Radiocarbon dating assumes that the carbon 14carbon 12 ratio

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Due to their different biosynthetic pathways, these components show differences in their carbon and oxygen isotope composition (see, e.g., Epstein et al. Although pioneering studies on the stable isotope composition of tree rings were performed using whole wood (e.g., Craig 1954, Libby et al. Although the use of a single constituent, such as cellulose, is justified on theoretical grounds, many studies have shown similar results when whole wood and purified cellulose are compared; in many cases a constant offset is observed, but the strength of the correlation to climate variation is conserved (Borella et al. Thus, correlations between cellulose and lignin might be strong when the amount of exchange between oxygen in lignin precursors and water is high, but weak when such exchange is low (Barbour et al. More work is needed to understand the way in which the oxygen isotope signal is transferred into lignin, and its variability with climate and among species.

1976, Gray and Thompson 1977, Leavitt and Danzer 1993, Borella et al. 1976), the analysis of a single chemical component (usually cellulose) was ultimately recommended by Epstein et al. The following reasons have been offered for focusing on cellulose. Third, whereas cellulose and hemicelluloses are deposited into secondary cell walls during cellular expansion, lignin is deposited after its cessation, resulting in a time lag between the incorporation of both compounds into whole wood (Boerjan et al. Finally, unlike cellulose, the lignin chemical composition of wood is not constant, and its complex biosynthetic pathway and associated isotope fractionations include several unresolved aspects (Boerjan et al. Consequently, cellulose has been the material of choice in most tree-ring studies (see references in Switsur and Waterhouse 1998, Mc Carroll and Loader 2004); this is despite the fact that cellulose purification is time consuming, and has no clear endpoint (Leavitt and Danzer 1993, Loader et al. We see potentially that after more intensive research it can be finally concluded that past climate signals are best recorded in δO of whole wood rather than with cellulose or lignin (Gori et al. In the meantime, the most sensible working procedure is to test cellulose extraction at least with a subset of samples, and use whole wood if the values are well correlated with those of cellulose.

On the other hand, the opposite occurs during precipitation, when the heavier isotopologues condense more readily due to the slightly stronger intermolecular bond strength (Gat 1996).

The amount of this fractionation varies according to temperature and altitude, resulting in a composition of precipitation that favours the heavier isotopes in positive relation with temperature, and in negative relation with altitude, distance from moisture sources and local precipitation amount (Dansgaard 1964, but see also Aggarwal et al. Therefore, analyses of precipitation δO can reveal intra-annual to multi-decadal information about regional weather patterns, and even large-scale atmospheric circulation dynamics, depending on geographical region and season (Baldini et al. Only recently, it has been shown that canopy processes can alter the isotopic composition of precipitation reaching the ground. (2008) observed that sublimation of snow intercepted by the canopy causes O isotopic enrichment of the snowpack below the canopy and the stream flow from forested areas. 2014), herbarium material (Beerling 1996, Köhler et al.

concentrations is expected to markedly affect plant growth and performance and, as a consequence, the composition and spatial distribution of species in terrestrial ecosystems (IPCC 2007). Being aware of such issues, analyses of stable isotopes in tree rings allow us to travel beyond the first-order aim of reconstructing paleoclimate, and towards the second-order aim of reconstructing responses in specific plant processes to paleoclimate.During the light period, starch is produced and accumulated in the chloroplasts of leaves, and the production of sucrose in the cytoplasm involves C may affect the carbon isotope composition of cellulose extracted from tree rings (Tcherkez et al. We know that there are diel variations in the expression of key enzymes involved in lignin (Rogers et al. If both prerequisites (diel pattern in δC of newly assimilated carbon depending on the proportion of carbon derived from the two sucrose pools (i.e., day versus night). A possible explanation for mixing of sugar pools during basipetal transport might be provided by the dynamic Münch mass flow model (see the review by Van Bel 2003).In the following section, however, we indicate that the day–night difference in the δC of leaf-exported sugars might be of minor or even no importance for the tree-ring isotope composition. (2008) observed dampening of short-term variations in δ R. The model proposes that while a proportion of the sucrose from sieve tubes is released during phloem transport, approximately two-thirds of it is recovered and transported back into the sieve tubes (Minchin and Thorpe 1987). At the beginning of the growing season, NSC concentrations increase and peak when resource demand is highest.First, mobile compounds such as resins or sugars might move along the parenchyma cells between several tree rings, potentially adding ‘noise’ to high-resolution climate reconstruction (Leavitt and Danzer 1993, Mc Carroll and Loader 2004, Harlow et al. Second, the relative contribution of the different compounds in wood might vary from one year to another, as well as within a single tree ring, causing additional uncertainties (Epstein et al. For downstream photosynthetic carbon isotope discrimination (cf.Table 1; Figure 1a), there are possible post-carboxylation fractionations at the leaf level that could cause the δC of extracted tree-ring cellulose to deviate from that predicted using leaf-scale models. 2010) biosynthesis with highest gene expression and thus cambial activity during night.

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