FOREST TREES OF THE PACIFIC SLOPE. 239 



equipped for effecting a wide distribution of its representatives by seeding. 

 The lightness and abundance of poplar seed doubtless accounts for the ubiqui- 

 tous presence of poplars wherever moist ground is available in all situations 

 from sea level to nearly 10,000 feet elevation, but especially in cold, far northern 

 regions. 



The wood of the poplars is light, soft, and straight-grained. The grain is 

 commonly fine, but is often coarse as a result of the rapid diameter growth 

 characteristic of these trees. Its color is from whitish to a light, sometimes 

 yellowish, brown. The wood of most species is brittle, unstable, and Lndurable, 

 but that of a number of them is nevertheless of great commercial value for 

 lumber and paper pulp. Formerly poplar wood had no economic importance, 

 but is now rapidly coming into wider and wider use, both for pulpwood and to 

 take the place of other woods, the supply of which is decreasing. "Yellow pop- 

 lar" (tulip-tree), which some of the poplars resemble in grain and in the ease 

 with which they can be worked, is one of the woods for which the poplars supply 

 substitutes. 



The poplars are important to the forester especially for maintaining tree 

 growth on stream bottoms where few other trees naturally grow. They produce 

 a forest cover and useful timber in from twenty-five to fifty years, while, like the 

 willows, the ease and convenience with which they can be grown from root and 

 branch cuttings and from cut stumps — even from stakes set in damp soil — 

 renders artificial propagation particularly simple. They attain maturity in 

 from 100 to 200 years, most of them within a century, and then begin to show 

 signs of arrested growth, but on account of their great vitality and recuperative 

 power some species may continue to grow for a much longer time, repairing 

 broken trunks and other injuries to which their brittle stems are subject. 



Ten poplars occur within the United States and adjacent Canadian territory, 

 and 4 of these inhabit the Pacific region ; but 3 of the latter also extend far out- 

 side the Pacific region, to the north and east. 



The poplars are of very ancient origin. Remains show that they existed 

 among the earliest tree forms of the Lower Cretaceous period in Greenland, and 

 that many different species inhabited the central portion of this continent in 

 the same epoch, and existed also in the Tertiary and Miocene periods in this 

 country and in Europe. 



Aspen. 

 Populus tremuloides Michaux. 



DISTINGUISHING CHARACTERISTICS. 



Aspen is the best known and most extensively distributed of our trees. Its 

 conspicuously whitish, smooth, straight trunks and small, trembling leaves dis- 

 tinguish it from its associates. It is from 60 to 80 feet high and from 14 to 20 

 inches in diameter, more commonly from 30 to 40 feet high and from 8 to 12 

 inches through. In high exposed places it is small, with bent or almost pros- 

 trate stems; elsewhere the trunks are straight, unbranched. except near the 

 sunfmit, and of an apparently uniform diameter for one-half or two-thirds of 

 their length. The short, slender, irregularly bent limbs stand out straight from 

 the Stem in a narrow dome-like crown, which is long in open stands and short 

 in dense growths, in which two-thirds of the stem may be clear of branches. 

 The hard, firm bark is little broken except near the ground. Near the ground 

 it is broken and blackish, and. on large trunks, is nearly 2 indies thick; higher 

 up it is thinner. Frequent black, rounded protuberances and curved, scar-like 

 marks characterize the trunks. 



