Vertical changes in cellulose oxygen and hydrogen isotope ratios along conifer trunks: Implications for physiological controls on tree-ring isotopes

In contrast to tree-ring width, oxygen isotope ratio (δ¹⁸O) 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 δ¹⁸O decreasing with age as well as the tree-ring width, making the long-term climate reconstruction difficult. In such trees, hydrogen isotope ratios (δ²H) of tree-ring cellulose usually show the opposite trend to that of δ¹⁸O, suggesting that the age trend in tree-ring δ¹⁸O and δ²H 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 δ¹⁸O and δ²H 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 δ¹⁸O time-series by integrating the δ¹⁸O and δ²H 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 δ¹⁸O and δ²H due to the post-photosynthetic process. However, it is quite difficult to elucidate the physiological relationship between δ¹⁸O and δ²H quantitatively by investigating the tree-ring δ¹⁸O and δ²H 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 δ¹⁸O and δ²H 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 δ¹⁸O and δ²H 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 δ¹⁸O decreased and the δ²H 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 “the top is youngest and the bottom is oldest”, 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 δ¹⁸O and δ²H.