-56 
roots far down between the fragments. In such cases they 
form nuclei for other pioneer speoies. Populus tremuloides 
and Populus grand!dontata are the pioneers along 3 ome parts 
of the forest front. 
as the cliff wears down and the talus elope becomes 
proportionately narrower, invasi on from both the bottom and 
top is accelerated (fig. 38). This is partially due to 
the fact that less new talus is put down, allowing the forces 
of weathering to establish a residual soil in which many 
speoies soon anchor themselves. One of the most efficient 
of these is Panicurn linear!foilurn Scribn. Evidence seems 
to show that it starts almost entirely by seeds from above. 
The warm nature of the talus soil, coupled with its moist 
condition during the early part of the season, must produce 
favorable conditions for germination of the many seeds which 
undoubtedly find lodgment there. Probably a very small 
percentage of tho seedlings are able to endure the very dry 
conditions which obtain during the summer. Other species 
of grass, such as Danthonia spioata , Qryzopsis Juneea, and 
Oryzopsis asperifolia also come in early. Arotostaphylos 
Uva-ursl and Aster maorophyllus L. form mats over the bare 
fragments, the latter plant being more prominent as con¬ 
ditions become shadier. Its large leaves form a very effect- 
