222 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



THE SUBALPINE FOREST CLIMAX. 



PICEA-ABIES FORMATION. 



Nature. The subalpine climax is the most variable of all the forests in its 

 ecological character. At its contact with the montane forest, the trees are 

 often 100 feet high, the canopy is closed, and a typical undergrowth is present. 

 In the ecotone between the two, the -respective dominants meet on nearly 

 equal terms to form an apparently homogeneous forest. At higher altitudes the 

 forest mass becomes more and more open or fragmented and nearer timber- 

 line is broken up into isolated groves and clumps. The individuals decrease 

 steadily in stature as the altitude increases and at timber-line they are either 

 greatly dwarfed or much deformed by the action of wind or snow. It is 

 exceptional that an actual forest community exists at timber-line when the 

 latter is due to cUmatic rather than local causes. The subalpine forest may be 

 bordered by a more or less complete zone of scrub, consisting of willows, birches, 

 or heaths, at its upper edge, or it may yield directly to alpine sedgeland. 

 The latter may extend down into the forest for considerable distances 

 along valleys or on rock or gravel slides, and as a consequence often furnishes 

 a large part of the undergrowth at the higher altitudes. There is generally a 

 marked tendency to form pure stands, as a result of the rigorous climatic 

 selection of species. The number of the latter is especially reduced toward 

 timber-line, which is often formed for long distances by one or two species. 



Extent. The subalpine forest is found from Alaska and Yukon to Mexico 

 and Lower California, wherever the altitude is sufficiently great. At the 

 north it extends somewhat into the plains east of the Rocky Mountains where 

 it meets the boreal Picea-Abies cUmax. South of the northern portion of 

 New Mexico and Arizona, and of the Sierra Nevadas, it is fragmentary and 

 usually represented by but one or two species. Its eastern limit lies along the 

 crests of the Front ranges in Colorado, and the western is found on the San 

 Jacinto, San Bernardino, and Sierra Nevada ranges north to the Siskiyous. 

 In Oregon and Washington, the western limit runs along the Cascades to the 

 Olympics and the peaks of Vancouver Island, from which it follows the Coast 

 ranges as far as Cook Inlet in Alaska. The northernmost dominant is PiniLS 

 contorta, which reaches latitude 64 in Yukon. Between the two great moun- 

 tain axes on which the subalpine formation attains its major expression, it is 

 found in reduced form on the higher ranges of the interior, such as the Blue 

 and Powder River Mountains of Oregon, the Charleston Mountains of Nevada, 

 and the Panamint and Inyo Ranges of southeastern California. 



Unity. The floristic unity of the subalpine climax is necessarily somewhat 

 less than that of the montane and coast formation, owing to the many barriers 

 offered by climate, topography, and vegetation to the species of high altitudes. 

 In spite of this fact, however, the formation exhibits a high degree of unity. 

 The two chief dominants, Picea engelmanni and Abies lasiocarpa, occur 

 throughout the formation, except in California. As a subalpine dominant, 

 Pinus contorta extends from the mountains of Yukon to the San Pedro Martir 

 of Lower California, and from the Front Range of Colorado to the Cascades 

 and the northern Coast ranges. Pinus flexilis and P. aristata also occur 

 practically throughout the entire formation, though each develops two dis- 



