A notable deterioration in mean tree crown density (from 72% in 1995 to 45% in 2005) has been observed in the oak trees at the Grizedale Intensive Forest Monitoring site. The decline in crown density is associated with severe defoliation by Tortrix viridana (oak leaf roller moth) and Operophtera brumata (winter moth) caterpillars. This is indicated by a significant reduction in the biomass of leaf litter compared with a pest-free year, and the amount of frass and number of pupae recorded in litterfall analysis. There is evidence that nitrogen deposition can promote insect defoliation (NEGTAP, 2001), although other influences on population numbers, including climate, cannot be ruled out.
A similar deterioration in crown density has been observed (from 75% in 1995 to 57% in 2005) at the Scots pine Intensive Forest Monitoring site (Lady Bower) in the historically polluted Pennines. Although acid deposition has fallen significantly at this site, indicators of the acidity and base status remain critical and could explain this trend.
In contrast, a modest increase has been observed in crown density at another intensively monitored Scots pine plot, at Thetford (see Tree nutrition).
Other sites show no significant changes in crown density, highlighting the importance of local factors rather than a widespread change in forest condition.
NEGTAP (2001).Transboundary air pollution: acidification, eutrophication and ground-level ozone in the UK. Report of the National Group on Transboundary Air Pollution. Defra, London.
Hendry, S.J., Boswell, R.C., Proudfoot, J.C. (2004). Forest Condition 2003 (PDF-300K) . Foresty Commission Information Note 62.