• Importance of Biospheric CO2 in a Subcanopy Atmosphere Deduced from 14C AMS Measurements

      Grootes, P. M.; Farwell, G. W.; Schmidt, F. H.; Leach, D. D.; Stuiver, Minze (Department of Geosciences, The University of Arizona, 1989-01-01)
      14C concentrations in the stem cellulose of a Sitka spruce from the Pacific coast of Washington respond to changes in atmospheric 14CO2 concentration within 5-6 weeks. Delta-14C values for cellulose were consistently lower than those of the corresponding clean troposphere during rapid increase in atmospheric 14C caused by nuclear weapons tests (1962-64). Possible reasons for this include: 1) a delay of days or weeks in incorporation of recent photosynthate, 2) the use of stored photosynthate, and 3) photo-assimilation of biospheric decay CO2. We estimate that the influence of process 1 is small or negligible. The respective contributions to the total carbon deposited as radial stem growth in our Sitka spruce then are 2) <15% (possibly 0), and 3) 10-23% (13%-28% if the possible effect of root respiration is included in the biosphere d:cay component). We plan to test this concept by looking for a vertical 14C gradient in the 1963 growth ring of a tree located in a dense forest canopy; we do not expect to find such a gradient in a similar tree from a strongly wind-washed location.