Abstract
Genetic resources are the foundation of American agriculture’s ongoing success—the diversity, security, health, and genetic integrity of these resources must be safeguarded. However, in contrast to other crops, protecting, managing, and using collections of woody landscape plant genetic resources present significant challenges. These include conservation of at-risk populations that have high genetic diversity, evaluation of taxa with an unknown potential to invade, and management of large specimens that have long generation times and often recalcitrant or difficult-to-store seeds. The wide diversity of taxa and the limited number of specialist curators and scholars present further challenges. Thus, effective collection, preservation, evaluation, and distribution of woody landscape plant germplasm require substantial and specialized resources. The most fundamental challenge is simple: too many built landscapes have low taxonomic diversity and are often dominated by a single taxon, or monoculture. In turn, these taxonomically and genetically depauperate landscapes are vulnerable to a host of biotic and abiotic threats and are less likely to provide ecosystem and societal services for which they were designed. To develop more resilient landscapes, there is an urgent need to expand taxonomic diversity, which requires the horticultural community to effectively acquire, curate, evaluate, distribute, and use diverse woody landscape plant genetic resources. The role of the US Department of Agriculture-Agricultural Research Service (USDA-ARS) National Plant Germplasm System (NPGS) Woody Landscape Plant Crop Germplasm Committee (WLPCGC) is to identify vulnerabilities and threats that compromise WLP crops in the United States and make recommendations for prioritization of germplasm acquisition, evaluation, research, and management priorities through regular interaction with stakeholders. Due to the breadth of genera and shifting plant genetic resource needs of the nursery industry, it is impractical to list specific threatened or priority genera in a broadly applicable crop vulnerability statement. Instead, the WLPCGC has identified themes of threats and vulnerabilities providing a foundation upon which to support prioritization as threats to specific genera emerge.
A growing body of knowledge demonstrates that landscape trees and shrubs are essential resources that enhance residential, public, and commercial environments, particularly in urban and suburban settings (Turner-Skoff and Cavender 2019). These plants provide key ecosystem services by cooling the air, mitigating poor air and water quality, sequestering carbon, decreasing runoff and erosion, improving stormwater management, and forming a critical ecological framework that supports vertebrate, invertebrate, and microbial diversity. Landscape plants increase the value of residential and commercial property, support local economies, and reduce energy and maintenance costs of buildings and roadways. They also play key roles in human well-being and recreational activities, and can generate revenue for municipalities through the use of public spaces and tourism (Nowak and Greenfield 2018; Wolf 2009). In addition to their broad impact on communities, landscape plants support the horticulture industry, across which diverse businesses contribute $196 billion to the US economy. Specifically, nursery growers contribute more than $4.2 billion in sales from 8000 operations, most of which are small (<$500,000 in annual sales) family-owned businesses (USDA-NASS 2020). The United States is the world’s largest producer of, and market for, woody landscape crops. Growers depend on a diverse and dynamic array of woody landscape plants for resistance to pests and diseases, tolerance to environmental extremes, adaptability in sustainable landscapes, and to meet changing industry trends and consumer demands. As the United States becomes more urbanized and urban forests are more greatly valued, the vital role of woody landscape plants in our communities and within society becomes more pronounced, and their social and economic value increases substantially.
Woody plants contribute an array of societal services that are linked to physical and mental health benefits related to access and exposure to green spaces or urban tree cover (UTC). The presence of vegetation has been linked to lower childhood obesity rates (Bell et al. 2008), reduced measures of stress, reduced cognitive fatigue (Kaplan and Kaplan 1989; Wolf and Robbins 2015), and other positive health and mental impacts (Maroko et al. 2009). Some of these benefits may stem from improved landscape aesthetics and noise reduction due to vegetation, as well as better community cohesion (Gaudon et al. 2022; Kuo 2003). Although safety is perceived by some as a problem associated with increased woody vegetation (Pataki et al. 2011), vegetation is generally linked to a decrease in overall crime rates (Kuo 2003; Wolfe and Mennis 2012). Concerningly, urban green spaces and the woody plants within them are often not equitably distributed in urban environments. For example, more UTC is likely to be found in high-income neighborhoods (Schwarz et al. 2015) and less UTC can be found in areas with low-income or renter communities (Landry and Chakraborty 2009). These and similar findings are cause for environmental justice and equity concerns, and also provide pathways to improvement (Foster et al. 2024).
