180 THREATENED ARIDITY ON THE PACIFIC SLOPE 



into Oregon. At its extreme southern end it is represented by 

 small scattered groups of trees — a few hundred individuals only 

 are reported — and a long gap intervenes before its appearance 

 farther north. The heaviest stands of the species are found in 

 Mendocino and Humboldt counties, in California. It thins out 

 toward the Oregon line and finall} 7 disappears a few miles north 

 of the boundary. The northward extension of the species is 

 evidently limited b} 7 a mean annual temperature lower than its 

 ultimate point of endurance. Southern extensions are impos- 

 sible, owing to an insufficiency of rainfall in that section of Cal- 

 ifornia, and its spread into the interior, away from the proximity 

 of the ocean, is precluded by adverse conditions of both temper- 

 ature and'humidity. The reproduction of the species is said to 

 be very low. Cut-over areas show no evidences of reforestation 

 with the same, species. Thus hemmed in by inimical climatic 

 conditions and unable to maintain its stands, its extinction seems 

 assured at no very remote period. 



SUMMARY 



The salient points brought out by a study of the forest condi- 

 tions in these regions, so far as they relate to the effects of climatic 

 aridity, can be stated concisely as follows : 



The arid, non-forested plains regions of eastern Oregon yield 

 silicified remains of arborescent vegetation identical or nearly so 

 with existing species on adjacent areas, proving the presence of 

 forest growth on these timberless lands at no very remote period. 



The forests on the semi-arid tracts, although consisting of 

 species capable of enduring dry climatic conditions, show every- 

 where a persistent and gradual dwindling in extent and density. 

 Their stands, consisting mostly of old trees, show a conspicuous 

 deficiency in seed production, an enormous percentage of the 

 ovules aborting, and a notable scarcity of seedlings. When, 

 from any cause, a tract of the old stands is deforested, reforesta- 

 tion does not occur, as a rule; this results in the formation 

 of detached groves and individuals whose reproductive powers 

 become even more limited and weakened and the extinction of 

 which is merely a matter depending on the age limit of the in- 

 dividual trees. 



In the subhumid forest there is a slow and apparently inef- 

 fectual adaptative evolution of smaller forms of the various 

 species to replace the larger ones, which require more moisture 

 for their growth. There is also a conspicuous shortage of cone 



