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

To test the hypothesis that canopy thinning might result from phosphorus li mitations induced by acidic pollutant deposition, paired stands with 'thin' and 'good' canopies were compared by examining (1) the phosphorus nutritio n 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 e ach of Sitka spruce, Scots pine and beech on a range of comparable soils we re investigated. A higher level of phosphorus stress in stands with the thi nner canopy compared with their corresponding 'partners' with the better ca nopy was demonstrated for eight out of the nine pairs; only the pair of bee ch stands on gley soils did not comply with this finding. A similar pattern was demonstrated with the soil available phosphorus measurements, namely t hat surface soils had lower amounts of resin-extractable phosphorus under t rees with the thinner canopy compared with their respective 'partner' stand with the better canopy; again the beech stands on gley soils did not compl y. Analysis of variance indicated that there were significant and complex i nteractions between tree canopy condition, tree species and soils, in phosp horus stress and surface soil available phosphorus. These results derived f rom stands distributed over a wide area of northern Britain suggest that th ere 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 cri tical load for acidic deposition appeared, from these estimates, to be exce eded for all the stands, irrespective of canopy condition. The demonstratio n of a possible link between tree canopy condition and phosphorus stress su ggests that there is a need for a wider investigation, possibly as an integ rated part of the UK Tree Health monitoring programme. The hypothesized ind uction by acidic deposition of phosphorus stress in trees, if true, will re quire actual monitoring, rather than prediction, of that deposition at each study site.