Growth patterns at the southern range edge of Scots pine: Disentangling the effects of drought and defoliation by the pine processionary caterpillar

Abstract

Dendrochronology and climate-growth modelling were used to assess to what extent defoliation by pine processionary moth (Thaumetopoea pityocampa) depresses growth in Scots pine (Pinus sylvestris) at the species’ southernmost distribution limit (southern Spain). Basal area increment (BAI) was measured in severely defoliated and non-defoliated trees, for a period of 20years, a period that includes three severe droughts and two severe outbreaks of pine processionary moth. Linear mixed-effects models of BAI as a function of climate were run for non-defoliated trees and thereafter the fit of the model was tested separately both for defoliated and non-defoliated ones. Severely defoliated trees showed pronounced growth reductions during defoliation, although their cumulative basal area and drought-sensitivity were similar to those of non-defoliated trees. The results were not related to tree size or age, while non-climatic residuals were significantly related to the defoliation percentage. The combined effect of drought stress and processionary caterpillar defoliation caused short-term growth reduction in affected trees, although defoliated trees presented higher growth recovery after the defoliation, displaying similar average growth. We conclude that, in Scots pine, the growth reduction induced by processionary caterpillar defoliation can be overcome to a greater extent than the impact of drought.