Importance of biospheric CO2 in a subcanopy atmosphere deduced from 14C AMS measurements

Abstract

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. Δ14C 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%. -from Authors