The phylogenetic impact on photosynthetic and post-photosynthetic hydrogen isotope fractionation in 73 tree species
Authors: Schuler Philipp , Vitali Valentina , Saurer Matthias , Gessler Arthur , Buchmann Nina , Lehmann Marco 
Published: March 2022

&lt;p&gt;While carbon (&amp;#948;&lt;sup&gt;13&lt;/sup&gt;C) and oxygen (&amp;#948;&lt;sup&gt;18&lt;/sup&gt;O) 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 (&amp;#948;&lt;sup&gt;2&lt;/sup&gt;H) isotopes and shaping the tree-ring &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H profile are barely understood and thus not widely applied yet.&lt;/p&gt;&lt;p&gt;To establish a first comprehensive comparison of the photosynthetic and post-photosynthetic &lt;sup&gt;2&lt;/sup&gt;H-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 &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H analysis. Leaf sugar and twig cellulose were measured with a newly established hot water vapour equilibration method.&lt;/p&gt;&lt;p&gt;Our findings show a wide variation in &lt;sup&gt;2&lt;/sup&gt;H-fractionation between species growing at a common site. The measured &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H values ranged from -63.5 to &amp;#8211;33.4&amp;#8240; for xylem water, between -22.3 and +28.5&amp;#8240; for leaf water, between -160.9 and +12.6&amp;#8240; for leaf sugar and between -79.1 and +6.9&amp;#8240; for twig cellulose. The biological fractionation between leaf water and leaf sugar ranged between -169.6 and +24.2&amp;#8240; and between leaf sugar and current year twig cellulose from -34.6 to +116.8&amp;#8240;. In general, sugar and cellulose of gymnosperms were significantly more &lt;sup&gt;2&lt;/sup&gt;H depleted than those of angiosperm species, with no impact of the leaf shedding behaviour to the measured &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H values. We observed significant differences in the &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H 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 &lt;sup&gt;2&lt;/sup&gt;H-fractionation are based on conservative metabolic reactions with a generally low mutation rate.&lt;/p&gt;&lt;p&gt;Additionally, the results from our diurnal sampling of leaf sugar are showing first evidence for a dynamic and species-specific nature of the photosynthetic &lt;sup&gt;2&lt;/sup&gt;H-fractionation, which is in contrast to current models, which are assuming the same constant &lt;sup&gt;2&lt;/sup&gt;H-fractionation processes for all plant species.&lt;/p&gt;&lt;p&gt;We conclude that the here presented results will help to improve our understanding of the mechanisms influencing the &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H values of leaf sugar and tree-ring cellulose and thus enabling the scientific community to use &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H in tree-ring cellulose as the third isotope-proxy for dendrochronological studies.&lt;/p&gt;