19 



PERMANEXCE OF DOUGLAS FIR STANDS. 



Both the pure and the mixed stands of Douglas fir in these regions 

 may be either temporary or fairly permanent. Temporary stands 

 usually result from the destruction of forests of shade-bearing species, 

 followed by a reseeding of the soil by all species, when Douglas fir 

 has an equal chance for light. Afterwards seedlings of the shade- 

 bearing species come up underneath, while the more light-demanding 

 Douglas fir undergrowth dies out. For instance, in the western hem- 

 lock forests of Washington and Oregon and in the Engelmann spruce 

 and alpine fir forests of the Rocky Mountains, Douglas fir may be 

 abundant in the first generation and almost unrepresented in the next. 



Permanent stands result when conditions of light, soil, moisture, 

 and temperature favor Douglas fir more than other trees. Altitude, 

 exposure, and topography influence these conditions and result in 

 changes in forest types. Douglas fir rarely grows in x^ermanent 

 stands in w^estern Washington and Oregon, except Avhere soil and 

 moisture conditions are more favorable to it than to hemlock, arbor- 

 vita?, spruce, or the true firs. The Kocky Mountain form often grows 

 in permanent stands w^ith ^^ellow pine and white fir. 



Fires which have left a preponderance of Douglas fir seed trees, 

 and have destroyed the humous la3^er, are often follovv^ed by dense, 

 pure stands of Douglas fir. Their permanence is favored by the 

 scarcity of seed trees of more tolerant species, but is jeopardized by 

 density of crown cover and the humus formed by the dead twigs and 

 leaves, which prevent the germination and growth of Douglas fir, 

 but which favor shade-enduring trees. 



SILVICAL CHARACTERISTICS AND REQUIREMENTS OF THE TWO 



FORMS. 



Some of the differences between the two forms of Douglas fir have 

 already been mentioned, but they also show more or less distinct 

 differences in many of their silvical characteristics and requirements, 

 and it is on a thorough knowledge of these that the intelligent man- 

 agement of the two forms must rest. Their respective soil require- 

 ments, tolerance of shade, reproductive characteristics, rate of growth, 

 longevity, and susceptibility to damage from fire, wind, and frost, all 

 have an important bearing on the treatment the two forms should 

 receive. 



SOIL AND ^rOISTURE. 



Little difference exists between the two forms w^ith respect to soil 

 requirements. Throughout its range Douglas fir accommodates itself 

 with facility to the character of the soil in the locality where it hap- 

 pens to strike root. But, other conditions being equal, it shows by 



[Cir. 150] 



