278 Eduard Riibel 



in the Salton Sink in southern California areas is influenced by 

 the level of underground water. For the greater part of the 

 year this scrub stands leafless, but when rain falls the bushes 

 produce very quickly their small compound acacious leaves. 

 Between these bushes I noted Atriplex canescens and Atriplex 

 lentiformis and, conspicuous in the undergrov^h, saltgrass, 

 Distichlis spicata. Important for the association are also Acacia 

 greggii, Acacia constricta, Flourensia cernua, and Larrea mexi- 

 cana. 



J 2. Aciculisilvae: Needle-leaved forests. — The dominant trees 

 bear needles. The needle leaf is small, linear, mostly with special 

 mechanical strengthening, and, with few exceptions (Larix), 

 evergreen. Many conifer forests belong in this association, but 

 I have also found conifers in the Laurisilvae and Taxodium in 

 the summer forests. The climate is cold and continental, the 

 vegetative season short. The forests grow in the subalpine belt 

 and the subarctic zone, in the inner parts of continents and in 

 inner alpine valleys, edaphically, where the soil is too poor for 

 broad-leaved forests — on sand, on flat ground, in swampy 

 ground poor in oxygen. Large parts of the northern hemisphere 

 between forty-five to fifty and between sixty-five to seventy 

 degrees of latitude bear such forests. The Piceion excelsae covers 

 enormous stretches in Europe and Asia. The unpretending 

 Pinus silvestris can form communities almost anywhere, but has 

 hardly the strength to compete with other trees for which the 

 habitat is good enough. The highest tree-bearing areas in the 

 Alps belong to larch and aroUa pine forests {Larix europaea, 

 Pinus c em bra). 



In the northwestern United States is a subalpine Aciculisilva 

 of Abies lasiocarpa and Tsuga pattoniana. In more continental 

 and more southern Colorado the formations lie higher and 



