Investigating the influence of sulphur dioxide (SO2) on the stable isotope ratios (δ13C and δ18O) of tree rings

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Abstract

This study reports the influence of a 20th century pollution signal recorded in the δ13C and δ18O of absolutely dated tree rings from Quercus robur and Pinus sylvestris from southern England. We identify a correspondence between the inter-relationship and climate sensitivity of stable isotope series that appears to be linked to recent trends in local SO2 emissions. This effect is most clearly exhibited in the broadleaved trees studied but is also observed in the δ13C values of the (less polluted) pine site at Windsor. The SO2 induced stomatal closure leads to a maximum increase of 2.5‰ in the isotope values (δ13C). The combined physiological response to high pollution levels is less in δ18O than δ13C. The SO2 signal also seems to be present as a period of reduced growth in the two ring-width chronologies. Direct, quantitative correction for the SO2 effect represents a significant challenge owing to the nature of the records and likely local plant response to environmental pollution. Whilst it appears that this signal is both limited to the late industrial period and demonstrates a recovery in line with improvements in air quality, the role of atmospheric pollution during the calibration period should not be underestimated and adequate consideration needs to be taken when calibrating biological environmental proxies in order to avoid development of biased reconstructions.

Introduction

In comparison to the pre-industrial period, results from the analysis of tree ring data from the 20th and early 21st centuries sometimes exhibit anomalous trends in their response, which require characterisation/correction when compared to earlier centuries (Freyer, 1986, Briffa, 2000, Loader et al., 2007). Although the focus of many tree ring based studies, global climatic change is not the only environmental consequence of human activity. Increased atmospheric CO2 concentration (ca), increase in biologically available nitrogen and increased incident UV-B have also been shown to influence tree growth directly. Similar increases in the quantity of associated anthropogenic atmospheric pollutants may also have significant impacts on the environment, which manifest themselves as disturbance events in plant response and ecosystem productivity (Savard et al., 2005, Wagner and Wagner, 2006). Where these influences represent a significant and non-random variation in wood formation, failure to identify such non-climatic anthropogenic signals in tree ring records used for palaeoclimatology may lead to biased regression coefficients in attempts to model and reconstruct past climate variability from tree ring records.

Of the common industrial pollutants, sulphur dioxide (SO2) can have a significant impact on the isotopic composition of plant material and on the ring-width growth. It has been shown through growth chamber experiments and field studies that gaseous SO2 can cause closure of stomata and in higher concentrations leaf injury and inhibition of photosynthesis (e.g. Linzon, 1972, Martin et al., 1988, Savard et al., 2004, Savard et al., 2005). Several mechanisms have been proposed that could regulate stomatal aperture changes during exposure (Olszyk and Tibbitts, 1981). The lowering of the ratio between intercellular and ambient CO2 concentrations (ci/ca) as a consequence of stomatal closure produces less negative carbon isotope composition (δ13C) in a C3 plant (Francey and Farquhar, 1982) (Eq. (1))δ13Cplant=δ13Cair-x-(y-x)ci/ca,where δ13Cplant and δ13Cair are expressed as ‰ differences from the VPDB standard, x is the fractionation due to diffusion of CO2 into the leaves (4.4‰) and y is the fractionation due to carboxylation (27‰). The inverse relationship between δ13C and ci was experimentally demonstrated by Evans et al. (1986). Ziegler (1972) suggested that the mechanism by which SO2 interfered with photosynthesis was due to the potent and competitive inhibition of ribulose 1,5-biophosphate carboxylase with respect to HCO3 (Parry and Gutteridge, 1984). Inhibition of photosynthesis has an opposite impact on the ci/ca ratio than stomatal closure and thus leads to more negative δ13C values (Eq. (1)).

The response of a plant to changing environmental conditions can be studied using intrinsic water-use efficiency values, which can be calculated from values of δ13C from tree organic matter (Ehleringer, 1993, Waterhouse et al., 2004):IWUE=Agw=pa(1-pi/pa)1.6=ca(y-Δ13C)1.6(y-x),where A is the net photosynthesis, gw the leaf conductance to water vapour, pa the atmospheric partial pressure, pi internal gas phase pressure and Δ13C the isotopic discrimination between carbon of atmospheric CO2 and plant carbon.

