Definitions 



Trees may be defined as woody plants having one erect 

 perennial stem or trunk at least 3 in (7.5 cm) in diameter 

 at breast height (d.b.h. or at 4.5 ft or 1.3 m), a more or 

 less definitely formed crown of foliage, and a height of at 

 least 13 ft (4 m) (75, p. 14). Some small trees that are 

 often shrubby and shrubby species rarely reaching the 

 minimum tree size have been included. 



Tree species are less numerous than those of smaller 

 plants on lists of proposed endangered plants. Of course, 

 tree species make up only a fraction of the seed plant 

 species listed in a local flora, as high as one-fourth in the 

 tropics and as low as one-tenth or less in temperate 

 regions. Trees are conspicuous and easily located, 

 watched, and mapped. Their large size and long life af- 

 ford some protection against grazing animals and various 

 hazards. They reproduce by seeds and from roots over a 

 period of years. If endangered, they can be propagated 

 readily by seeds or vegetative shoots and distributed to 

 botanical gardens, parks, and arboreta. 



A rare species is one that has a small population in its 

 range, which may be restricted or widespread. The term 

 local species is used here for one of relatively small range, 

 but which is sufficiently common not to be called rare. A 

 species whose range is limited to a particular named area, 

 usually small, is cited as endemic. 



Border or peripheral species reach the limit of their 

 natural range a short distance into the United States, 

 where they may also be classed as rare or local. They may 

 be rare at the edges of their ranges but not in danger of 

 extinction because of occurrence in greater numbers else- 

 where. A species could become extinct in the United 

 States and still be common in a neighboring country. 



Some rare and local tree species are also on the lists of 

 proposed endangered and threatened species compiled by 

 the Smithsonian Institution (21). However, border or 

 peripheral species generally are excluded by definition. 



Two administrative or legal definitions may be quoted 

 from the Endangered Species Act. "The term 'en- 

 dangered species' means any species which is in danger of 

 extinction throughout all or a significant portion of its 

 range. ..." The report by the Smithsonian Institution 

 (21, p. 17) adds: "Existence may be endangered because 

 of the destruction, drastic modification, or severe curtail- 

 ment of habitat, or because of overexploitation, disease, 

 predation, or even unknown reasons. Plant taxa from 

 very limited areas, e. g., the type localities only, or from 

 restricted fragile habitats usually are considered en- 

 dangered. Survival of an endangered species requires 

 assistance and additional protection." 



"The term 'threatened species' means any species 

 which is likely to become an endangered species within 

 the foreseeable future throughout all or a significant por- 

 tion of its range." Included are species categorized as 

 rare, very rare, or depleted. These species are to be 

 watched. 



Recently extinct or possibly extinct species are "those 

 species of plants no longer known to exist after repeated 

 search of the type localities and other known or likely 

 places. Some species may be extinct in the wild but 

 preserved by cultivation in gardens. . . ." (27). (Of 

 course, the reference is to the modern or recent flora 

 named from collections of living plants and not to the 

 wholly extinct fossil floras.) Species that have disap- 

 peared from one island, but not others, are not extinct. 



