The impact of green spruce aphid, Elatobium abietinum (Walker), and root aphids on the growth of young Sitka spruce in Hafren Forest, Wales: effects on height, diameter and volume

In 1992 the UK Forestry Commission established a field experiment in Hafren Forest, Wales, to determine the impact of green spruce aphid ( Elatobium abietinum) and root-aphids ( Pachypappa spp.) on the growth of young, 3–5 year-old Sitka spruce. Four experimental treatments were created by applying insecticides to maintain or control E. abietinum and root-aphids, together or separately, on replicated plots of trees. A fifth treatment of high E. abietinum densities was produced by infesting trees with aphids artificially in spring 1994. Aphid populations and tree growth were monitored closely from September 1993 until 1997. Root-aphids at the low, but typical densities present had no detectable effect on growth. Natural populations of E. abietinum also were low during the study period, but numbers on the artificially infested trees reached densities equivalent to a moderate–severe outbreak by June 1994 before declining (Straw, N.A., Fielding, N.F., Green, G., Coggan, A., 1998. The impact of green spruce aphid, Elatobium abietinum (Walker), on the growth of young Sitka spruce in Hafren Forest, Wales: pattern of defoliation and effect on shoot growth. For. Ecol. Mgmt. 104, 209–225). The high densities of E. abietinum caused 38.5% defoliation of older needles and reduced height increment (leader length) by 22.4% in 1994 ( p < 0.001), 11.5% in 1995 ( p < 0.01) and by about 6% in 1996. Stem diameter and volume increments were not affected in 1994, but were reduced by 12.2% and 23.8% , respectively, in 1995. No effect was apparent in 1996. Overall differences between treatments in diameter and volume increment were not significant on account of smaller sample sizes and greater variability in parameter estimates, but regressions of increment against peak aphid densities recorded in 1994 were significant ( p < 0.05) and confirmed the association of E. abietinum with these growth parameters. The reductions in increment observed could not be explained as a consequence of defoliation, but appeared to be caused by an effect of E. abietinum on shoot extension at the time of infestation and a related influence on the formation of winter buds. The disruption of bud-set appears to reduce the biomass and/or quality of foliage produced in the following year and, as a result, the supply of current photosynthate available for stem diameter and volume growth.