90 A Study of the Vegetation of 



xerosere to its climax community, we will next consider briefly 

 a hydrophytic adsere, after which we will take up the secondary 

 succession or subsere. 



The Adsere. Alnus-Salix Associes 



Distinctly hydrophytic plant communities characterized by 

 Alnus sinuata, Salix scouleriana, and Pyrus sitchensis occupy 

 broad wet ravines and springy places often high up on the moun- 

 tain sides. The weight of the great banks of snow which drift 

 over the trees and sometimes almost bury them, pushes them into 

 a semi-procumbent position leaning down the slope. The alder, 

 which is usually dominant, reaches a diameter of 2-5 inches and 

 a vertical height of only 12 or 15 feet, while not infrequently it 

 attains a length greater than twice its height. Likewise, the 

 willow and mountain ash and also the mountain maple, Acer 

 glabrum douglasii, which accompany it in this habitat, are 

 bent down the slopes by the weight of the snow. Because of 

 the late melting of the snow in such situations and the resulting 

 springy nature of the soil, coniferous tree seedlings are excluded. 

 Engelmann spruce, as already indicated, often borders such alder 

 thickets and in turn gives way to the cedar. 



In this tangle of semi-prostrate trees, shrubs are nearly ex- 

 cluded, although Sambucus melanocarpa and Ribes lacustre occur 

 rather infrequently. A dense herbaceous undergrowth which 

 almost completely conceals the ground is made up of the follow- 

 ing mesophytic species : 



Aconitum columbianum Disporum oreganum 



Actea spicata arguta Mertensia membranacea 



Athyrium cyclosorum Osmorhiza divaricata 



Circaea pacifica Senecio triangularis 



Claytonia asarifolia Streptopus amplexifolius 



Clintonia uniftora Thalitfrum occidentale 



Disporum ma jus Trillium ovatum 



The preceding plant species together with certain other meso- 

 phytic shrubs and herbs and hydrophytic mosses border the moun- 



90 



