The impact of pine looper moth, Bupalus piniaria L. (Lepidoptera; Geometridae) on the growth of Scots pine in Tentsmuir Forest, Scotland

Abstract

Pine looper moth ( Bupalus piniaria) is an autumn defoliator of pines in Europe, but its impact on tree growth is not well established. Full stem analysis of 40 Scots pine trees felled close to their harvest date in Tentsmuir Forest, Scotland, was used to relate annual growth increments to cyclic fluctuations in B. piniaria densities as recorded by Forestry Commission pupal surveys. Between 1954 and 1989, five peaks in the moth population occurred at Tentsmuir, and radial, cross-sectional area (CA) and volume increments were reduced significantly 1–2 years following each peak. Height increments were reduced after a delay of 3 years. These reductions in increment occurred even though defoliation in the stand was light and B. piniaria densities never exceeded 33 pupae m −2, a density below that normally considered damaging. Temperature variables had some influence on radial, CA and volume increments, and rainfall had an effect on height growth, but the role of climatic factors was small and largely overridden by the greater effect of defoliation. Radial, CA and volume increments were reduced by 17–28% for 2–3 years after peak moth densities, but height growth showed only a small and variable response. Final diameter at breast height, CA, volume and height of the trees at felling were reduced on average by 3.1%, 4.6%, 5.6% and 1.5%, respectively. Wood production at the final crop spacing of 513 stems ha −1 was reduced by 28.3 m 3 ha −1, a loss which would have required a further 2.2 years of unaffected growth at the end of the rotation to have been replaced.