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 Level Network II site. The decline in 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 of 2003-2005.
The incidence of moth outbreak is still poorly understood, but winter temperature, the quality of the foliage and the occurrence of masting may all be influencing factors. There is evidence that nitrogen deposition can also promote insect defoliation (NEGTAP, 2001).
Intensive monitoring at the Grizedale plot since 1994 provides an important record through time of tree nutritional status, growth and crown condition that can be uniquely matched with levels of Tortrix infestation at the site on an annual basis.
- Determine if there are cause-effect relationships that are apparent between climatic factors and the outbreak of moth infestation.
- Assess the effect on tree growth of caterpillar attack, by using the annual record of tree diameter at breast height (DBH) measurement taken at the Level II plots, which give a gross measure of the retardation of tree growth. Since these attacks are reflected in the formation of the internal tree rings, it is possible to look back into the past by using dendrochronological techniques to determine how frequently these attacks have occurred in the recent past.
- Potentially link historical meteorological data to the identified years of insect attack.
Ten dominant trees were cored in the vicinity of the Grizedale Level II network plot. Cores were taken in two directions on each tree, approximately North and South, to a depth of 20 cm to sample recent growth increments (going back 50 years).
Funders and partners
This research is funded by the Forestry Commission.
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.
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