Between 1200 and 1500 genera constitute the array of woody landscape plants in the United States (Rehder 1940), of which ∼200 are commonly planted in US landscapes and are considered specialty crop species. The introduction of new plants has been heavily weighted toward international rather than domestic collections, due in part to the floristic richness of areas such as eastern Asia, where climatic and edaphic similarities fostered the successful introduction, cultivation, and integration of novel plant germplasm into American horticulture. Although non-native woody plants remain a staple in ornamental horticulture, consumer demand for species endemic to local ecosystems is increasing (Kauth and Pérez 2011). This increase in demand stems, in part, from consumer perception that natives are better adapted to local site conditions, require less maintenance, and benefit birds and other wildlife more compared with non-natives (Brzuszek et al. 2007, 2010; Tallamy 2009). A leading barrier to the widespread use of native plants is low commercial and retail availability. In a nationwide assessment of the US native plant industry, White et al. (2018) reported that only 26% of the ∼25,000 vascular plant taxa native to the United States were commercially available for purchase. In addition, the most recent of the recurring national survey, “Production and Marketing Practices and Trade Flows in the United States Green Industry,” showed that native plants contributed 9.1% of total nursery sales in 2018, a decrease from 17.1% reported in 2013 (Khachatryan et al. 2020). Nursery adoption of a new plant species, including natives, requires the plant to be economical to propagate and grow to salable size. Collection, characterization, and storage of genetically diverse germplasm can aid in identifying aesthetically superior phenotypes that are also amenable to conventional nursery production methods.
Compared with other crop commodities in the United States, breeding efforts of woody landscape plants are disproportionately small in scale and scope, especially considering the value of woody landscape plants to the US economy. A 1994 survey published as the “National Plant Breeding Study-I” in 1996 by Iowa State University identified 18 ornamental breeders at universities (11.7 woody), five ornamental breeders in the USDA-ARS (four woody), and 64 ornamental breeders in the commercial industry (23.9 woody, omitting pine). Because of long generation times and the high costs involved in breeding woody plants, most small nurseries cannot afford to operate breeding programs and many nurseries simply select and introduce new cultivars through chance seedling selection or cloned sport mutations. Hobbyist breeders tend to focus on the aesthetic qualities of a few genera, such as rhododendrons and roses, and do not typically involve multigenerational crossing programs to incorporate resistance to insect pests and diseases or tolerance to other abiotic stresses. This is largely because of capacity constraints relative to the time and space necessary to evaluate selections effectively in multisite trials. In addition, in the 30 years since the survey on plant breeding, decreased levels of support have led many public institutions to reduce or eliminate their breeding programs, and very few botanic gardens have active breeding programs focused specifically on woody landscape plants.
Urgency and Extent of Crop Threats and Vulnerabilities
As summarized previously, woody landscape plants are a vital component of the US economy, the environment, and society. However, a host of pressures and concerns threaten these plants and their managed landscapes. One of the most serious concerns is the low genetic diversity of landscape plants, which undermines landscape resiliency in the face of biotic and abiotic threats. Relatively few genera and species within them dominate managed landscapes in the United States, and the genetic base of individual species under cultivation is often narrow. For instance, green ash (Fraxinus pennsylvanica), redtip photinia (Photinia
Such a narrow taxonomic and genetic base increases a landscape’s vulnerability to catastrophic losses by insects and diseases, as illustrated by several examples in the past 70 years. Extensive and uniform plantings of American elm (Ulmus americana) resulted in widespread losses of urban street trees to Dutch elm disease beginning in the 1950s. During the same period, the lethal yellowing phytoplasma decimated vast numbers of Atlantic tall coconut palms (Cocos nucifera ‘Atlantic Tall’) in Florida. More recently, the imported wooly adelgid has caused widespread destruction of hemlocks (Tsuga spp.) in the eastern United States. Ashes (Fraxinus spp.), many of which were planted as street trees to replace American elm, are now under attack by the emerald ash borer with devastating losses to natural and urban forests from the Atlantic coast to the Pacific with the most recent introduction in Oregon (Pokorny 2023). Multiple genera in the Lauraceae, including Sassafras, are threatened by an introduced ambrosia beetle that causes a lethal fungal infection. Last, numerous woody genera are threatened by the spotted lanternfly, which has spread rapidly from the mid-Atlantic region to the Northeast and Midwest. In addition to these and many other well-known pests (Table 1) that pose significant risks to woody landscape plants, monitoring for future biotic threats must be ongoing as host preferences change and novel pests continue to be imported.
Known insect and disease pests and woody plant species hosts used in developed landscapes.
Abiotic stresses are expected to become more pronounced as a result of climate change and pose a threat to woody landscape plant germplasm in forests. We are already witnessing these changes occurring in real time. The annual 2023 summary report (NOAA 2024) assessed climate of the United States and noted that mean temperatures of 2023 were consistently and significantly higher than the average temperature of the 20th century with exceptions in some montane and arid regions of the west and southwest United States (Fig. 1). This made 2023 the fifth warmest year on record in the contiguous United States. The US Climate Extremes Index (USCEI) was 65% above average and ranked 11th highest over a 114-year period. These extremes are attributed to both changes in temperature (hot and cold) and precipitation (wet and dry). As weather patterns fluctuate, environments and the plants growing in them will continue to experience extremes in precipitation (drought and flooding) and temperature (absolute highs and lows and atypical seasonal fluctuations).
