Southeastern Washington and Adjacent Idaho. 79 



nized. Many of the seeds which are overlooked by rodent popu- 

 lations never germinate. Of those that do germinate many dry 

 out before they have become well established. Other seeds are 

 prevented from reaching the soil because of the sod. However, 

 under a combination of favorable conditions a few seedlings be- 

 come fairly established. Surface wash may lodge the seeds 

 against a rock or bush or other obstacle and at the same time 

 cover them with soil. Opportune showers may so favor their 

 growth that they become well established. Having gained a per- 

 manent foothold they may reach maturity. These pioneer trees 

 establish a. more congenial habitat for their offspring. The fallen 

 needles and branches catch more surface water and debris, and 

 decaying, aid the grasses in increasing the humus content and 

 consequent water holding capacity of the soil. Likewise, shading 

 prevents the rapid oxidation of humus and at the 'same time 

 lowers the evaporating power of the air. It is not unusual to 

 find parent trees in prairie openings establishing about themselves 

 just such areas of ecesis. In this way forests may replace 

 prairies. 



The invasion of pines into chaparral is accomplished with less 

 difficulty, especially if the latter is of not too dense a growth. 

 The shrubs not only modify the edaphic condition in a manner 

 already indicated but also the aerial plant parts are greatly shel- 

 tered, especially from drying winds. I have noted scores of situa- 

 tions where shrubs were advancing up butte slopes into the prairie, 

 while numerous pine trees of all sizes and ages had established 

 themselves in the chaparral. However, the chaparral is often 

 so dense as to prevent the establishment of the pines. This 

 happens when the light under the shrubs falls to an intensity of 

 0.15. But usually, even in such situations, there are open places 

 in which forest growth may occur provided tree seeds are present. 



It is probable in this semi-arid region that during the more 

 favorable years of a climatic cycle the forest trees are able to 

 gain a foothold in situations where growth conditions would be 

 too severe ordinarily. An investigation along this line would 

 probably throw considerable light upon the puzzling problem of 

 an isolated stretch of yellow pine in the canyon of Union Flat 



79 



