Comparing carbon isotope composition of bulk wood and holocellulose from Quercus cerris, Fraxinus ornus and Pinus radiata tree rings

Tree-ring δC is widely employed in ecophysiological studies, because it represents an integra-ted proxy of the ratio between photosynthesis (A) and stomatal conductance (g), which expresses the intrin-sic water use efficiency (iWUE), strongly affected by the environmental conditions experienced by the plant during its life span. Tree-ring δC also reflects long term variations of atmospheric CO2 concentration and of its carbon isotope composition, partly due to increasing anthropogenic emissions. Carbon isotope abundances in tree rings can be assessed on bulk wood as well as on wood biochemical components, wich show different δC values because of secondary discrimination during biosynthesis. We present the results of a comparison between δC values of bulk wood and holocellulose samples obtained from the last three (1999, 2000 and 2001) annual growth rings of two hardwood (Quercus cerris L. and Fraxinus ornus L.) and one conifer (Pinus radiata D. Don) species. We found that δC values differed significantly among tree species, both in the case of holocellulose and bulk wood, but only in the case of P. radiata bulk wood samples tend to provide more negative δC values than holocellulose, as reported in the literature. We suggest that, at least for the two hardwood species studied, bulk wood is a suitable material to work with for δC assessment, whilst in P. radiata holocellulose could provide a more stable and reliable index, when studying plant ecophysiological responses to changing environmental conditions.