A key initial criterion for UK UWMN site selection was that there should be minimal agricultural activity within the catchments, resulting in a focus on sites in moorland settings or where moorland had been afforested. The dominant use of the land across the network since the UK UWMN was set up has been rough grazing, predominantly by sheep, and forestry. During the last twenty years gradual change in grazing practices have become evident, with cattle introduced at a number of sites since monitoring began (e.g. Burnmoor Tarn, Dargall Lane, Scoat Tarn, Llyn Llagi and Round Loch of Glenhead). This appears to be a rather widespread change in conservation managed sites and the UK UWMN data should prove invaluable in future in assessing impacts on water quality. At the five afforested catchments, normal forestry management practices have continued, including felling and re-planting. Although changes in livestock stocking rates in grazed catchments can lead to an impact on surface waters, the impact of planting and removing conifers are likely to have been the most significant land-use changes experienced by UK UWMN catchments during the period of monitoring.
The decision to include a number of afforested vs non-afforested comparisons in the UK UWMN was based on results from previous work which demonstrated that afforested catchments were more vulnerable to acid deposition than those without forestry as a result of "scavenging" by the forest canopy (Mayer & Ullrich, 1977), increased uptake of base cations as nutrients (Miller, 1981) and reduced dilution of pollutants (Neal et al., 1986). Analysis of ten years of data from the afforested vs non-afforested comparison sites in the UK UWMN suggested that the former exhibit higher acid anion concentrations and are more acidic (Monteith & Evans, 2000). Subsequent analysis of the longer time-series (15 years) showed that whereas three of the five forested sites showed no significant trends in pH or alkalinity, all with the exception of the Afon Hafren showed a positive trend for AB-ANC (alkalinity based acid neutralising capacity) (Davies et al., 2005). With the changing age structure of the plantation forestry, it is likely that trends at these and other afforested sites will be complicated by the variation resulting from the growth and felling cycles that influence rates of pollutant interception and nutrient uptake particularly (e.g. see Tetzlaff et al., 2007). Harvesting results in a loss of base cations from the system as timber is removed from the catchment. However base cation uptake may also be increased by new planting and the net effect of these processes is difficult to predict (Davies et al., 2005).
There are a number of key questions associated with land use change and recovery:
- Are there significant differences in the extent to which chemical and biological recovery is occurring in afforested, as opposed to moorland catchments?
- Will new forest plans to decrease the amount of conifer planting in future lead to a more rapid recovery?
- Will future climate change, especially as a result of increased rainfall and sea-salt deposition, affect sites with afforested catchments differently from those with moorland catchments?