Given the array of current threats to biodiversity (including habitat destruction, pollution, climate change, and invasive species), it is no surprise that roughly one out of every three plant species in the world is threatened with extinction. As native habitats are changing and disappearing, ex situ conservation (preservation of species outside their natural habitat as living plants, seeds, or other viable tissue) efforts are even more vital to the successful conservation of plants.
Public gardens, numbering more than 700 in the United States alone (BGCI 2012), offer valuable resources, facilities, and horticultural expertise that support conservation efforts. These ex situ refuges also allow visitors a unique chance to learn about and observe threatened species firsthand. While these important plant collections serve as insurance policies against extinction for many species, the recent North American Collections Assessment found that only 39% of North America’s threatened species are currently cultivated in public gardens; clearly, there is much opportunity to increase rare plant conservation collections (Kramer et al. 2011). In addition to increasing the number of threatened species in ex situ collections, broadening genetic diversity within those collections will support meaningful conservation applications.
During my Putnam Fellowship from 2008 to 2010 I worked with Arboretum Curator of Living Collections Michael Dosmann to assess the conservation potential of the living collections in the Arnold Arboretum. We started with the formal conservation collections maintained as a partnership with the Center for Plant Conservation (CPC) (Hird and Dosmann 2010). We knew the CPC collections were well-constructed collections of threatened species, but we didn’t know how they had fared since their establishment nor what priorities we should keep in mind for future collections development. The following is a summary of our findings and the lessons we learned along the way.
Collecting, maintaining, and preserving plant biodiversity was a founding principle of the Arnold Arboretum in 1872 and remains at the core of the Arboretum’s activities today. Charles S. Sargent, the Arboretum’s first director, initiated the creation of one of the most extensive temperate woody species collections in the world. This long-standing commitment to collections development remains a core value of the Arboretum. Today, the Arboretum’s rich and dynamic botanical collections serve active research and education programs, and represent an invaluable repository of preserved genetic resources.
The CPC was founded in 1984 through a collaboration among 18 botanical gardens and arboreta as a network aimed at the “establishment of a permanent, well-documented, and accessible collection of rare and endangered native plant taxa of the United States” (CPC 1984). The Arnold Arboretum played a key leadership role in jump-starting the effort by housing the first CPC office and building the first CPC collections. Now headquartered at the Missouri Botanical Garden in St. Louis, the CPC has grown to a nationwide network of 38 botanical institutions. This network works to preserve 772 critically endangered North American species that compose the CPC National Collection of Endangered Plants. Each participating institution is assigned species relevant to their institution and region, monitors remaining wild populations, and collects and maintains genetically sound, long-term ex situ collections to support research, education, and, ultimately, species survival.
As a CPC participating institution, the Arnold Arboretum is obligated to follow a set of eight management guidelines (facing page) described in the CPC Handbook (CPC 2007). The Arboretum meets and exceeds most CPC guidelines, especially in the areas of collections data management and research. These guidelines are associated with detailed information including original wild collection data, germination and propagation protocols, health conditions of each specimen through time, and cultural requirements for each species. However, a variety of challenges have prevented the Arboretum from fulfilling certain guidelines, such as meaningful seed storage and reintroduction of species.
CPC Collections Management Guidelines (CPC 2007)
1. Taxa should be proposed and accepted by the CPC Science Advisory Council for inclusion into the National Collection.
2. Propagative materials should be collected from the wild in accordance with CPC guidelines and should be maintained in protective storage.
3. A usable seed storage and germination protocol should be developed for the taxon and initial seed viability should be determined if possible.
4. Horticultural techniques for ex situ cultivation should be established and documented, and the taxon should be successfully raised to reproductive maturity.
5. Adequate propagules and data should be stored in at least two separate secure sites
6. An initial baseline germination test should be conducted on stored seed accessions of the taxon, and viability should be retested at appropriate intervals, using enough seed if possible to detect statistically valid declines in viability.
7. Collaborative research agreements to be established for taxa as necessary and appropriate.
8. Legitimate reintroduction programs or experimental reintroductions are encouraged.
In total, the Arboretum has assisted in the conservation efforts of 24 threatened CPC species by collecting wild germplasm and maintaining those plants in the living collections (see Table 1). A majority of the Arboretum’s CPC collections began via collecting expeditions to the southeast and northeast regions of the United States in the late 1980s and early 1990s by Rob Nicholson, then plant propagator for the Arnold Arboretum (Nicholson 1996). At the time of this assessment there were 13 species assigned to the Arnold Arboretum. Many of the current and historic CPC species collected by the Arboretum originate from the southeastern United States, and some have experienced cold hardiness issues in the Northeast. In the past 15 years, due to significant decline in health and numbers of living accessions, about half of the original CPC species have been transferred to more appropriate institutions closer to their native range and with more compatible climates.
