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CCRG Undergraduate Dissertation Prize 2012 awarded to: Christopher Jones, University of Leeds



Physiological response of Cedrela salvadorensis to variations in climate and increasing atmospheric carbon dioxide concentration – A dendrochronological study with analysis of δ13C isotope ratios



Stable carbon isotopes (δ13C) in tree rings can be used to measure the physiological response of trees to variations in climate, tree age and atmospheric carbon dioxide concentration (ca). Since the industrial revolution the rapidly increasing atmospheric carbon dioxide concentration has impacted plant physiology through a reduction in stomatal conductance. This has resulted in an increasing intrinsic water use efficiency (Wi) trend. Dendrochronological techniques were used to isolate annual tree ring cellulose for analysing the stable carbon isotopes (δ13C) of Cedrela salvadorensis, a tropical dry forest tree species from Nizanda, Southern Mexico (1639′N, 9500′W). Intrinsic water use efficiency (Wi) and intercellular carbon dioxide (ci) were calculated from δ13C values to represent both long term trends and inter-annual variation, respectively. Results showed extreme increases in Wi over the past 118 years by 533%. When compared to an additional dry site, a moist site and two wet sites the factor driving the gradient of the Wi trend could be attributed to drought stress; because there is a linear relationship between the gradient of the Wi regression line and mean annual precipitation, with significantly different regression line gradients observed. A strong correlation was observed with ci and total annual cloud cover (0.54), and moderate correlations with mean annual minimum temperature (0.45), total annual number of days with thunderstorms (0.43) and total annual precipitation (0.35). The results demonstrate a complex relationship between strong annual variations in climate and intercellular carbon dioxide, with evidence suggesting that the increasing atmospheric carbon dioxide concentration is partially driving long term trends in intrinsic water use efficiency, and drought stress determining the gradient of this trend.


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