2023 Mean temperature departures from the average temperature of the 20th century in degrees Fahrenheit depicted in a color coded heat map where shades of red indicate temperatures above average as reported by the National Oceanic and Atmospheric Administration.
Citation: HortScience 59, 10; 10.21273/HORTSCI18010-24
The survival of woody landscape plants in developed urban and suburban environments is also dependent on their tolerance to a host of human-induced abiotic stressors including soils that are heterogeneous, compacted, calcified, or laden with salts; air pollution; vandalism; and lack of care. The impacts are compounded when novel, introduced biological stressors inflict trees growing in marginal, stress-prone urban landscapes. Unlike most other crop plants, woody landscape plants contribute extensively to the long-term health and stability of natural and constructed ecosystems and are often used to ameliorate or remediate stress-prone environments. If woody plants—keystones to those environments—fail, it triggers a cascade of further degradation. Therefore, it is critical that we identify, conserve, evaluate, and distribute a diverse palette of germplasm that can tolerate, and even improve, these conditions, or develop tolerant selections through targeted breeding efforts.
Woody landscape plants are not only threatened by, but can act as, introduced biological stressors. The authors of this paper recognize that although plant improvement and introduction are the foundation of modern horticulture yielding a diversity of useful plants for managed landscapes and gardens, a small proportion of introduced plant species escape cultivation and may cause negative impacts on natural systems and biological diversity (Richardson and Rejmánek 2011; Williamson and Fitter 1996). The issue is complicated by the fact that species can be invasive in some regions, but not in others, and the impacts of biological invasions can occur at times and places far removed from the site of introduction. These principles have been reiterated by several groups, including the National Invasive Species Council established by Federal Executive Order No. 13,112 (1999) and the St. Louis Declaration on Invasive Plant Species (Fay 2001). The National PLANTS Database (http://plants.usda.gov), maintained by the USDA Natural Resources Conservation Service (NCRS), publishes lists of invasive, threatened, and endangered plants. Thus, it is essential that known or potential invasiveness be considered when making decisions regarding the prioritization of woody landscape plant genera, collection and exchange proposals, and curation.
On the other end of the spectrum are plants threatened with extinction in nature. Concerns about these plants have resulted in federal, state, and local legislation including the Endangered Species Act of 1973 (ESA). The ESA was enacted to protect endangered plant and animal species and their habitats. Ex situ germplasm collections not only preserve endangered woody plants and their populations but also provide essential material for research and reintroduction programs. Maintenance, distribution, and research on rare plants also support the nursery industry, as numerous species of woody plants of conservation value in the United States and abroad are also recognized ornamentals (e.g., Hamamelis ovalis, Rhododendron vaseyi, Kolkwitzia amabilis). A growing number of commercial entities play an active role in the preservation and reintroduction of threatened and endangered plants. In some cases, nurseries may provide propagation expertise and other resources for cultivation while being an avenue for genetic preservation within the developed landscape. In one particular case, a major arboriculture company has dedicated resources, including space, to the development of conservation groves for threatened oak species (Amy Byrne, Global Tree Conservation Coordinator and Manager, personal correspondence). Many activities related to US threatened and endangered species are coordinated by Botanic Gardens Conservation International (BGCI), the Center for Plant Conservation (CPC), as well as in coordination with federal, state, and regional conservation alliances. International transport of endangered species is governed by the Convention on International Trade in Endangered Species (CITES).
The combination of the preceding challenges is coupled with other obstacles that affect the status of woody landscape plant crop wild relatives in natural environments. Global and accelerated changes in land use, as well as degradation in soil, water, and air quality directly affect populations’ existence and viability. Geopolitical changes shift attitudes regarding how natural resources are preserved, and also the ability for scientists to access populations for study and germplasm acquisition. These will ultimately result in decreased genetic diversity of natural populations in and ex situ. Thus, it is imperative that the genetic base of vulnerable target species be protected and expanded as soon as possible while the genetic diversity exists and opportunities allow.
