Impacts of pollutant inputs on forest canopy condition in the UK: possible role of P limitations

Summary To test the hypothesis that canopy thinning might result from phosphorus limitations induced by acidic pollutant deposition, paired stands with ‘thin’ and ‘good’ canopies were compared by examining (1) the phosphorus nutrition of the trees using a root bioassay and (2) availability of phosphorus in surface soil using a resin-extraction procedure. Three pairs of stands of each of Sitka spruce, Scots pine and beech on a range of comparable soils were investigated. A higher level of phosphorus stress in stands with the thinner canopy compared with their corresponding ‘partners’ with the better canopy was demonstrated for eight out of the nine pairs; only the pair of beech stands on gley soils did not comply with this finding. A similar pattern was demonstrated with the soil available phosphorus measurements, namely that surface soils had lower amounts of resin-extractable phosphorus under trees with the thinner canopy compared with their respective ‘partner’ stand with the better canopy; again the beech stands on gley soils did not comply. Analysis of variance indicated that there were significant and complex interactions between tree canopy condition, tree species and soils, in phosphorus stress and surface soil available phosphorus. These results derived from stands distributed over a wide area of northern Britain suggest that there may be a general link between canopy condition and phosphorus stress in trees. However, no link was found between phosphorus stress and estimates of net acidic deposition inputs derived from broad-scale modelling. The critical load for acidic deposition appeared, from these estimates, to be exceeded for all the stands, irrespective of canopy condition. The demonstration of a possible link between tree canopy condition and phosphorus stress suggests that there is a need for a wider investigation, possibly as an integrated part of the UK Tree Health monitoring programme. The hypothesized induction by acidic deposition of phosphorus stress in trees, if true, will require actual monitoring, rather than prediction, of that deposition at each study site.