Divergent Biochemical Fractionation, Not Convergent Temperature, Explains Cellulose Oxygen Isotope Enrichment across Latitudes

Abstract

Recent findings based on the oxygen isotope ratios of tree trunk cellulose indicate that the temperature of biomass production in biomes ranging from boreal to subtropical forests converge to an average leaf temperature of 21.4°C. The above conclusion has been drawn under the assumption that biochemically related isotopic fractionations during cellulose synthesis are not affected by temperature. Here we test the above assumption by heterotrophically generating cellulose at different temperatures and measuring the proportion of carbohydrate oxygen that exchange with water during cellulose synthesis and the average biochemical fractionation associated with this exchange. We observed no variation in the proportion of oxygen that exchange with different temperatures, which averaged 0.42 as it has been observed in other studies. On the other hand, the biochemical oxygen isotope fractionation during cellulose synthesis is affected by temperature and can be described by a 2nd order polynomial equation. The biochemical fractionation changes little between temperatures of 20 and 30°C averaging 26‰ but increases at lower temperatures to values of 31‰. This temperature sensitive biochemical fractionation explains the pattern of cellulose oxygen isotope ratios of aquatic plants encompassing several latitudes. The observed temperature sensitive biochemical fractionation also indicates that divergent biochemical fractionation and not convergent leaf temperature explains the increase in oxygen isotope enrichment of cellulose across several biomes.

Modelling tree ring cellulose <i>δ</i><sup>18</sup>O variations in two temperature-sensitive tree species from North and South America

“…In accordance with DeNiro and Epstein (1981), Sternberg (1989) and Yakir and DeNiro (1990), the biochemical fractionation ε 0 is assumed here to be lower than 30 ‰. However, a recent study has demonstrated that this parameter, nearly constant between 20 to 30 • C, increases at lower temperatures to values of 31 ‰ (Sternberg and Ellsworth, 2011). During the growing season, maximum temperatures can reach 20 • C in western Argentina and 30 • C in northeastern Canada, which suggests that the high mean δ 18 O TR levels in N. pumilio may be due to biochemical fractionation higher than 30 ‰ due to temperature generally lower than 20 • C. However, when the prior range of ε 0 is extended to 31 ‰ in the optimization process, the mean δ 18 O TR levels of N. pumilio are still too low in comparison with the observations.…”

“…Ideally, daily δ 18 O P longterm records from meteorological stations in the study region should be used as an input of MAIDENiso. Simulations from high-resolution regional circulation models, such as REMOiso, which has a 0.5 • × 0.5 • (∼ 55 km) horizontal resolution (Insel et al, 2013;Sturm et al, 2005Sturm et al, , 2007, may produce reliable local δ 18 O P values. Such a dataset has proven to be quite helpful with MAIDENiso in the Fontainebleau forest (France) (Danis et al, 2012).…”

Oxygen isotopes as a valuable tool for measuring annual growth in tropical trees that lack distinct annual rings

“…2) Model parameters The equilibrium (ε*) and kinetic fractionation factors (ε k ) are regarded as being relatively constant, and have been reported to be approximately 9‰ and 29‰, respectively (Allison et al, 1985). The proportion of isotopic exchange between xylem water and carbohydrate oxygen (f o ) does not vary at different temperatures (Sternberg and Ellsworth, 2011), and we adopted a previously reported value of 0.42 as the f o value for our calculations (Roden et al, 2000;Sternberg and Ellsworth, 2011). Although the net biological isotopic fractionation factor between xylem water and exchanged oxygen (ε o ) is temperature-sensitive, ε o changes little within a temperature range from 20 to 30°C (Sternberg and Ellsworth, 2011).…”

“…The proportion of isotopic exchange between xylem water and carbohydrate oxygen (f o ) does not vary at different temperatures (Sternberg and Ellsworth, 2011), and we adopted a previously reported value of 0.42 as the f o value for our calculations (Roden et al, 2000;Sternberg and Ellsworth, 2011). Although the net biological isotopic fractionation factor between xylem water and exchanged oxygen (ε o ) is temperature-sensitive, ε o changes little within a temperature range from 20 to 30°C (Sternberg and Ellsworth, 2011). Monthly temperature ranged from 18 to 26°C at our sampling site, and ε o was 27‰, as has been observed in previous studies (Sternberg et al, 1984;Sternberg and DeNiro, 1983;Yakir and DeNiro, 1990).…”

Climate history of the Southern Hemisphere Westerlies belt during the last glacial–interglacial transition revealed from lake water oxygen isotope reconstruction of Laguna Potrok Aike (52° S, Argentina)

“…The application of both equations in the investigation period is primarily based on the assumption that biochemical oxygen isotope fractionation during cellulose synthesis is almost constant under different temperatures. However, Sternberg and Ellsworth (2011) proposed a temperature effect on cellulose oxygen isotope enrichment relative to source water, especially at temperatures below 20 • C. Accordingly, isotopic enrichment would increase with decreased temperature and the mean of increased enrichment between 4 and 15 • C is about 2 ‰ (Sternberg and Ellsworth, 2011). This value is, however, derived by summarizing various field studies under different analytical conditions and has not been further confirmed by the latest modern calibration data set from sites in southern Patagonia , which shows no apparent effect of host water temperature on the fractionation between aquatic cellulose and host waters.…”