Maintaining woody landscape plant germplasm exposes additional logistical vulnerabilities that are distinct from many other crops. Species categorized as having desiccation-intolerant (recalcitrant) seeds cannot be stored using traditional methods. In such instances, germplasm preservation may require bud wood storage, tissue culture, or cultivation of living plants within ex situ collections. Ex situ conservation of living organisms offers convenient access to material for research activities and a source for further collection and distribution of seed and clonal germplasm. However, these collections require large amounts of space (particularly for trees), represent only a portion of extant genetic diversity, and plants may outcross with related taxa producing seeds with unknown pollen parentage. For some species, requirements to break seed dormancy and tolerance of seed desiccation are unknown and years of study may be required before reliable storage conditions or germination can be achieved. Given that all seed has a finite storage time, all seed accessions ultimately require recollection at the taxon or population level, or regeneration ex situ. For woody plants, regeneration from seed may take several decades and sometimes requires additional steps to achieve successful pollination and isolation from outcrossing.
Status of Woody Landscape Plant Genetic Resources Available for Reducing Genetic Vulnerabilities
No single organization has the resources to acquire and manage the woody landscape plant genetic resources of the world. Collaboration among partners—federal, state, private, and nonprofit—is crucial to ensure the long-term conservation of germplasm. Woody landscape plant germplasm is conserved by numerous institutions, both within and beyond the NPGS. Within the NPGS, woody landscape plant germplasm is held by the following entities.
The National Plant Germplasm System (NPGS), whose mission is to collect, document, maintain, evaluate, enhance, distribute and preserve the plant genetic resources necessary for improving the quality and production of economic crops important to US and global agriculture.
The National Germplasm Resources Laboratory (NGRL) facilitates the acquisition, exchange, and documentation of crop genetic resources important to world food security.
Plant germplasm repositories within the NPGS, a network of active sites (Table 2) located throughout the United States that manages working collections of a diverse array of plant germplasm and associated information.
National Plant Germplasm System active sites responsible for maintaining living collections of woody landscape plants across the United States in the form of plants and seeds.
The National Laboratory for Genetic Resource Preservation (NLGRP) secures NPGS working collections through intentional duplication in a base collection designed for long-term conservation.
Outside the NPGS, organizations that play active roles in landscape plant germplasm preservation include the following entities.
The USDA Natural Resources Conservation Service operates a Plant Materials Program that collects plants and develops technologies for their successful conservation and use.
The USDA-Forest Service (USFS), whose mission is to achieve quality land management under the sustainable multiple-use management concept to meet the diverse needs of people.
The American Public Gardens Association (APGA) represents nearly 300,000 accessions growing in the collections of North American botanical gardens and arboreta that represent taxonomically and genetically diverse resources originating throughout the world, including rare or endangered species, documented wild provenances, and modern and historic cultivars.
The Plant Collections Network of the APGA (PCN, previously called the North American Plant Collections Consortium, NAPCC) was established in the early 1990s both to coordinate a continent-wide effort among botanical gardens for the conservation of plant genetic resources and biodiversity and to elevate curatorial standards for the management of plant collections.
Private, industry, and university collections hold significant unreported woody landscape plant crop germplasm outside the formal plant germplasm preservation community, with variable quality and completeness of documentation.
Herbaria affiliated with arboreta, botanic gardens, universities, and museums, constitute a valuable resource for woody landscape plant germplasm by serving as critical repositories for vouchers of living collections and natural history collections documenting natural distributions and plant invasions.
Exploration and exchange.
Before germplasm can enter any of the NPGS repositories or other collections, it must be acquired. The acquisition of foreign and domestic plant germplasm through exploration and exchange is vital to expanding collected plant diversity and drives the sustained growth of the nursery and landscape industries. Plant exploration activities have not always been coordinated with preservation objectives; consequently, the germplasm and the associated information on many of these resources are no longer available. In the past decade, the NPGS has developed effective protocols for obtaining access to plant genetic resources in many countries, established ethical and standardized plant collecting methods, and has continued to support numerous explorations. In addition, successful explorations have been conducted by botanic gardens and arboreta, as well as under commercial, university, and individual sponsorship. In some cases, international law now dictates that genetic resources are a sovereign resource of the country of origin; whereas in other cases, genetic resources are recognized as common heritage of humankind and thus ownership of genetic resources and subsequent limitations created by intellectual property rights is prohibited. Consequently, access to plant genetic resources in many countries can be limited and can depend on the development of agreements that outline clear boundaries. New strategies that include increased collaboration and additional resources are now required to continue international plant exploration and exchange activities.
