THE RELATIONSHIP BETWEEN CROWN CONDITION AND SOIL SOLUTION CHEMISTRY IN OAK AND SITKA SPRUCE IN ENGLAND AND WALES

In recent years the critical loads concept has become the primary appr oach for formulating research on pollutant impacts and setting targets for pollution abatements. For forest soils critical loads have been s et on the assumption that base cation uptake and tree growth an decrea sed when ratios of Ca + Mg to Al in soil solution fall below 1.0. Fore sts have been monitored in the UK since 1984 under EU Air Pollution Re gulations and the UN-ECE Convention on Long Range Transboundary Air Po llution (LRTAP) in order to identify any pollutant-related decline in forest condition. The soil solution chemistry of six oak and six Sitka spruce plots from these surveys were analysed in the work reported he re. (Ca + Mg)/Al was found to be above 1.0 at all sites in the organic and mineral soil horizons. Large ratios resulted from both large base cation concentrations and generally low Al values. Surprisingly, poor est tree condition (crown density) was associated with large Ca + Mg c oncentrations and large (Ca + Mg)/Al for spruce. This correlation cont rasts in direction with the relationships between (Ca + Mg)/Al and tre e growth which have been used to calculate forest soil critical loads. However, UK values of this ratio lay entirely above the threshold for damage.