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 