Many botanic gardens and arboreta are involved in domestic and international plant exploration activities, including Arnold Arboretum of Harvard University, Atlanta Botanical Garden, California Botanic Garden, Chicago Botanic Garden, Dawes Arboretum, Denver Botanic Gardens, Desert Botanical Garden, Fairchild Gardens, Holden Arboretum, Huntington Botanical Gardens, Longwood Gardens, Lyon Arboretum of University of Hawaii, Minnesota Landscape Arboretum, Missouri Botanical Garden, Montgomery Botanical Center, Morris Arboretum of the University of Pennsylvania, Morton Arboretum, JC Raulston Arboretum of North Carolina State University, National Tropical Botanical Garden, North Dakota State University Dale E. Herman Research Arboretum, Sonoma Botanical Garden, Polly Hill Arboretum, Santa Barbara Botanic Garden, UC Davis Arboretum, University of California Botanical Garden at Berkeley, University of Washington Botanic Gardens, and the US National Arboretum. In turn, most of these institutions share their germplasm with other gardens and repositories or universities, including those within the NPGS. Plant exploration and collection have also been improved by the formation of consortia. Examples include the North America-China Plant Exploration Consortium (NACPEC) and the Plant Collecting Collaborative (PCC).
The WLPCGC must work with NPGS curators, as well as managers at non-NPGS collections, to determine if priority genera are represented by known sources and if the collections include representative genetic diversity. A strategy for developing future plant exploration and exchange objectives can be based on this assessment to ensure sufficient coverage of diversity within a species relative to provenance coverage or specific target traits such as insect or disease pest resistance.
Evaluation.
The value of germplasm is often not fully realized until it has been characterized or evaluated in field or landscape settings. It can then be further enhanced by breeders, other scientists, and recipients of germplasm after distribution. Evaluation of landscape plant germplasm is conducted by numerous institutions encompassing a range of evaluation, breeding, and selection projects. Many botanic gardens, universities, and nurseries conduct these evaluations, and it is often difficult to standardize the data because of genotype × environment effects, or difficulties in linking provenance data to evaluation results; however, these varying phenotypes due to site variance may actually reveal traits that may otherwise remain cryptic.
Many woody landscape plants are evaluated based on the value of an accession as a potential cultivar and not on its genetic potential in a long-term breeding program. The long maturation period for woody plants, coupled with the fact that most woody landscape plants evaluated are not genetically far removed from their wild crop relatives, exacerbates the issue. Thus, plant lineages that may possess unique genetic traits are often discarded prematurely because initial evaluation reveals limitations in specific adaptations or other characteristics. Ideally, plant germplasm should be characterized for all important traits, including disease and insect resistance, tolerance to various environmental stresses, amenability to production, and aesthetic qualities such as habit and foliar and floral characteristics.
Provenance trials and biosystematic studies are also used to evaluate woody plant germplasm. Although valuable, one limitation of these reports is a lack of evaluation of aesthetic or landscape characteristics. Many studies are often limited in scope, not associated with germplasm preservation, or do not have data linked to other projects that have examined similar accessions and provenances.
Fortunately, there are projects under way to develop standardized databases containing pertinent evaluation data; however, little effort has been made to coordinate their activities. Examples of such projects include the USDA Natural Resources Conservation Service Woody Plant Evaluations made by Plant Materials Centers for conservation purposes; the WLPCGC development of a framework of basic descriptors for characterizing and evaluating woody landscape plant germplasm in Germplasm Resources Information Network (GRIN); and the description systems established by the Malus, Pyrus, Prunus, Juglans, Vitis, and Small Fruits Crop Germplasm Committees for GRIN. Of these examples, only the GRIN-Global system is directly connected to germplasm preservation.
Two programs stand out concerning the evaluation of woody landscape plant material: the North Central Regional Ornamental Plant Trials (NC-7 Trials) and the Southern Extension and Research Activities-Information Exchange Group-27 (SERA-IEG-27). The NC-7 Trials began in 1954 to gain knowledge about useful plants for the nursery trade of the central and upper Midwestern states by emphasizing detailed, long-term evaluations of plant survival, growth, and aesthetic characteristics across a broad range of sites. The Trials rely on a network of cooperators at ∼30 sites located throughout the North Central region and in other states with similar climatic characteristics. Cooperators are located at land-grant universities and experiment stations, public gardens, and the region’s four USDA Natural Resource Conservation Service (NRCS) Plant Materials Centers. There are now nearly 50 years of data collected from 10-year trials of more than 500 accessions of landscape trees, shrubs, and vines, including a much wider range of genera that are maintained at the North Central Regional Plant Introduction Station. Data on plants distributed for testing since 1984 are held in a database available at www.ars-grin.gov/ars/MidWest/Ames/trialhmpge.html and are also linked to accessions in the GRIN database when possible.
The Southern Extension and Research Activities-Information Exchange Group-27 (SERA-IEG 27) was formed in 1996 and is sponsored by the Southern Association of Agricultural Experiment Station directors and Southern Extension directors. Representatives from extension and research programs from 13 land-grant universities and the US National Arboretum cooperatively evaluate ornamental plant germplasm adaptable to the southeastern United States. The objective is to identify, evaluate, select, and disseminate information on superior environmentally sustainable landscape plants for nursery production and landscape systems in the southeastern United States. The group is currently evaluating ∼20 woody plants, although evaluation reports are not yet available.
