Keyword searches of genus and species using Web of Science, AGRICOLA, and CAB Abstracts were conducted in January and February 2020. Search queries were conducted in English, using contemporary taxonomy (genus and species identifiers and their surrogates), as suggested through Silvics of North America (Burns and Honkala 1990) and the U.S. Department of Agriculture PLANTS database. The initial set of tree species were derived from SNA and supplemented with species that were identified as having a tree-like growth quality from the U.S. Department of Agriculture PLANTS database. Duplications of genus and species identifiers were resolved by carefully examining each genus/species combination. Some tree species were combined as a result into one instance of a species, when they used the same genus/species combination. Other tree species had to be carefully evaluated to avoid double-counting search results. For example, Pinus monophylla (western white pine) is also known as a variant of another species, Pinus edulis var. fallax. Therefore, the query for western white pine contained a criterion using the Boolean operator OR (“Pinus monophylla” or “Pinus edulis var. fallax”). However, when Pinus edulis (twoneedle pine) was assessed, it required a criterion using the Boolean operator NOT (“Pinus edulis” not “Pinus edulis var. fallax”). Common names were considered for inclusion in the queries using the Boolean operator OR (e.g., OR “common name”), however several tree species shared common names and the query results could not be disentangled. The term “North America” was not used as a keyword (as topic or in “all fields” in these queries as it acted to severely limit the results. In fact, in limited tests using this term, no results were returned when queries included AND “North America”.

A ranking of North American tree species based on keyword searches of genus and species using Web of Science, AGRICOLA, and CAB Abstracts. A bibliographic analysis using three bibliographic databases was conducted to understand the importance of North American tree species in literature published since 1900 and since the development of the last Silvics of North America in the 1980s. The Silvics of North America is the most comprehensive guide to North American tree-like species containing about 200 of the 696 tree-like species found in the literature. Depending on the bibliographic database, over 100,000 to over 600,000 publications on the 696 plant species can be obtained. At least half of the publication records are from 1985 forward. The ranking in this database is based on the average rank of six bibliographic searches (three bibliographic databases and two time frames: 1900-2020 and 1985-2020).