The stomatal response to ambient SO2 concentrations also has an impact on the δ18O (and δ2H) values of plant material. The SO2 signal is imprinted in the leaf water as a result of decreased loss of the lighter isotopes owing to decreased transpiration (Dongmann et al., 1974). However, theoretical models predict more pronounced leaf water enrichment than is generally observed. This discrepancy increases as transpiration rates increase. The phenomenon is caused by the Péclet effect: a backward diffusion of H218O molecules collecting at the site of evaporation is opposed by the convection of isotopically lighter source water to the sites of evaporation (Flanagan et al., 1991, Barbour et al., 2000, Barbour et al., 2001). The impact of high ambient SO2 concentrations is expected to have a smaller effect on δ18O than on δ13C values, because the leaf water enrichment signal is further muted when an average of 42% of the produced sucrose oxygen are exchanged with xylem (source) water during cellulose synthesis in non-photosynthetic tissues (Hill et al., 1995, Roden et al., 2000).

Considering the strong industrial history of UK and other parts of Europe, SO2 could potentially influence tree δ13C and δ18O values more significantly than has been recognised. In the UK the main source of pollution until the 1950s was domestic and industrial fossil-fuel (coal) combustion (POST, 2002). Since 1970 there has been a substantial reduction of around 68% in the national SO2 emissions (Salway, 1998). Today rural Britain is largely free from SO2 concentrations or pollution events that are capable of causing damage to or reduced productivity in higher plants (NEGTAP, 2001).

We report that stable carbon isotopic analysis of tree ring series from southern England reveal co-occurring deviations and a weakening in their relationship with climatic parameters, which appears to reflect late 20th century trends in SO2 pollution. The nature of the signal and its influence on isotopic fractionation in broadleaved deciduous oak and coniferous pine trees is discussed and the wider implications of this study considered with specific reference to the application of the tree ring archive as a proxy for recent environmental change.

Section snippets

Site description

The two sites investigated in this study are located in southern England (Fig. 1). One species was analysed in each location: pedunculate oak (Quercus robur) in the Deer Park of Woburn Abbey Estate and Scots pine (Pinus sylvestris) in Windsor Great Park. The climate of this region is humid maritime temperate, being dominated by southwesterly depressions from the Atlantic during the winter and experiencing a more continental climate influence from continental Europe during the summer.

The total

Tree ring data

This study formed part of a larger-scale research project and network study (ISONET) for which there was a common sampling strategy to ensure comparability of results across the consortium (Treydte et al., 2007, Loader et al., 2008). Samples for dendrochronology and isotopic measurement were collected at breast height using 5 mm and 12 mm diameter increment borers. At Windsor, disc samples were also obtained from the forest as part of local management works.

In Woburn the four oldest trees, which

The δ13C and δ18O chronologies of woburn and windsor

Fig. 3 compares the carbon and oxygen chronologies from Woburn (1612–2003) and Windsor (1763–2003). δ-Values have been converted to z-scores to aid comparison. For Woburn normalisation is based on the years prior to 1850 to avoid potentially “non-baseline” conditions that could hamper identification of post industrial (late 20th century) disturbance trends. Owing to the relative shortness of the Windsor records the period used for normalisation was extended to 1899. Prior to 1785 the Windsor

Conclusions

The 20th century δ13C and δ18O records of Woburn and the δ13C record of Windsor exhibit trends that we attribute to SO2 pollution. This association was identified as a divergence in isotopic series and instrumental climate data and was more pronounced in carbon than oxygen isotopes. If undetected, such data could bias calibrations and limit their usefulness in palaeoclimatology. Since high ambient SO2 concentrations have been a rather common problem across Europe particularly before the

Acknowledgments

We would like to thank David Hardie for permission to sample the Woburn oak trees and Graham Sanderson for the help and access to the pine site in Windsor. Several people provided invaluable assistance in fieldwork. Thanks are due to Trevor Emmett, Debbie Hemming, Joanne Kelly, Iain Robertson, Paula Santillo and Jonathan Woodman-Ralph for coring and to Robert Evans for the soil descriptions. We would like to express our gratitude to Rashit Hantemirov in the Institute of Plant and Animal

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