The unknown third - exploring the climatic and non-climatic signals of hydrogen isotopes in tree-ring cellulose across Europe
Authors: Vitali Valentina , Martínez-Sancho Elisabet , Treydte Kerstin , Andreu-Hayles Laia , Dorado-Liñán Isabel , Gutiérrez Emilia , Helle Gerhard , Leuenberger Markus , Loader Neil J. , Rinne-Garmston T. Katja , Schleser Gerhard , Allen Scott , Waterhouse John , Saurer Matthias , Lehmann Marco 
Published: March 2022

&lt;p&gt;Stable carbon (&amp;#948;&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;c&lt;/sub&gt;) and oxygen (&amp;#948;&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;c&lt;/sub&gt;)&lt;sub&gt;&lt;/sub&gt;isotopes measured in tree-ring cellulose, together with tree-ring width (TRW), have been used extensively to investigate the effects of past climatic conditions on tree growth. By contrast, the information recorded by the third major chemical component of tree-ring cellulose, the non-exchangeable carbon-bound hydrogen, has been explored far less due to methodological drawbacks and lack of understanding of &lt;sup&gt;2&lt;/sup&gt;H-specific fractionations. In this first Europe-wide assessment we investigate the signals stored in the hydrogen isotope ratios in tree-ring cellulose (&amp;#948;&lt;sup&gt;2&lt;/sup&gt;H&lt;sub&gt;c&lt;/sub&gt;), from a unique collection of 100-years records, from two major genera (&lt;em&gt;Pinus&lt;/em&gt; and &lt;em&gt;Quercus&lt;/em&gt;) across 17 sites (36&amp;#176;N to 68&amp;#176;N).&lt;/p&gt;&lt;p&gt;The climate correlation analyses showed weak climate signals in the &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H&lt;sub&gt;c&lt;/sub&gt; high-frequency chronologies, compared to those recorded by &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;c&lt;/sub&gt; and &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;c&lt;/sub&gt;, but similar to the TRW ones. The &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H&lt;sub&gt;c&lt;/sub&gt; climate signal strength varied across the continent and was stronger and more consistent for &lt;em&gt;Pinus&lt;/em&gt; than for &lt;em&gt;Quercus&lt;/em&gt;. The &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H&lt;sub&gt;c&lt;/sub&gt; inter-annual variability was strongly site-specific. Focusing on the effect of extreme climatic conditions during years with extremely dry summers, we observed a significant &lt;sup&gt;2&lt;/sup&gt;H-enrichment in tree-ring cellulose for both genera. Our findings clearly indicate that &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H&lt;sub&gt;c &lt;/sub&gt;registers information about hydrology and climate, but it also records non-climatic signals such as physiological mechanisms associated with carbohydrates storage remobilization &lt;sup&gt;2&lt;/sup&gt;H-specific fractionations and growth.&lt;/p&gt;&lt;p&gt;To disentangle the climatic and non-climatic signals in &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H&lt;sub&gt;c&lt;/sub&gt;, we investigated its relationships with &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;c&lt;/sub&gt; and TRW. We found significant relationships negative between &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H&lt;sub&gt;c&lt;/sub&gt; and TRW at 7 sites and positive between &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H&lt;sub&gt;c&lt;/sub&gt; and &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;c&lt;/sub&gt; at 10 sites, while the rest of the sites did not show any significant relationships. The agreement with the TRW chronologies confirms the relationship between growth and &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H&lt;sub&gt;c&lt;/sub&gt;, while the divergence&lt;sub&gt;&lt;/sub&gt;from &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O suggests a loss of the hydrological signal in &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H&lt;sub&gt;c&lt;/sub&gt;. These highlights, once again,&lt;sub&gt;&lt;/sub&gt;a stronger physiological component in the &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H signature independent from climate. Advancements in the understanding of &lt;sup&gt;2&lt;/sup&gt;H-fractionations and their relationships with climate, physiology, and species-specific traits are therefore needed to improve the mechanistic modeling and interpretation accuracy of &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H&lt;sub&gt;c&lt;/sub&gt; in plant physiology and paleoclimatology. Such advancements could lead to new insights into trees&amp;#8217; carbon allocation mechanisms, and responses to abiotic and biotic stressors.&lt;/p&gt;