Cellulose Isotope Research Articles

Reconstruction of summer droughts using tree-ring cellulose isotopes: a calibration study with living oaks from Brittany (western France)

Reconstruction of summer droughts using tree-ring cellulose isotopes: a calibration study with living oaks from Brittany (western France)

Reconstruction of summer droughts using tree-ring cellulose isotopes: a calibration study with living oaks from Brittany (western France)

The aim of this study is to establish a calibration of the late wood cellulose carbon and oxygen isotopic inter-annual variability measured on four living oaks (1879–1998) in the Atlantic area (Rennes Forest, Brittany, western France) to meteorological (beginning in 1885) and hydrological (beginning in 1951) data. We find a better tree-to-tree consistency of the δ18O ratio, compared with that of the tree-to-tree variability of the ring width and the δ13C possibly affected by individual competition effects.On a century-long time scale, the δ13C ratio in the cellulose reflects the globally decreasing trend of δ13C in atmospheric CO2, which is mainly due to fossil fuel burning. In contrast with the ring width, which here shows a weak and complex dependence on meteorological parameters, the isotopic composition of the cellulose enables a reliable reconstruction (R2 > 0.45), mainly due to the δ18O signal, of selected summer climatic parameters: relative humidity, soil moisture deficit and temperature. The reconstructed parameters capture both low-frequency variations and extreme dry years (summer droughts). While both summer temperature and annual mean precipitation have a long-term increasing trend, the reconstructed water stress indicators do not show a significant trend during the 20th century. On average one summer drought occurs every seven summers, but this frequency varies in parallel to decadal changes in mean summer temperature, with fewer droughts in the 1930s and 1960s–1970s and more droughts in the 1900s, 1940s and 1990s.

A mechanistic model for interpretation of hydrogen and oxygen isotope ratios in tree-ring cellulose

A mechanistic model is presented to quantify both the physical and biochemical fractionation events associated with hydrogen and oxygen isotope ratios in tree-ring cellulose. The model predicts the isotope ratios of tree-rings, incorporating both humidity and source water environmental information. Components of the model include (1) hydrogen and oxygen isotope effects associated with leaf water enrichment; (2) incorporation of leaf water isotope ratio values into photosynthetic carbohydrates along with the biochemical fractionation associated with autotrophic synthesis; (3) transport of exported carbohydrates (such as sucrose) from leaves to developing xylem in shoots and stems where cellulose is formed; (4) a partial exchange of oxygen and hydrogen isotopes in carbohydrates with xylem sap water during conversion into cellulose; and (5) a biochemical fractionation associated with cellulose synthesis. A modified version of the Craig–Gordon model for evaporative enrichment adequately described leaf water δD and δ 18O values. The leaf water model was robust over a wide range of leaf waters for both controlled experiments and field studies, far exceeding the range of values to be expected under natural conditions. The isotopic composition of cellulose was modeled using heterotrophic and autotrophic fractionation factors from the literature as well as the experimentally derived proportions of H and O that undergo exchange with xylem water during cellulose synthesis in xylem cells of tree-rings. The fraction of H and O from carbohydrates that exchange with xylem sap water was estimated to be 0.36 and 0.42, respectively. The proportions were based on controlled, long-term greenhouse experiments and field studies where the variations in the δD and δ 18O of tree-ring cellulose were measured under different source water isotopic compositions. The model prediction that tree-ring cellulose contains information on environmental water source and atmospheric vapor pressure deficit (related to relative humidity) was tested under both field and greenhouse conditions. This model was compared to existing models to explain cellulose isotope ratios under a wide range of source water and humidity conditions. Predictions from our model were consistent with observations, whereas other models showed large discrepancies as soon as the isotope ratios of source water and atmospheric water deviated from each other. Our model resolves the apparently conflicting and disparate interpretations of several previous cellulose stable isotope ratio studies.

Reducing uncertainties in δ13C analysis of tree rings: Pooling, milling, and cellulose extraction

Recent developments of on‐line methods have provided another boost to the determination of stable isotope ratios in organic material. Along with a significant increase in sample throughput, the sample sizes decrease, both of which are necessary conditions to acquire long time series from limited wood amounts. In view of this new technique we reconsidered the most important factors influencing the measured isotopic signature which are (1) pooling, (2) homogeneity, and (3) cellulose extraction. In most cases, pooling (i.e., mixing wood of the same year from different trees) can be made in a simple way by mixing the whole wood available because mass‐weighted and unweighted isotope measurements were the same within the error. More attention must be paid in homogenizing the sample. Theoretical considerations underpinned by experimental results suggest a fineness of 0.15 mm (115 mesh) if cellulose is extracted and 0.1 mm (165 mesh) for direct wood analysis. Many of previous studies did not achieve this fineness. We find that wood is as good a climate proxy as cellulose. This is shown by comparing correlations of wood and corresponding cellulose isotope values with meteorological data, which are identical within the uncertainty.

