10 MISCELLANEOUS CIRCULAR 92, U. S. DEPT. OF AGRICULTURE 



NATURALIZED TREES 



It has become necessary from time to time to add to our indigenous 

 arborescent flora foreign trees that have escaped from cultivation 

 and established themselves without the aid of man. Such new- 

 comers are termed " naturalized." A number of them are very 

 aggressive and persistent growers. Notably so are the Ailanthus, 

 Paper Mulberry, White Mulberry, Pear, Apple, Peach, Cherry, 

 European White Poplar, and others, some of which are veritable 

 nuisances. Many such adventive trees have found suitable soil and 

 climatic conditions within our borders and are abundantly able to 

 take care of themselves in competition with native trees. Later 

 observations may show that some of these exotics, now believed to 

 be naturalized, are unable permanently to maintain themselves here. 



A few authorities are somewhat reluctant to admit as members of 

 our tree flora all veritably naturalized exotics; particularly is this 

 true of exotics that have found apparently permanent lodgment in 

 tropical portions of the United States. For the purpose especially of 

 acquainting students with the existence of all of the trees to be met 

 with in the wild state, the writer has, with few exceptions, included 

 all exotics that reliable evidence shows to be naturalized here. It 

 would seem that this rather extensive admission of foreign species 

 does not in any way detract from the importance or standing of our 

 native trees. As here catalogued no confusion of foreign and native 

 trees can arise, because the adventive nature of foreign trees is uni- 

 formly noted in their "Range." 



DEFINITION OF A TREE 



It is difficult satisfactorily to draw the line between a tree and a 

 shrub. Difference of opinion regarding this mooted question has, 

 as the case may be, increased or decreased the number of recorded 

 arborescent species and varieties. Judgment as to when a plant is 

 to be called a tree and when a shrub appears to be based chiefly 

 on tha size, height, and diameter attained. Of course, differences 

 of judgment are likely to arise only with regard to woody plants that 

 are close to the imaginary line drawn between trees and shrubs. 

 In order to have a workable definition of a tree it has been necessary 

 arbitrarily to assume a minimum height, diameter, and form of 

 development, outside of which a woody plant can not be considered 

 arborescent. The general rule applied by the author in defining a 

 tree includes woody plants having one well-defined stem and a more 

 or less definitely formed crown, and attaining a height of at least 8 

 feet and a diameter of not less than 2 inches. Most truly arborescent 

 plants produce a single erect or ascending trunk. Some species of 

 trees, however, have the habit of producing several trunks from the 

 same root. Examples of this type of growth are to be found among 

 the willows, some of which, on account of their large size, obviously 

 are properly classed as trees. 



The fact that some species admitted as arborescent are generally 

 shrubby in size has not seemed to the writer a sufficient reason 

 for excluding them from our tree flora, for they occasionally become 

 trees in some part of their range, even though it be outside our border. 

 A notable example of this type in many respects is the so-called 

 Dwarf Juniper {Juniperus communis Linnaeus), which is world-wide 