International Cultivar Registration Authorities.
Although not directly associated with germplasm collection, evaluation, or distribution, the International Cultivar Registration Authorities (ICRAs), appointed by the International Society for Horticultural Science, promote stability in the nomenclature of cultivars and cultivar groups within designated plant groups, and produce and promote authoritative checklists and registers of all names known to have been in use in such groups. Each ICRA records and registers cultivar and cultivar group names and collects information on the cultivar, including illustrative materials and evaluation data. An ICRA registrar may be an individual with superior knowledge of the taxon or may be a member of a plant society, governmental organization, botanic garden, or university. Some genera of woody ornamental landscape plants have their own registrar (e.g., boxwood, holly, and camellia), although most fall under the registration authority for unassigned woody plants. Although the ICRA does not conduct trials or judge the distinctness and superiority of one cultivar over another, the organization that sponsors the ICRA often participates in an evaluation of plant materials. A list of ICRAs can be found at www.ishs.org/icra/index.htm.
Priority Issues and Prospects for Future Development
Intellectual property
In recent decades, breeders and developers of new plants have sought and obtained greater protection of their intellectual property. In the United States, this protection is offered through the US Patent and Trademark Office (USPTO) through plant and utility patents and through the USDA as plant variety protection (PVP) certificates. The University of Tennessee Extension Institute of Agriculture published a bulletin titled, “A Green Industry Guide to Plant Patents and Other Intellectual Property Rights” (Fulcher et al. 2020). This extension bulletin provides detailed information, including examples concerning how intellectual property affects the germplasm used by the green industry.
Plant patents, plant variety protection, and utility patents can be used to protect various aspects of woody ornamental plants. Of these, utility patents, which protect genes or specific traits, could have the most impact on the acquisition, characterization, and enhancement of woody landscape plant germplasm in the future. This includes wild-origin germplasm collected under specific access agreements with domestic or foreign government bodies that prohibit patenting or protecting. Plant breeding efforts will be stymied if efforts to control the use of protected plants, genes, or traits continue.
Nature of woody landscape plants, particularly trees
Although not a monolithic crop, woody landscape plants are characterized by a suite of traits that present challenges to their curation, access, and development. Large and long-lived trees have space requirements that far exceed the typical crop plant. Many tree species require a decade or longer to reach reproductive maturity, delaying opportunities for breeding and seed regeneration. For some species, traits such as functional size require decades and several mature specimens planted in diverse environments to be properly characterized. Thus, there remain acute challenges for those cultivating or studying them: long-term commitments to large swaths of land that span several human careers.
Outreach
Germplasm is most valuable when it is actively used. The WLPCGC recommends raising awareness of germplasm resources available to researchers, breeders, and industry through the Plant Collections Network, BGCI, public gardens, and the USDA. In addition, communication about funding opportunities for the collection and evaluation of woody landscape plant genetic resources should be enhanced.
Germplasm inventory, acquisition, and distribution
Before recommendations can be made about which genera and species require collection, comprehensive inventories need to be developed, including those from NPGS (in GRIN-Global) and holdings associated with the PCN. This could also include a gap analysis of at-risk genera held by accredited collections. Standards for seed/plant collecting and distribution should be developed and disseminated to ensure that collecting trips result in germplasm of adequate genetic diversity, quality, and quantity to support long-term research and conservation.
Germplasm characterization and research
The WLPCGC recommends that evaluation and development efforts focus on underutilized North American trees and shrubs that have landscape potential. Phylogenetic research resulting in taxonomic resolution is also needed to facilitate effective breeding strategies, and population genetic studies are needed to evaluate diversity within and among populations of important woody landscape plant taxa. In addition, many important genera are missing information on fundamental attributes with immediate application to breeders, such as genome size and chromosome numbers. Characterization and evaluation of woody landscape plants focusing on broad adaptability, disease and pest resistance, and fecundity should also continue.
Maintain and enhance partnerships
APGA–PCN.
The task of maintaining woody plant germplasm cannot be fulfilled by any one organization. Continuing the strong partnership between ARS and APGA has several important benefits, including a more complete representation of the germplasm of managed taxa, more efficient germplasm management, back-up of germplasm at multiple sites, and coordinated efforts to develop continent-wide meta collections. However, at present there is not an up-to-date inventory of all accessions maintained across all Nationally Accredited Collections. In the future, resources should be directed toward increased support for coordination, database linkages with distributed querying, expanded institutional participation, and continued improvement of curatorial and evaluative expertise. We recommend that GRIN nomenclature be used if there is no established synonymy (www.ars-grin.gov/npgs/tax/index.html).
BGCI.
