Digging new pools: How an experiment on Georgia granite mountains is increasing endangered and threatened plants
Since 2007, the U.S. Fish and Wildlife Service’s Georgia ecological services office started counting the tiny green plants that can only be found in vernal pools on granite outcrops during the rainy season from December through March and during mid-summer rain events. (Vernal pools are temporary pools of water that provide habitat for distinctive plants and animals.) Stone Mountain, Arabia Mountain and Heggie’s Rock, all in Georgia, are granite outcrops. Because they are protected, they are some of the only granite outcrops where rare endemic green plants, such as the federally-listed black-spored quillwort, mat-forming quillwort, and pool sprite, can still be found.
Granite outcrops are large expanses of rock, interspersed with islands of vegetation and vernal pools. They are bare, windy, and cold in the winter, and hot (up to 130 degrees) and dry in the summer. These beautiful moonscapes possess a great concentration of globally rare species that can survive in a harsh environment. Black-spored quillwort, mat-forming quillwort, and pool sprite have adapted to life on the granite in pools that have taken thousands of years to form through erosion and acid leaching from fungi, another outcrop survivor.
During the winter rains, shallow, flat-bottom depressions fill with water and the quillwort species and the pool sprite come to life and cover the pools, if other conditions are right. The pools are about three feet in diameter with a rim and only deep enough to hold four to six inches of rain water, which allows these three species to grow without much competition from other plants.
After March when the pools dry up, the two quillwort species go into dormancy and are not seen again until summer rains start when they will produce leaves and spores. These spores will remain in the soil or travel as water flows to new pools. The pool sprite is an annual that leaves its seeds in the soil before it completely dies back. The seeds of the pool sprite will start growing again the following winter when the environmental conditions are right. All three plant species are known mostly from Georgia with a few populations also in Alabama and South Carolina.
As Service biologists, we investigate why plant and animal populations decline, and we search for management options that can help endangered and threatened species thrive and recover into growing populations once again. All three of these granite outcrop species have suffered loss of pool habitat mainly from mining that leaves behind flat sheets of granite with no pools. Changes to the pool’s habitat through sedimentation, recreational vehicles, trash, fires, and foot traffic degrades pools, making them unsuitable for these rare plants. The lack of suitable vernal pools is one of the most significant limiting factors to the survival of these species.
One of the tasks identified in the recovery plan for these three species is to “reestablish populations and augment populations at protected locations, if deemed necessary.” Surveys in 2007 and 2008 found the federally endangered black-spored and mat-forming quillwort species at only five and seven sites, respectively, and the federally threatened pool sprite at only 38 sites.
To increase the redundancy and resilience of these populations, a technique to deepen existing pools has been done successfully at Stone Mountain and Plum Creek at Greensboro South. Because of hundreds of acres of granite mining on the Davidson-Arabia Mountain Nature Preserve (Arabia Mountain), there are only two remaining pools of black-spored quillworts found on top of the mountain.
The Service decided to reclaim habitat for the black-spored quillwort and pool sprite by creating some entirely new pools on Arabia Mountain’s old quarry site. In 2013, with funds provided by the Partners for Fish and Wildlife program, our partners at Arabia Mountain hired highway contractors with diamond bladed saws and jack hammers who sliced through four feet of solid granite to make 10 pools, four by four feet in size and four to six inches deep. We removed the granite debris and dust from the new pools and then went to the two natural black-spored quillwort pools. We removed a small amount of soil from the bottom of the pools and small samples (ramets) of the plant. We put the soil in the bottom of the created pools. Our partner, the State Botanical Garden of Georgia, took the ramets back to their lab and propagated enough plants for all 10 pools over the next two years. We then placed these back into the new pools in 2014-2015.
We had to wait a full year after each planting, and approached the created pools every time with trepidation. Would there be any plants there at all? Would the pools have been vandalized? To our gratification, all the pools had plants, and the experiment seemed to be working. We have done follow-up surveys every year and, in the 2017 survey, we had about 40 to 85 percent coverage of the pools with black-spored quillwort. Instead of only two pools of this endangered plant on Arabia Mountain, we can now confidently say we have 12 pools.
With the help of Atlanta Botanical Garden, we decided to try a modified version of this technique in 2014-2015 at two other protected sites, Greensboro South (now owned by Weyerhaeuser) and Heggie’s Rock (owned by The Nature Conservancy). We brought back our highway contractors, but this time had them deepen a four by four foot area in the bottom of three existing pools at each of the two locations. This technique was less costly and looked more natural. We followed the same procedure of borrowing soil and plants from nearby mat-forming quillwort populations and growing out the plants.
The pools at Weyerhaeuser have been successful. The plants’ population has expanded within the three pools and also in one more that we had deepened in 2012. We placed plants into the pools at Heggie’s Rock in January 2017 and will have to wait patiently until next year to see how they do. Many of our pools at all locations also have pool sprite growing in them, which we think will continue to increase.
These projects all together have added 17 new pools at three protected locations and have given our three endangered and threatened granite outcrop plants some security and a start towards recovery.