Plants in the Arnold Arboretum’s CPC Collections
Abies fraseri: The Fraser fir is well-known in the Christmas tree industry due to its spirelike crown and fragrant foliage. Reaching heights of up to 25 meters (82 feet), this species is native to the Smoky Mountain Range and is unique because it grows at high elevations. It is severely threatened in the wild by the invasive balsam wooly adelgid (Adelges piceae) introduced from Europe. In 1876, Asa Gray first collected a wild plant for the Arboretum (accession 1522), which did not survive. Since then, several specimens were unsuccessfully introduced to the the Arboretum. A collecting trip in 1985 supported the establishment of the CPC collection of this species, which have suffered excessive losses due to incompatible climate and spider mite infestations. The Arboretum maintains 6 specimens from 3 states (Virginia, North Carolina, and Tennessee).
Amelanchier nantucketensis: The Nantucket shadbush is a stoloniferous shrub which forms dense colonies in its restricted native habitat along the northeastern Atlantic coast. Flowers usually open in May but are small and hard to notice. Threats to this species include overcrowding by other plant species, harmful management practices such as fire suppression, and uncontrolled land development of coastal habitat. The Arboretum maintains specimens collected in the 1980s from New York, Massachusetts, and Maine. A large group was successfully transplanted during the Bradley Rosaceous Collection renovations in 2009 and now thrives near Dawson Pond.
Diervilla rivularis: The mountain bush honeysuckle is an arching shrub 1 to 2 meters (3.3 to 6.6 feet) tall and wide, very similar in appearance to D. sessilifolia, and forms colonies by rooting where the tips of its branches touch the ground. Small yellow flowers appear in July and attract insect pollinators. Its native range extends from the Blue Ridge to the Appalachian Plateau of the southeastern United States. This species is threatened in the wild by habitat destruction from logging and crowding by invasive species. The Arboretum maintains vigorous specimens from one location in Georgia, which are cut back every 2 to 3 years to maintain them as individual plants.
Diervilla sessilifolia: The southern bush honeysuckle is a sprawling shrub, 1 to 2 meters (3.3 to 6.6 feet) tall and wide, very similar in appearance to D. rivularis, except that the leaves are sessile (stalkless) on its arching branches. Native from Georgia to the Blue Ridge in Virginia, this species is threatened by construction, development, and crowding by invasive species. The Arboretum currently maintains healthy specimens from Georgia, North Carolina, and Tennessee in the Leventritt Shrub and Vine Garden, and near the top of Bussey Hill.
Fothergilla major: The mountain witchalder, or large fothergilla, is a dense, colonizing shrub known to reach up to 6 meters (19.7 feet) tall in the wild. Fragrant, creamy white bottlebrush-like flowers emerge in May and make this a popular landscape plant. It is native to six states in the southeastern United States, and is threatened by land development. The Arboretum has successfully cultivated this species since 1876. Specimens from North Carolina and Georgia currently thrive here.
Gaylussacia brachycera: The box huckleberry is a slow-growing evergreen shrub native to the Mid-Atlantic United States; it grows in dense, self-incompatible clonal colonies. The oldest plant in North America is a colony of this species growing in Pennsylvania, thought to be about 5,000 years old. Small white tubular flowers resemble other species in the heath family. This species is threatened by irresponsible land development and management practices, and invasive species. Charles Sargent first brought this species to the Arboretum from Pennsylvania in 1905. Since then, several specimens have grown in the collections. The Arboretum currently maintains two specimens including one collected from Tennessee; both are growing well in the Leventritt Shrub and Vine Garden.
Ilex collina: The longstalk holly is a multi-stemmed shrub to 3 meters (9.8 feet) tall that produces large, red to yellow berries on female plants. This species is native to North Carolina, Virginia, West Virginia, and possibly Tennessee, and is threatened by land development. There is taxonomic doubt as to whether this is a synonym of I. longipes or not. The Arboretum maintains specimens from all states except Tennessee, and this species thrives in cultivation.
Magnolia pyramidata: The pyramid magnolia grows 3 to 7 meters (9.8 to 23.0 feet) tall, and produces creamy white flowers that give it potential as an ornamental landscape plant. It is native to a limited range along the coastal plain of the southern and southeastern United States, and is threatened by land development. The Arboretum does not currently have specimens in the living collections, but had grown two lineages from Texas that were removed in 2001 when it was determined that the specimens were not M. pyramidata.
