Horticulture as a conservation tool for rare plants: examples from North America

Horticultural use of uncommon trees and shrubs in managed landscapes represents a conservation tool for species that are rare and/or threatened in their natural habitats. Researchers in North America with dual interests in horticultural science and ecology have focused on indigenous species that meet several aesthetic, physiological, and ecological criteria. The goals are to protect wild populations, to raise the profile of these species in horticulture, and to gain insights regarding the ecology and genetics of populations that often are disjunct or otherwise fragmented. My objective for this paper is to summarize research conducted in collaboration with students and colleagues on the ecology, genetics, propagation, and horticultural merits of rare North American trees and shrubs. Work with Alnus maritime led to a revised taxonomic treatment of the species, to an understanding and protection of its niche in nature, to characterization of its association with nitrogen-fixing Frankia bacteria, and to selection of a cultivar, ‘September Sun’. Population genetics, reproductive biology, cold hardiness, and propagation have been the foci of studies with rare shrubs in the genus Dirca. The ecological status of the wetland species Leitneria floridana was characterized in its disjunct populations, and protocols for propagating this monotypic species from seeds were determined. The potential for unusually pronounced cold hardiness of a small, northern-disjunct population of Styrax americanus is being examined along with its photosynthetic responses to heat. Research with these species illustrates how scientists can blend horticultural and ecological objectives to facilitate the conservation of rare trees and shrubs in the wild while exploring the potential for use of these plants in landscaping.