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Evidence of Reduced Photosynthetic Rates in Old Trees

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Wood production generally declines as forests age. The decrease in production has been attributed to higher respiration associated with increasing biomass, but direct measurements fail to support this assumption. An alternate hypothesis is that the growth decline is caused by reduced net photosynthesis. To test this hypothesis, we compared the net photosynthetic rates of foliage from trees that had ceased height growth with foliage from actively growing, mature trees. Results showed that net photosynthesis per unit area of 1-yr-old foliage from old Pinus contorta and P. ponderosa averaged 14-30% lower than the same-aged foliage from younger trees. Computer simulations from a previous study indicated that differences of this magnitude are sufficient to explain the lower wood production in old trees. The cause of the photosynthetic decline is not clear, but it did not appear to be due to a reduction in the maximum capacity for photosynthesis. Ratios of stable carbon isotopes from leaves and wood indicated a greater stomatal limitation of photosynthesis in older trees. A possible relationship between lower hydraulic conductance in the vascular systems of older trees leading to greater stomatal closure is discussed. For. Sci. 40(3):513-527.

Keywords: Pinus contorta; Pinus ponderosa; hydraulic conductance; photosynthesis; senescence

Document Type: Journal Article

Affiliations: Rocky Mountain Experiment Station, USDA Forest Service, 240 West Prospect RD, Fort Collins, CO 80526

Publication date: 1994-08-01

More about this publication?
  • Forest Science is a peer-reviewed journal publishing fundamental and applied research that explores all aspects of natural and social sciences as they apply to the function and management of the forested ecosystems of the world. Topics include silviculture, forest management, biometrics, economics, entomology & pathology, fire & fuels management, forest ecology, genetics & tree improvement, geospatial technologies, harvesting & utilization, landscape ecology, operations research, forest policy, physiology, recreation, social sciences, soils & hydrology, and wildlife management.
    Forest Science is published bimonthly in February, April, June, August, October, and December.

    2015 Impact Factor: 1.702
    Ranking: 16 of 66 in forestry

    Average time from submission to first decision: 62.5 days*
    June 1, 2016 to Feb. 28, 2017

    Also published by SAF:
    Journal of Forestry
    Other SAF Publications
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