Rhododendron prunifolium: The plumleaf azalea is one of the showiest native azaleas, and may reach up to 6 meters (19.7 feet) tall in the wild. It has glabrous leaves and bears clusters of red-orange flowers in July and August. It is native to Alabama and Georgia and is threatened by logging and low seedling numbers in the wild. The Arboretum currently has specimens from two locations in Georgia, and is responsible for introducing this species into cultivation in the early 1900s via plant collector T. G. Harbison.
Rhododendron vaseyi: The pinkshell azalea is an upright shrub known to grow up to 5 meters (16.4 feet) tall in the wild. Scentless (and frost resistant) pink flowers emerge in April prior to leaf bud break, providing striking ornamental value. This species is native to North Carolina, and is threatened by land development and illegal collecting in the wild. The Arboretum introduced it to cultivation in 1880, and maintains several specimens from North Carolina which thrive in the Boston climate.
Spiraea virginiana: The Virginia meadowsweet is a 1- to 2-meter-tall (3.3 to 6.6 feet) shrub that forms dense clumps of upright, arching stems with cream colored inflorescences in May. This species is endemic to the central and southern Appalachians, where its sporadic populations are threatened by competition with fast growing herbs and vines, habitat destruction including dam construction, and lack of sexual reproduction. Plants were first collected by the Arnold Arboretum in 1919 by T. G. Harbison in North Carolina (accession 10160), grown at the Case Estates, and then repropagated via cuttings and brought to the main Arboretum grounds in 1988, where the lineage still exists today. When this CPC collection was established in the mid1980s and 1990s, the Arboretum amassed one of the most extensive ex situ collections of this species in the world, composed of plants from all states where it is currently known to grow. Two groups were recently transplanted to beds near the South Street and Mendum Street gates.
Torreya taxifolia: Once a towering tree of 15 meters (49.2 feet) or more, the stinking cedar (named for its pungent, sharp needles) is native to Georgia and Florida, and is now one of the most threatened conifers in the world because of a fungal disease. The few remaining wild individuals have been reduced to root suckers. Until 2010, the Arboretum maintained 33 specimens from known remaining populations. This collection suffered severe attrition because the species is poorly adapted to the Boston climate. In 2010 the collection was successfully transferred to the Atlanta Botanical Garden (see textbox on page 22). Viburnum bracteatum: A shrub to 3 meters (9.8 feet) tall with spreading branches and sharply-toothed leaves, this viburnum species is well adapted to the Boston climate. There is some taxonomic question whether this species should be included as part of V. dentatum. It is native to Alabama, Georgia, and Tennessee, and has long been noted as naturally rare in the wild. It is currently threatened by limestone quarrying. The Arboretum first introduced it to cultivation in 1904 and maintains specimens from Tennessee and Georgia.
CPC Collections Review
Prior to this collections review, we had only anecdotal information about the CPC collections at the Arnold Arboretum. Staff members had made annual field checks of the CPC specimens, so there was a lot of information stored in BG-BASE (the curation database). We knew basic information such as which species grew well in certain areas of the Arboretum and which were problematic for maintenance, but we did not know certain things such as which CPC species were truly thriving (versus barely surviving) in the Arboretum’s cultivated environment, and which CPC species were adequately represented as a collection to support conservation activities. We also wanted to develop directions for the care and maintenance of the CPC collections, an important step in determining curatorial and horticultural priorities for future development. Curatorial reviews of each CPC species were done by compiling all relevant plant records, conservation, and historical information in order to accurately assess the current value and future conservation potential of each collection. This process brought to light several ways to guide the management and development of the Arnold Arboretum’s CPC collections (Hird and Dosmann 2010). Described in more detail below, we looked at the major collections factors: lineages, accessions, plants, and supporting documentation
The number of unique genetic lineages (i.e., plant material collected from one or a few individuals in one location; clonal reproduction can extend a lineage through time) of a species in an ex situ collection reflects the potential genetic diversity available for research and conservation efforts (including reintroduction of plants to the wild). Genetic diversity allows for evolutionary adaptation of a species, and healthy plant populations typically have high levels of genetic diversity, allowing them to survive a variety of environmental pressures. Conservation collections should be managed with the aim of preserving as much genetic diversity as possible, as an insurance policy in case the gene pool of natural populations diminishes or disappears. Sampling standards have been developed to ensure the greatest genetic diversity is captured in ex situ collections of rare species (see Table 2; Falk and Holsinger 1991). For rare species with three or fewer populations remaining in the wild, 100% of these populations should be sampled and preserved in ex situ collections. For rare species with four or more populations remaining in the wild, approximately 80% of the populations should be preserved in ex situ collections. The Arnold Arboretum’s CPC species mostly fall into the latter category, so an appropriate sample size for a majority of these collections is at least four or five unique populations. This can guide lineage development both within and among populations.
