Recent strengthening of correlations between tree-ring δ13C and δ18O in mesic western China: Implications to climatic reconstruction and physiological responses
Introduction
High-resolution climatic reconstructions, calibrated and verified using meteorological measurements, are generally obtained from proxy climate information contained within tree-ring archives (e.g., ring width, density and stable isotopes) (McCarroll and Loader, 2004, Liang et al., 2008, Wang et al., 2010). Growth rings from trees in ecologically limited environments are highly sensitive climatic proxies, widely used to reconstruct past climatic parameters for periods prior to instrumental data. However, 20th century global warming, seems to have contributed instability in the climatic signal recorded in tree-ring archives as reported in many studies (Briffa et al., 1998, Cullen et al., 2008, D'Arrigo et al., 2008, Hilasvuori et al., 2009, Salzer et al., 2009, Esper et al., 2010, Ram, 2012), which is partly from the changes in the balance of energy and water over the region. If the climatic signals recorded in tree-ring parameters are non-stationary (Reynolds-Henne et al., 2007, Hilasvuori et al., 2009, Coppola et al., 2012, Ram, 2012) or there is potential shift during certain environmental conditions and this phenomenon remains a widespread feature of tree growth or isotope composition, this would limit the skill of tree-ring based climate reconstructions to properly estimate climatic variations during preinstrumental warm periods, such as the Medieval Warm Period (Lamb, 1965, Esper et al., 2002, Büntgen et al., 2012) and recent warming.
The vast forest resources of western China offer abundant opportunities to investigate paleoclimate using tree-ring analysis. Several temperature reconstructions have been obtained for different seasons in this region using tree-ring width and density data (Shao and Fan, 1999, Fan et al., 2008, Liang et al., 2008, Wang et al., 2010). Besides tree-ring width and density, stable isotope ratios measured in tree rings are another important source of climate information for the period before instrumental records became available in the region (Liu et al., 2011, An et al., 2012, Liu et al., 2012, An et al., 2013, Liu et al., 2013). Measurements of the stable isotopes of carbon (δ13C) and oxygen (δ18O) in tree rings are increasingly used to reconstruct climates of the recent past and their influence on physiological processes (Treydte et al., 2006, Gagen et al., 2011, Young et al., 2012). A number of authors (Scheidegger et al., 2000, Cullen et al., 2008, Roden and Farquhar, 2012) have pointed out that simultaneous measurements of both δ13C and δ18O in plant material may be particularly useful, as external factors influence both isotopes (Danis et al., 2006, Loader et al., 2008). Furthermore, numerous studies have successfully used simultaneous measurements of δ13C and δ18O in tree rings to investigate relative importance of stomatal limitations versus photosynthesis by inferring stomatal behavior (Scheidegger et al., 2000, Barbour et al., 2002, Cullen et al., 2008, Roden and Farquhar, 2012). Barbour and Farquhar (2000) suggest that the relative response of both δ13C and δ18O can be related to the sensitivity of a plant to evaporative conditions. Thus, a conceptual model (Scheidegger et al., 2000) was developed to constrain the interpretation of δ13C variation by measuring δ18O in the same material (the dual-isotope approach). Tree-ring δ13C records the balance between stomatal conductance and photosynthetic rate, dominated at dry sites by relative humidity and soil water status and at moist sites by summer irradiance and temperature (McCarroll and Loader, 2004, and the reference therein). Where the water stress is low, photosynthetic rate should be dominant, and annual δ13C values are likely correlated with climatic variables such as temperature, sunshine or cloud cover. At xeric locations and where trees suffer from moisture stress, stomatal conductance may play a more important role in carbon isotopic fractionation, leading to statistically significant relationships with antecedent or growing season moisture supply (Leavitt, 1994, Feng and Epstein, 1995, Gagen et al., 2004, McCarroll and Loader, 2004, Bale et al., 2011). Tree-ring δ18O records both meteoric source water that contains a temperature signal, and leaf transpiration controlled dominantly by vapor pressure deficit (McCarroll and Loader, 2004, Grieβinger et al., 2011, Liu et al., 2013). Therefore, δ18O is often negatively correlated with stomatal conductance (gs), because changes in gs and transpiration with humidity alter leaf temperature and evaporation enrichment (Barbour and Farquhar, 2000). Variable exchange with xylem (source) water during wood synthesis determines the relative contribution of the source water and leaf enrichment signals in tree-ring δ18O (Roden et al., 2000, Gessler et al., 2009, Offermann et al., 2011).
