22 MISC. PUBLICATION 42 3, U. S. DEPT. OF AGRICULTURE 



Castilleja spp., Cordylanthus wrightii, Aster commutatus, Erigeron spp., 

 Antennaria rosulata, Achillea lanulosa, Artemisia dracunculoides. 



In many localities in the northernmost part of the State Artemisia 

 tridentata occurs in extensive colonies in the xeric or mesic forest, often 

 accompanied by Chamaebatiaria millefolium, Cowania stansburiana, 

 and Fallugia paradoxa. 



The type of northern mesic evergreen forest dominated by spruce 

 and fir is represented by very small areas on the desert mountains of 

 southern Arizona, by a large area in the White Mountains, by small 

 areas on the San Francisco Peaks, and by an area of considerable 

 extent on the Kaibab Plateau. This is a closed forest of large trees 

 reaching heights of 60 to 100 feet with a floor, which is sometimes 

 open, sometimes occupied by shrubbery. At elevations of 10,500 to 

 11,000 feet, the size of the trees becomes less and the stand more open. 

 The lowest bodies of this forest occupy north slopes at 7,000 to 7,500 

 feet elevation, and it is found on level ground or south slopes only 

 above an elevation of about 9,500 feet. 



The dominant trees of this type of forest in Arizona are Pseudotsuga 

 taxifolia (Douglas -fir), Abies concolor (white fir), Picea engelmanni 

 (Engelmann spruce), Picea pungens (blue spruce), Pinus flexilis (lim- 

 ber pine), Abies lasiocarpa, and A. arizonica (corkbark fir). The 

 Douglas-fir is found at lower elevations and in drier situations than the 

 other forms mentioned. Pinus flexilis is confined to the San Francisco 

 Peaks and Navajo Peak. The spruces are found only at the highest 

 altitudes. Abies lasiocarpa is abundant on the Kaibab Plateau and 

 on a few high mountain peaks. On the whole, the spruce-fir type of 

 mesic forest exhibits greater variety in its composition than the pine 

 type does (pi. 8). 



South of the Mogollon Mesa, Douglas-fir and white fir are the dom- 

 inant trees, with Mexican white pine playing a minor role. Above 

 9,500 feet in the Pinaleno Mountains the dominant trees are Picea 

 engelmanni and Abies sp. (A. arizonica?) . Near the summit of these 

 mountains (10,500 feet) the forest is somewhat open, the trees retain 

 their branches down to the base of the trunk, and the crowns are 

 pointed, all indicating an approach to the limiting conditions for tree 

 growth. The highest elevation in the White Mountains (Baldy 

 Peak, 11,470 feet) is forested to the summit by Picea engelmanni, 

 Pseudotsuga taxifolia, and Abies concolor. The summits of the San 

 Francisco Peaks are surrounded by a belt of spruce-fir forest, in which 

 the limber pine - is found in addition to the trees last named. On the 

 summit of the Kaibab Plateau there is an area of spruce forest esti- 

 mated to be about 375 square miles in extent. The dominant trees 

 are Picea engelmanni and P. pungens. The aspen (Populus tremu- 

 loides) occurs in scattered or nearly pure stands, and Quercus gambelii, 

 Betula fontinalis (water birch), and Acer negundo var. interius are the 

 commonest deciduous trees. The prostrate shrubby mountain juni- 

 per, Juniperus communis var. montana, is locally abundant on the 

 Kaibab Plateau. 



Alpine Summits. — The summit of the San Francisco Peaks is the 

 only truly alpine area in Arizona. The timber line varies from about 

 11,000 to 11,400 feet, according to slope exposure, above which the 

 slopes are steep, poorly watered during the latter part of the growing 



