EFFECTS OF NITROGEN ADDITION ON THE STRESS SENSITIVITY OF CALLUNA-VULGARIS

The effects of low levels of nitrogen addition (7.7 and 15.4 kg N ha(- 1) yr(-1)) on plant sensitivity to biotic and abiotic stress were Stud ied at a lowland heath in the south of England that has received N tre atments since 1989. Larval growth rates and adult weights of heather b eetles were found to be significantly higher when insects were reared on plants that had received additional N, with implications for insect survival and reproductive success. Electrolyte leakage measurements f ailed to reveal any significant impact of N addition on plant sensitiv ity to frost episodes in early winter. In April, however, there was so me evidence of slightly decreased frost hardiness in plants receiving additional N. Accelerated spring bud burst also suggested earlier phys iological activity in N-treated plots. The rate of water loss from exc ised shoots of Calluna vulgaris (L.) Hull was significantly faster in plants receiving additional N, although no difference in plant water p otential was measured in the field after a prolonged dry spell. Whilst experimental N addition had only a small effect on plant sensitivity to abiotic stress, the relationship between enhanced deposition and in creased insect performance was clear, with the potential for substanti ally increased insect damage at deposition rates around the critical l oad contributing to the formation of gaps in the Calluna canopy.