The Vital Role of Woody Landscape Plants
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.
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. 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.
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.
Threats and Vulnerabilities
Despite the vital role of woody landscape plants, 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.
“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.“
Multiple genera are now under attack from introduced pests and diseases, with devastating losses to natural and urban forests. Examples include the emerald ash borer, the imported woolly adelgid, and the spotted lanternfly, which have spread rapidly across the country. In addition to these well-known threats, monitoring for future biotic risks must be ongoing as host preferences change and novel pests continue to be imported.
Abiotic stresses are also expected to become more pronounced as a result of climate change, posing a threat to woody landscape plant germplasm in forests. The annual 2023 climate summary reported that mean temperatures were consistently and significantly higher than the 20th-century average, making 2023 the fifth warmest year on record in the contiguous United States. As weather patterns fluctuate, environments and the plants growing in them will continue to experience extremes in precipitation (drought and flooding) and temperature (highs, lows, and atypical seasonal fluctuations).
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 compacted and saline soils, air pollution, and lack of care. These impacts are compounded when novel, introduced biological stressors inflict trees growing in marginal, stress-prone urban landscapes.
Maintaining Genetic Diversity
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 Woody Landscape Plant Crop Germplasm Committee (WLPCGC) plays a key role in identifying vulnerabilities and threats, and making recommendations for prioritizing germplasm acquisition, evaluation, research, and management.
Maintaining woody landscape plant germplasm exposes additional logistical vulnerabilities that are distinct from many other crops. Species with recalcitrant seeds that cannot be stored using traditional methods may require alternative preservation techniques, such as 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, but these collections require large amounts of space and represent only a portion of extant genetic diversity.
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. Within the USDA’s National Plant Germplasm System (NPGS), entities like the National Germplasm Resources Laboratory, National Laboratory for Genetic Resource Preservation, and various plant germplasm repositories play active roles. Outside the NPGS, organizations such as the USDA Natural Resources Conservation Service, USDA Forest Service, American Public Gardens Association, and private/university collections also hold significant woody landscape plant germplasm.
Evaluation and Development Efforts
The value of germplasm is often not fully realized until it has been characterized or evaluated in field or landscape settings. Many botanic gardens, universities, and nurseries conduct these evaluations, often focused on the potential value of an accession as a cultivar rather than its genetic potential in a long-term breeding program. Standardized databases containing pertinent evaluation data, such as the North Central Regional Ornamental Plant Trials and the Southern Extension and Research Activities-Information Exchange Group-27, help to coordinate these efforts.
Breeders and developers of new plants have also sought greater protection of their intellectual property through plant patents, plant variety protection certificates, and utility patents. While these legal protections can incentivize breeding programs, they may also limit access to germplasm and stymie efforts to expand the genetic diversity of landscape plants.
Recommendations and Priorities
The WLPCGC has identified several key recommendations to address the threats and vulnerabilities facing woody landscape plants:
- Raise awareness of germplasm resources: Increase communication about the availability of germplasm resources and funding opportunities for collection, evaluation, and research.
- Develop comprehensive inventories: Create detailed inventories of woody landscape plant germplasm holdings, including those from the NPGS, public gardens, and private/university collections, to identify gaps and prioritize acquisition efforts.
- Establish standards for collecting and distribution: Develop and disseminate guidelines to ensure that collecting trips result in germplasm of adequate genetic diversity, quality, and quantity to support long-term research and conservation.
- Focus on underutilized North American species: Prioritize evaluation and development efforts on promising native trees and shrubs that have landscape potential but are currently underutilized.
- Expand collaborative partnerships: Strengthen existing partnerships and explore new collaborations to leverage limited resources and ensure the preservation of valuable germplasm.
By addressing these priorities, the horticultural community can work to safeguard the genetic diversity of woody landscape plants and develop more resilient, sustainable landscapes that continue to provide invaluable ecosystem services and societal benefits.
Conclusion
Woody plants are the foundation of our managed landscapes and natural ecosystems, providing a wealth of social, environmental, and economic benefits. However, these vital resources face a multitude of threats, from pests and diseases to abiotic stressors driven by climate change. To secure the future of our forests, both urban and rural, we must invest in the long-term sustainability of woody landscape plant genetic resources through collaborative efforts in collection, evaluation, and conservation.
By working together to expand taxonomic diversity, characterize germplasm, and develop resilient cultivars, the horticultural community can ensure that the trees and shrubs that populate our landscapes continue to thrive and deliver their essential services for generations to come. The Local Builder London blog is committed to sharing practical insights and inspiring readers to create more sustainable, resilient built environments that harmonize with the natural world. For more information, please visit our website.
