SEED VITALITY AS A FACTOR IN FOREST TYPES 15 



short period required for germination of yellow pine when condi- 

 tions are favorable is its greatest asset in gaining possession of 

 severe sites or maintaining itself on such sites. If in the spring 

 there is a period of favorable moisture and temperature for 2 

 or 3 weeks such species as are able to respond quickly to those 

 conditions are the ones which have a fair chance for success, 

 because such conditions may not be duplicated again throughout 

 the season and the species which germinate slowly may be only 

 beginning to respond when the favorable period closes. Hence 

 the inherent germination characteristics of tree seeds have an 

 important bearing on the limitations of the species. 



SEEDLING ESTABLISHMENT. 



Establishment involves the ability of the seedling concerned 

 to take advantage of the favorable periods of soil moisture, soil 

 temperature and light. First of all, as shown above, the seed 

 must germinate quickly when the favorable conditions obtain in 

 order to gain its greatest possible development before the ad- 

 verse conditions overtake it. A favorable period of 6 weeks 

 may suffice to germinate and establish a seedling when the seed 

 germinates in 10 days or 2 weeks, but one which requires 4 to 

 6 weeks to germinate will have developed just far enough to be 

 the victim of the first unfavorable period of drought or tempera- 

 ture. The surface soil often dries out quickly on exposed sites 

 and the seedling with only a short radicle is soon caught by such 

 dry periods, while the seedling which produces a deep root early 

 in its development can resist such short unfavorable periods. 

 This is a fundamental principle which often determines the 

 success or failure of a species. The size of the seed is the 

 important factor because on the stored food in the endosperm 

 depends the early rapid development of the plant. These points 

 are graphically shown in plates 1-5. It is these facts which 

 give the yellow pine the advantage over the hemlock and its 

 associates on the severe sites in the forests of western Montana 

 and Idaho. For the same reasons the Douglas fir is able to es- 

 tablish itself on the drier slopes of the Cascades, while the cedar 

 and hemlock fail. A south slope covered with yellow pine or 

 Douglas fir and a north slope covered with hemlock, white pine, 

 cedar and other species does not necessarily mean that each of 

 these species is in its optimum habitat, but that these are instances 

 of competition and establishment. The yellow pine would produce 

 even a better forest on some of the slopes occupied by the other 

 species if it could established itself there, but conditions un- 

 favorable to its establishment prohibit its presence. Its develop- 

 ment on such sites is shown by the magnificent single yellow 

 pine trees found among the north and east slope types. On the 

 other hand the hemlock and cedar do very well under the condi- 

 tions of the south slope wherever they can get conditions favorable 