A few species such as Spiraea virginiana have a fairly wide genetic base at the Arboretum, but a few of the more-threatened species such as Abies fraseri, Gaylussacia brachycera, and Viburnum bracteatum are not adequately represented to support effective conservation. Notably, lower lineage numbers for some CPC species like Abies fraseri are a result of high attrition from lack of adaptability to the Arboretum’s climate.
The number of living accessions (i.e., plants from a single lineage, acquired by one means of propagation at one time) for each species further demonstrates the depth of each collection and sheds light on lineage redundancy within the living collections. With limited resources and space, the Arboretum sets a collections goal to have 2 to 3 accessions per unique lineage for most types of plants (species, etc.) in the living collection (Living Collections Committee 2007). When comparing the total number of living accessions to the total number of living lineages, each CPC species is represented by 1 to 2 accessions
per lineage. This assessment showed that both of the Diervilla species had a higher number of accessions per lineage, demonstrating redundant clones within the same lineages. To maintain appropriate accession-to-lineage ratios for the CPC collections, we identified lineages and accessions that could be bulked up via clonal propagation and others that could be “thinned” by sending back-up material to other institutions.
Health conditions through time and total numbers of living plant specimens give an indication of how well a species grows in the Arboretum and can provide guidance for collections management. At the time of this assessment in 2009, most CPC specimens were healthy. However, management needs were further considered for species with significant proportions of specimens in fair or poor condition, such as Torreya taxifolia, Amelanchier nantucketensis, and Abies fraseri. Also, Magnolia pyramidata, with no living plants represented in the collection, was prioritized for a collection transfer or germplasm acquisition.
By using the Arboretum collections standard of maintaining an average of 2 plants per unique accession (Living Collections Committee 2007), we identified collections redundancy or deficiency for each CPC species. As we analyzed accession-to-lineage ratios, we also compared the total number of living accessions with the total number of living plants per species, also taking into account specimen health. This allowed us to identify specific plants in need of repropagation, removal, or relocation. One particularly successful example of making management decisions to improve plant health is the Amelanchier nantucketensis specimens in the Bradley Rosaceous Collection (BRC). Poor health had been recorded for these plants for several years, and during bed renovations in the BRC they were transplanted to new beds near Dawson Pond. This location’s higher soil moisture has resulted in improved health for the plants. A common issue identified for Amelanchier nantucketensis, both Diervilla species, and Spiraea virginiana was maintaining individuals of these mass-forming species. As a result, these specimens were put on a pruning schedule to prevent uncontrolled spreading and suckering.
Since the Arboretum’s primary goal with the CPC collections is preservation of living germplasm, long term survival of the CPC plants is a top priority. Collections management at the Arboretum includes the preservation of unique lineages through clonal repropagation if needed. Sometimes plants brought to the Arboretum are not well-suited to survive in the collections for reasons such as lack of compatibility to cultivation or the local climate. Species whose records show high levels of lineage or plant loss, such as Torreya taxifolia, likely represent poor compatibility with Arboretum conditions, making them potential candidates for transfers to institutions better able to cultivate them.
The geographic, temporal, and environmental details about the source of an accessioned living plant are referred to as the passport data, which are curated in the Arboretum’s plant records. Passport data can make collections more valuable for conservation, education, horticulture, and research by associating valuable habitat or biological information with each specimen. For wild-collected plant material the value of a collection increases with the amount of passport data. This can range from coarse geographic information such as country and state to highly local information such as soil type or altitude of an original collection location.
Additional supporting documentation may include observations, voucher herbarium specimens, images, verifications, and recorded instances of collections use (for tours, publications, and educational projects involving a species). Herbarium specimens and images offer long-term genetic and biological information that can enhance understanding and aid in conservation of a threatened species. The Arnold Arboretum Cultivated Herbarium sets a goal to document the living collections with vegetative, flowering, and fruiting material per unique lineage (Curatorial Department 2009).
This CPC assessment identified gaps in passport data and supporting documentation for each species. In addition to augmenting geographic passport data for many CPC accessions, we also established herbarium specimen and image collecting targets, as well as past verifications that could be entered into the plant records database.
Collections Enhancement Priorities
The Arboretum has taken a number of positive steps following this assessment to improve and more effectively manage the CPC collections, making them more valuable and accessible for research, education, and conservation. Individual species reviews allowed us to create a prioritized master list of recommended curatorial and horticultural actions based on collections goals and needs.
