Pine growth response to processionary moth defoliation across a 40-year chronosequence

Estimating the impact of pest insects on forest productivity requires a better understanding of host tree responses. While many studies have focused on juvenile trees, studying the impacts of defoliation on trees of increasing age helps to better characterize underlying mechanisms regulating growth responses to defoliation. During winter 2009–2010 a large outbreak of pine processionary moth occurred in Southwestern France. We established a field experiment to examine the effects of pine processionary moth defoliation varying from 25% to 100% on the growth of Pinus pinaster in stands ranging from 3 to 40 years old. Our results showed that pine processionary moth defoliation resulted in significant loss of radial growth for at least two years following defoliation. Stem growth loss in the first and second years was proportional to defoliation intensity and ranged from 32% to 93% in year 1, and from 17% to 68% in year 2. Stem growth was most reduced in older trees. Carbohydrates and nitrogen contents in needles and stem sapwood were also affected by defoliation. Our results suggest that defoliation affects stem growth through nitrogen and carbon resource limitation and that stem growth would be a lower priority sink for resources than other physiological processes in pine trees.