distribution within the seepage and adjacent woodland. The numbering system is as follows: 1) hydric seepage, 2) seepage ecotone, 3) xeric woodland, and 4) grading randomly through the study area.

TAXA CODE

LYCOPODIACEAE

Lycopodium obscurum L. * 1, 2

OSMUNDACEAE

Osmunda regalis L. 1

Osmunda cinnamomea L. 1

POLYPODIACEAE

Pteridium aquilinum (L.) Kuhn 4

PINACEAE

Pinus virginiana Mill. 1, 2

CYPERACEAE

Carex spp. 1

ARACEAE

Symplocarpus foetidus (L.) Nutt. 1

LILIACEAE

Medeola virginiana L. 1, 2

Smilax glauca Walt. 1, 2

Smilax rotundifolia L.

ORCHIDACEAE

Cypripedium acaule Ait. 4

Isotria verticillata (Willd.) 2

JUGLANDACEAE

Carya tomentosa Nutt. 2, 3

CORYLACEAE

Alnus serrulata (Ait.) Willd. 1, 2

FAGACEAE

Quercus alba L. 2, 3

Quercus prinus L. 3

Quercus rubra L. 1, 2

Quercus velutina Lam. 2, 3

MAGNOLIACEAE

Magnolia virginiana L. * 1

Liriodendron tulipifera L. 1

LAURACEAE

Sassafras albidum (Nutt.) 4

HAMAMELIDACEAE

Hamamelis virginiana L. 4

ROSACEAE

Aronia arbutifolia (L.) Ell. * 1

Amelanchier arborea (Michx. F.) Fern. * 1, 2

Prunus serotina Ehrn. 4

ACERACEAE

Acer rubrum L. 1, 2

NYSSACEAE

Nyssa sylvatica Marsh. 4

CORNACEAE

Cornus florida L. 4

PYROLACEAE

Chimaphila maculate (L.) Pursh. 3

Monotropa hypopithys L. * 1, 2

Monotropa uniflora (L.) Gray 1, 2

ERICACEAE

Rhododendron nudiflorum L. Torr. 1, 2

Rhododendron viscosum L. Torr. 1, 2

Kalmia latifolia L. 4

Lyonia mariana L. 3

Lyonia ligustrina L. DC. 1, 2

Leucothoe racemosa (L.) Gray 1

Epigaea repens L. 3

Gaylussacia frondosa L. T.&G. 2, 3

Gaylussacia baccata (Wang) K. Koch 2, 3

Vaccinium staminium L. 3

Vaccinium pallidum Ait 3

Vaccinium corymbosum L. 1

OLEACEAE

Chionanthus virginicus L. * 1, 2


References:

1. Allaby, M. 1989 Dictionary of the Environment. 3rd ed. New York University Press. Washington Square, NY, NY. 532 pp.

2. Darton, N.H. 1947. Geologic Map of Washington D.C. and Vicinity. United States Geological Survey. 1947.

3. Dietrich, R.V. 1970. Geology of Virginia. University of Virginia Press. 185 pp.

4. Harvill, A.M. Jr., T.R. Bradley, C.E. Stevens, T.F. Wieboldt, D.M.E. Ware, D.W. Ogle. 1986. Atlas of the Virginia Flora. 2nd Ed. Virginia Botanical Associates, Farmville, VA. 135 pp.

5. Johnston, P.M., 1964. Geology and Groundwater Resources of Washington D.C. and Vicinity. United States Geological Surver, Water Supply Study #1776. 97 pp.

6. Malcolm, H.R. 1986. Winkler Botanical Preserve Hydrologic Analysis. Unpublished. 31 pp.

7. Reed, J.C. Jr., Sigafoos, R.S., Fisher, G.W. 1980. The River and the Rocks: The Geologic Story of Great Falls and the Potomac River Gorge. Unit4ed States Geological Survey Bulletin #1471. 73 pp.

8. Strausbaugh, P.D. and E.L. Core. 1978. The Flora of West Virginia. 2nd Ed. Seneca Books, Inc., Grantsville, W.V. 1075 pp.

9. Sutton-Kennerly Associates. 1987. Winkler Botanical Preserve, Interstate Tributary StreaImprovement Map. Greensboro, N.C.

10. Wentworth, C.K. 1930. Sand and Gravel Resources of Coastal Plain of Virginia. University of Virginia. 146 pp.

11. Woodward-Clyde Consultants. 1987. Geotechnical Investigation of Small Dam Structures in the Winkler Botanical Preserve. Unpublished. 20 pp.

*Aquiclude (or Aquitard) is defined as a geological rock or soil formation that is sufficiently porous to absorb water slowly, but does allow water to pass quickly enough to furnish a supply for a well or spring. In this case the aquiclude forms a linear spring or seepage in the forest floor.