Beyond CO2-fixation by Rubisco -: an interpretation of 13C/12C variations in tree rings from novel intra-seasonal studies on broad-leaf trees

Evidence is presented for a very specific, seasonally recurring tri-phase carbon isotope pattern in tree rings of broad-leaf deciduous tree species. It is derived from highly resolved intra-annual measurements of C-13/C-12 ratios of wood and cellulose from tree rings of Fagus sylvatica, Populus nigra, Quercus petraea and Morus alba. Investigations on delta(13)C from buds and leaves of Fagus sylvatica revealed a similar tri-phase delta(13)C pattern. At the very beginning of a growing season, the delta(13)C trend of tree rings and foliage shows a marked increase of up to 5parts per thousand. The maximum delta(13)C-value of each vegetation period always occurs in young heterotrophic leaves shortly after bud burst and persistently in the early wood of each tree ring, when growth depends on carbon reserves. Thereafter, delta(13)C profiles represent the autotrophic stage of the leaves, which show different patterns of variation, by and large characterized by a decline. The minimum delta(13)C-value always shows up in the late wood of each tree ring. At the very end of each tree ring delta(13)C-values start rising again. This increase in delta(13)C marks the gradual switch-over to storage-dependent growth and can also be observed in senescent leaves. Seasonal changes of more than 4parts per thousand were measured, whereas contiguous delta(13)C values rarely differed from each other by more than 0.3parts per thousand. This tri-phase pattern cannot be explained by the common model of carbon isotope fractionation during photosynthesis. It appears to be primarily an indication of seasonal changes in down-stream processes of the carbohydrate metabolism. Environmental influences on the carbon isotope fractionation during photosynthesis are presumably of secondary importance and expressed by certain peculiarities showing up during the autotrophic phase, i.e. the mid-section of the seasonal delta(13)C pattern.