Correlating δ13C and δ18O in cellulose of trees

ABSTRACTWe measured the carbon and oxygen isotopic composition of stem cellulose of Pinus sylvestris, Picea abies, Fagus sylvatica and Fraxinus excelsior. Several sites along a transect of a small valley in Switzerland were selected which differ in soil moisture conditions. At every site, six trees per species were sampled, and a sample representing a mean value for the period from 1940 to 1990 was analysed. For all species, the mean site δ13C and δ18O of stem cellulose are related to the soil moisture availability, whereby higher isotope ratios are found at drier sites. This result is consistent with isotope fractionation models when assuming enhanced stomatal resistance (thus higher δ13C of incorporated carbon) and increased oxygen isotope enrichment in the leaf water (thus higher δ18O) at the dry sites. δ18 O‐δ13C plots reveal a linear relationship between the carbon and oxygen isotopes in cellulose. To interpret this relationship we developed an equation which combines the above‐mentioned fractionation models. An important new parameter is the degree to which the leaf water enrichment is reflected in the stem cellulose. In the combined model the slope of the δ18O‐δ13C plot is related to the sensitivity of the pi/pa of a plant to changing relative humidity.

Oxygen isotopes in cellulose from modern and quaternary intertropical peatbogs: implications for palaeohydrology

Cellulose δ 18O-values were measured in modern peatbog plants, in recent peats and in mountain peatbog cores from Burundi in a study of the hydrologic control of the | frsol| 18O/ 16O ratio in intertropical peat cellulose. Modern plants and waters were collected under various altitudes (800–2500 m a.s.l.) with mean annual relative humidity ranging from 0.72 to 0.82. The water δ 18O-values (δ i) vary between −4 and −2.5‰. The cellulose-water enrichment is of +25.3±0.5‰ for aquatic plants, of +24.6 emergent plant species. The scatter in the cellulose σ 18O-values ( δ in) vary between −4 and −2.5‰. The cellulose-water enrichment is Cellulose δ 18O-values of emergent plantss tend to decrease with increasing elevation (−0.6 to −0.07‰ per 100 m) and relative humidity (−0.5 to −0.1‰ per %). Mean emergent plant δ 18O-values are approximated within ±0.6‰ by available cellulose δ 18O models relating cellulose δ 18O to the relative humidity ( h) and the environmental water δ 18O, except in a large swampy system. Peats were subsampled in core Ka-2 (Kashiru, Burundi, 3°28′S, 29°34′E, 890 cm long, 2240 m a.s.l. altitude) and, for control, in the topmost 50 cm of core Ku-2 (Kuruyangwe, 3°35′S, 29°41′E, 2020 m a.s.l. altitude). Similar δ 18O-vaues (+26 to +28.5‰) were found in the topmost 50 cm of both cores. They compare well with the mean δ 18O for modern emergent plants at Kashiru. AMS- 14C dating showed that core Ka-2 spans the last 30 ka. Celluloses are mainly derived from emergent plants and yield δ 18O-values ranging from +19 to +29‰ with the highest values between 2.1 and 0 ka and the lowest values (+19 to +23.5‰) between 6 and 4.5 ka. For the period between 6 and 4.5 ka, the models yield a decrease in δ in of −4 to −2.5‰ relative to present, for an h increase of 6% to 11%. Before 11.7 ka, the models yield a δ in-value close to the present value, for an h increase of 6%.

Tree cellulose 13C/12C isotope ratios and climatic change

Because tree cellulose is formed from carbon acquired through photosynthesis, its 13C/12C isotope ratio reflects that of atmospheric CO2. Fractionation changes related to photosynthetic reactions also influence 13C/12C values, and an offset between atmospheric and tree cellulose isotope ratios results. Here, we measured the pre-1850 cellulose 13C/12C isotope ratios of 19 North American coniferous trees and found a strong dependency of the ratios on latitude. Relative humidity and perhaps temperature appear to be important for this relationship. Assuming climate change to be the dominant factor influencing pre-1850 tree cellulose isotope ratios, we compare a 2,000-yr-long record of 13C/12C isotope change with existing records of ice acidity and temperature1. When accepting a lag in 13C/12C response to climatic change of 70 to 90 yr, correlation coefficients of 0.8 are found for the 1100–1850 interval.