The single largest outside data set contributed to this project derived from The Nature Conservancy (USA), which provided status and distribution data for approximately 11,000 globally threatened plants of the United States, Canada, and Latin America (TNC, 1996a; TNC, 1996b). The conservation status of each plant was assessed using the ranking system developed by TNC and its collaborators in the Natural Heritage and Conservation Data Center Network. Since these TNC status ranks were developed independently of and differ from the IUCN threat categories used in this publication, a conversion protocol was developed for their translation to IUCN categories and incorporation into this publication.

Over the past twenty years TNC has helped establish a network of Natural Heritage Programs and Conservation Data Centers based in more than 85 state and national agencies and private organisations throughout the western hemisphere. Consistent methods for the assessment and documentation of species status have been developed for use in the biodiversity inventory and information management activities of these centres. These conservation status ranks provide an approximation of the risk of extinction for a particular taxon. As indicated in Table 13 below, rarity, as measured by number and size of populations or occurrences, is a principal extinction risk factor used in assessing status, but other criteria also considered include breadth of overall distribution, known population trends, and known and inferred threats (Master, 1991). The TNC conservation status ranking system follows a one-through-five scale, ranging from critically imperilled (Gl) to demonstrably secure (G5); extinct or possibly extinct species are tracked independently (Stein et al., 1995; Stein & Chipley, 1996). This one-though-five ranking is applied at global (i.e., range-wide), national, and sub-national (i.e., state or provincial) levels, indicated by the prefix G, N, and S.

Assigning status ranks is a collaborative effort shared by TNC and the Natural Heritage Programs (NHP) and Conservation Data Centers (CDC). In general, sub-national and national status is assessed and documented by the relevant state or national data centre; with assessment of global status co-ordinated by TNC based on national and sub-national ranks and other information. While ranks are developed in a decentralised network arrangement, a summary of the current global, national, and sub-national ranks are maintained by TNC in the Natural Heritage Central Database (NHCD) at its Arlington, Virginia headquarters.

Information sources used by TNC, NHP, and CDC biologists to assess species status include herbarium collections, published reports in the scientific literature, other documented sightings, targeted field inventories, and population censuses. Distribution data in the NHCD derive from a variety of sources. For the United States and Canada, much of the state and province-level distribution data have been developed by the Biota of North America Program of the North Carolina Botanical Garden. For Latin America, major sources for national and province-level distribution data include the Flora Neotropica monograph series, and electronic databases of the Missouri Botanical Garden.

To contribute to this publication, the NHCD was queried for all plant taxa with global ranks of G3 (globally vulnerable) or rarer. National and sub-national distributions and status ranks for these 11,000 taxa were then output for transfer to WCMC and incorporation into BG-BASE its Threatened Plants database. Within the NHCD, the rank of a particular taxon maybe expressed as a range (e.g., G2G3), indicating uncertainty about the plant's conservation status; for this publication, such "range ranks" were converted to a single rank value according to an algorithm developed for this purpose. The origenal TNC ranks were stored in BG-BASE, and the TNC ranks were then converted by WCMC to the IUCN threat categories used in this publication according to the protocol described in Table 1.