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ARS Home » Northeast Area » Washington, D.C. » National Arboretum » Floral and Nursery Plants Research » Research » Research Project #425386

Research Project: Taxonomy of Landscape Trees and Shrubs

Location: Floral and Nursery Plants Research

2015 Annual Report

The long-term objectives of this project are 1) to improve the understanding of interrelationships and the distribution of genetic variation (especially genome size and ploidy level) in understudied groups of specialty crops (primarily woody landscape plants), 2) to further develop the National Arboretum as a scientific resource, and 3) to investigate emerging and suspected invasive woody plants among groups of interest to the nursery industry and to National Arboretum plant introduction programs. These goals will be achieved through the following specific objectives: Objective 1: Identify and characterize genetic resources in understudied priority groups of woody landscape plants, including, but not limited to, Ulmus, Celtis, and Carya, to elucidate their genetic relationships and variation within species. Sub-objective 1.A. Determine taxonomic and genetic relationships among selected species of elms (Ulmus). Sub-objective 1.B. Determine ploidy level and comparative genome size in Carya using flow cytometry. Sub-objective 1.C. Determine the distribution of apomixis and polyploidy in North American species of hackberry (Celtis) and their taxonomy and relationships. Sub-objective 1.D. Resolve taxonomic problems and determine the amount and apportionment of genetic diversity in selected white oaks (Quercus sect. Quercus). Sub-objective 1.E. Carry out research projects to meet needs for taxonomic research expressed by stakeholders, such as the production of regional floras and botanical manuals in collaboration with other scientists and horticulturists. Objective 2: Maintain and enhance the National Arboretum Herbarium as a scientific resource and a collection documenting important agricultural research and germplasm. Sub-objective 2.A. Maintain and enhance the U.S. National Arboretum Herbarium as a scientific resource and a collection documenting important agricultural research and germplasm. Sub-objective 2.B. Maintain and enhance the U.S. National Seed Herbarium as a scientific resource and a collection documenting important agricultural research and germplasm. Objective 3: Determine the identity and basic biological characteristics of selected invasive woody plants in the United States. Apply herbarium records of woody plant introduction and naturalization in the mid-Atlantic area to test statistical models for predicting characteristics contributing to invasive behavior in woody plants.

Investigations will utilize a variety of different data types, primarily morphological data, DNA sequence variation, and variation in single-locus DNA markers within variable sites in the genome, analyzed using phylogenetic analyses and other multivariate statistical methods. Organisms will be studied in the field, herbarium, laboratory, and garden.

Progress Report
For elms (Objective 1A), additional SSR data was gathered for representative diploid and tetraploid American elm populations, and flow cytometry was carried out on 180 seedlings from trees in an additional natural mixed stand of the two ploidy levels, confirming previous studies indicating that diploid and tetraploid plants are genetically distinct and do not interbreed with one another when they grow together. Our improved understanding of the Ulmus americana complex allows us to selectively target promising populations for future disease-resistance testing. In addition, the first full data set for phylogenetic analysis of the genus Ulmus as a whole, a RADseq analysis, was produced under a cooperative agreement with a collaborator at the Morton Arboretum. This project will improve our ability to use hybridization and selection to produce nursery trees with desirable characteristics. For the genus Carya (Objective 1B), data on nuclear DNA content was acquired for representative plants of 16 of the 19 species of Carya (pecans and hickories), plus several related genera, in collaboration with an ARS scientist at the USDA Pecan Germplasm Repository. These data improve our understanding of relationships and potential for crossbreeding in this economically important group of trees. For the genus Celtis (Objective 1C), flow cytometry was carried out on representative populations, but planned herbarium visits had to be postponed because of agency-mandated travel restrictions. For the oaks (Objective 1D), field work was carried out in the southeastern and south-central states to collect material of selected white oaks (Quercus subgenus Quercus) for preliminary analysis, and assess some underutilized species for traits of horticultural significance. A cooperative agreement was established with the Morton Arboretum to work jointly on genetic analyses of white oak groups of interest. For other projects (Objective 1E), treatments of two families were completed for the Flora of Oregon. We also worked on treatments for the Flora of North America and Vascular Plants of the Northeastern United States; monitored phenology of representative woody plants in collaboration with an intercontinental consortium of botanical gardens; and discussed collaboration on research to better characterize variation and classification of North American Fraxinus species threatened by the Emerald Ash Borer. A revision of the taxonomy of Catalpa worldwide is in progress. Activity in the herbarium and seed herbarium (Objective 2) included continuing exchange with other herbaria worldwide, loaning of our specimens to qualified scientists, hosting visiting scientists, and continued mounting, filing, and databasing of specimens. For weed risk assessment (Objective 3), extensive data was collected on biological characteristics and native ranges of known invasive and non-invasive exotic woody plants cultivated in the mid-Atlantic states.