A Calcareous Ravine Garden

I just went to hear a talk by Dr. Donna Ware and Ralph Will at the recently installed calcareous ravine display at the Williamsburg Botanical Garden in Freedom Park. The ravine display is home to a selection of mountain disjunct plant species that can be found growing locally. Disjunct species are plants that have their primary range in one location, and a secondary location separated by a large gap, sometimes hundreds of miles.

In Virginia, the separation typically occurs between plants growing in the Blue Ridge Mountains and fossil shell-filled ravines in the coastal plain. In the picture below, from the Virginia Digital Atlas, you can see an example of this distribution. The counties colored light blue are ones where you can find dog violets (Viola labridorica). Mind the gap!
The calcium carbonate that leaches from the fossil shells makes the soils in the ravines more alkaline than the acidic soils you normally find in our area. In these alkaline soils you can find plants like wild sasparilla, American bellflower, and zig zag goldenrod that, if you didn't know better, might lead you to think that you were in another part of the country.
Donna Ware, Emeritus Research Associate Professor of Biology and Curator Emeritus at William & Mary, had an experience something like that when she first arrived at the here. On her first field trip through the college woods she recognized many species that she knew from her home in the Ozarks, such as alternate leaf dogwood (below) and wild coffee. So how did these disjunct species end up here so far from home? You have to go back a long, long time ago to find out. Here's how Donna explained it:

Way back around 4.5 million years ago, this area was covered by shallow oceans where countless shellfish lived and died, accumulating on the sea floor. After hundreds of thousands of years, the seas retreated and the shells were buried under later soil deposits. Then more recently, as the glaciers of the Wisconsinan Ice Age retreated, melt-water streams carved ravines through the soil, exposing a layer of fossil shells like the one in the picture below. The shells are typically found at an elevation of 50' above sea level and interestingly, the shells fossilized as discrete specimens, not locked in rock, so it's quite easy just to pull them out of the soil.That explains the calcareous soils, but how did the plant populations become separated? Donna filled us in:

After the last ice age, there was a continuous band of mountain species along the east coast. Jack pine and spruce at first, and later hemlocks, white pines and northern hardwoods. As the climate gradually warmed, plants from farther south spread into our area, displacing the mountain species. But some of the retreating species were able to hang on in the shell-filled ravines. It's not that these mountain species needed the calcareous soils to survive, but rather that they could out compete the more southerly intruders in the alkaline soils.

The discovery of our areas disjunct species has occurred slowly over the years. Some of the first species were recorded in John Claytons Flora Virginica back in 1700's. In the 20's botanist E. J. Grimes listed 16 species and discoveries continued with Harvard's M. L. Fernald in the 30's and 40's. By the 60's A. M. Harvill listed 36 species.

In the 80's Donna Ware studied plants in a 2 mile section of the Grove Creek watershed found 32 additional species. And recently, Leah McDonald came up with a total of 90 species while writing her master's thesis.

For some reason, our area of the coastal plain seems to be a hot spot for disuncts. A county by county analysis by Donna revealed the highest concentrations of species in James City County with 67 species, followed by Surry County with 48, and York County with 34. This is significantly more than surrounding counties. Donna said one theory about why there are so abundant in our area somehow has to do with the Chesapeake Bay impact crater.
Also on hand at the talk was Ralph Will, who operates a native plant rescue organization with his wife Carolyn Will. Ralph spent a lot of time helping to build the ravine exhibit and talked about how it was conceived and constructed. As Ralph explained, it was immediately apparent that you couldn’t just dig a ravine in the garden. It would just fill up with water. So William & Mary Professor Emeritus, Dr. Jerre Johnson suggested building the ravine above ground and facing north, as many of the ravines naturally occur. And he wasn't just talk —Jerre built the stone wall himself, following a clay model sculpted by Donna. The walls are reinforced against the weight of the soil with metal stakes and chicken wire, and soaker hoses have been built in to help with keep the plants watered. It's kind of an experimental approach. As far as Donna knows, no one else has attempted such a garden.

Jerri also suggested planting things within the crevices of the walls. Earlier this year I found nice clumps of bloodroot (below), hepatica, columbine, and robin's plantain blooming in between the stones. While not disjuct species themselves, they are companion plants that are often found alongside the disjuct species.Other companion plants planted around the ravine are putty root orchid, leatherwood, and maidenhair fern. All the plants used in the garden are rescued or donated plants. Donna describes them as hyper-local; she wanted to get plants as genetically appropriate as possible. By the way, Donna's looking for an umbrella magnolia to add to the garden. Can anyone donate one from their yard?

If you stop by to see the garden, don't be suprised if it seems a little bare. It's still young, but it will be great to see it fill in over the next several years. You can see more pictures of the ravine here.