50 FORESTRY INVESTIGATIONS U. S. DEPARTMENT OF AGRICULTURE. 



developed although far from being the most varied of this continent, is characterized by the red- 

 wood, the tidelaud or Sitka spruce, hemlock, and giant arbor vita 1 . 



Broad-leaved trees are not absent, but so little developed in comparison with the mighty conifers 

 that they play no conspicuous part except along the river bottoms, where maples, cotton wood, ash, 

 and alder thrive, and in tlie narrow interior valleys and slopes an open growth of oak is found. 

 Toward the south and on the lower levels the broad-leaved trees again become evergreen as on 

 the Atlantic side, and with a new tribe of pines, with large hooked cones added, form a subtropic 

 flora. 



Finally to the south, analogously to the extension of the tropical West Indian flora in Florida, 

 we find a northern extension of the Mexican forest, mingled with which species from the Pacific 

 forest on the west and from the Atlantic on the east. The mesquite and some acacias, the tree 

 yuccas, and the giant or tree cactus are perhaps the most characteristic and remarkable species 

 of the deserts of this region, while the high mountains support dense forests of firs and pines. 



This distribution of forest types is exhibited on the accompanying map. Besides the botanical 

 and geographical interest it has an eminently practical interest to the forester, because it shows 

 him the limits within which he may expect to produce satisfactory results with the species of trees 

 composing the forest in each section. 



While a vast territory on the Atlantic side and a narrower belt on the Pacific coast, connected 

 by a broad belt through the northern latitudes, bears the forest growth thus differentiated, with 

 the crest and slopes of the Hocky Mountains forming an intermediate extension from the Northern 

 belt, there is a vast empire in the interior without forest growth, although not entirely without 

 tree growth, the prairies and plains. 



Of parts of this territory we feel reasonably certain from strong evidences that the forest once 

 occupied them, but has been driven off by aboriginal man, the firebrand taking sides with the 

 grasses, and the buffalo probably being a potent element in preventing reestablishment. In other 

 parts it is questionable whether the lines along the river courses, the straggling trees on the pla- 

 teaus and slopes, are remnants of a vanquished army or outposts of an advancing one. In some 

 parts, like the dry mesas, plateaus, and arroyos of the interior basin, and the desert like valleys 

 toward the southern frontiers, it may reasonably be doubted whether arborescent flora has more 

 than begun its slow advance from the outskirts of the established territory. 



Certain it is that climatic conditions in these forestless regions are most unfavorable to tree 

 growth, and it may well be questioned whether in some parts the odds are not entirely against the 

 progress of the forest. 



Temperature and moisture conditions of air and soil determine ultimately the character of 

 vegetation, and these are dependent not only on latitude, but largely on configuration of the laud, 

 and especially on the direction of moisture-bearing winds with reference to the trend of mountains. 



The winds from the Pacific Ocean striking against the Coast Range are forced by the compres- 

 sion and subsequent cooling to give up much of their mojsture on the windward side; a second 

 impact and further condensation of the moisture takes place on the Cascade 1'ange and Sierra 

 Nevada. On descending, with consequent expansion, the wind becomes warmer and drier, so that 

 the interior basin, without additional sources of moisture and, no additional cause for condensation, 

 is left without much rainfall and with a very low relative humidity, namely, below 50 per cent. 

 The liocky Mountains finally squeeze out whatever moisture remains in the air currents, which 

 arrive proportionally drier on the eastern slope. This dry condition extends over the plains until 

 the moist currents from the Gulf of Mexico modify it. Somewhat corresponding, yet not quite, to 

 this distribution of moisture, the western slopes are found to be better wooded than the eastern, 

 and the greater difficulty of establishing a forest cover here must be admitted; yet since the forest 

 has the capacity of creating its own conditions of existence by increasing the most important 

 factor of its life, the relative humidity, the extension of the same may only be a question of time. 



Temperature extremes, to be sure, also set a limit to tree growth, and hence the so-called tim- 

 ber line of high mountains, which changes in altitude according to the latitude. 



If, now, we turn our attention from the phyto- geographic consideration of the forest cover to 

 the botanical features, we may claim that the North American forest, with 450 or more arborescent 

 species, belonging to 158 genera, many of which are truly endemic, surpasses in variety of useful 

 species and magnificent development any other forest of the temperate zone, Japan hardly excepted. 