BGCI currently coordinates the Global Conservation Consortia (GCC) whose mission is to accelerate effective conservation of plant diversity and works very closely with APGA in recruiting member gardens to participate in the development of coordinated and global metacollections of woody species including Acer, Conifers, Dipterocarps, Ebenaceae, Erica, Magnolia, Nothofagus, Oak, Rhododendron, and Rosaceae. BGCI’s mission in action is aligned with the mission of the WLPCGC as they have identified strategic priorities and actions to conserve North America’s crop wild relatives and wild used plants (Khoury et al. 2019).
NRCS.
Although the Plant Materials Program already cooperates with the NPGS, further enhancement of this relationship is possible in the following areas: preservation of NRCS collections, submission of existing collections to NPGS, and collaborative funding and planning for collection and evaluation.
Other partnerships.
Several additional partnerships should be explored to leverage limited resources. These include strengthening collaboration with the US Forest Service, continuing to build partnerships with nursery growers and the arboriculture industry, and enhancing collaboration with universities. These partnerships could ensure the preservation of valuable germplasm that has been characterized and may otherwise be “retired” with staff turnover.
Conclusion
Without any doubt, there is value in the woody plants that comprise our managed landscapes—these plants provide social, physical, mental, environmental, and economic benefits. Based on our experience, what has been demonstrated historically, and what we forecast for the future, the trees and shrubs that populate our landscapes and natural ecosystems are vulnerable to a multitude of threats that include the humans they benefit. Plant genetic resources developed, curated, studied, enhanced, and shared through living collections held by the NPGS and public gardens around the world are positioned to meet and address existing and future challenges; however, success hinges on our willingness as a society to invest resources in the long-term sustainability of our natural and built ecosystems. Protecting urban, suburban, and natural forests depends on our capacity to maintain long-lived germplasm collections. For this, we must sustain widespread collaborative networks with diverse and broad expertise. To save and retain our forests—both the source and the product of our germplasm acquisition, research, and development—we need to better understand the trees and shrubs that comprise them.
References Cited
Aiello AS, Bachtell KR, Dosmann MS, Wang K. 2021. Tree of the year: Acer griseum, the paperbark maple. International Dendrology Society, Yearbook. 2020:26–54.
Bachtell KR, Aiello AS, Dosmann MS, Wang K. 2019. Paperbark maple (Acer griseum) conservation project. Comb Proc Int Plant Propagators Soc. 69:87–93.
Bell JF, Wilson JS, Liu GC. 2008. Neighborhood greenness and 2-year changes in body mass index of children and youth. Am J Prev Med. 35(6):547–553. https://doi.org/10.1016/j.amepre.2008.07.006.
Brzuszek RF, Harkess RL, Kelly L. 2010. Survey of master gardener use of native plants in the southeastern United States. HortTechnology. 20(2):462–466. https://doi.org/10.21273/HORTTECH.20.2.462.
Federal Executive Order No. 13,112. 1999. 3 CFR 159–163. https://www.govinfo.gov/content/pkg/CFR-2000-title3-vol1/pdf/CFR-2000-title3-vol1-eo13112.pdf.
Foster A, Dunham IM, Bukowska A. 2024. An environmental justice analysis of urban tree canopy distribution and change. J Urban Aff. 46(3):493–508. https://doi.org/10.1080/07352166.2022.2083514.
Fulcher A, Fessler L, Stackhouse T. 2020. A green industry guide to plant patents and other intellectual property rights. University of Tennessee Institute of Agriculture Bull. PB 1882.
Gaudon JM, McTavish MJ, Hamberg J, Cray HA, Murphy SD. 2022. Noise attenuation varies by interactions of land cover and season in an urban/peri-urban landscape. Urban Ecosyst. 25(3):811–818. https://doi.org/10.1007/s11252-021-01194-4.
Kaplan R, Kaplan S. 1989. The experience of nature: A psychological perspective. Cambridge University Press, New York, NY, USA.
Kauth PJ, Pérez HE. 2011. Industry survey of the native wildflower market in Florida. HortTechnology. 21(6):779–788. https://doi.org/10.21273/HORTTECH.21.6.779.
Khachatryan H, Hodges AW, Hall CR, Palma MA. 2020. Production and marketing practices and trade flows in the United States green industry, 2018. South Coop Ser Bull. 421 2020.
Khoury CK, Greene SL, Krishnan S, Miller AJ, Moreau T. 2019. A road map of conservation, use, and public engagement around North America’s crop wild relatives and wild utilized plants. Crop Sci. 59(6):2302–2307. https://doi.org/10.2135/cropsci2019.05.0309.
Kuo FE. 2003. The role of arboriculture in a healthy social ecology. AUF. 29(3):148–155. https://doi.org/10.48044/jauf.2003.018.
