Southeastern Washington and Adjacent Idaho. 107 



Often the gregarious growths almost exclude other plants. Its 

 large, dark-green, shiny, resinous leaves, more than a foot long 

 and as many as 20 on a single individual, are no less characteris- 

 tic than its numerous, large yellow flowers. A single plant may 

 occupy an area of more than 2 square feet although 20-30 plants 

 often occur in a single quadrat. It possesses a strong tap root, 

 5-9 inches in circumference, which may penetrate the soil to a 

 depth of over 6 feet. 



The wet meadow is gradually replaced upon its higher lying 

 borders by typical prairie species. 



It should be mentioned in this place that in the drier regions 

 large areas of low-lying ground are dominated by dense growths 

 of rye-grasses, especially Elymus condensatus. In low ground 

 such as dried basins of shallow ponds and where the soil is alka- 

 line, socies of salt grass, Distichilis spicata, or of Spartina gracilis 

 are to be found. 



The Flood-plain Forest Associes 



The flood-plains along the streams, because of the seasonal dis- 

 tribution of the precipitation, are peculiarly apt to be subjected to 

 periodical submergence. Growing in these rich, well-watered, 

 alluvial soils are to be found dense thickets of various trees and 

 shrubs and occasional groves of poplars and aspens. Since the 

 larger streams soon cut canyons into the basalt, the flood-plains 

 are usually not extensive and the zone of tree growth is often 

 limited, especially westward, to a mere fringe along the bank of 

 the streams (Fig. 35). Likewise many of the smaller streams 

 are intermittent and along these thickets of woody species are 

 few or entirely absent. A close relationship exists between the 

 protection from, wind, afforded flood-plain areas by canyon walls 

 and hills, and the presence or absence of trees and shrubs. 



Although these flood-plain 'species usually occur in various 

 mictia, however, two communities are rather well defined the 

 Populus consocies and the Salix-Crataegus mictium. 



The Populus Consocies. Small groves of poplars consisting 

 of Populus tremuloides, or more rarely of Populus trichocarpa 

 are frequent along the flood plains, especially in the less arid parts 



107 



