Fig. 3. (a) Pearson correlation matrix of the main parameters potentially influencing the ¹³C of stem cellulose, where GI is growth index (df = 51, critical value = 0.35 at a confidence level of 99%). (b) Smelter feed along with particulate and SO₂ emissions. (c) Average ¹³C_VPDB results relative to the 1920–1921 value, and average summer precipitation through time at the test site. The term VPDB indicates the international standard which was used for reporting the relative abundances of ¹³C via the delta notation; VPDB is a carbonate provided by IAEA in Vienna (V) as a replacement of the former international standard Peedee Belemnite (PDB).