Strength properties and density of SO2 polluted spruce wood

Spruce wood from two sites in the East Ore mountains in Germany with more and less strong SO2 pollution during approximately 1935 and 1992 respectively, was studied as to its mechanical properties (fracture toughness, acoustic emission (AE)) as well as its density and annual ring width changes. Earlywood and latewood width and density were measured with X-ray densitometry equipment. The mechanical tests were performed with specimens taken from the same trees using the wedge splitting technique. Using notched, rectangularly shaped specimens, the experiments allowed to determine the notch-tensile strength sigma(NTS) Of each yearring in RL orientation separately by differentiating earlywood and latewood. The results show clear correlations between earlywood density and notch-tensile strength, good, inverse correlation to some extent between ring width and sigma(NTS) and also between AE emission activity and earlywood width for the less polluted trees. Data scattering with resulting less pronounced trends was found for all parameters measured for the more severely polluted trees. The results and correlations allow assuming that sigma(NTS) is influenced by the SO2 pollution to some extent via treering growth (ring width) and density. The results point to a more pronounced influence of SO2 pollution between approximately 1970 and 1985 than before that time, and a subsequent recovery of the tested wood properties in the trees that had survived this period.