Hydrogen and oxygen isotope ratios of tree-ring cellulose for riparian trees grown long-term under hydroponically controlled environments

Abstract

Saplings of three riparian tree species (alder, birch and cottonwood) were grown for over 5 months in a hydroponics system that maintained the isotopic composition of source water in six treatments, ranging from -120 to +180‰δD and -15 to +10‰δO. The trees were grown in two greenhouses maintained at 25°C and at either 40 or 75% relative humidity, creating differences in transpiration rates and leaf water isotopic evaporative enrichment. The cellulose produced in the annual growth ring was linearly related to source water with differences in both slope and offset associated with greenhouse humidity. The slope of the isotopic composition of source water versus tree-ring cellulose was less than 1 for both δD and δO indicating incomplete isotopic exchange of carbohydrate substrate with xylem water during cellulose synthesis. Tests using the outer portion of the tree-ring and new roots were similar and showed that the tree-ring values were representative of the cellulose laid down under the imposed environmental conditions. The fraction of H and O in carbohydrate substrate that isotopically exchange with medium water was calculated to be 0.36 and 0.42 respectively, and biochemical mechanisms for these observed fractions are discussed. A mechanistic model of the biochemical fractionation events for both δD and δO leading to cellulose synthesis was robust over the wide range of cellulose stable isotope ratios. The experimental results indicate that both water source and humidity information are indeed recorded in tree-ring cellulose. These results help to resolve some of the disparate observations regarding the interpretation of stable isotope ratios in tree-rings found in the literature.

Leaf δ18O of remaining trees is affected by thinning intensity in a semiarid pine forest

“…O in semiarid forest with more open canopies (Burk & Stuiver 1981;Yakir 1992;Roden & Ehleringer 1999;Barbour 2007). However, we found that the remaining trees in low-density stands (heavily thinned) with more open canopies showed consistently lower foliar d…”

Multi-isotope labelling of organic matter by diffusion of <sup>2</sup>H/<sup>18</sup>O-H<sub>2</sub>O vapour and <sup>13</sup>C-CO<sub>2</sub> into the leaves and its distribution within the plant

“…Thus biochemical reactions lead to different isotopomers of organic compounds (Augusti and Schleucher, 2007). The proportion of O and H exchanged can be considerable; for example, during cellulose synthesis, around 40 % of O and H is exchanged with the tissue water (Roden and Ehleringer, 1999;Yakir and DeNiro, 1990). The exchange with water explains to some extent the stronger relative 18 O and 2 H signal in the leaf OM compared to the stem and root OM, since the leaf water was labelled, while the stem and root water was not.…”

Regional And Watershed-Scale Coherence In the Stable-Oxygen and Carbon Isotope Ratio Time Series In Tree Rings Of Coast Redwood (Sequoia sempervirens)

“…humidity effects on leaf evaporative enrichment, Lipp et al 1996;Roden and Ehleringer 1999b). The strong coherence of between-site d 18 O time series highlights their potential as proxy records that can be used for paleo-climate reconstruction.…”

“…Through a series of known fractionation events, d 18 O (and d 2 H) variation derived from hydrologic inputs is incorporated into the organic matter that makes up tree rings (Yapp and Epstein 1982;White et al 1994;Roden and Ehleringer 1999b;Roden et al 2000;Treydte et al 2006;Gessler et al 2009). Tree-ring cellulose d 18 O values can be modified by variation in atmospheric humidity and the unique isotope values it possesses, by vapor pressure deficit and therefore the magnitude of leaf evaporative enrichment associated with transpiration (Edwards and Fritz 1986;Roden and Ehleringer 1999a;Wright and Leavitt 2006;Kahmen et al 2008), by variation in source water (Wright et al 1999;Roden andEhleringer 1999b, 2007) and by post-photosynthetic but pre-biosynthetic factors shown to dampen the original isotope values of sucrose made at the leaf-level before it becomes incorporated into tree-ring cellulose (Brandes et al 2006).…”

“…In addition, it is important to determine if specific locations at any site are more sensitive to climate variation than others. For example, organic matter d 18 O values are primarily influenced by source water d 18 O variation of meteoric waters (Roden and Ehleringer 1999b;Robertson et al 2001). However, if a tree obtains most of its water from a perennial stream because of its location in the riparian zone, then it may not be sensitive to changes in meteoric water d 18 O.…”