The unknown third - exploring the climatic and non-climatic signals of hydrogen isotopes in tree-ring cellulose across Europe

<p>Stable carbon (&#948;¹³C_c) and oxygen (&#948;¹⁸O_c)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 ²H-specific fractionations. In this first Europe-wide assessment we investigate the signals stored in the hydrogen isotope ratios in tree-ring cellulose (&#948;²H_c), from a unique collection of 100-years records, from two major genera (<em>Pinus</em> and <em>Quercus</em>) across 17 sites (36&#176;N to 68&#176;N).</p><p>The climate correlation analyses showed weak climate signals in the &#948;²H_c high-frequency chronologies, compared to those recorded by &#948;¹³C_c and &#948;¹⁸O_c, but similar to the TRW ones. The &#948;²H_c climate signal strength varied across the continent and was stronger and more consistent for <em>Pinus</em> than for <em>Quercus</em>. The &#948;²H_c 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 ²H-enrichment in tree-ring cellulose for both genera. Our findings clearly indicate that &#948;²H_cregisters information about hydrology and climate, but it also records non-climatic signals such as physiological mechanisms associated with carbohydrates storage remobilization ²H-specific fractionations and growth.</p><p>To disentangle the climatic and non-climatic signals in &#948;²H_c, we investigated its relationships with &#948;¹⁸O_c and TRW. We found significant relationships negative between &#948;²H_c and TRW at 7 sites and positive between &#948;²H_c and &#948;¹⁸O_c 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 &#948;²H_c, while the divergencefrom &#948;¹⁸O suggests a loss of the hydrological signal in &#948;²H_c. These highlights, once again,a stronger physiological component in the &#948;²H signature independent from climate. Advancements in the understanding of ²H-fractionations and their relationships with climate, physiology, and species-specific traits are therefore needed to improve the mechanistic modeling and interpretation accuracy of &#948;²H_c in plant physiology and paleoclimatology. Such advancements could lead to new insights into trees&#8217; carbon allocation mechanisms, and responses to abiotic and biotic stressors.</p>