The vertical gradient of bark pH of twigs and macrolichens in a Picea abies canopy not affected by acid rain

Abstract

The pH of 192 thin, even-aged twigs from 4 height levels of 12 randomly selected trees within a boreal Picea abies canopy naturally exposed to rainfall with a high pH (>5·2) was measured. The largest variation in bark pH was due to the height above the ground. However, a highly significant horizontal variation between trees was also found, apparently due to small-scale soil variations. The biomass of alectorioid lichens increased with increasing height above die ground to at least 12 m, a height interval with fairly constant pH values. The uppermost twigs had an unusually high pH and an abnormal species composition for P. abies, with dominance of the foliose Melanelia exasperatula. The canopy hosted several cyanobacterial lichens, but these were scattered and had low biomass, restricted to lower branches of the trees with the highest bark pH.