The phylogenetic impact on photosynthetic and post-photosynthetic hydrogen isotope fractionation in 73 tree species

While carbon (δ¹³C) and oxygen (δ¹⁸O) isotopes in tree-ring cellulose are widely used as climatic and physiological proxy in dendro sciences, the processes that are affecting the fractionation of non-exchangeable hydrogen (δ²H) isotopes and shaping the tree-ring δ²H profile are barely understood and thus not widely applied yet.

To establish a first comprehensive comparison of the photosynthetic and post-photosynthetic ²H-fractionation of northern-hemisphere trees, we sampled leaves and twigs of 152 trees representing 73 species, 48 genus, 19 families and 12 orders containing both evergreen and deciduous angio- and gymnosperms in a common garden, as well as diurnal cycles (6 species from 6 families) of leaf sugar. We extracted leaf water and sugar, as well as twig water and the current year twig xylem cellulose for δ²H analysis. Leaf sugar and twig cellulose were measured with a newly established hot water vapour equilibration method.

Our findings show a wide variation in ²H-fractionation between species growing at a common site. The measured δ²H values ranged from -63.5 to –33.4‰ for xylem water, between -22.3 and +28.5‰ for leaf water, between -160.9 and +12.6‰ for leaf sugar and between -79.1 and +6.9‰ for twig cellulose. The biological fractionation between leaf water and leaf sugar ranged between -169.6 and +24.2‰ and between leaf sugar and current year twig cellulose from -34.6 to +116.8‰. In general, sugar and cellulose of gymnosperms were significantly more ²H depleted than those of angiosperm species, with no impact of the leaf shedding behaviour to the measured δ²H values. We observed significant differences in the δ²H values between different orders and families, but not between genus and species within a family or genus, respectively. This pattern indicates that the photosynthetic and post-photosynthetic ²H-fractionation are based on conservative metabolic reactions with a generally low mutation rate.

Additionally, the results from our diurnal sampling of leaf sugar are showing first evidence for a dynamic and species-specific nature of the photosynthetic ²H-fractionation, which is in contrast to current models, which are assuming the same constant ²H-fractionation processes for all plant species.

We conclude that the here presented results will help to improve our understanding of the mechanisms influencing the δ²H values of leaf sugar and tree-ring cellulose and thus enabling the scientific community to use δ²H in tree-ring cellulose as the third isotope-proxy for dendrochronological studies.