Some Ecological Studies on Bristlecone Pine in the White Mountains of California

Bristlecone pine, Pinus aristata, was studied in the White Mountains of California. The climate is dry, with annual precipitation in the bristlecone zone averaging 12 to 13 inches. The trees are found in a zone from approximately 9,500 feet to 11,500 feet elevation. Three geologic substrates are widely exposed in the bristlecone zone: dolomitic limestone, sandstone and granite. Vegetation was sampled on these substrates, using line transects. Bristlecone pine is restricted principally to dolomite. Sagebrush, Artemisia tridentate and A. arbuscula, is distributed in a complementary pattern, restricted largely to sandstone and granite. Bristlecone pine and sagebrush constitute the bulk of the vegetation. Dolomite in the White Mountains is a nearly white rock, whereas sandstone and granite are dark. The white rock reflects more solar radiation than do the other substrates, and as a result the dolomite soil averages several degrees centigrade cooler than sandstone soil. This lower temperature acts as a moisture conserving mechanism on dolomite, delaying soil drought. Dolomite also has higher moisture capacity than sandstone and granite. Through use of an infrared gas analyzer, the effect of soil drought on photosynthesis of bristlecone pine was measured. Photosynthesis was depressed by soil drought in the same range as the attained in field soils during dry periods in summer. Photosynthesis of sagebrush as a function of soil drought was also measured. Sagebrush was found more tolerant of drought than bristlecone pine. Drought tolerance may be one factor contributing to maximum development of bristlecone pine on dolomite, and of sagebrush on sandstone and granite. Bristlecone pine reaches maximum development on north slopes, and sagebrush reaches maximum development on south slopes. This supports the conclusion that drought tolerance is a decisive factor in determining substrate-oriented distribution patterns. Sagebrush and bristlecone pine seedlings both grew poorly on dolomite in pot trails. It was suggested that the high pH of dolomite soil, averaging 8.1, results in low mineral nutrient availability, and that sagebrush, with its shallow root system, is less efficient in obtaining mineral nutrient requirements than is the deep rooted pine. Photosynthesis measurements demonstrated that bristlecone pine is tolerant of shading. Furthermore, it was shown by growth measurements that bristlecone pine seedlings grow much more slowly than sagebrush seedlings. These findings indicate that the pine seedling would not succeed in the shade of sagebrush seedlings, another response that contributes to the substrate-oriented distribution of bristlecone pine. Maximum elevation of bristlecone pine is the same on all substrates, and was found to be limited by air temperature during the growing season. Minimum elevation is lower on dolomite than on the other substrates, and is under the control of soil drought. Attainment of great age was shown to be associated with death of the tree trunk around most of its circumference. Lack of krummholz at tree line shows both genetic resistance to deformation, and the influence of a very dry climate.