Importance of Biospheric Co2 in a Subcanopy Atmosphere Deduced From 14C AMS Measurements
Abstract
14 C concentrations in the stem cellulose of a Sitka spruce from the Pacific coast of Washington respond to changes in atmospheric 14 CO 2 concentration within 5–6 weeks. δ 14 C values for cellulose were consistently lower than those of the corresponding clean troposphere during rapid increase in atmospheric 14 C 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 CO 2 . 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 decay component). We plan to test this concept by looking for a vertical 14 C 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.