Vertical changes in cellulose oxygen and hydrogen isotope ratios along conifer trunks: Implications for physiological controls on tree-ring isotopes
Authors: Nakatsuka Takeshi , Kato Minori , Kato Yoshikazu , Li Zhen 
Published: February 2023

In contrast to tree-ring width, oxygen isotope ratio (&#948;18O) of tree-ring cellulose does not usually show clear age trend, indicating that it can be utilized to reconstruct past climate variation not only for the high frequency domain but also for the low frequency domain. However, in some conifer trees growing in dense forests, there are predominant trends in the tree-ring cellulose &#948;18O decreasing with age as well as the tree-ring width, making the long-term climate reconstruction difficult. In such trees, hydrogen isotope ratios (&#948;2H) of tree-ring cellulose usually show the opposite trend to that of &#948;18O, suggesting that the age trend in tree-ring &#948;18O and &#948;2H are owing to age-related changes in the rate of post-photosynthetic isotope exchanges between carbohydrate and xylem water before the cellulose synthesis.Based on an assumption that tree-ring cellulose &#948;18O and &#948;2H vary in positive and negative relationships due to climatological and physiological factors respectively, Nakatsuka et al (2020) proposed a simple method to separate climatological and physiological components in original tree-ring cellulose &#948;18O time-series by integrating the &#948;18O and &#948;2H data in order to reconstruct past climate variation for all frequency domains seamlessly. In this method, it is very important to fix the quantitative relationship between the long-term age-related changes in &#948;18O and &#948;2H due to the post-photosynthetic process. However, it is quite difficult to elucidate the physiological relationship between &#948;18O and &#948;2H quantitatively by investigating the tree-ring &#948;18O and &#948;2H time-series solely because they are influenced not only by the physiological mechanism but also by climate variations.Here, we propose a new strategy to study physiological controls on the tree-ring cellulose &#948;18O and &#948;2H quantitatively. It is based on vertical changes in tree-ring isotope ratios along trunks, those have been seldom investigated in previous dendrochronological studies. We measured inter-annual variations in tree-ring cellulose &#948;18O and &#948;2H at different heights in individual trunks of cypress (Chamaecyparis obtusa) and cedar (Cryptomeria japonica) in central Japan and found that the cellulose isotope ratios showed clear vertical trends together with the ring width. From top to bottom along the trunk, the &#948;18O decreased and the &#948;2H increased, suggesting that we can monitor the age trend in tree rings not only in the horizontal direction but also in the vertical direction, such that &#8220;the top is youngest and the bottom is oldest&#8221;, without being influenced by the climate variation. Based on the fact that leaves are located only at the top of trunk in the dense conifer forest, we will discuss the effects of post-photosynthetic isotopic exchanges quantitatively and elucidate the mechanism underlying the apparent age trends in the tree-ring cellulose &#948;18O and &#948;2H.