Accomplishments include enhancements in plant records information through the addition of county names, latitude and longitude, or other location information when possible. Voucher and image collection has also been a priority for the curatorial department, and over 350 herbarium specimens have been collected to further document the CPC collections. Recommended repropagations, removals, and relocations have been completed, including repropagation of two Abies fraseri specimens which are failing in the collection; addition of new lineages (Rhododendron vaseyi); removal of non-wild-origin plants and acquisition of new wild-origin lineages (Rhododenron prunifolium); planting out of nursery stock (Torreya taxifolia); and removal of redundant specimens to send as back-up material to other institutions (Amelanchier nantucketensis, Spiraea virginiana). Successful relocations of Spiraea virginiana and Amelanchier nantucketensis to other locations in the landscape have improved plant conditions and horticultural management of these collections. Horticultural practices identified and implemented during this assessment include applying horticultural oil to Abies fraseri to control spider mite outbreaks, cutting back of both Diervilla species and Spiraea virginiana to maintain specimens as individuals, and pruning of suckering roots to maintain individual specimens of Amelanchier nantucketensis. There were also several wild-origin lineages that were historically not included in the Arboretum’s CPC collections, so these valuable specimens were formally reported to CPC and added to the annual inventory process for close monitoring and care in the future. After failed viability tests, we discarded a short-term seed collection of several CPC species (Amelanchier nantucketensis, Diervilla sessilifolia, Ilex collina, Rhododendron vaseyi, Spiraea virginiana) and storing garden-origin seed was discontinued. We streamlined the annual CPC collections inventory and data reporting processes by setting up automatic reports in BG-BASE. We also identified future acquisition targets for under-represented populations of several of the CPC species. To encourage broader awareness of the Arboretum’s CPC collections we created a web page of CPC species highlights and have given several public tours focused on CPC species. Perhaps our biggest success was the official transfer of the Torreya taxifolia CPC collection to a more appropriate location and garden.
Going forward, there are a lot of exciting opportunities for the Arboretum to maintain and enhance genetic diversity of the CPC collections and further meet the CPC collection management guidelines. This may include wild-collecting additional plants or seeds for long-term seed storage and continuing to identify institutions that could receive backup germplasm of the Arboretum’s CPC collections. Within the Arboretum, archival research for additional wild-collection information and digitization of species verification records may enhance plant records data. Incorporation of specific horticultural needs into the Landscape Management Plan (Horticulture Department 2012) would encourage close monitoring of CPC collections by staff horticulturists. Opportunities to further share information about CPC collections through classes, tours, and web applications present exciting possibilities.
Of top priority are additional collection transfers for Abies fraseri, Gaylussacia brachycera, and Magnolia pyramidata. These collections have not thrived at the Arboretum so transfer to a more appropriate institution or region of the United States would ultimately support the long-term survival of these species. Appealing to the CPC and identifying potential receiving institutions, are the first steps. Once remaining collections are stabilized and appropriate species are transferred, new acquisitions of Northeastern threatened woody species can be considered as potential CPC collections in the future. This regional focus for CPC species would bring conservation work closer to home and likely result in increased success for threatened species grown, maintained, and utilized at the Arnold Arboretum.
BGCI. 2012. GardenSearch database. Accessed online: www.bgci.org/garden_search. php. Botanic Gardens Conservation International.
CPC. 1984. The Center for Plant Conservation Archives. The Arnold Arboretum of Harvard University.
CPC. 2007. Management Guidelines for Participating Institutions. Participating Institution Handbook. CPC.
Curatorial Department. 2009. Cultivated Herbarium Collection Policy. The Arnold Arboretum of Harvard University.
Falk, D. A. and K. E. Holsinger (eds.) 1991. Genetics and conservation of rare plants. New York: Oxford University Press.
Hird, A. and M. Dosmann. 2010. CPC Collections Analysis. The Arnold Arboretum of Harvard University.
Horticulture Department. 2012. Landscape Management Plan, 2nd Edition. The Arnold Arboretum of Harvard University.
Kramer, A., A. Hird, K. Shaw, M. Dosmann, and R. Mims. 2011. Conserving North America’s threatened plants: Progress report on Target 8 of the Global Strategy for Plant Conservation. Botanic Gardens Conservation International U.S.
Living Collections Committee. 2007. Living Collections Policy. The Arnold Arboretum of Harvard University.
NatureServe. 2012. NatureServe Conservation Status. Accessed online: http://www. natureserve.org/explorer/ranking. htm
Nicholson, R. 1996. CPC file archives. The Arnold Arboretum of Harvard University.
Abby Hird is a Research Associate with Botanic Gardens Conservation International’s United States office, based at the Arnold Arboretum.