Landry SM, Chakraborty J. 2009. Street trees and equity: Evaluating the spatial distribution of an urban amenity. Environ Plan A. 41(11):2651–2670. https://doi.org/10.1068/a41236.
Ma B, Hauer RJ, Wei H, Koeser AK, Peterson W, Simons K, Timilsina N, Werner LP, Xu C. 2020. An assessment of street tree diversity: Findings and implications in the United States. Urban Forestry & Urban Greening. 56:126826. https://doi.org/10.1016/j.ufug.2020.126826.
Maroko AR, Maantay JA, Sohler NL, Grady KL, Arno PS. 2009. The complexities of measuring access to parks and physical activity sites in New York City: A quantitative and qualitative approach. Int J Health Geogr. 23:1–23.
NOAA. 2024. Assessing the U.S. climate in 2023: A historic year of U.S. billion-dollar weather and climate disasters; 2023 was the fifth-warmest year on record for the contiguous U.S. National Centers for Environmental Information website, https://www.ncei.noaa.gov/news/national-climate-202312#:~:text=The%20average%20annual%20temperature%20of,third%20of%20the%20historical%20record.
Nowak DJ, Greenfield EJ. 2018. US urban forest statistics, values, and projections. J Forestry. 116(2):164–177. https://doi.org/10.1093/jofore/fvx004.
Pokorny K. 2023. “Don’t cut down that ash tree just yet–Oregon won’t see full effect of emerald ash borer for years.” Today, Oregon State University. https://today.oregonstate.edu/news/don%E2%80%99t-cut-down-ash-tree-just-yet-%E2%80%93-oregon-won%E2%80%99t-see-full-effect-emerald-ash-borer-years. [accessed 3 Oct 2023].
Pataki DE, Carreiro MM, Cherrier J, Grulke NE, Jennings V, Pincetl S, Pouyat RV, Whitlow TH, Zipperer WC. 2011. Coupling biogeochemical cycles in urban environments: Ecosystem services, green solutions, and misconceptions. Front Ecol Environ. 9(1):27–36. https://doi.org/10.1890/090220.
Rehder A. 1940. Manual of cultivated trees and shrubs hardy in North America: exclusive of the subtropical and warmer temperate regions (2nd ed). The Macmillan Company, New York, NY, USA.
Richardson DM, Rejmánek M. 2011. Trees and shrubs as invasive alien species – a global review. Diversity Distrib. 17(5):788–809. https://doi.org/10.1111/j.1472-4642.2011.00782.x.
Schwarz K, Fragkias M, Boone CG, Zhou W, McHale M, Grove JM, O’Neil-Dunne J, McFadden JP, Buckley GL, Childers D, Ogden L, Pincetl S, Pataki D, Whitmer A, Cadenasso ML. 2015. Trees grow money: Urban tree canopy cover and environmental justice. PLoS One. 10(4):e0122051. https://doi.org/10.1371/journal.pone.0122051.
Tallamy DW. 2009. Bringing nature home: How you can sustain wildlife with native plants. Timber Press, Portland, OR, USA.
Turner-Skoff JB, Cavender N. 2019. The benefits of trees for livable and sustainable communities. Plants People Planet. 1(4):323–335. https://doi.org/10.1002/ppp3.39.
USDA-NASS. 2020. Census of horticultural specialties (2019). https://www.nass.usda.gov/Publications/AgCensus/2017/Online_Resources/Census_of_Horticulture_Specialties/HORTIC.pdf. [accessed 3 Oct 2023].
USDA. National PLANTS Database. https://plants.usda.gov/home. [accessed 3 Oct 2023].
White P. 2002. Linking ecology and horticulture to prevent plant invasions: an introduction to the St. Louis declaration and the codes of conduct. Southeast Exotic Pest Plant Council Wild Winter. https://se-eppc.org/pubs/ww/stlouisWinter2002.pdf.
White A, Fant JB, Havens K, Skinner M, Kramer AT. 2018. Restoring species diversity: Assessing capacity in the U.S. native plant industry. Restoration Ecol. 26(4):605–611. https://doi.org/10.1111/rec.12705.
Williamson M, Fitter A. 1996. The varying success of invaders. Ecology. 77(6):1661–1666. https://doi.org/10.2307/2265769.
Wolf KL. 2009. More in store: Research on city trees and retail. Arborist News. 18:22–27.
Wolf KL, Robbins A. 2015. Metro, environmental health, and economic value. Environ Health Perspect. 123(5):390–398. https://doi.org/10.1289/ehp.1408216.
Wolfe MK, Mennis J. 2012. Does vegetation encourage or suppress urban crime? Evidence from Philadelphia, PA. Landscape and Urban Planning. 108(2-4):112–122. https://doi.org/10.1016/j.landurbplan.2012.08.006.