Posted on 7 September 2017
Torreya taxifolia, known as the Florida Torreya, is one of the rarest conifers in the world. Once found as a canopy tree, Torreya is an evergreen dioecious tree endemic to a narrow range of bluffs and ravines adjacent to the Apalachicola River in northwest Florida and extreme southwest Georgia. In the mid-20th Century this species suffered a catastrophic decline as all reproductive age trees died. In the decades that followed, this species did not recover. What remains is a population approximately 0.3% of its original size, which is subjected to changes in hydrology, forest structure, heavy browsing by deer, loss of reproduction capability, as well as dieback from fungal disease. Atlanta Botanical Garden’s dedication and efforts to protect Torreya furthers understanding of its ecology and life cycle as well as the decline of this once majestic species. The ongoing research to meet recovery objectives for this species is presented here, as well as how they relate to other rare species conservation and evolution under predicted climate change.
In 1984 this species was listed Endangered under the U.S. Endangered Species Act. It is currently listed as Critically Endangered on the IUCN Red List. In 1990, Atlanta Botanical Garden received 155 clones of T. taxifolia propagated from the remaining natural population by Arnold Arboretum and the Center for Plant Conservation. This material has been safeguarded at the Atlanta Botanical Garden since that time, and propagation efforts have increased the collection to include almost 1,000 plants, including nearly 500 distinct vegetative clones from the wild. The distribution of material across institutions has decreased; only two of the original ten gardens still maintain collections. The number of clonal lineages has decreased only slightly in the past 25 years, but only half of cultivated material across multiple institutions remains. The Center for Plant Conservation (CPC) awarded sponsorship to the Garden for Torreya taxifolia in 2010.
David Ruland, Greenhouse Manager, collecting cones for propagation and distribution. David is an expert at propagating Torreya and has produced hundreds of trees from cuttings and seed, from both wild and cultivated material
Allozyme analysis was used to evaluate multiple populations within the five ravines in which Torreya taxifolia occured in 1994. Genetic diversity detected using allozyme markers in the ex-situ population of Torreya maintained at Atlanta Botanical Garden was found to be higher than that in wild populations. Differences are likely due to the bottleneck effect and decline in genetic variation in the wild. Diversity of cultivated collections was close to the average level found in gymnosperms and typical of long-lived species with mixed-mating systems.
One of the limiting factors to ex situ conservation of this species is the inability to use conventional seed storage techniques for preserving germplasm. Torreya taxifolia produces recalcitrant wet seeds that cannot be dried for storage in freezers. Therefore, until recently the only way to maintain ex situ germplasm was through living collections.
In collaboration with Georgia Institute of Technology, a somatic embryogenesis tissue culture system was developed to initiate cultures, produce somatic seedlings and cryogenically store cultures of T. taxifolia. Large numbers of somatic embryos and resulting seedlings can be developed in culture from a single seed. One of the lessons learned was that the water potential (-MPa) of T. taxifolia gametophyte tissue rises greatly, in contrast to many other coniferous tree seeds, during seed after-ripening, and mimicry of this rise in vitro is necessary to continue development of somatic embryos to produce new seedlings in culture. All of the genotypes tested for cryopreservation were successfully recovered after retrieval from liquid nitrogen and can provide material for disease research, restoration or establishment of seed nurseries for conservation.
Timothy Putzke, Conservation Horticulturist, inspects Torreya fruit prior to harvest
Garden staff began collaborating with biologists and researchers at the Florida Park Service and the University of Florida in 2008. Efforts have included evaluation and mapping of wild trees. Research has been conducted identifying the disease causing agent as a new species of Fusarium (Fusarium torrayae). Field surveys have found that stem damage from deer antler rubbing is a significant source of stress in addition to disease, and is causing severe impacts to more than 50% of trees.
Efforts at understanding ecological requirements of this species for reintroduction include caging the trees to protect them from deer damage. To date 21.6% of surveyed wild trees have been caged for protection. Although the majority of habitat for T. taxifolia is protected in state parks or by The Nature Conservancy, until damage from deer and stem canker can be controlled, recovery of the species is dependent on ex situ conservation efforts.
Collaboration with the Georgia Plant Conservation Alliance facilitated partnership with the University of Georgia. This relationship led to the establishment of an experimental planting of Torreya at a University research station in 2000. The research site has now developed into a seed orchard, and nearly 5,000 cones were harvested in 2016. Seed has been shared, and seedlings are being distributed to other botanical gardens to use in developing additional ex situ collections and another seed orchard.
Torreya taxifiolia seed orchard (a collaborative project between ABG & UGA) - This female tree was grown from lateral cuttings and, along with 20 others, was planted in 2000. Nearly 5,000 fruit were harvested in 2016
Over the past ten years significant progress has been made in developing a variety of techniques for conservation of this critically imperiled species. These collaborative projects have resulted in scientific publications, presentations and educational materials for the public. Future research will determine the host range of the disease and offer insights on its origins. Current efforts include evaluation of the current status of the species, estimation of remaining genetic variability in the wild, and efforts to reintroduce the species into areas where it has been lost. Seed production in ex situ collections has increased dramatically, increasing diversity of this species in cultivation and availability of plant material for safeguarding and research.
Please contact Carrie Radcliffe (Restoration Coordinator & Safeguarding Database Manager, Department of Conservation & Research - Atlanta Botanical Garden) for more information about this project.