Here, we used a dendrochronological (tree-ring δ13C) approach, combined with our recently developed δ18O series (controlled mainly by relative humidity) (see An et al., 2013), to interpret temporal correlations between δ13C and δ18O in tree rings of fir (Abies georgei) growing in the typical wetter environment of western China. Our objective was to determine whether control of photosynthesis in fir varies through time in response to changes in climate over the past. There has been a regional-scale warming/drying trend in the past 150 years over this broad region, where the climate is mainly influenced by the strength of Indian Summer Monsoon (Grieβinger et al., 2011, Zhu et al., 2011, Xu et al., 2012, An et al., 2013, Liu et al., 2013). We hypothesized that relative stomatal control of photosynthesis, as indicated by a positive correlation between δ13C and δ18O in tree rings, is enhanced during drier periods in the normally moist environment. Conversely, during wetter periods, stomatal conductance will be high and photosynthetic capacity more limiting, so the two isotopes will be either negatively related or show no relationship (Scheidegger et al., 2000).
Section snippets
Location and methodology
Sampling was carried out on the Batang–Litang Plateau, in western China's Sichuan Province (An et al., 2013). The region belongs to the eastern part of the Qinghai–Tibetan Plateau, and experiences a typical plateau climate, which is characterized by a cold and dry winter and a warm and wet summer. Meteorological data from the Batang and Litang stations indicate that during the instrumental period (1960–2009), mean monthly temperatures range from − 0.6 °C in January to 15.7 °C in July, with an
Tree-ring δ13C
The long-term mean δ13C value over its entire interval 1750–2009 was ca. − 21.8‰, with a standard deviation of 0.5‰ (Fig. 2a). The δ13C values ranged from − 23.7‰ to − 20.9‰. It appears that after 1970, tree-ring δ13C began decreasing, approximately matching the rate of decline of atmospheric δ13C in CO2. The corrected δ13Cpin values are slightly higher than those of δ13Ccor (Fig. 2b). The climatic-response patterns of δ13Cpin and δ13Ccor are rather similar, and the signal strength recorded in δ13C
Climatic response of δ13C and δ18O over the instrumental period
Tree-ring δ13C and δ18O are now widely used in dendrochronological studies and provide information on trends in temperature, rainfall and relative humidity, as well as the effects of climate on ecophysiological processes (Barbour et al., 2002, Liu et al., 2013). In the period with observed meteorological data, both tree-ring δ13C and δ18O showed negative correlations to moisture parameters and positive correlations to temperature (Fig. 3; An et al., 2013), even though there are differences in
Conclusions
Our findings identify a shift in correlation of δ13C–δ18O in tree rings under the background of global warming in western China, with the climatic-response pattern during recent decades being similar. Moisture conditions dominantly influence both δ13C and δ18O, which are seen in the significant negative correlation with precipitation and relative humidity in growth season. Comparison of reconstructed relative humidity deduced by δ13C and δ18O in tree rings, respectively, reveals that in the
Acknowledgments
This research was supported by the National Natural Science Foundation of China (41121001, 41171167), by the self-determination project of the State Key Laboratory of Cryospheric Sciences (SKLCS-ZZ-2013-01-03) and the Knowledge Innovation Project of the Chinese Academy of Sciences (KZCX2-YW-QN308). We gratefully acknowledge the journal's three anonymous reviewers for their constructive comments on earlier versions of this manuscript.
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