Field measurements were made of the rates of transpiration and of net photosynthesis and dark respiration at controlled temperatures on plants of bristlecone pine (Pinus aristata) and big sagebrush (Artemisia tridentata). These plants were growing close to one another at 10,150 ft in the subalpine zone of the White Mountains of California. The bristlecone pine plants (saplings) were relatively insensitive to the change in environmental conditions during the course of the summer of study, which included an extended drought period. Carbon dioxide exchange rates of terminal branches, expressed on a leaf-dry-weight basis, were comparatively stable from the initiation of seasonal bud elongation until the end of the growing season. The photosynthetic rates of needles of different ages included in the measurement sample were similar. Transpiration rates, during the active growth period including the drought, were relatively constant. The sagebrush plants, however, showed marked fluctuations in metabolic responses during the growing season. The differential reactions of plants of this species and of bristlecone pine correlate with apparent differences in available water supply. These findings are related to the natural distribution of these plants.
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