Variations in bark acidity and conductivity over the trunk length of silver fir and Norway spruce

Abstract

 Before using bark to determine the degree of acidity in the environment due to atmospheric pollution, a better understanding of the internal factors which are involved in the evolution of the physico-chemical characteristics of bark is necessary. In this paper, variation of acidity and bark conductivity over the trunk length of silver fir and Norway spruce are examined. First, we show that it is more accurate to measure average bark pH and conductivity in each 2 m section of a tree by starting at the top than by starting at the base of the tree. A positive pH and conductivity gradient is observed over the trunk length; bark thickness decreases towards the top of the trunk. In the upper part of both silver fir and Norway spruce there are good correlations between acidity, conductivity, bark thickness and height, but in the lower part of the trees, silver fir and Norway spruce show contradictory behaviour. In silver fir, pH is linked with bark thickness and distance from the ground, while conductivity is not correlated with these two criteria. In Norway spruce we observe opposite results. It appears that conductivity measured on the external part of bark is directly influenced by the proximity of internal tissues, which are rich in ions, while acidity depends on the length of exposure to leaching due to water flowing down the trunk.