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

The pH of 192 thin, even-aged twigs from 4 height levels of 12 randomly sel ected trees within a boreal Picea abies canopy naturally exposed to rainfal l 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 horizonta l variation between trees was also found, apparently due to small-scale soi l variations. The biomass of alectorioid lichens increased with increasing height above the ground to at least 12 m, a height interval with fairly con stant pH values. The uppermost twigs had an unusually high pH and an abnorm al species composition for P. abies, with dominance of the foliose Melaneli a exasperatula. The canopy hosted several cyanobacterial lichens, but these were scattered and had low biomass, restricted to lower branches of the tr ees with the highest bark pH. (C) 2001 The British Lichen Society.