δC-variations of leaves in forests as an indication of reassimilated CO from the soil.

An attempt has been made to evaluate the contribution of soil respired CO to the total assimilation of a forest tree, by heeding the C-concentrations of CO from the free atmosphere and from mineralization processes within the soil respectively. An expression has been derived, according to which the assimilated fraction of CO from the soil at a particular height of a tree is given by the δC-value of the corresponding leaves, δC of atmospheric CO, δC of soil respired CO and the physiological state of the leaves expressed as the ratio of total respiration over gross photosynthesis and internal over external CO-concentration. In the particular case investigated, a δC-difference of 5‰ has been determined from bottom to top of a beech tree which results in a CO contribution from the soil of about 22% for the lower forest strata, while the total contribution of soil respired CO accounts for about 5% of the overall assimilation.