Wintertime Photosynthesis of Bristlecone Pine (Pinus Aristata) In the White Mountains of California

Winter rates of photosynthesis and respiration in bristlecone pine were measured at controlled temperatures at 3,094 m elevation. Following the onset of severely cold temperatures in autumn, net photosynthesis declined sharply. By midwinter net photosynthesis became zero and remained there through early spring; during this time only low gross photosynthesis could be measured. Respiration rates remained high until late winter and then dropped significantly in early spring when temperatures increased and the soil thawed. The absence of net photosynthesis and the continued high respiration results in a large negative CO 2 balance by the end of the winter. Approximately 117 hr of photosynthesis during summertime at peak rates are needed to equalize the winter loss of 140 mg CO 2 per gram dry weight of photosynthetically active tissue. These findings are related to the growth of bristlecone pine at timberline.